Wireless... · EXperi ent Wireless A JOURNAL OF RADIO RESEARCH AND PROGRESS VOL. I, No. 3....

EXperi entWireless

A JOURNAL OF RADIO RESEARCH AND PROGRESS

VOL. I, No. 3. DECEMBER, 1923. is. NET.

Experimental Topics.Wireless Television.

/N the course of an address given beforethe Royal Society of Arts last monthM. Edouard Belin disclosed the fact

that he had recently succeeded in trans-mitting real photographs in half -tones bywireless. M. Belin has been making ex-tended researches in his laboratory atLa Malmaison, and he is convinced that thesolution of the problem of wireless televisionis near at hand. The Belin system of tele-photography had been developed not onlyto permit of the transmission of ordinaryhandwriting and of shorthand, but also toprovide absolute secrecy in telegraphic andradiotelegraphic transmissions. Autographmessages have been transmitted by wirelessboth in France and America. This is afascinating field of research, and it does notrequire much imagination to visualise theenormous possibilities of wireless televisionwhen achieved on a commercial scale.

The Wireless Exhibition.The All -British Wireless Exhibition at

Shepherd's Bush which has formed thecentre of radio interest during the middlefortnight of November came exactly at theright moment. The Postmaster -General, byhis recent licensing decisions had cleared theway for a revival of business, and a largesection of the public, fortified by the know-ledge that they had made their peace withthe law, or could do so quite easily at thenearest post -office, flocked to see the latest

developments in British wireless practice.In one respect the crowd was very differentfrom that which thronged last year's Exhibi-tion at the Royal Horticultural Hall. Itwas a much better informed crowd. Lastyear the attendants at every stand in theshow were bombarded with requests forexplanations of how " it was done." Thisyear the crowd came armed with knowledge :questions of an elementary nature weremuch less in evidence, and reasoned praiseor criticism of construction and design muchmore general. This is all for the good ofwireless, and the Exhibition itself hasundoubtedly still further advanced theeducation of the radio public. The Exhibi-tion, taken as a whole, was a very repre-sentative display, though broadcasting inter-ests certainly predominated and somevery fine examples of complete receivinginstallations 'roved one critic to describeit as being more of a cabinet-making than ascientific exhibition. This is not altogetheran unfair comment for the time is withinsight when every well-appointed home willhave a receiving equipment en suite with therest of the furniture, just as it now includesa book -case, or a sideboard, or a piano.From the experimenter's point of view theoutstanding feature of interest was thedisplay of dull -emitter valves by variousmakers, although there were many firmsshowing other components and accessoriesworthy of close attention. Elsewhere inthis issue we deal in some detail with the

2

December, 1923.

more interesting items which came to ournotice ; it is outside our province to attempta complete description of all the exhibits,and we have contented ourselves withcommenting on those matters of especialinterest to experimenters. We would addour congratulations to the National Asso-ciation of Radio Manufacturers, and toMr. Bertram Day, on the success of theExhibition, and hope that it has given tothe -wireless trade in general an impetuswhich was very badly needed.

The Transatlantic Tests.This year the Transatlantic Tests organised

in conjunction with the American RadioRelay League are due to commence onDecember 22. Although many experimentersoccupy their time with Transatlantic workthroughout the greater part of the winter,the official tests always arouse considerableinterest amongst amateurs as a whole, andit is anticipated that the number of listenerson this side will be greatly increased by manynewcomers in the field of experimental work.To those we make a special appeal. Theradiating properties of a receiving aerialon 200 metres are very good, and unlessparticular care is exercised in the use ofoscillating circuits, it is highly probablethat the work of an experienced experi-menter may be completely spoiled by radia-tion from a neighbouring aerial. In thesubsequent pages of this issue will be foundseveral articles particularly relating to short-wave reception, and it is sincerely to behoped that many will adopt the sugges-tions given, to the benefit of all concerned.So far as our own transmission is con-cerned, the -granting of. no and I,000 -wattlicences to certain experimenters certainlyincreases our chances of " getting acrossthe pond," but the ultimate success of theexperiment is, unfortunately, wholly depen-dant upon the prevailing conditions duringthe period of the tests.

Very Variable Condensers.We publish in our correspondence columns

this month a letter from a reader whocomplains of the unsatisfactory state ofaffairs in the trade in regard to the capacityrating of variable condensers. While it istrue that there are firms to whom ourcorrespondent's criticisms do not apply, wethink it is equally true that there are good

z8 EXPERIMENTAL WIRELESS.

grounds for complaint in other directionsIt is easy to see the harm which may becaused to the wireless industry by the saleof unreliable or of mis-represented goods ;the disappointed constructor or experimentermay prove to be a very bad advertisementfor wireless in general, and for unreliablefirms in particular. Possibly the troublecomplained about arises in some cases fromwant of thought, or from the lack of realtechnical knowledge, on the part of thevendor. A good many people have rushedinto the wireless business with only a slightsmattering of the science underlying theapparatus they sell, and they do not alto-gether appreciate the need for somethingmore than approximate accuracy in theirinstruments or in their descriptive matter.Day by day wireless is becoming more andmore of an exact science, 'and indeed onehas only to move among the best firms inthe trade to realise how much true scientificknowledge and research is being appliedto the advancement of the industry. Itis important that the lesser firms too shouldrealise the need of putting their products ona sound basis of scientific accuracy, and theimportance of making impossible such criti-cism as our correspondent expresses in theletter we have referred to.

Our Progress Abroad.We are glad to be able to report that

EXPERIMENTAL WIRELESS iS making friendswith wireless workers overseas just as readilyas at home. Although the time for thereceipt of return mails has in many casesbarely elapsed, we have already receivedsubscriptions and letters of congratulationfrom experimenters in the United States,Brazil, South Africa, Switzerland, France,Portugal, Austria, Belgium, Holland, Ger-many, and Egypt. Some of our corres-pondents have promised us notes on ex-perimental equipment and methods in theirown countries, and this friendly co-operationcoupled with other arrangements for over-seas information we have put in hand willensure for our readers that internationalexchange of experience and knowledge whichis essential for the advance of every branchof science. Our readers may be interestedto know that one letter we received duringthe past few days from Germany carriedtwenty-one postage stamps, each of the valueof two thousand million marks

EXPERIMENTAL WIRELESS. 219 December, 1923.

Directive Radio Telegraphy and Telephony.BY R. L. SMITH -ROSE,' Ph.D., M.Sc., D.I.C., A.M.I.E.E.

During the last few years there has been considerable development in directionalwork, particularly with the use of extra short waves. There are obviously manyapplications of directional transmission, and we are giving below a general sum-mary of modern methods and practice.

I.-THE EMPLOYMENT OF SHORT WAVES UP TO ABOUT20 METRES IN LENGTH.

(a.) !'Early Experiments of Hertz andMarconi.

THE classicalcarried outnow known

tions of Maxwell'sThese experiments

experiments of Hertzfrom 1885 onwards areas verifying the predic-electro-magnetic theory.of Hertz* demonstrated

Fig. 1.-An early short wave directional transmitter.

the essential properties of the waves whichare created by the electrical discharge of acondenser, these properties being exactlysimilar to those of the much shorter wavescommonly associated under the terms lightand heat. The means for the production ofthe waves was found in the type of openoscillator developed by Hertz, a similaropen oscillator or a nearly closed ring resona-tor being used for reception purposes. Thewaves generated were shown to penetratethrough bodies which are generally classedas dielectrics, and to be subject to refractionin such transmission, the angle of refractionbeing found to agree with that calculatedfrom the usual optical formula, using theappropriate values for the refractive indicesof the media traversed. Other bodies classedas conductors were shown to be opaque to

* H. Hertz, " Electric Waves," translated byD. E. Jones, 1900, p. 172.

the radiation, this being completely reflectedfrom metallic surfaces. The superpositionof the reflected waves from a metallic surfaceon the incident waves was shown to give riseto a set of stationary waves, which provedthe finite velocity of travel of the waves andalso provided a means of determining thelength of the waves in air. The wave-lengths used by Hertz and other workersat that time were of the order of 1 or 2metres down to a few centimetres.

By arranging the reflector in parabolicform, with its axis parallel to the oscillatoremployed, the radiation could be concen-trated in a roughly parallel beam for pro-jection in any desired direction, and a similarreflector at the receiving end served toconcentrate the radiation on the detectinginstrument placed at the focus. The radia-tion so produced was shown to be planepolarised with the electric force parallel tothe oscillator, and experiments were carriedout by Hertz with wire grid screens whichare exactly analogous to the transmissionand absorption of polarised light by Nicolprisms.

Many scientific workers were immediatelyattracted to Hertz's discoveries, but themajority of these were chiefly concernedwith the researches opened up by thedemonstration of the electro-magnetic natureof luminous radiation and the correspondingdevelopment of optical theories. Otherworkers concentrated their attention onthe production of a more efficient generatorof the waves, and a more sensitive andreliable detector of the oscillations producedat the receiving end. Confining our atten-tion, however, to directive radiation, itappears that Marconi was the next to makeany use of reflectors for the concentrationof waves in the required direction in about

2*

December, 1923.

49

Fig. 2.-An arrangement comparable with Fig. 1, adapted forrecept on.

1895*. The general plan of the transmittingand receiving apparatus is shown by Figs.and 2. An induction coil c (Fig. 1)was used to charge the spheres d, e ofa Righi oscillator placed in the focal line ofa suitable cylindrical parabolic reflector,f. On pressing the key b an oscil-lating discharge occurs across the system ofspheres which thus become the source ofradiated waves. At the receiving end, twoshort conducting strips k are placed inthe focal line of a similar reflector f,connected to a filings coherer j, whichacting as a detector on the arrival of wavescaused the operation of the relay it andthe recorder h. The length of the copperstrips k was carefully determined to bein tune with the transmitted waves.

By means of apparatus of this descriptionMarconi was able to transmit intelligiblesignals over distances up to i miles, andthe concentration of the beam was such that,at this distance, its effective width in whichthe receiver could be operated was onlyRio feet. Marconi immediately appreciatedmost of the advantages to be derived fromthe directional selectivity accompanyingthis arrangement and the possible applica-tion of a rotating wireless beacon as an aidto ship navigation in a similar manner tothe use of lighthouses. Owing, however,to the greatly increased signalling rangewhich was obtainable by earthing one sideof the Hertzian oscillator and extending theother as a long vertical wire up to Too feetin height, further experiments on the use ofthe reflecting system were discontinued.

* G. Marconi, " Wireless Telegraphy," JournalI.E.E., 1899, Vol. 28, p. 273 ; G. Marconi, " RadioTelegraphy," Proc. Inst. Radio Eng., 1922, Vol. 10,p. 215.

120 EXPERIMENTAL WIRELESS._

At that time the development of the subjectwas being concentrated upon its applicationto communication over the greatest dis-tances possible, and with the generatingapparatus then available this was foundto be most easily accomplished by the use ofelevated antenme at both transmitting andreceiving ends. Now the application ofoptical principles shows that in order thatthe parabolic metal sheet should act as areflector its length must be equal to severaltimes the length of waves employed and thefocal distance should be not less than one-quarter of a wave -length. Hence, in orderthat the reflector system shall be practicable,particularly if the reflector is to be used togive a revolving beam, the length of wavesemployed must be limited to a few metres,

Fig. 3.-A short wave generator in which the inductances takethe form of single turns, inductively coupled to the aerialsystem

whereas those emitted by the elevatedantennw ranged from 5o metres upwards.Research on the utilisation of the waves forthe continued extension of the range for


transmitting purposes led to the use ofincreasingly longer waves, up to the present -.day limits of about zo kilometre, which,.apart from being subject to less absorptionby the atmosphere and surface of the earth,were much more easily generated in thehigher powers necessarily involved. Alsothe first practical application of wireless,namely, for communication to and betweenships, demanded " all round " transmissionrather than a directional system. Thisaspect of the reflector system was apparentlyso discouraging that, except for the filingof one or two patents covering the use ofseparate vertical wires in place of sheetmetal for the reflector, no research workof any description was carried out on shortwave propagation between 1896 and 1916.

(b.) Recent Work of Marconi andFranklin.

In the latter year experimental work inthis direction was resumed by Senatore Mar-coni, assisted by Mr. C. S. Franklin,* thewaves employed being 2 or 3 metres inlength. Preliminary experiments showedthat on these short wave -lengths one of thegreat difficulties in modern wireless tele-graphy, viz., interference, is very greatlyreduced. The only source of interferenceapparent was that due to the ignition sys-tems of motor cars and motor boats in thevicinity, which emit waves of from i to 4ometres in length. Similar difficulties wereexperienced at the introduction of wirelessreception on aeroplanes, and the remedythere adopted was to efficiently screen thewhole of the ignition system.

The early experiments showed that theuse of reflectors considerably increased theworking range of the apparatus for commu-nication purposes, and good directive proper-ties were also obtained with reflectorsproperly proportioned to the wave -length.With the waves of 3 metres length used insome experiments at Carnarvon the attenua-tion was found to be very great even oversea, and the range of transmission was cor-respondingly limited to about 4 to 6 miles.The strength of the electric field, however,increases with the height above the groundat a rate dependent on the wave -length, and,

* C. S. Franklin. " Short -Wave DirectionalWireless Telegraphy," Journal LE.E., 1922, Vol. 6,p. 930.

although with the long waves commonlyemployed this is not perceptible, the increaseis very rapid with short waves. Franklinfound, for example, that by raising trans-mitter and receiver to a height of aboutJo wave -lengths above the interveningground the range could be increased to sixor seven times its former value.

The usual arrangement of these directionaltransmitters is in the form of a single Hertzianrod placed vertically in the focal line of acylindrical paraboloid reflector. The rod iscoupled by a single turn loop at its centreto an oscillating valve circuit, which while

10:

Fig. 4.-illustrating the menthodF of using the oscillator shownIig. 8.

of quite the usual form has very smallelectrical constants for the extremely highfrequency required. Quite frequently theinterelectrode capacity of the valve is

sufficient for a circuit whose inductancecomprises a single turn loop of a few inches.Such a generator is shown in Figs. 3 and 4,taken from a Bureau of Standards paper* in

* F. W. Dunmore and F. H. Engel, " DirectiveTransmission on a Wave -length of 10 Metres,"Bureau of Standards Scientific Paper No. 469, 1923.

December, 1923. 122 EXPERIMENTAL WIRELESS.which sufficient details are given to enableconstruction to be carried out. The reflectoris formed of a framework supporting anumber of straight vertical wires around itssurface (Fig. 5). The length of the individualwires is adjusted to bring them in resonancewith the wave -length in use, the length ofthe wires being usually somewhat less thanhalf a wave -length owing to the mutualcapacity between them. The effect ofalteration in the length of wires on the polarradiation curves is shown in Figs. 6 and 7.

Fig. 5.-A parabolic reflector for a wavelength of 10 metres.

In the case of Fig. 6 all the wires have beencarefully adjusted in length to be in tunewith the waves. In Fig. 7 alternate wireshave been removed from the screen, andowing to the resulting decrease in mutualcapacity the remaining wires are nowslightly out of tune and some radiation isnow seen to be transmitted behind thescreen.

For reception, either a loop or another rodoscillator may be employed suitably adjustedto the wave -length, and of course with thestraight rod a parabolic reflector may againbe used to concentrate the received energyon the rod.

(c.) Application of Reflector Systems toCommunication.

One of the uses to which this directionaltransmission has been put experimentallyby the engineers of the Marconi Companyis that of radio telephony. Since in theemployment of a rod oscillator with reflectora fairly concentrated beam of radiation isproduced, it is evident that no wirelessreceiver placed outside the beam will receiveany signals whatever. Within limits the" spread " of the beam can be reduced byincreasing the aperture of the reflector asshown in Fig. 8. This, combined with thegreat difficulty of tuning a receiver to sucha short wave -length when this is not accu-rately known, makes the system very muchmore secret for telephonic purposes than theordinary broadcast mode of transmission.Tests carried out over both sea and land showthis short-wave reflector system of radiotelephony to be quite practicable at distancesup to about ioo miles on a wave -length of15 metres. In some of Franklin's experi-ments carried out between Hendon andBirmingham in 1921, local measurementsof the polar curves taken round the stationshow that the electric field is increased aboutfour times and that the same order of increaseis obtained during reception. The increaseof energy due to each reflector will, therefore,be sixteen times, giving a total increase of256 with reflectors at each end. Thiscalculated figure agrees well with actualmeasurements made with the reflectorsboth up and down. By using two medium-size power valves in parallel receiving aboutboo watts input, about 300 watts radiatedpower could be obtained. To get the samesignal strength without the use of reflectorswould require a 14o -kw. transmitter workingat the same efficiency. Although the fre-quency of 20 million cycles per second givesrise to some problems in the structure andoperation of the valves employed, thereappears to be no reason why input powers ofseveral kilowatts may not be used in sucha system.

(d.) The Revolving Wireless Beacon.Reverting now to the directional properties

of the reflector systems, the general idea isto use a revolving beam transmitter as akind of wireless lighthouse. The beam willbe invisible, but if a suitable receiver be


Fig. 6.-The Radiation Characteristic Curve of a parabolic reflector when the aperture is equal to one wave -lengthThe deflecting wires are all in tone with the source.

Fig. 7.-The Radiation Characteristic Curve of a parabolic reflector when the aperturelis equal to/one wave -length(10 metres), but with every other reflecting wire removed, the remaining twenty wires being detuned.

Dec nilbor, 1923.

10°

15°

0.28 Metre WaveAperture 2.57 Waves:

124

5-34 IletreWaveAperture 1.99 Waves:

EXPERIMENTAL WIRELESS.

614' Pfere WaveAperture 1.83 Waves:

Fig. 8.-Polar Curves of the Inchkeith reflector, measured at a distance of 4 miles from the transmitter.

used at a distance no signals will be hearduntil the beam flashes past the receiver inthe course of its rotation.step in this direction has been the erectionof such a beam transmitter at InchkeithIsland in the Firth of Forth, while a secondis in course of erection at the South Fore-land.* At Inchkeith the rod oscillatoremits a wave of about 6 metres length, thisbeing concentrated into a beam by a wireparabolic reflector. The whole system makesa complete revolution about once in twominutes. As the beam rotates distinctiveMorse signals are sent out as shown in thecompass diagram in Fig. 9. To each secondpoint of the compass is allocated a specialletter, and the intervening points and half-points are marked as T and I respectively.As the beam flashes past the receiver thesignal strength rapidly increases to a maxi-mum and then diminishes, and if the suc-cession of Morse signals received during thistime is read the direction of the transmittercan be obtained by reference to the diagram.The direction is that of maximum signalstrength or midway between the commence-ment and cessation of the signal heard.For example, when listening at the receiver

* J. A. Slee, " Recent Developments in theApplication of Wireless Telegraphy to Shipping,"Engineering, 1923, Vol. 116, p. 410.

the succession of signals IKITI wouldindicate a direction of half a point east ofN.E. Confirmation of the can beobtained on another rotation of the beamat an interval of two minutes. The receiveron the ship consists merely of a half -waverod, erected at the end of the bridge andconnected up with a detector and valveamplifier to telephones or a loud speaker.With the receiver is provided a disc, markedas in Fig. 9, into which pegs may be insertedat the points corresponding to the commence-ment and end of the signal, and the bearingof the transmitter is thus obtained as mid-way between the pegs to an accuracy of aquarter point or about 2.8 degrees. Whentwo or more beam transmitters are put intouse the same characteristic signals may be,retained for the two -point positions, but theintervening points may be characterisedby some letter other than T as at Inchkeith,so that the train of signals ITI or, say, ISIwill identify the transmitters, the letter Iindicating half -points as before.

It will be appreciated that this systemis fairly straightforward in use, and witha slight knowledge of the Morse code anynon -wireless officer can operate it from thereceiver -end. The method is independentof the time of revolution of the beam andso accurate timing and the use of a stop-


watch are obviated. The working rangeof Inchkeith transmitter is about 10 miles,and as this is in daily use it will be inter-esting to watch for the results obtainedfrom the point of view of its utility to navi-gation. Judging by experience in the propa-gation of wireless waves on wave -lengthsof from 30o metres upwards, it is probablethat the system will be entirely independentof prevailing weather conditions, and itshould, therefore, be particularly useful infoggy weather when other lighthouses areinvisible. In propagation over sea it is alsoto be expected that no other serious errorswill be encountered, but when land inter-venes the possibility of very serious errorsdue to reflection from cliffs and hills orrefraction in crossing the boundary betweenland and sea is by no means remote. Infact, it has been suggested that the locationof the reflected beam arising from a short-wave transmitter might provide a useful

Fig. 9.-Compass bearings with letter designations for directionfinding.

means of locating a near -by ship or evenicebergs in the dark or thick fog.

The Sheffield Relay Station.The Sheffield relay station was officially

opened in the middle of November, and,no doubt, some details will be of in-terest to many readers. The circuits

'VVVVV's

employed arestraightforward.control system

11-7

ployed, the schematic diagram of whichis shown in Fig. i.

The present aerial consists of a four -wirecage about 120 feet high at the distant end ;

00

-

Fig. 1-Illustrating the fundamental

exceedingly simple andA perfectly normal chokeof modulations is em-

Land Zino or.771cro phone

elreult of the Sheffield Relay Station.

the earth screen consists of the iron frame-work of the building over which it is sus-pended. The set is rated at ioo watts.

De her, 1923. 126 EXPERIMENTAL WIRELESS.

Simultaneous Broadcasting.By ALEXANDER J. GAYES.

Much interest is now centred round the subject of simultaneous broadcasting, and inorder that our readers may be familiar with the methods employed we outline belowthe general mode of operation.

THE wireless broadcasting of a speechor a musical item simultaneouslyfrom several transmitting stations in

different cities, as now so satisfactorilyaccomplished, has been rendered possiblelargely by the great advance made in recentyears in the study of attenuation charac-teristics as applied to land line telephoniccommunication. The problems which presentthemselves on any such undertaking aremany and varied, and, further, the necessity,for commercial reasons, of employing existinglines and cables often make the solutionsunduly complex. Without taking a specific

Fig. 1.-The push-pull microphone employs a stretched silkdiaphragm, which is heavily air damped. The generalform of construction should be apparent.

case, however, it is interesting to reviewthe subject in the light of the most moderndevelopments in the telephone art, in whichrespect credit must be given to the WesternElectric Co. for the development of boththe principle and the apparatus describedin this article.

The two most important factors whichenter into every problem of telephonictransmission are the attenuation and thedistortion of the comparatively high-fre-quency waves which constitutes telephonicvoice currents. By attenuation is meantthe loss of energy of the waves of the tele-

phonic currents, and distortion is theimpairment of the many component wavesof which the voice currents are composed.In a telephonic circuit the waves are slowlyaltered, both in form and in amplitude,as they progress along the lines, usuallydue to the wire -to -wire capacity, but bythe addition of suitable inductance to thecircuit this alteration can be very consider-ably reduced. The inductance is added atcalculated intervals along the line in theform known as loading coils, and by meansof these coils the original wave form and theinitial wave amplitude can be preservedto such an extent as to bring the transmissionefficiency of a cable under certain conditionsup to six times that of an unloaded cable.More recently thermionic valves have beenintroduced in trunk line telephone circuits,and, owing to the amplification thus possible,the standard land -line speech transmissionhas now reached a very high level.

The requirement of the commercial tele-phone service, however, is intelligibility,whereas the transmission of high-grade musicdemands a naturalness which imposes farmore severe electrical requirements, not onlyon the lines and cables, but on all associatedapparatus. This is best expressed by sayingthat, whilst excellent speech transmissioncan be obtained with circuits and apparatuswhich function without material distortionbetween the frequency range of 400 to2,000 periods per second, freedom fromdistortion over a frequency band of fromi6 periods to 5,000 periods per second, oreven higher, is necessary with music if eachinstrument in a first-class orchestra is to bediscernible.

Obviously the first piece of apparatus tobe considered in an extensive transmissionscheme is the microphone. It is essentialthis be of a type which will faithfully convertevery feature of the original sound waves intoelectrical energy ; that is, it must functionuniformly between i6 and 5,000 periods.


An air -damped stretched diaphragm con-denser microphone gives excellent results,and these are being constructed with a thinsteel diaphragm about 2 ins. in diameter,stretched until its period of vibrationapproaches 8,000 periods per second. Thediaphragm forms one plate of a condenser,the other being a rigid disc, the dielectricconsisting of a film of air approximatelyI-L000th inch in thickness. The highnatural frequency of the diaphragm, coupled

127 December, 1923.

A more manageable microphone, and onenow in extensive use for high quality trans-mission, is the granular carbon type instru-ment with two buttons, one on each side ofa diaphragm. This is commonly known asthe push -and -pull microphone, and hasnearly the same high quality characteristicsas the condenser microphone owing to theuse of a tightly -stretched diaphragm andthe same principle of heavy air damping.The energy output of this microphone is,

Fig. 2.-The push-pull microphone Is normally suspended in a metal ease by a system of springs so arranged as to reduce the effect ofmechanical vibration.

with the damping effect of the film of air,results in a very high quality of reproductionuniform in intensity throughout the musicalrange. The condenser microphone, althoughhighly responsive, is extremely insensitive,needing a voltage amplification of 50,000times (corresponding to an energy amplifi-cation of 2.5 X Io9) to bring its output upto that of an ordinary microphone, and,further, considerable technical knowledge isnecessary in adapting this microphone tothe input circuit.

roughly, one -millionth part of that usualwith ordinary microphones, and can beexpressed as x x t0 +8 watt under averageconditions. A part section view of thepush -and -pull microphone is shown in Fig.whilst Fig. 2 is a view of the microphonein its mounting with one side of the latterremoved. It will be seen that the micro-phone proper is suspended on four hooks,which engage with spiral springs on themounting in such a manner as to eliminatethe effects of small mechanical jars and

December, 1923. 128 EXPERIMENTAL WIRELESS.

vibration. An outer drum, having perfora-tions on all sides protected by fine brassgauze, serves to form a cover, and it will benoted that no horn cr sound collectingMechanism whatever is used with thismicrophone. In passing, it might be men-tioned that the double button constructionreferred to above almost completelyeliminates the distortion caused by thenon-linear nature of the pressure resistancecharacteristics of granular carbon.

After leaving the microphone, and beforethe speech energy is sent out on the line,it is amplified, and if, for example, it were

special three -valve amplifier illustrated. Themicrophone control apparatus consists ofswitches and other mechanism which enablesthe operator to switch quickly from onemicrophone to another, as with some publicfunctions the speeches are made at differentpoints during the ceremonies. Special pre-cautions are taken to prevent clicks whenswitching from one microphone to another,and, if necessary, provision is made for twoor more microphones being connected tothe amplifier at one time. This latterfeature is desirable when solo singers areaccompanied by an orchestra in a theatre, for

Fly. 3.-A g I view of a speech amplifier panel with central devices. The screening of the valve is clearly seen.

desired to broadcast a theatrical performance,a very suitable apparatus would be thespeech input amplifier, an illustration ofwhich is shown in Fig. 3. The exact designof the amplifier would depend on conditions,but the aim is to produce an energy outputapproximating in value that of ordinary linetelephones.

The circuit diagram of a speech inputamplifier, suitable for microphones whenthe latter are arranged to collect soundswithin a radius of several feet, is given inFig. 4. For close talking a less powerfulamplifier would suffice, but it is interestingto follow for a moment the operation of the

example, where by proper adjustment ofthe respective volumes very pleasing resultscan be obtained.

It will be seen that the feeble currentsfrom the microphone enter the amplifierthrough a differential input transformer onto the first valve. This valve is similar tothe one following it, and by a clever seriesconnection the filaments of both are fedfrom a common 12 -volt supply. Advantageof this method of connection is taken tosecure the necessary negative bias on thegrid of each valve with the exception ofthe third valve, where a special grid batteryis provided for the purpose. The third valve


Micro onecontrol

annaraas.

High,9aalitmicro, one.

129

Infiat High and low Gant regyulatingtransformer fain switch.. fiete/d-io/rzel-er.

t

>2 V: AVair

//J

0 +350 V A 5' u/z/z1,y.

December, 0923.

Out'zutTrans,

Fig. 4.-The microphone currents are applied through a differential input transformer to the first valve, thegrid bias of which is determined by a series resistance. The subsequent valves are choice coupled, thegrids being controlled by special potentiometric devices.

is of a different type to the other two, andcurrent for filament heating is drawn directfrom the 12 -volt supply in this case. Need-less to say the valves are of the latest oxide -coated dull emitter filament pattern, as withno other type would it be possible to secureso great a degree of amplification with entireabsence of noise. From the last valve theoutput is taken to a special transformer,or repeating coil, as the telephone engineerwould prefer to call it, out to the line througha winding, the centre point of which isearthed. This relieves the line of any staticchange, and generally makes it far lesssusceptible to interference effects by theemployment of windings accurately balancedwith regard to inductance, self capacity andspeech -frequency resistance. From this pointonward the transmission becomes a telephoneproblem of the highest class, and every effortmust be made to maintain the value or powerof the voice currents at a certain desirablelevel throughout the scheme. If the powerbe allowed to become too weak, the ex-traneous power induced from parallelingcircuits would tend to obliterate the trans-mission. On the other hand, excessiveamplification would overload any iron -coredinductances in the transmission line, andthus alter their characteristics, as well as

-0To lines.

giving rise to overhearing troubles onneighbouring circuits.

Referring now to the telephone trans-mission lines, as previously stated, thesehave been designed to function most effi-ciently between a frequency range of 400to 2,000 periods per second. In other words,the attenuation at frequencies beyond thisrange is an uncertain factor, and thus wehave the peculiar case of the modern tele-phone cable fitted with loading coils orrepeater stations being inferior to an ordinaryopen wire for the transmission of the highestgrade of music ; that is, from a distortionpoint of view only, of course.

To understand the subject fully it isnecessary to have a clear conception ofattenuation and distortion as applied totelephone cables. These two effects areinter -linked, and the latter results from theformer, but it is possible to eliminate distor-tion if only sufficient skill is exercised. Thispoint can be made clearer if considerationbe given for a moment to the study of thetransmission characteristics of a telephonecable. Referring to Fig. 5, line A shows thetransmission equivalent of a io-mile lengthof ordinary _dry -core paper -insulated cable.It will be seen that at about 800 p.p.s. theequivalent of the length u roughly to miles.

December, 1923. 130

In other words, the cable under considerationapproximates closely the accepted standardcable, on the basis of which all comparisonsare made. At 2,50o p.p.s., however, thesame length of cable exhibits propertiessimilar to those of a 20 -mile length ofstandard cable, assuming the latter to befunctioning at 800 p.p.s. throughout, whilstat 5,000 p.p.s. the same io-mile length ofcable gives the effect of a 3o -mile lengthat a unifcrm 800 p.p.s. From this it willbe obvious that the attenuation taking placein a given length of cable varies accordingto the frequency of the impressed waves.In the case under consideration the loss at2,50o p.p.s. is twice as great as it is at 800p.p.s. In such a cable, with the energy lossvarying with the frequency, the speech

40

Cocr)

30

20

ti10

C.?

Atteetzation e aalceee:

A

0 1000 2000 3000 4000 5000l'reftlency in Periods izer seconds.

Fig. 5.-Illustrating the transmission equivalent withoutequalisers at A, and with equalisers at B.

currents would suffer distortion, the degree ofwhich would increase rapidly with wideningfrequency ranges.

By the addition of loading coils the attenu-ation, taken as a whole, can be reduced, andcan be made more uniform throughout thespeech range, but here the previously -mentioned wide range of from i6 to 5,000p.p.s. must be considered. To secure uni-formity throughout such a range is no meanfeat, but it can be accomplished by theintroduction of attenuation equalisers. Suchan equaliser, as shown in the diagram (Fig. 5),consists of a calculated combination andarrangement of inductance, resistance andcapacity. In a general way it could besaid that such an equaliser increases the lossat the lower frequencies and results in a


levelling up of the losses to such an extentas practically to eliminate distortion alto-gether. Curve B shcws this clearly, and,although the nett result is a fall in the overallefficiency of transmission, this can easilybe corrected by suitable amplification.

To maintain the desired level in the circuitsby proper adjustment of the amplifiersit is found necessary to have some means forquickly indicating the volume of the trans-mission. This is accomplished by a " volumeindicator " consisting of an amplifier detectoroperating a direct reading milliammeter.By adjusting the amplifier in such a manneras to keep the deflections of these instrumentsreasonably close to a point determined byprevious calibration it is possible to maintainthe volume or power of transmission betweenthe required limits. In the case of anextensive distribution scheme small ampli-fiers can be fitted on all limbs of bridgedor teed lines, and by means of volumeindicators at these points the requisiteamount of power or volume can be dis-tributed to each station.

The volume indicators can be used inaddition to compensate partially the largerange in volume which occurs with musictransmission ; this wide range being particu-larly noticeable if a theatrical performanceis being broadcast. Consideration of thispoint will show that, were the amplifierspermanently set with sufficient volume tobring fully into prominence the words ofa distant speaker, the volume of the loudmusic would be such as to overload theapparatus in the land lines, as well as thewireless transmitting and receiving appara-tus, whereas by judicious use of the controls,based on the reading of the volume indicator,a more natural and pleasing effect can besecured.

The diagram will indicate the arrange-ment of apparatus involved in a typicalsimultaneous broadcast scheme. Thespeech input amplifier would be located inthe concert room or in an adjacent room inthe same building. A volume indicatorwould be provided, and, during a perform-ance, an operator, who might also be pro-vided with means of listening on a monitoringcircuit, would regulate the amplifier to giveapproximately the predetermined outputvolume. This operator would also controlthe microphones if more than one were fitted.


Keg:-crephoite.

AYfieech infiat am/124(1er.

El Volatile indicator.Amplifier.

E Attenaati on, eRaalizer.Radio Transmiltingiltation.

n Additional afivaifier atRadio AVtation.

fle

gy

At the main distributing centre (marked A)the incoming speech currents would be sentdirect to the radio -transmitting apparatus,and also several amplifiers, as shown, wouldhave their input circuits bridged on to theincoming lines. This is a fixed and steadyload, which would remain unaltered by thedemands of the several stations, thesedemands being met by amplifiers Controlledby volume indicators as shown. The attenu-ation equaliser E in area A might proveunnecessary, as usually the concert roomwould be within a short distance of themain distributing point, but it is includedto show the principle.

131

\borzy,

December, 1923.

Area :-A._BroadcastAVtation and main

distrihatiny /toe nt.B. _Broadcast AV -al -ton and slob

distributing /to int.C. _Broadcast Nteztion.

_E. Distantifiroadcas7Vration.

In actual practice many difficulties areencountered in attempting simultaneoustransmission which cannot be detailed here ;for example, the linking of cables to openlines, or the coupling of cables havingdifferent attenuation characteristics. If per-fection is desired throughout the very widefrequency range of from 16 to 5,000 p.p.'sall such matters become difficult problems,and the satisfactory accomplishment of anextensive scheme of simultaneous broad-casting undoubtedly reflects great credit onall concerned with so complex a venture.


Some Experiments on the Fading of Signals.BY J. ALLAN CASH (2GW).

In the October issue of "Experimental Wireless" some information was given relatingto some of the probable causes of the fading of signals. Below will be found some datawhich has recently been collected, and the results should be of considerable interest.

VERY little is known about the fadingof signals, and investigation of thisphenomenon is likely to lead to the

solution of many problems in radio. Thewriter does not propose to put forth any newtheories in this article, but rather to indicatethe lines on which other experimenters maypursue their investigations, and to give someidea of his own experiences.

It is surprising how wide a field of investi-gation is opened up when one attempts tofind out something about this interestingsubject. Many theories have been pro-pounded to explain why signals on shortwave -lengths should vary in strength, butnone of them can be said to be really satis-factory. When the atomic theory of matterwas first propounded it became evident, astime went on, that many problems in sciencecould be solved by it. The more it wasused, the more evident it became that itwas more than a mere guess, until to -dayit is universally adopted as the true con-ception of matter, and has been provedbeyond doubt to be correct. What is wantedin fading is a similar theory, which, oncepropounded, would probably solve manymore problems than fading alone.

Fading only occurs in any marked degreebetween 15o metres and 600 metres. Below15o metres and above 600 metres signalsremain steady during a transmission period,although they may vary in strength fromday to day. There is really no way ofovercoming fading at the moment. It isno fault of either the receiver or the trans-mitter, but there is something between thetwo which has a very big influence on thesignals. As is generally the case in elec-tricity, there is no visible indication of thecause, so one is compelled to start guessingand to see how the information already inone's possession fits in with the suppositions.

Last winter the writer burnt the midnightoil with Mr. W. R. Burne and others many

times. After the first excitement of hearingAmerican amateurs had died down, certainpeculiar features about these and othersignals began to impress themselves on thelisteners' minds. Fading was hardly everspoken of then, but it soon became evidentthat signals from any American station werevery difficult to copy in their entirety owingto the fact that they varied in strengthso persistently.

At first regarded as a nuisance, this phe-nomenon soon began to interest the writerand his friends, and, while one read themessages, another would time the fading.Then things began to hum. It soon became.evident that there were two distinct kindsof fading-one irregular and the otherregular. This seemed something new, andenthusiasm ran high. Many signals were.carefully timed, and soon another pointmade itself known.

Signals coming mostly over land fadedvery erratically-sometimes slowly, some-times quickly, and at times remaining steadyfor various periods of time. There seemedto be no system about this, so attention wasturned to signals which faded regularly.These were found in all cases to have travelledover large stretches of water, e.g., MalinHead, Valencia Island, and Americanamateurs. These stations hardly ever fadedirregularly.

The first thing to be noticed about thisregular fading was that the time for a com-plete swing (i.e., from maximum to minimumand back to maximum) varied with differentstations. Next it was discovered that thetime for a complete swing was directlyproportional to the distance between thetransmitting and receiving stations. Fromobservations made last winter, and partlycorroborated this autumn, it seems that oneswing takes two and a half seconds perhundred miles. This, of course, requiresfurther proof, and is at the moment only a


guess from comparatively few observations(see Fig. 1).

Malin Head was noticed to fade andreturn to normal strength in about six anda half seconds, being 25o miles away.Valencia took about 9 seconds, being 350miles distant. A ship off Anglesey, ioo milesaway, took 24 seconds to fade. Other ships(C.W. harmonics on about 200 metres) allswung at the rate of 24 seconds per hundredmiles. It was, of course, necessary to findtheir exact positions, but once this had beendone these signals formed excellent subjectsfor investigation. American amateur station2ZL, belonging to Mr. J. 0. Smith, of LongIsland, New York, with which station specialtests were carried out by Mr. Burne and the

R9B8

. BYRR5

tht 119

14iZeR

t/I,

"AA 2100

133 December, 1923.

between 2 and 3 a.m., by station U.S. 2ZL.His strength at full maximum made itimpossible to wear the 'phones. When atfull minimum his signals were just readable.His normal swing varied between R.5 andR.9 or more.

It is very difficult to observe this modifica-tion of fading, as a station would have totransmit consistently for at least a quarterof an hour in order to demonstrate all thefading without doubt. This is very rarelydone by American amateurs. Perhaps itwould be easier to watch broadcasting.There are many things to be discoveredyet about this strange double fading.

Perhaps it would be as well to mentionhere that fading varies from night to night.

*NAT

fj\

20 so so 80 100

AVe coatis- 8001ifaes --- 1,600 lifiles----3,100 MilesFig. 1.-Fading curves of signals from 800, 1,800, and 8,400 miles.

writer, took 8o seconds to fade and return.This station is 3,200 miles away, so the samething applies in this case as in the others.Stations in the 5th and 9th Districts ofAmerica, i.e., 4,000-5,000 miles away, tookIoo-125 seconds to swing. Further, when-ever the fading of these signals was timedit was always the same.

A curious modification of this regularswinging was soon noticed (Fig. 2). Afterso many regular swings, a signal, beginningto fade as before, would be suddenly checkedand begin to increase in strength again.This increase would be continued until thesignal was far above its usual maximumstrength. Then it would fade off again toits usual minimum value. After a fewmore regular swings, this process would berepeated only in the opposite direction,i.e., fading away to far below minimumafter increasing only about half way towardsnormal maximum strength.

An excellent demonstration of this modifi-cation was given on January 17, 1923,

'so /50 $00 /80

Last winter the fading of American amateursseemed consistently to follow the aboveconditions, but since the summer their fadingseems to have been influenced by someunknown cause. Very often quite irregularfading has taken place, and on many occa-sions the time for a complete swing has beenwrong. However, bearing in mind lastwinter's results, it is quite easy to see thatthe fading tends to follow the same rules.

Now a word to those who have noticedregular swinging on English broadcastingstations. Several experimenters have writtento various wireless periodicals saying thatsignals from London at a few hundred milesdistant fade regularly every so many minutes.Quite right, but does London always fadein the same time ? If so, by all means takeas many observations as possible and tryto find the cause of it. But if, when swingingregularly, the time for a complete swingvaries from day to day, it seems to the writerthat it would be far better to watch stationswhich always or nearly always fade regularly,

3


find out something definite about regularfading, and then compare the results withthe occasional regular swinging of London.In this way it should be much easier to detectthe influences which cause irregularities inthe fading of signals coming over land. Bywatching stations which only occasionally

B9

toy RO

0,, ti R5/A..".7..111

t).,R3714

c4 .4 REb..\ \ \ 1V\ \/ /

, .R80 M

ti RepN rV

0 20 00 60 80 /00 /20 11 /00Seconds

Fig. 2.-Fading curve of signals from 800 miles, showing double Increase and double fade.

whether radiation is taking place or not bytuning in a C.W. signal and then movingthe aerial tuning condenser, or, if a coupledcircuit is used, the secondary tuning con-denser. If the set is radiating the beat-noteof the signal will change, but if no radiationis taking place only the strength of the signal

swing regularly one is apt to get lost in amaze of observations, in which irregularswinging predominates.

To those who have observed fading onEuropean stations, and compared it withthat of American stations, there are somany which seem to influence signalscoming over land that for convenience sake,for the time being anyway, it seems betterto class all " local "' stations as fadingirregularly, and to concentrate on the bestof all stations to observe on-Americanamateurs and broadcasting. As signals on400 metres seem to fade in exactly the sameway as those on 200 metres, American broad-casting is very useful to those who requirea good " steady " signal to work on.

A most important point to rememberwhen observing fading on a telephonytransmission is-not to take the strength ofthe signal as being that of the music orspeech, but to heterodyne the carrier waveand observe on that. Music is alwaysvarying in strength regardless of radioconditions. Speech, if consistent, is lesslikely to lead to mistakes, but better thaneither of these is the carrier wave. Greatcare must be taken if a self -heterodyne typeof receiver is used to see that the set doesnot radiate, as this would probably spoilreception for others in the vicinity. It isfar safer to use a separate heterodynealtogether, coupled, if necessary, as far fromthe aerial as possible. If a self -heterodynereceiver is used, it is easy to determine

f80 200

will change, the beat -note remaining thesame.

The writer's present receiver consists ofsix valves -4 H.F., D., and i L.F. Anynumber of valves can be used at a time.The H.F. valves are transformer coupled.When the nearest the detectoris used alone, the set radiates when oscillat-ing, but when any more H.F. valves arebrought into action they do not oscillatealong with the first one. The heterodyningpart of the set remains confined betweenthe detector and the first H.F. valve, andwith two, three or four H.F. valves in actionthe set does not radiate. This enviablestate of affairs is only made possible by usingreversible H.F. transformers-i.e., with thefour pins fixed in the form of a square andnot in the usual valve -pin way. It oftenhappens, when the writer brings anotherH.F. valve into play, that the whole setoscillates, and therefore radiates, but bypulling out the last transformer to be added,giving it a quarter turn, and again insertingit, the general oscillation ceases and thereis no radiation. Signals are every bit asgood as when the set radiates. The con-denser tuning the transformer nearest to thedetector is the only thing which will alterthe beat -note of a signal when the set isadjusted correctly. By using reversible H.F.transformers, self -oscillation between H.F.coupled valves has been entirely eliminatedat 2GW.

It will be seen that the writer pins great

EXPERIMENTAL WIRELESS. 135

faith on H.F. valves. Perhaps some readerswill question the necessity for so manyvalves. Let it be stated at once that signalsfrom America can be heard quite easily onone, two and three valves, but in order toobtain accurate data on fading it is highlyadvisable to more than merely hear a signal.A signal should not fade to inaudibility atall, and, if possible, should be easily readablewhen at its weakest. It has been foundthat, when QRN is bad, the L.F. valve ismore trouble than assistance in reception,but by bringing in one or two more H.F.valves, QRN is but little worse, while signalsare considerably louder. It is very rarelynecessary to use all the H.F. valves, butthey are there for use when the occasionarises.

The writer has not troubled very muchwith record -breaking achievements, althoughAmerican signals on short waves have beenread on one valve, and various other" stunts " have been achieved " just to

December, 1923.

During September of this year greatnumbers of American amateur and broad-casting stations were received in England,but for the first four weeks of October no one,to the writer's knowledge, has heard morethan a solitary one or two American signals.Why ? Is it weather conditions, wind,temperature, clouds, humidity, or what ?Further, is the cause of signals going offduring last year's Transatlantic Tests thesame thing that prevented signals comingacross during October this year ? Theseand many more questions remain to beanswered. The only way to do it is toimagine everything that could possiblyinfluence signals, obtain regular informationabout each thing, draw graphs if possible(Fig. 3), and compare them with the resultsobtained. Perhaps in this way some experi-menter will come across a feature of Naturewhich influences signals, and which wouldaccount for fading of all kinds. Anyway,there is a vast field of research open to all

1010k 1030

1000_to ti >010

k 1000rki el 990`..1's 980

1).%) 970960s0.

70 60 SO° 00* 30° 30° 10° 0°New .Brttish.Degrees ty Zonsraade

York west of Gree/uoic7t.Fig. 3.-Chart showing barometric pressure over the Atlantic Ocean on December 20,

very good conditions for reception existed at this time.

show there is no ill feeling ! " Much moreimportance is attached to the scientificside of reception than to record breaking.After all, what one fool can do, anothercan !

During last year's Transatlantic Testssignals came through very well until Decem-ber 15, when they began to go off. OnDecember 16 only two American amateurswere reported among all listeners in GreatBritain. December 17 saw signals returning,and December 18 was a very good night.Here is a chance for investigation. Signalsfrom America often go off like this, butgenerally return after a day or two. Localsignals remain apparently the same as usualduring 'these periods.

1922. It was found that

who are interested, and no unusual or costlyapparatus is necessary to begin with at least.Perhaps other experimenters will makeknown their views on this most fascinatingsubject.

British Amateur Call Signs.It is understood that the supply of call

signs commencing with the figure 6 is nowexhausted. It seems that other figures arenot to be introduced, and the call signs willbe continued by another series of " twos,"followed by various three -letter combinations.It needs little imagination to appreciate theconfusion which is likely to arise whenlistening for American amateurs.

3*


Some Original Notes on Selectivity.BY G. L. MORROW (6UV).

Since the introduction of broadcasting the experimenter has turned his attention to thequestion of selectivity with a view to separating the various broadcasting stations.Most time seems to have been devoted to the use of rejector or acceptor circuits, and thefollowing notes which approach the subject from a different viewpoint are of greatinterest.

IN the early part of the present yearthe writer was engaged upon thecompilation of comparative data with

respect to the fading of certain 600 -metremarine traffic shore spark stations, and forthe purpose of these investigations a receiver,comprising two aperiodic H.F. stages, rectifierand one note magnifier, was employed ;the tuner being a standard, calibrated MarkIII*.

To those who are conversant with 600-metre reception, it will be appreciated thatanything in the nature of accurate signalstrength measurement on any given stationis practically impossible on this wave -lengthowing to interference.

as compatible with signal strength ; the other,by the use of rejector circuits or what arecommonly known as " wave traps." As itis well known that by far the greater amountof 600 -metre spark interference is causedby the " double -wave " radiation of suchstations, it was decided to concentrate onobtaining as high a degree of selectivitythroughout the receiver as was possible,and it was proposed to use the minimumcoupling between the aerial coil and thesecondary, together with a high degree ofresonance in the H.F. stages, according tothe particular station whose fading wasto be observed.

For some months previous to the com-

0+HrFig. (-Small fixed condensers are placed between the primary and secondary of the transformers-that is, between the

anode of one valve and the grid of the next valve.

It is hoped, therefore, that the methodsto be described by which this interferencewas overcome may be of interest, openingout, as they do, an interesting field forresearch in amateur DX working in thiscountry.

In considering the means by which thisinterference might be reduced, one of twomethods only appeared to hold out reasonablechances of success, one being to strive for theutmost degree of selectivity obtainable byas small a coupling percentage in the tuner

mencement of these investigations it hadbeen noticed that the weather report trans-mitted nightly from GLD at 22.3o G.M.T.was subject to marked periodic fading, andit was, therefore, decided to start observa-tions on this station. Owing to jammingby FFU and GNI such observations were oflittle value, and the writer therefore triedthe effect of putting small variable con-densers of o00025 mfd. maximum capacityin shunt to the secondary windings of thetransformers. It was found that a capacity


Fig. 2.-The longitudinal axes of the coils were gret placed at 45°, u shown.

of o0003 mfd., together with the limitingaction of the rectifying valve-anode recti-fication being employed-was sufficient tobring up GLD's strength from an averageof R6 to R8 withcut increasing that ofFFU or GNI. Two fixed condensers ofthis capacity were then made up with theintention of connecting them permanentlyacross the transformer secondaries ; this wasdone, and a strange effect was immediatelynoticeable : whereas previously it had onlyrequired two divisions of the secondary(tuner) condenser to bring in FFU and GNIsufficient to jam GLD, it now required twelvedivisions, and when tuned to GLD the twoformer stations were of very much reducedstrength.

As no lcgical explanation of this effectwas apparent, the wiring of the amplifierwas examined, more by custom than in thehopes of finding an explanation, and it wasthen found that by mistake the small fixedcondensers had been connected, not acrossthe secondaries of the transformers, butbetween primary and secondary, as shown inFig. i. Thus the writer is bound to admitthat what he might have striven for un-successfully for weeks was, through careless-ness, discovered in a few hours.

At this stage it was decided to leave theamplifier as it was and employ the loosestcoupling possible in the tuner, and it wasdiscovered that, when the longitudinal axisof the secondary was at 45° with that ofthe primary, as shown in Fig. 2, GLD wasquite inaudible, while FFU and GNI were

December, 1923.

reduced to strength R3 and R2 respectively.When, however, the coupling was still furtherreduced to approximately 85°, GLD becameaudible again at strength R4, FFU and GMbeing both reduced to strength R2.

As these results were promising in theextreme, the primary and secondary coilswere removed from a spare Mark III* tunerand mounted in such a manner that thesecondary coil could be removed to a distanceof 2 ft. from the primary, the longitudinalaxis of both coils being the same, and,profiting from the experience of the H.F.transformers, a small variable condenser ofo0005 maximum capacity connected asshown in Fig. 3.

At this point it may not be out of placeto investigate the function of the condensershown in Fig. i. It should be rememberedthat the high -frequency transformers wereof the aperiodic type wound with resistancewire, and thus, in their original condition,giving a comparatively speaking blunt,resonant peak on 600 metres. The effect ofadding a small condenser in the positionshown in the diagram is to increase thecapacity between primary and secondary,thereby still further flattening out theresonance curve on the optimum wave-length. This, combined with the selectivityobtained by very loose -tuner coupling andthe limiting properties of the rectifying valve,enabled the two interfering stations to bevery much reduced in strength. In passingit may be of interest to mention that thetrue wave -length of GLD as measured by a

Deoe,nber, 1923. /38

standard wave -meter was 605 metres, withonly a weak radiation on 585 metres, whilstthe true wave -length of FFU and GNIwere noted as 595 and 62o respectively.

Reverting now to the special tuner, it wasfound that, with the secondary coil removedto i4 times its own length, and connectedas shown in Fig. 4, when the coupling

-00111/d.


that the shorter the wave -length with anygiven coupling, so the static componentincreases, and on wave -lengths below 300metres the writer uses a vernier condenserof o0002 mfd. maximum capacity.

This method of what may, perhaps, betermed a combination of inductive andcapacity coupling was sufficient to enable

000SM d.

Cods ca/zahle of beingmoved Zia° feet afitzrZ:

Fig. 3.-Subsequently the coils were separated by a distance of 2 feet, the axes being aligned, and the soils wereelectro-statically coupled.

To Potentiometer

ToAmplifier.

.To Grid

had been loosened sufficiently to obtain thedead point on GLD a slight movement onlyof the condenser was required to bring thisstation in again at quite good strength, atthe same time eliminating the two interferingstations.A Potentcometer!'

Primary. Xecondary.Fig. 4.-The secondary is removed from the primary to a

distance of 1i times Its own length.

It would appear that, at the silent point,the coupling is so reduced that the magneticand static components are equal, and thatany further reduction in coupling results inthe static component being the greater,thus giving in effect a capacity couplingwhich is actually accentuated by the smallvariable condenser. It would also appear

the necessary data on fading to be tabulated,and the results obtained were considered tobe sufficiently interesting to warrant similarobservations being taken on several Britishamateur stations operating round 20o metres.On this much shorter wave -length it wasfound, however, that this method was notwholly satisfactory on C.W. transmissions,as certain stations continually broke throughand rendered observations almost impossible.

This was finally overcome by replacingthe aperiodic secondaries of the H.F. trans-formers with tuned copper windings, andat the same time arranging variable couplingbetween the primaries and secondaries ofthese transformers. This gave all the selec-tivity necessary, and the final circuit is thatshown in Fig. 5.

This, at first sight, would appear tonecessitate several critical adjustments, butin practice the writer has found that oncethe " feel " of the circuit has been obtainedstations can be tuned in and interferingstations eliminated with comparative ease.

In any case, the remarkable degree of


Fig. 5.-The filial circuit employed, with the selective device applied both to the aerial circuits and the amplifier circuits.

selectivity which can be obtained by thesemeans appears amply to justify the extraadjustments required. It will be noticedthat throughout these notes no mention ofreaction has been made. Reaction wastried, but, as would be expected, renderedthe circuit unstable, and, since the signalstrength was quite sufficient, and no tendencyto instability apparent without its use, itwas not adopted. Finally, as a matter ofinterest, using this arrangement, the 450 -metre ship' and shore D.F. traffic, which sofrequently cuts through the broadcast trans -

English 2KF.DEAR SIR,

We have completely verified your report ofSeptember 19 on our 'phone signals, and we wishto thank you for same.

The report has been officially recognised byQ.S.T., the leading American amateur wirelessmagazine.

Perhaps you do not realise it, but you havehelped us to establish a world's record. The'phone set we were using at the time incorporatedfour 5 -watt tubes in a Hartley circuit, using Heisingmethod of modulation, two tubes acting as oscilla-tors and two as modulators, the input being 500volts D.C. at 180-200 milliamperes, or about 100watts.

This set radiates from 2.3 to 2.5 amps. on 'phone,

missions from 5IT, can be completelyeliminated, as' also can the interference ofany one broadcasting station with anotherat a position 3o miles N.W. of London.

In conclusion, the writer has made noattempt to deal with the theoretical explana-tion of the various phenomena observed, buthas given these notes in the hope that theywill prove of interest as showing in whatmanner a definite problem arose and how itproved capable of solution to such an extentthat the observations on fading were ableto be completed.

Copy of letter received from U.S.A. 8KG.Niles, Ohio.

October 21, 1923.

the voice being amplified by a special two -stageamplifier before it is impressed on to the modulatortubes.

As you will now realise, this is some record forlow -power 'phone set such as this one is, and weshould like to arrange tests with you at some timein the near future

Again we wish to thank you for your very kindreport.- Yours truly,

(Signed) J. WM. KIDD,Oppr. 8KG.

Note.-This station was received when workingtelephony to 2RS on the morning of September 19,1923, at 4.50 a.m., using two valves (detector and1 L.F.), and the speech was just audible, the callsbeing easily recognised. The transmission wassteady, and no fading was observed. 2KF.


General Efficiency of' Reception onShort Waves.

BY HUGH N. RYAN (5BV).

A recent examination of a number of amateur receiving stations intended tooperate on short wave lengths shows in a very large number of cases very poorexamples of design. In view of the coming trans -Atlantic tests it is likely thatmany will be working on very short wave -lengths and those to whom the subjectpresents any difficulty will find the following notes helpful.

IN reading reports of the reception resultsobtained by various stations, onecannot but notice the great differences

exhibited in the logs of different stations,which differences are too great to beaccounted for entirely by the location ofthe stations. It is very interesting to lookthrough the " Calls Heard " columns ofseveral radio periodicals. One can " place "each station in its right class immediately.First look at the end of the list. Either therewill be a string of O's and 8's, or there willbe none at all. In the latter case, you canbe pretty sure that every station in the listwill be a 440 -metre station. In some casesthere may be some very good work indicatedon the 440 wave -length, in the way of distantstations received, this showing that thestation in question is not entirely " dud,"in spite of his evident inability to receiveon 200 metres. The chief object of thisarticle, therefore, is' to give a few hints tothose who are experiencing difficulty withthe short waves. I can hear many of yousaying that a man who cannot receive aswell on 200 as on 440 does not deserve hints.I would ask those to consider the time whentheir "own 200 -metre reception was, perhaps,not all that might be desired. It is evenpossible that, in a few cases, there is stillroom for improvement. Certainly, in theaforementioned lists, the results on shortwaves, when there are any recorded, are notalways comparable with the receivers used.So, you who get everything there is to geton short waves, why not try getting the sameresults on one valve less ? (In a few casesI know of, I should like to suggest four orfive valves less !)

First of all, we will deal very briefly withthose who cannot receive at all below about30o metres. Their troubles can be reduced

to three main headings. Too much dead-end.Too large a reaction coil. Too much straycapacity.

The remedies for the first and third areobvious, and have been emphasised too oftento need any further remarks. The fault ofhaving too large a reaction coil is probablyresponsible for 90 per cent. of short-wavetroubles. Those whose receivers work onlyon the longer waves find that, as they tunedownwards beyond a certain point, they needmore and more reaction to make the circuitoscillate. A little further down the circuitwill not oscillate at all. Strong spark andtelephony can still be received fairly well,but it is noticed that, however much furtherthe A.T.I. is reduced, the wave -length doesnot decrease. Now the point below whichthe reaction coupling has steadily to beincreased is the natural wave -length of thereaction coil (or, more correctly, of thecircuit comprising the reaction coil and thevalve capacity). No circuit will oscillatefreely at a frequency higher than its ownnatural frequency. Therefore, the oscilla-tions which are obtained a little below thepoint are forced and unstable, and ceasealtogether a little lower down. The reasonthat reducing the A.T.I. fails further toreduce the wave -length is that the reactioncoil has now, taken charge of the tuning.It must be remembered that, although whenwe say that a set is " oscillating," we usuallymean that it is generating oscillations ofitself, yet the circuits of a receiver are oscil-lating whenever it is receiving signals, theoscillations being sustained by the distanttransmitting station instead of the receiver.So, for the same reason that the circuitcannot generate oscillations below the naturalwave -length of its largest coil, it cannotrespond to signals of a lower wave -length


than that. This explains why the reactioncoil takes charge of the tuning as soon asthe natural frequency of the grid circuitin the ordinary receiver is increased (i.e., itsnatural wave -length reduced) above thenatural frequency of the plate circuit.

Now, many experimenters do not realisethis, and so, when they observe that, belowa certain point, they require more reaction,and that a little lower still they cannotobtain oscillation at all, they assume thatthey have not sufficient reaction for shortwaves. They therefore put in a largerreaction coil, and so make matters worse.

Assuming that the coils are arranged soas to allow of a reasonable (but not necessarilytight) coupling, and that the damping of thegrid circuit is not too great, a very smallcoil indeed will suffice for reaction. I havevery often heard this excellent advice metwith the reply that the set might be persuadedto oscillate a bit with a small reaction coil,but the oscillation would be pretty feeble.May I, therefore, interpolate at this pointa few sentences dealing with popular ideasabout oscillation. Oscillation is the fetishof the average non -technical experimenter.His first thought about any receiver is not" Will it receive ? " but " Will it oscillate ? "

The guiding rule of many receiving men is" if the set will oscillate it must be working,if it will oscillate hard it is working well,if it will howl violently it is working per-fectly." Hence the often -heard objectionto a small reaction coil, namely, that it willnot make the set oscillate hard enough.Let us try to explode this fallacy once andfor all. A good autodyne set of any ordinarytype is working at its best for the receptionof C.W. when it is just oscillating. Theoreti-cally, the signals heard in the 'phones areloudest and purest when the local oscillationsin the circuits of the detector valve are ofexactly the same amplitude as those producedby the received signals. Now, with theslight degree of overlap present in themajority of receivers (even if it is too slightto be noticeable) the weakest stable oscilla-tions which our receiver can generate arestronger than those which we are receiving.Therefore, always try to keep the receiveronly just oscillating for C.W. reception, andon no account blame your receiver becauseit will not oscillate hard enough. Anothervery important point must be mentioned

141 December. 1923.

in this connection. Whatever the text-books may say about it, it is an indisputablefact that if a receiver is connected to anaerial, and any valve of that receiver isoscillating, some energy will be radiatingfrom the aerial. The tuned anode H.F.amplifier, with reaction coupled to the anodecoil, is no more free from this trouble thanany other. If the set be tuned correctly,and the detector valve is generating oscilla-tions, the H.F. valve will oscillate also andenergise the aerial. So there is anotherimportant reason for keeping the localoscillations subdued. It has been said thatthe amplitude of the local oscillations shouldbe equal to that of the received oscillationsfor the best results. In connection withthis, it will be understood that the remarksabout the local oscillations in an autodyneset always being in practice stronger thanthose received only apply when receivingweak signals, but, of course, the set does notneed to be at its most sensitive adjustmentfor receiving a station in the next street.

If this ideal of equalising local and receivedoscillations is achieved, then the interferencecaused by the receiver is minimised. Itmeans that, even if reaction direct on to theaerial coil were used, the interfering wavesreceived by any other station, howevernear, could never be stronger than those ofthe distant station. Now, if all the usualprecautions are adopted, and aerial reactionbarred, the interference can be eliminated,and reception improved at the same time.In connection with the last two matters, onecannot too strongly urge the use of a separateheterodyne. It is the only method by whichthe strength of the local oscillations can beregulated to a nicety, and the most effectivemethod of minimising the so-called re -

radiation.Now, having reduced the size of the

reaction coil, it is possible that the set willstill not oscillate on short waves. Thismay be because the reaction coil has not beensufficiently reduced. In this case the receiverwill exhibit the same symptoms as before,but on a lower wave -length. But assumingthat the reaction coil is now small enough,the trouble is probably due to an excessivedead-end, or too fine a gauge of wire on theA.T.I. or grid coil. For the best results onshort waves, use a coil which just coversthe required range and no more. From

December, 1923 142 EXPERIMENTAL WIRELESS.

150 to 220 metres is a reasonable range forone coil, with, say, three or four tappings.Use plug-in coils if you like them, but makethem yourself, using thick wire for winding.The universal fault with all short-waveplug-in coils on the market is that the wireused is far too thin.

All coils for short-wave work should bewound at least i8 or 20 gauge wire. Seethat all the connections are made with heavygauge wire, as short as possible, well spaced,and all joints soldered.

Use a well insulated counterpoise in prefer-ence to an earth connection wherever possible.

Now, having reduced all damping to aminimum, adjust the set to the highestwave -length on which it is required to work.It should just oscillate on this wave -length.If it oscillates readily with very little reactioncoupling, take more turns off the reactioncoil. After adjusting everything in thecircuit to its best value, the reaction coilshould be just large enough to produceoscillation over the whole range of wave-length.

I have already pointed out that it isessential for the natural wave -length of thereaction coil to be below that of the receivedsignals, but so far it has been assumed thatit does not matter how close to its naturalwave -length we work so long as we keepabove it. As a matter of fact, it is bestto keep its natural wave -length as far aspossible below that on which we wish toreceive, for two reasons. The first is justa matter of mass, and has nothing to dowith frequency. We do not want to intro-duce more matter, especially metal, into thefield of the grid coil than is necessary, owingto eddy current losses, which may be quitehigh on these frequencies. The secondreason has to do with smoothness in tuningthe set. If the curve, obtained by plottingwave -length against the degree of reactionrequired to produce oscillation, is examinedit will be found that it is fairly flat at adistance from the natural wave -length ofthe reaction coil, becoming steeper as itapproaches that wave -length. In otherwords, if we want to keep the set just oscil-lating, while varying the wave -length, thenecessary adjustment of the reaction couplingwill be slight if we are working well abovethe natural wave -length of the reactioncoil, but becomes greater as we approach

that wave -length. This means that " search-ing " becomes very troublesome if we areworking only just above this natural wave-length, since, if the set is just oscillating,raising the wave -length slightly brings theset " miles off oscillation " and loweringthe wave slightly causes it to oscillate muchtoo hard. Thus, for every reason, it is bestto have the reaction coil as small as possible.The last reason applies with still more forcewhen a separate heterodyne is used, as thereceiver is then used " just off oscillation "instead of just oscillating, and, if thesesuggestions are followed, the set will be inthis state over a small band of wave -lengthswithout adjustment of the reaction coupling,thus reducing the number of controls.

Finally, if this article helps anyone toimprove his results on short waves, will heplease remember that there are others tryingto receive the same signals as he is, and that,by yielding to the temptation to let the setoscillate hard, he is spoiling other people'sreception as well as his own. If people inthe same neighbourhoods would co-operatein avoiding interference, things would gomuch better. I was listening in a few nightsago and heard about 4o American amateurs.The call signs and some of the traffic of 25of these were completely blotted out byone local heterodyne.

If you must use autodyne receivers, usethem carefully. Don't swish " up anddown anyhow, and remember that if onestation in a neighbourhood is listening witha radiating aerial to a distant station, and hekeeps still, all the others can keep just offoscillation point and receivefairly well, usingthe other man's radiation as a separate hetero-dyne, but one man "swishing" will ruin every-thing. So use a separate heterodyne ifyou possibly can, and whatever happens,if you hear anything, go for it carefully andsteadily. Don't get wildly excited and" swish " up and down on the signal. A littlefriendly co-operation is worth a lot of fancyanti -radiation circuits.

American Broadcasting Stations.It is interesting to note that at the time

of going to press, the American broadcastingstations are now being received some 5o percent. louder than during the last month.


Some Notes on the Sources of Energy Loss inCondensers.

BY PHILIP R. COURSEY, B.Sc., F.Inst.P., A.M.I.E.E.

The occurrence of losses in condensers, and their resultant effect upon a particularcircuit. is a subject of which the experimenter has had little chance of investigating.Below will be found the first part of a simple discussion, and should prove ofconsiderable value to those to whom the subject is new.

WHEN, as in the early days of electricalwork, the use of condensers wasconfined to the storage of small

high -voltage electrical charges, absence ofleakage was a property of prime importance.Condensers were then of the Leyden jarform with a glass dielectric, and probablytheir chief application was to Wimshurst andsimilar electrical machines.

The phenomena of dielectric absorptionand residual charge gave then an indicationthat the properties of the dielectric in suchcondensers were more complex than was tobe suspected from the straightforward charge

3Fig. 1.-A typical arrangement of a smoothing condenser for

double wave rectification.

and discharge effects, while this complexityhas only been further emphasised by recentdevelopments in high -power continuous -waveradio.

Considering for a moment this questionof insulation resistance, it is evident thatwhen a condenser is maintained charged bya source of E.M.F.-such as in the case ofthe example quoted above of the Leydenjar charged by an electrical machine-there

will be some loss of charge by leakage, thisloss taking the form of a leakage currentflow through the condenser insulation. Ifthe condenser is charged to a potentialof, say, Io,000 volts, and its insulationresistance is only as low as 10 megohms,the leakage current through the dielectricwill evidently be-

y 104

Ii 12/ 107 =0431 ampere.==

Such a leak would be a very serious one,and if the condenser potential were main-tained it would involve a not inconsiderableexpenditure of energy.

This energy loss is-v2 108

W R 107-10 watts.

The whole of this energy would be expendedin heating up the dielectric in the condenser,and any other insulation-such as at theterminals, etc.-where the leakage wastaking place. Although an energy loss ofro watts is not much if it is in the open,if it occurs in a closed -up space, as in thedielectric of a condenser, it may lead in asmall condenser to a considerable tempera-ture rise. In actual practice the loss shouldbe much less than this figure.

This type of condenser loss is one that is,or may be, encountered in the case of smooth-ing condensers used in connection withrectifying valves, as at C in Fig. i, which isthe conventional arrangement for " two -wave " rectification. If the rectifying equip-ment is doing nothing beyond charging thecondenser C to a potential of approximately0.7 V, where V is the R.M.S. voltage acrossthe outer ends of the secondary windingof the transformer T (assuming that theconnections are arranged as shown in Fig. i,with one terminal of the condenser connected

December, 1923. 144

to the mid -point tapping of the transformersecondary), the above source of loss will bethe only one when once the condenser hasbeen charged up to the above -mentionedvoltage. The energy loss will be as before -

2vWc = - watts (1)

Where W, = the energy loss due to D.C.leakage, and V,= the actual voltage acrossthe condenser C.

The extent of this loss can obviously bedetermined if the insulation resistance ofthe condenser is known. The value of thisquantity It1 which should be used in equation

600

r

500

400

,,`" 300

ti

200tt

/00

0


changes as when the whole of the leakagetakes place through the dielectric of thecondenser. Some idea as to the extent ofthis variation may be seen from Fig. 2, whichshows the apparent D.C. resistance of acondenser plotted against time as measuredfrom the instant of first application of acharging voltage. Curves are also given inthe diagram for different values of theapplied charging voltage, as this effect ofchanging apparent resistance is not inde-pendent of the voltage.

The changes in resistance with time aredue to the absorption of the charge by the

ear-oes ofApparenliesisternceCiondenser

220 Volts

0 20 4G0 80Time in Oeconds

Fig. 2.-Illustrating the apparent resistance of a condenser plotted against time.

(1) should be determined by measurementwith a direct -current voltage of the valueV, under which the condenser will be operat-ing. The reason for this statement is that,with most dielectrics used in commercialcondensers the apparent value of R1 is byno means constant, but varies with themethod of measurement. If the leak is avery serious one across terminal insulators,etc., there will not, as a rule, be such large

80 >00

dielectric-an absorption which not onlyshows itself in this way, but also by theresidual charges which are given out by thedielectric after the condenser has beendischarged.. While the charge is soakinginto the dielectric in this manner the currentflowing into the condenser is really mainlymade up of the charging current which issupplying these absorbed charges, and thiscurrent does not, therefore, accurately give


any indication of the true insulation resist-ance of the condenser.

This latter is the value that the apparentresistance takes up after an infinitely longcharging period. Expressed otherwise, itis the asymptotic value of the apparentresistance curves of Fig. 2. It is frequentlydesirable to be able to specify some value ofthe apparent insulation resistance of acondenser which can be readily determinedwithout the necessity of a very prolongedcharging period, and consequently in manycases the apparent value of the insulationresistance after a charging period of oneminute at a certain voltage is specified.Such a definition, while not, of course,giving the true insulation resistance, gives

145 December, 1923.

are usually quite unreliable on account offluctuations of voltage from the generator.

In actual practice, when a condenser isused for smoothing out the ripples from arectifying valve circuit for feeding a valvetransmitter, the load put upon the smooth-ing condenser by the oscillator valvesprevents the condenser from being chargedup to the full no-load voltage, and conse-quently there is a pulsation of voltage at thecondenser terminals. This state of affairsentails additional losses, as compared withwhat has already been considered. In thefirst place, the charging time during eachpulsation of the charging voltage is small, sothat the effective D.C. leakage loss is in-creased due to the apparent decrease of

Fig. 8.-Illustrating the condition of a condenser when a sine wave voltage is applied across it.

a figure which can be used for comparisonpurposes between a number of similarcondensers.

In connection with determinations ofinsulation resistance by measuring the flowof current through the condenser under animpressed D.C. voltage on the lines indicatedabove, it is desirable to bear in mind thenecessity for a truly steady voltage. Anyfluctuation of voltage, such as may be dueto commutator ripples in a generator supply-ing the circuit, may cause violent fluctuationsin the reading of the microammeter or galvano-meter which is used to measure the leakagecurrent, due to the comparatively large cur-rents flowing into or out of the condensercharging or discharging it in accordancewith the voltage fluctuations. Such effectsmay entirely mask the readings which it isdesired to obtain. For a similar reasonreadings taken with an ordinary " megger "

insulation resistance caused thereby. ThisD.C. leakage loss may thus be regarded asmade up of two parts, the first being thesteady D.C. leakage loss due to the portionof the rectified voltage which does notpulsate, and the second due to the similarloss arising through the short chargingperiod during each pulsation of the totalvoltage. Reference to Fig. 3 may make thisclearer. In this diagram the sine curve AAis intended to represent the applied voltage,while the curve B, C, D, E, F is the curve ofvoltage across the condenser terminals assum-ing single -wave rectification. This lattercurve rises during the periods B, C and D, E,during which the condenser is being rechargedby the supply at a greater rate than it isbeing discharged by the load circuit of theoscillator valves.

During the periods C, D and E, F thecondenser is not being charged by the supply

December, 1923. 146

circuit, but its potential is falling under thesteady discharge of the load circuit. Thusthe condenser voltage curve can be regardedas made up of two parts, a steady part ofvalue Vo and a pulsating part V1. Hencewe may also look upon the D.C. energy lossin the condenser as made up of two parts :the first VoYi,,, where R10 is the true D.C.insulation resistance of the condenser understeady applied voltage V0, in the sense thathas been defined in this article ; and thesecond V12/412/ where R1, is the apparentinsulation resistance of the condenser underapplied voltage V1/2 for a short time ofapplication of the voltage represented by theperiod of the A.C. supply, and V1/2 is themean voltage during this period. Thissecond quantity is one which it is, perhaps,easier to measure than to define, as thedetermination of R11 is likely to prove adifficult experimental feat. The effectiveenergy loss could more readily be determinedby difference between the total loss and thatdue to "%re/Rh .

Unfortunately, however, this is not theend of the story, as the determination ofthis portion of the energy loss by difference,as just indicated, would be likely to give afalse value to this quantity, for the reasonthat other sources of energy loss might beincluded. Thus, for instance, it is possibleto look upon this extra loss in other ways,and to consider it as partly due to the energyloss consequent upon the soaking in and outof the absorbed charges. The cause of theabsorption effects which have here beendiscussed may be looked upon as the chargingup of particles of the dielectric material(particles possibly of molecular dimensions)in the interior of the mass of the dielectric,the charges having to filter their way throughthe molecules of the dielectric in this process.This conduction of the charges into theinterior entails an energy loss due to the flowof the charging current through the resistanceof the body of the dielectric. As we haveseen from the curves in Fig. 2, the magnitudeof these charging currents may be muchgreater than that of .the final steady currentsthrough the dielectric, and in consequencethe actual energy loss due to them may becorrespondingly greater. This may be clearerfrom Fig. 4, which expresses similar measure-ments to those plotted in Fig. 2, but in this


case given in terms of the current flowthrough the condenser dielectric.

The magnitude of this source of energy losswill evidently depend upon the frequencyof the pulsations of voltage applied to thecondenser, for each time the condenser ischarged and discharged these absorptioncharge currents must flow into and out of thedielectric. Hence, on this basis, this losswill increase approximately in proportionto the frequency. As the frequency is raised,however, another effect begins to come intoplay. We have seen that this absorptionof the charges is a process requiring time,and therefore it follows that if the actual .charging period is very short (as must neces-sarily be the case if the frequency of thecharges and discharges becomes large) lesscharge will be absorbed than would be thecase for longer charging times.

At ordinary commercial supply circuitfrequencies this reduction of the absorptioneffect does not seem to be very pronounced,but there is evidence indicating that for thehigher frequencies there is a marked reductionin the absorption, so much so that thereduction of absorption may overtake theincrease of loss due to the higher frequency,with the result that there is a nett reductionof loss at very high frequencies. The experi-mental difficulties surrounding measurementsof this nature render an exact determinationof these changes by no means easy, but atleast in the case of some materials whichshow considerable absorption losses at com-paratively low frequencies (in the neighbour-hood of i,000 y ) there is certainly much lessloss due to this cause when they are subjectedto radio -frequency voltage pulsations.

This effect may perhaps more simplybe expressed by saying that at radio fre-quencies there is no time for the charges tosoak into the material, with the result that,under these conditions, the materials behaveas much more perfect dielectrics. Whetherthese results are truly general for all dielec-trics, or are confined to a few only, canonly be settled by exhaustive experimentalinvestigations.

Still another effect needs consideration ifthe voltage pulsation (V1 in Fig. 3) is of anyconsiderable amplitude on a condenser usedfor smoothing purposes with a valve rectifier.This is the A.C. energy loss in the condenserdielectric due to the voltage pulsations. The


3

6

o4

3

0

147 December, 1923.

Curves of Current flow through Condenserunder various afifilied .D.e.VoZtages.

9000 Volts.

500 Volts.

220 Volts.

0 20 60Time inA_Sleconds.

Fig. 4.-Showing how the current through a condenser at various voltages varies with the time.

application of an E.M.F. between the platesof a condenser implies that the condenserdielectric is subjected to a mechanical strainas well as to an electrical one. This strainwill be of an alternating or pulsating nature,depending upon the type of potential varia-tions to which the condenser is subjected.In a condenser built up of a stack of alternatesheets of tinfoil and paper fastened togetherwith a suitable binding, such as of tape,it is quite possible to obtain audible evidenceof these mechanical strains. By applyingan alternating E.M.F. to such a condenserthe varying attractions and repulsionsbetween the condenser plates set up anactual mechanical vibration of the electrodeplates, which will give rise to a sound if theA.C. frequency is a suitable one. Suchmechanical stressing of the dielectric andmechanical vibration of the electrode platesentails the expenditure of energy, which,

80 100

since it is drawn from the electrical circuitin which the condenser is connected, repre-sents another source of energy loss in thecondenser.

In a properly designed condenser this lossshould be a very small one, but its magnitudewill depend very greatly upon the nature ofthe material used for the dielectric, and uponthe method employed for the construction ofthe condenser.

To summarise, then, the main energy lossin a condenser used for D.C. smoothingpurposes may be regarded as made up ofthree parts :-

(r) The leakage loss W1 , which may itselfbe resolved into two components-

---V02/1240 ±V12/4R/, (la)

(2) The absorption loss W a, which may bewritt en-

Ws =V f (n) (2)

December, 1923. 148

(3) The vibration loss due to voltagepulsation-

(V,2n) (3)

So that we may therefore write-Wa Wv (4)

The exact type of the functions expressedby f and f' will vary with different dielec-trics, and probably also with the numericalmagnitude of the frequency, as has alreadybeen indicated. The vibration loss W hasbeen put down as a function of Via (whereV1 is the applied pulsating voltage), since

EXPERIMENTAL WIRELESS

the mechanical attraction between theoppositely charged plates will thus varywith the voltage between them.

In a badly constructed condenser theremay also be some small additional loss dueto the flow of the charging currents throughthe conducting plates themselves, but thisloss should be negligible in a condenser usedsolely for smoothing purposes. It maybecome serious in some cases when thecondenser is carrying considerable high -frequency currents, but this case will beconsidered in a later section.

(To be continued.)

Artificial Aerials.MANY applicants for transmitting

licences have received from theP.M.G. authority to transmit on a

non -radiating artificial aerial circuit, andconsequently a few notes on artificial aerialworking will no doubt be of interest. Theartificial aerial is defined as a closed non -earthed oscillatory circuit containing in-ductance, capacity and resistance. Nowmany amateurs say on receiving such alicence that this is not any use to them.However, if any experimental work is really

Ifeter 0- 5

Pic?activeresistance

5 Ohms.10.1nyamrox

A.

Transmitter.

I.0003

Fig. 1-Showing the connections of the artificial aerials tothe transmitter.

intended, a great deal can be done. Iconsider that my own most useful work hasbeen accomplished with an artificial aerial,and many others will agree that they findit of equal use.

The artificial aerial is so called because itis a closed circuit which is intended to shuntthe aerial earth terminals of the trans-mitter, thereby replacing the aerial earthsystem. It is meant to have just the samecharacteristics as an outside aerial, so thaton replacing one by the other the operation

of the transmitter is not interfered with inany way. The usual amateur aerial con-stants are approximately as follows :-Capacity .0003 mfd., resistance, say ro ohms,inductance ro mhys. These figures of coursereally only apply to one particular wavelength, but serve as a guide for a suitablecircuit. If a .5 amp. max. aerial currentmeter (hot-wire) is used we have in this aresistance of say, 5 ohms. The other 5 orso ohms can be made of about No. 3o S.W.G.Eureka on a 2 -in. former, which will providea small inductance to suit the conditions.The condenser should be as good a one aspossible. Two or three large copper orbrass plates carefully air -spaced and madenearly self-supporting, so that little insula-tion is needed, will be found very satisfac-tory. These three components are con-nected in series all across the transmitteroutput terminals. The transmitter is thenadjusted in the normal way as if the usualaerial and earth system were connected tothe transmitter. The efficiency of the setcan be found as the resistance of the arti-ficial circuit is known and also the currentin it (see EXPERIMENTAL WIRELESS No. 1,Efficient Transmission).

By using such a circuit as this a greatdeal of useful information may be obtained,and I hope that many will now conduct someof their testing on artificial circuits so as toleave a clearer ether for others who haveexperiments which of necessity have to becarried out on a radiating aerial.

" 2 SH."


The Design and Construction of Filters.BY FREDERIC L. HOGG (2SH).

The design of a smoothing circuit seems to be a subject which presents considerabledifficulty when the frequency is rather low. Many amateur experimenters spoiltheir telephony by generator hum, and it is hoped that the following article will helpin the solution of the problem.

IN radiophone transmitters it is essentialthat the anode voltage supply shouldbe perfectly pure D.C. Now, when

this supply is obtained from a D.C. generatoror from rectified A.C. there is a considerableripple on the steady D.C. voltage which

rn-6-6M A_L

ti

gerc;es _B

Fig. 1(a)-A series resonant circuit.

completely spoils any telephony. The usualprocedure in amateur stations is to grab holdof half a dozen old spark coils and Mansbridgecondensers and hope for the best. Needlessto say, the best is usually the worst. It

1(b)-A parallel resonant circuit.

would not be far out to say that there areprobably not more than half a dozen amateursusing A.C. for their 'phone sets whose smooth-ing is absolutely perfect in this country. Inthis article it is proposed to deal with thedesign and construction of wave filters fromthe amateurs' standpoint.

An electric filter is an apparatus whichenables us to separate into its various partsan electric current made up of a combina-

tion of different frequencies. For instance,a small condenser of, say, oor mfd. willdiscriminate between a radio -frequencycurrent of 1,500,000 cA and a speechfrequency of Boo The action of aninductance is the reverse. The simplestpractical filter is a resonant circuit acrossthe supply, either series or parallel (seeFig. r). If we apply an A.C. voltage to

\To lg.

I/veclatuot

Impedance

,rOCurrent

o SO /00 150 200 250 300

Frefueney.Fig. 2-illuttratIng the relation between current and tre-

quercy applied to a series circuit.

one of these circuits, and vary the frequency,we find that in the series circuit the currentwill increase as we increase the frequencyup to a certain point, and then will fall offagain on further increase (see Fig. 2). Theimpedance or effective resistance is shownalso in Fig. 2. It will be seen that theimpedance and current are respectively atminimum and maximum at a certain point.This is called the resonance point. For aparallel circuit we get just the reverse of

4

December, .1923. 15o 'EXPERIMENTAL WIRELESS.

what a series circuit gives us. In thesecases we have assumed that the filter is ofinfinitely low resistance. We will nowinvestigate the case when the filter containsa resistance, or, in other words, has a loadtaken from it. Let us consider, again, theseries circuit. We take our load acrossA, B. The effects of various loads are shownin the diagram (Fig. 3). From elementaryA.C. theory the voltage across the condenserwill be :-

eE

1[1- (2 wf)2 LC] -1- [27-1IT

Where E is the supply voltage, f its frequency,and L, C and R the constants of the circuit.

140

120

100

80

,.41. o0

20

0

1000w

800 w

It SOO w

50 100 150.Frefteency.

Fig. 8-Illustrating the effect of various loads applied to aswiss filter.

200

In the diagramthe curves have been workedout for E=ioo volts, L =1 henry, C=f mfd.,and R L000, Boo and 500 ohms. It will beseen that any filter works best for a particularload. This difficulty, however, does notarise very much in practice, except when asmall filter is overloaded. However, it mustnot be overlooked in building a filter. Now,it will be seen that this series circuit willtransmit current below a certain frequency,and above this frequency the current rapidlydiminishes, or, rather, the circuit increases

rUIRT(515ZTh

Fig. 4-An Inductance Is connected In series with the load.

in resistance and so cuts down the current.This type of circuit is known as a " low-pass "filter. It will be obvious that if we put theload across the inductance we should get thereverse effect, and this is really, in effect,a parallel circuit. This type is known asa high-pass " filter.

The simple series circuit mentioned abovedoes not give a very sharp separation offrequencies. This can be improved con -

f

Fig. 5-The " TC" type of filter

siderably by inserting an inductance inseries with the load. This is called a Tsection (Fig. 4). If we were to re -drawFig. 3 for this section we should find thatthe cut-off would be very much sharper,and that it would be moved to 141(zoo x Also for heavy loads it wouldbe found that the voltage first falls and thenrises again to normal at the cut-off frequency,falling once more as the frequency rises.Note that the two inductances have the samevalues. In all types of filters the inductancesor condensers should all be of the samesize except the end sections, which should

Fig. C-A "high pass" filter circuit.


Fig. 7-A "band-pass" filter circuit.

be of half the capacity or inductance as thecase may be. The reason for this is fairlyobvious. The addition of further sectionsto the filter will have the effect of sharpeningthe cut-off and increasing the number ofmaxima and minima in the portion of thecurve before the cut-off frequency.

Another type of section is the I:. This isshown in Fig. 5. The same remarks applyhere. Now, we can build either a filterwhich will pass all below a certain frequency,and, under the same conditions, one which

Fig. 8-A "band -elimination" filter circuit.

will have the reverse effect. Now, if weinsert a low-pass filter in our line, cuttingoff at, say, 200 u) , and follow this up bya high-pass filter with a cut off of zoo r ,we should get what is known as a band-passfilter, passing all frequencies between iooand 200 , and by reversing the cut-offfrequencies we get a filter of the bandelimination type between ioo and 200However, as we are only interested in thelow-pass type, no further mention of theothers will be made. Figs. 6, 7 and 8 show

Fig. 9-A two -section T type filter.

151 December, 1923.

complete high-pass band-pass and band -elimination filters respectively.

To get to the more particular design oflow-pass filters, the simple T and rc typesdealt with above will give us a fairly gradualcut-off, but the attenuation increases withthe frequency, i.e., they become better andbetter as filters. Now, by inserting resonantcircuits in series with or across the line wecan make our filter give us a very sharpcut-off, but the attenuation will decreaseas the frequency rises. Thus we could makea filter of this type to cut off at ioo

T T,Fig, 10-A two section It type filter.

and give maximum attenuation at 120 cy)for some special purpose. The calculationsin the design are rather complicated, andwould be of little use, so will not be given here.Figs. 9 and 10 show two -section T and 7rfilters giving distribution of inductance Land capacity C.

To find L and C for any particular set ofconditions, if Z is the resistance of the load

2

30-50 H./1111111

2Mfd.

Fig. 11-A practical filter for 50 cycles.

in ohms, and w is 27c times the cut-offfrequency -

2ZL = -w and C = 2w Z

Z would be the effective internal resistanceof the valve in the cases we are considering,or the total resistance of one or more in

parallel. This would be given byanode volts

anode currentwhen actually working, and not the steady

4*

December, 1923. 152 EXPERIMENTAL WIRELESS,

internal resistance value, as the valve isosc Hating and is transferring energy to theaerial circuit. In designing a filter for anexperimental set the filter should be ofgenerous dimensions, as a large filter onlow power is better than a small filter onhigh power.

.9H.000

611:fd

>0-3H.

4351Y:id. 61Y:fd.

filter such as Fig. 13 will really filter andgive almost perfect D.C. under the moststringent tests. Values are shown againstthe various components.

The average amateur will say, perhaps,that he has already 5o to ioo henries forhis filter. But, unfortunately, iron core

10.3H.1111

CT.osz-1-.PC)

14.S H4 I 1 I 1

8216rd.

3.9H(bbd`

.4351frd

Fig 12-illustrating the arrangement of a correctly designed filter circuit for 50 shales.

In our case, when we are dealing principallywith rectified A.C., we can take our waveform as consisting of a large number ofharmonics, or we can consider it as wavein which there are gaps to be filled in Inthe first case a certain amount of brain isused, whereas in the second we get D.C.because the A.C. cannot help itself when weput huge capacities and inductances in. Afilter for 5o v) A.C. should be designed tocut-off at about 20 (4 . It will be found,

12.6H.

chokes must be designed just as much asanything else. First of all, a closed corechoke must not be used. A closed corefosters the harmonics and often does moreharm than good. Also if care is not takenthe steady D.C. current present will saturatethe core. The choke should be made witha small air gap and sufficient iron to ensurethat it is never near saturation. A goodtransformer iron, preferably stalloy, about.015 in. thick, should be used. Fig. 14 shows

30 -SOX.(6oszoon'\----.

-113111j-e d31iff d.

13-6H.(Rio

Fig. 13-A resultait Alter which Is a modification of Figs. 11 and 12 two tuned traps being Insetted.

unfortunately or fortunately, whicheverway you look at it, that dealing with theirregular wave forms which we get the" brutal " method is best, though a judiciouscombination of both helps. For an averageset Fig. ix gives an excellent filter.Fig. 12 gives a most excellent design,but works not quite so well as Fig.Ir ! An improvement can be made byinserting a few tuned traps, and a

type of core and windings. All the wireshould be placed on one limb, and the twoends should be butt -jointed instead oflapped. The other limb must be adjustableand rigidly fixable in any position. Thetable gives sizes of core (square) and otherdetails for various currents and inductances :

" X " and " Y " are the actual dimensionsof space occupied by wire. I in. or morespace must be left for cheeks, according to


size, so that the length of longer laminationswill be X + (depth of core) to in. spacefor cheeks. Enamelled wire is quite suitable,but the winding must be well insulatedfrom the core. On the larger sizes a layer of

1.53December, 1923.

iron core choke very tightly so that the airgap cannot alter. Considerable force existsin such a choke. When choke control isused for telephony it is best to use anotherchoke in the positive lead apart from the

Current Herfries. Air Gap. Turns. S.W.G. X. Y. Core. Volt

amp. in. in. in. in. Drop.

-05 1 1/64 2,300 .5 .33 7

-05 5 1/40 3,500 -62 -42 ixt 14

.0505

1020

1/323/64

3,8005,700

36-64.78

.43

.52lx1lx1

1829

05 50 7/64 11,000 140 -75 lx1 li4

-05 100 f 8,900 j .97 -65 2x2 76

1 1/50 1,500 -53 .37 /xi 6

1-1I

51020

1/401/323/64

2,6001,9002,900 33

.7160

-

-75

.49-42-51

lx12x22x2

141626

1 50 7/64 5,300 1.00 -70 2x2 50

-1 100 i 8,900 t. 1'33 -90 2x2 90

25 1 1/50 1,100 8752 .5 lx1 6

25 5 1,300 I .53 2x2 10

2525

1020

5/163/64

1,300 I1,900 27

-821.00

.53-65

3x33x3

1624

.25 50 1/3 5,0001 1.60 1.1 3x3 70

-25 100 19/32 8,400 2.1 1.4 3x3 120

waxed paper should be placed over everyfew layers. The inductance can be variedby adjusting the air gap or by changingthe number of turns. It must be borne inmind that the inductance will vary approxi-mately as the square of the number of turns.The air -gap figures are meant merely toindicate the order of magnitude of the gap.It should be varied for best results. A good

1--Y1

/.; / :

Air svaii.

Winding,Isfuice.

179. 14 --Constructional details of a smoothing choke.

method of testing is to put a loud speakerin series with a good ooi condenser across theoutput on load and to vary chokes, etc.,till hum is least. Don't use a pair of 'phonesand wear them. If you use 1,50o volts orso a shock on the head is unpleasant, tosay the least of it ; and if you use 2,50o voltsor so your filter will be no use to you, howevergood it is, because you would probably bedead ! Care must be taken to clamp the

usual filter chokes. The size of chokenecessary is given by-

L = 2 of I,where E=anode volts,

I=anode current,f=mean speech frequency,=800 Co.

though usually about 10-20 henries is foundsufficient for all-round purposes. This canbe designed from the tables given above.

In this short article it has only beenpossible to touch on a few points of the sub-ject, but I hope sufficient information willbe found herein to enable anyone to getrid of any ripple in his plate supply. If butone of the stations who appear to do tele-phony on raw A.C. is cured of trouble Iam satisfied !

The Construction and Manipulationof Wavemeters.

Readers may have noticed that in Fig. 7,Page 74, showing the wiring lay-out of aheterodyne wavemeter, the positive andnegative poles of the high tension batterywere shown reversed. This point should beremembered when making the instrument.

December, 1923. 154 EXPERIMENTAL WIRELESS_

Notes on High Tension ElectrolyticRectifiers.

B. E. H. ROBINSON (2VW).

As the chemical rectifier is now used for obtaining the high anode voltage for transmis-sion purposes by many experimenters, it is thought that the following notes by one whowas the first to develop it in these directions will be of considerable value.

IN an issue of the Wireless World andRadio Review last December the writer

described an aluminium electrolyticrectifier suitable for rectifying high -voltagecurrents to produce D.C. suitable for valvetransmitters. Since then a number of in-quiries for further details have been receivedconcerning the construction and workingof such rectifiers and in view of the fact thatmany experimenters are now adopting thismethod of obtaining their H.T. supply thefollowing notes may be found useful.

Suitable A.C. Frequency.In the first place it should be noted that

the electrolytic rectifier is only suitable towork on an alternating current supply whoseperiodicity does not exceed about 120 cyclesper second. A certain amount of rectifica-tion may be obtained on periodicities up to200, but not much higher, as the rectificationbecomes extremely poor. Electroly tic recti-fiers cannot be used with 50o cycle suppliessuch as are obtained from some DisposalsBoard alternators. Similarly there is nohope of using electrolytic rectifiers with aT.V.T. unit, as such units give an unsuitablewave form at too high a frequency.

Electrolytic rectifiers work best onsinusoidal A.C. at the usual commercialfrequencies between 5o and 90 cycles persecond. On these low frequencies very goodresults may be obtained at quite a favourableoverall efficiency.

Electrodes.For the cathodes of the rectifier cells

aluminium is almost invariably used, andas it is at the surface of the aluminium thatall the rectification takes place, too muchattention cannot be given to the properdesign and maintenance of these aluminiumelectrodes. In the original Nodon valvethe aluminium was alloyed with a certain

amount of zinc or other metal, but ordinarycommercial sheet aluminium is quite suitableprovided that it is properly cleaned. It isa good plan thoroughly to clean the surfaceof the aluminium electrodes the last thingbefore mounting them in situ in the rectifiercells. This may be done by immersing themin a solution of caustic soda for about tenminutes or until the surfaces are slightlybut uniformly corroded ; they are rinsed freefrom caustic soda, drained and immersedin strong nitric acid for a few minutes.Nitric acid attacks practically all impurities,but not the aluminium, so that after a secondthorough rinsing in clean water the alumin-ium should have a matt silvery -whiteappearance.

The actual metal used as anode in thecells is not very important as its only func-tion is to make contact with the electrolyteand takes no part in the rectifying action.The only important thing is that the 'anodeshould not be attacked by the electrolyte.The most common substances used are iron,carbon, tin and lead, the latter being thebest from practical considerations. Ironshows a tendency to slow corrosion whenammonium salts are used for the solution,whereas lead is absolutely permanent. Inuse a brown deposit of lead peroxide formson the surface of the lead, but this does notappear to be in any way detrimental.

The Electrolyte.The choice of electrolyte is one of the

most important factors. A number ofneutral salts give solutions in which analuminium electrode shows a rectifying actionto a greater or less extent, but only a fewsalts combine efficient rectification withclean working. Of these, pure ammoniumphosphate is one of the most readily pro-curable and, in the writer's experience, themost efficient. Sodium phosphate is con-


siderably cheaper and gives fair results, butis distinctly inferior to the ammonium salt.When the ammonium phosphate is usedcare should be taken that the solution doesnot give acid reaction ; it should be neutral,or, if anything, very slightly alkaline. Thecheaper commercial grades of ammoniumphosphate are apt sometimes to be ratherstrongly acid. It is therefore advisableto buy the pure phosphate if the extraexpense is not objected to. Even the latteris sometimes strongly enough acid to turna piece of blue litmus paper red, and whenthis is found to be the case the solutionshould be rendered neutral before using.This may be done by adding either somestrong ammonia solution (sp. gr. .88o), or a

155 December, 1923.

properties of the surface. Also a saturatedsolution seems to favour the formation ofsludge and colloidal aluminium hydroxidemore than a weaker solution does.

The following data about sodium andammonium phosphates are useful whensolutions are being made up.

AMMONIUM PHOSPHATE.

Chemical formula of the salt manufacturedcommercially (N1-1,),HPO4.

Solubility :-One part in four parts ofwater (by weight).

Weight of ammonium phosphate requiredfor half -saturated solution is

125 gms. per litre of water, or21 ounces per pint (very nearly).

Fig. 1.-An electrolytic rectifier, employing the connections shown in Fig. 3, capable of delivering 50 milliamps. at 1,000volts of ful. cycle rectified D.C. The small quantity of sediment which forms at the bottom of the cells does net matter aslong as it does not reach the aluminium strips.

half -saturated solution of sodium phosphate,and stirring until the phosphate solutionwill just turn a piece of red litmus paper blue.If the neutralising is done with sodiumphosphate the solution obtained will, ofcourse, contain a mixture of the two phos-phates, but this is found to work quite well.

With regard to the strength of solution thewriter recommends that whichever salt isused as electrolyte the solution should behalf or three-quarters saturated. The fre-quent practice of using a fully saturatedsolution is not to be recommended becauseas soon as a little of the water in the solutionevaporates some of the dissolved salt crystal-lises out, the crystals not infrequentlyforming on the aluminium surfaces, therebycausing leakage and spoiling the rectifying

SODIUM PHOSPHATE.

Formula of commercially manufacturedsalt M.81)04.'21-120.

Solubility one part in five parts of water(by weight).

Weight of sodium phosphate crystals tomake a half -saturated solution-

ioo per litre of water, or2 ounces per pint.In comparing the performance of sodium

and ammonium phosphates it is interestingto note that sodium phosphate becomesstrongly alkaline after continued use whereasammonium phosphate tends to lose ammoniaand become acid. Partially used sodiumphosphate is found often to contain freecaustic soda, which accounts for the fact

December, 1923. 156

that the rectification degenerates and thealuminium becomes corroded when this saltis used. This does not seem to happen withammonium phosphate.

Other electrolytes than the two mentionedabove may be used, but none seem to surpassammonium phosphate. Sodium bicarbonateand borax are two commonly recommendedsubstitutes. Borax, in particular, is fre-quently recommended in American periodi-cals for high-tension rectifiers, but thewriter has tried it several times and foundit hopeless. In the first place, boron is onlysparingly soluble in water and forms acomparatively poorly conducting solution.Secondly, the rectification is not particularlygood and the borax tends to crystallise outon to the aluminium electrodes. Even if

4:4

Fig. 2.-Showing the Grata connections for full -cycle rectification.

the freshly made up cell rectifies the alu-minium becomes covered in a short timewith an incrustation of goodness knows what,and the rectified output goes down to anegligible quantity. Similar remarks applyof sodium bicarbonate.

Voltages and Currents Allowable.If a rectifier cell is connected in series with

an ammeter across a source of direct currentquite a large current will be registered whenthe supply is connected so as to make thecurrent pass from lead to aluminium. Onreversing cell, however, so as to make thecurrent pass from aluminium to lead onlya very small current flows. This reversecurrent may be only a few milliampereswhen the voltage of the D.C. source doesnot exceed about 15o volts. If the appliedvoltage is raised much above 200 the insula-ting film which is formed on the submergedsurface of the aluminium begins to breakdown, the cell heats up and passes a largecurrent. A very good cell may hold upagainst 240 volts of reverse e.m.f. for quite

EXPERIMENTAL -WIRELESS.

a considerable period. For rectifying 5oor 6o cycles A.C. such high voltage shouldnot be allowed per cell. A good workingbasis is to allow ioo volts per cell of A.C.That is, if we wish to rectify i,000 volts(R.M.S.) A.C. input with the four-group(Gratz) connections we should have ten cellsin each group to sub -divide the potentialstrain. As a matter of fact it has beenfound that when in good condition rectifiercells will often work quite happily at 120to 14o volts apiece without unduly largeback currents occurring. The higher thevoltage that can be allowed per cell the lesswill be the total number of cells requiredand the higher will be the overall efficiencyof the rectifier, provided always that heatingand excessive reverse currents are not setup. Owing to the fact that aluminiumelectrolytic rectifiers show a maximumefficiency when working at about Rio voltsper cell they are more satisfactory for high-tension low current work than for largecurrents at only a few volts as in accumulatorcharging. Thus the fact that the rectifier hasrather fallen into discredit for batterycharging should. not unduly bias our opinionwhen judging it for H.T. purposes.

The size of the aluminium electrodesshould be properly proportioned to thecurrent which is to be rectified. If thesurface area is too big the current densitywill be small and the electrodes will notpolarise rapidly enough to keep out reversecurrents. An analogy may be drawn witha pump whose valves are too sluggish to actif the plunger is moved too gently. If, onthe other hand, the surface area is too smallthe resistance of the cell in the right directionwill be too high and sufficient output willnot be obtained. The correct current den-sity is about 5 milliamps. per square centi-metre of submerged aluminium surface ;

this includes both back and front surfacesin the case of fiat strips or plates. Forrectifiers to deal with very small currentssuch as are wanted for receiving H.T.supply or for transmission on powers below5 watts aluminium wire of about No. 14gauge may be used with satisfactory results.For powers of fo watts or over it is moresatisfactory to use strip electrodes cut fromaluminium and lead sheets. The aluminiumshould be as thick as can be convenientlycut with a pair of metal shears. Aluminium


and lead strips may be connected togetherin pairs by drilling or punching holes at theirupper ends and clamping with small boltsand nuts. Fig. x clearly illustrates themethod of assembly. As at two points(output) there are lead -to -lead and alumin-ium -to -aluminium connections extra longstrips of lead and aluminium are bent overto may the two adjacent electrodes withoutthe necessity of clamping two separatepieces together.

The rectifier shown in the diagram willreadily deliver 5o milliamps. of D.C. (fullcycle rectification) at a voltage of 900 toi,opo, this being much more than is requiredby the ro-watt transmitter.

157 December, 1923.

When heating occurs the hot electrolyterises to the surface and it is just where thealuminium enters the solution that the mostrapid corrosion occurs ; bad electrical leakagefrequently occurs here also. This troubleis often met by sheathing the aluminiumwith a piece of tightly -fitting rubber tubingover the parts which extend above andabout half -an -inch immediately below thesurface of the solution. It is simpler,however, to float about an inch of paraffinoil on the surface of the solution. Thewriter has always found this effective, andit serves materially to prevent " creeping "and evaporation.

A.C:InAut.

0 0 0 0 0 0 0 0 0 0 0 00 0 0 0 0 0 0 0 0 0 0 0

-o 0+JleOut/zu.

Fig. 3.-Showing how the old Gratz connections are modified to handle voltages above 100.In each of the four groups.

One of the chief limitations of the. loadwhich may be put on to such a rectifier isthe heating up of the electrolyte. The onlysimple practical remedy is to make the cellsof ample size so that the bulk of liquid is toogreat to allow any serious rise in temperatureat the normal working current. On noaccount should the temperature be allowedto exceed 40° C. as the rectification is verypoor above this temperature and there is amarked tendency for the aluminium to becorroded, with the consequent formation ofa precipitate of aluminium hydroxide. Therectifier shown in the photograph employslarge test tubes (" boiling 'tubes "), 6 incheslong by Z inch diameter, which are filledwithin an inch of the top with half -saturatedammonium phosphate solution. The cellsget just slightly warm when run on a loadof 4o milliamps. for about half an hour.On smaller loads the rectifier may be runindefinitely. Very small rectifiers to handlea few milliamps. for reception purposes maybe quite satisfactorily made up in theordinary r 5i" test -tubes.

Allow one cell for every 100 volts

" Forming " the Electrodes.A freshly made up electrolytic rectifier

seldom rectifies immediately the A.C. inputis first applied, but the cells may for a timepass a heavy current in both directions.It seems as if some " forming " process withthe aluminium electrodes is necessary beforethe proper rectifying action sets in. Forthis reason it is not advisable to put a newrectifier straight across the full high-tensionvoltage, but the voltage should be cut downto begin with or a safety resistance insertedin series with the supply. Usually a rectifierforms' nearly completely after the currenthas been passing for about a quarter of anhour, after which period full voltage maybe applied. Sometimes, however, the recti-fier proves more refractory and a moredrastic method of gingering up the cellsmust be resorted to. The following plan isvery effective where zoo-volt D.C. mainsare available. Each cell in turn is connectedin series with a high -consumption lamp(about i amp. normal rating) across the20o -volt D.C. mains with the aluminium

December, 1923. z5

electrode connected to the positive mainlead. If, on switching on, the aluminiumis not formed the lamp will light up and arelatively large current will pass throughthe cell. As the aluminium surface formsthe current drops rapidly and the lamp goesout and if the cell is a good one the currentwill fall to a negligible value. If the cellis a dud the lamp will remain alight and thesolution will get hot and if on several suc-cessive tests the cell still refuses to formit may be taken that either the solution isat fault or the aluminium requires chemicallycleaning as described above. If only A.C.is available put two aluminiums in oppositionin one cell and apply about 120 volts inseries with a lamp ; this tests two aluminiumsat once. In the event of there being onlya 220 A.C. supply available two test cellswill have to be treated in series at a time.

If each cell has responded properly to thismethod of gingering up the rectifier as a wholeshould at once work satisfactorily off thenormal high-tension A.C. supply.

Once a rectifier is properly formed itshould work for weeks without furtherattention, provided that it is not undulyoverloaded.

Sometimes it is found that a cell refusesto rectify owing to a grey or black depositwhich forms on the surface of the aluminium.

re/zre

De. Out/1.a:

TransformerFig. 4.-Connections for obtaining full -cycle rectification,

using a centre -tapped transformer.

Strong nitric acid will usually clean this offwithout the previous use of caustic soda.Aluminium electrodes that are doing theirwork properly always have a very cleanwhite appearance.

Indications of Correct Working.If an electrolytic rectifier is viewed in the

dark when connected across the H.T. trans-former but with no load across the D.C.output a luminosity is seen on the submerged

S. EXPERIMENTAL WIRELESS..

surfaces of the aluminiums. Under idealcircumstances this takes the form of auniform glow, but more frequently it hasa sparkling appearance not unlike a starrysky on a very clear night. This scintillationis no cause for great anxiety-it only showsthat the cell is functioning well and is,perhaps, working near its voltage limit.It is the cells that show no luminosity inthe dark that are the black sheep in thefold ; in these either the solution is impure

0

N CI

ort

ditrhyral.

o

CIji

04tip'17::;

0Fig. 5-Double condenser connections for full -cycle rectifica-

tion. A doubling of voltage is obtained.

ED

or the aluminium needs forming or cleaning.It not infrequently occurs that all the cellsin a set are made up at the same time andunder apparently identical conditions, yetafter a short period of working some of thecells refuse to rectify while others work well.This is due, no doubt, to contamination onparts of the original sheet of aluminiumfrom which the electrodes were cut, and theremedy has already been indicated.

It is important that the bolts which holdadjacent strips together should be wellaway from the surface of the electrolyte.

Various Rectifier Connections.It is always worth while to use full -cycle

rectification in preference to half -cycle asa higher output is obtainable for a giventransformer voltage and it is very mucheasier to smooth the D.C. output from afull -cycle rectifier. Half -cycle rectified 50 -period A.C. gives a ripple frequency of 5o,whereas full -cycle rectification gives a ripplefrequency of Zoo. Other things being equal,the higher the ripple frequency the moreeasily is it smoothed out with chokes andcondensers.


Fig. 2 shows the original four -cell or Gratzconnections for full -cycle rectification asused for accumulator charging with large -capacity rectifier cells. Fig. 3 shows howthese connections are modified for high-tension purposes by replacing each of thecells in Fig. 2 by a bank of cells in series.This system of connections is used in therectifier shown in Fig. i and is very satis-factory for general purposes.

Fig. 4 shows the centre -tapped transformerarrangement which is well known and does notcall for much comment. It is well to note,however, that although only half the numberof cells is required to give the same voltageas the arrangement in Fig. 3, it has thepractical disadvantage that a special trans-former is needed with twice the number ofturns required in the ordinary way. Thusif one is working on r,000 volts each halfof the transformer secondary in Fig. 4 musteach give 2,000 volts, the total voltagebetween the outer ends being 2,000 volts.Thus not only is it necessary to use twice theamount of wire in the secondary, but it isnecessary to insulate for 2,000 volts insteadof 2,000. Readers who have attempted tomake efficient H.T. transformers will agreethat the task of getting well -insulatedsecondary into the small space available isone which does not admit of unnecessarymagnification.

Fig. 5 is one of the most ingenious andconvenient rectifier connections. The writercan thoroughly recommend it where it ispossible to obtain a couple of condensers ofat least 4 mfds. apiece which will stand thevoltage. The arrangement not only givesfull -cycle rectification, but doubles thetransformer voltage. Also for a given D.C.output only one -quarter of the number ofcells used in the Fig. 3 arrangement isrequired, thus reducing the labour involvedin constructing the rectifier as well as re-ducing the space taken up by the apparatus.Actually the D.C. output voltage on no loadwill be nearly twice the peak voltage of thetransformer or nearly 2.8 times the R.M.S.voltage. On load this will fall to a littleover twice the R.M.S. transformer voltage,depending upon the capacity of the con-densers and the efficiency of the rectifiercells. If one requires, say, D.C. at r,000volts with this arrangement only aboutro cells are required and the transformer

December, 1923.

secondary need only give 500 volts. Thearrangement in Fig. 5 should be particularlyuseful to men who have 220 -volt A.C. mains asit will give them nearly 500 volts D.C. with-out the use of any transformer at all. Thebigger the two condensers are the better. 6mfds. each is quite a good value, but if largercondensers are available so much the better.

In the event of any reader being fortunateenough to have a 3 -phase supply the con -

tiMy

O

Dee/a/Mi.0

Fig. 6.-Scheme of connections for rectifying a 3 -phase supply.

nections for a 3 -phase rectifier is shown inFig. 6. For simplicity each group is repre-sented by one cell. The great advantage ina three-phase rectifier is that the D.C. outputnever falls to zero owing to the overlappingof the phases, so that without any smoothingarrangements at all there is always a steadycontinuous current component with onlyabout 20 per cent. ripple. What ripplethere is has a frequency of six times thesupply frequency and is consequently veryeasily smoothed.

oQa

St. Dunstan's Broadcast.In connection with the St. Dunstan's

Carol League, a selection of Christmas Carolswill be simultaneously broadcast from 2L0between 8.3o and 9 p.m. on December 23by four gentlemen from St. Paul's CathedralChoir. St. Dunstan's wish to ask everyreader of EXPERIMENTAL WIRELESS toarrange listening -in parties amongst theirfriends, at which they will make a collectionon behalf 'of the funds which they so badlyneed. We have little doubt that readers willbe only too willing to comply with therequest, and special collecting envelopesmay be obtained from St. Dunstan's Head-quarters, Regent's Park, London, N.W.i.

December, 1923. 16o EXPERIMENTAL WIRELESS.

The Heterodyne Reception of ShortContinuous Waves.

BY CAPTAIN ST. CLAIR-FINLAY, B.Sc.E. (Laus.),Late Chief Experimental Officer, Inter -Allied Research Commission, etc.

It appears that certain difficulty is experienced in the reception of weak short-wavesignals. The following notes deal adequately with the subject, indicating how bestto obtain efficient amplification without complicated circuits, and at the same timesecuring the minimum of interference.

IN view of the forthcoming TransatlanticTests, for which it now behoves usseriously to consider the preparation

of our stations, a few notes upon the subjectof suitable receiving arrangements mayserve as a " refresher," and, may not beinopportune.

The essentials of a receiver suitable forsuch a purpose may be summed up brieflyas follows :-

(I) High sensitivity ;. (2) Ease of searching and tuning ;

(3) Self -silence ; and(4) Non-interference.

Selectivity being a matter of course, as willbe shown, whilst the fourth will speedilybecome obvious when two or more stationsin a given area commence searching for givensignals with energised aerials.

In order to design and effectively operatesuch a receiver the principles and con-siderations underlying the beat reception ofC.W. signals must be appreciated, and itis here proposed, firstly, to discuss theserelative to the special object in view, and,secondly, to describe certain forms of circuitwhich fulfil in considerable degree thenecessary requirements.

It is known, firstly, that this system ofreception is based upon the fact that whenwaves of different frequencies are allowedto interact, a third or " beat " frequencyresults which will'be equal to the differencebetween the originating frequencies andmust be lower than either, thus :-

fi3=f -f" or f" -f'where f'= frequency of incoming or received oscil-

lations,f'= frequency of local or heterodyning

oscillations,fo = frequency of resultant or beat oscillations,

so that, although the originating frequencies

may themselves be far above the limitsof audibility, a resultant beat of any desiredaudible frequency may be produced accordingto the difference given to the former.

It is also known that such resultant beatsmust be greater in amplitude than thelesser of the originating oscillations, and will,in fact, be equal to the sum of the latter,since-

= (I' + (I"- I') =21',where I'=amplitude of incoming oscillations,

V=amplitude of local oscillations,Io= amplitude of resultant beats,

so that an amplification of the originalsignals amounting to -f results from theheterodyne effect quite independently ofthe amplifying action of the valve or othermagnifier, and this ratio will hold good solong as the local oscillations are not inferiorin amplitude to the incoming oscillations.

If, however, the latter are allowed toexceed the former a somewhat differentstate of affairs results, with very far-reachingeffects. The production of beats, of course,remains unaltered, and the resultant ampli-fication also remains unaltered, except inthe important respect that, whereas in theformer case it was the incoming oscillationsthat were amplified, it will now be the localoscillations that will be amplified, thus :-

4=(1'+I")- (IV- -= 2I'',

with the result that the incoming signalsmay not now be amplified at all, but may,on the contrary, actually be limited by theheterodyne action, as would occur werethey of more than twice the amplitude.

The consequence of this being manifestlya loss in amplification of the desired signals,the importance of making the local oscilla-tions strong enough will be evident.

EXPERIMENTAL WIRELESS. 16x

Neither is this by any means all.reconsider the first case, where

Ip= gm +11- (Is-F)=21'.Now, under these conditions the selectivityof the arrangement will be at a maximum,since It3 will depend entirely upon I' andnot upon I", so that the heterodyne ampli-fication will be greatest under conditions ofresonance (because I' will then be at itsgreatest), as will naturally apply to desiredsignals, and will fall off greatly as signalsbecome detuned, as will apply to undesiredsignals unless these be of exactly the samefrequency.

But now, if I" be inferior to I', as in thesecond case, then the reverse condition

It1=-(r+I")-(V-I1=21"

Let us

obtains, which means that Ift is now entirelydependant upon I" and not at all upon I',so that variations in I'-whether of desiredor undesired signals-produce no effect uponresultant signal strength, and undesiredsignals, although mistuned and thereforerelatively weak, will consequently be heardas strongly as the desired signals so longas their amplitude be not less than that of I",selectivity which is normally so valuablea characteristic of heterodyne reception beingthus to a great extent lost. (Actually,this selectivity will be at a maximum whenI"=I', since under these conditions theresultant amplitude of undesired signals-however great their initial amplitude-willbe limited by I" and cannot exceed 21",which will only be the same as that ofthe desired signals, so that these will usuallyremain readably selectable to within a smallfraction of a metre-on short waves-byseparation of the respective beat-notes-thisbeing of considerable impertance wheninterference is experienced, fcr example,from high -power cr local stations operatingor producing harmonics at cr near the samewave-length-whereas if I" exceed I' thesemay greatly exceed and completely jam thedesired signals, it being thus sometimes ofadvantage to limit I" to I'.)

But this is not the only consideration.It will have been noted from the foregoingthat the amplification directly due to hetero-dyne action reaches a maximum when I'and I" are equal and cannot be increasedby increase of I" beyond this point. Actually,however, a secondary but important effect

December, 19237

of the local oscillations is to increase theefficiency of the detector by polarisation,so that an additional amplification effectindependent of the actual heterodyneamplification and due mainly to improvedrectification, is obtained, which increases(within limits) with the amplitude ofI", so that, in practice, best result willusually be obtained when I" is materiallyin excess of I', and is variable, which should,therefore, be provided for in the design of ourreceiver, whether this be of the separate orauto -heterodyne type.

0' A

0"

I 1.11

ff1111111

1.11111.11'

08

Ft& 1.-Illustrating the interaction of two oscillations of equalamplitude and different frequency.

The relative merits of these will be dis-cussed later, the next point to be touchedupon being that of the actual sensitivity ofthe receiver itself to weak signals.

This-given that the aerial system, etc.,has been made as efficient as possible-willdepend mainly upon :-

(a) The circuital resistance ;(b) The inter -electrode and circuital

capacities ;(c) The static inertia of the circuits ; and(d) The sensitivity of the detector as such

to feeble impulses.(a) and (b) have already been discussed andtheir vital importance emphasised by thewriter elsewhere in these pages (Vol. I,No. 1), and may, therefore, now be dismissedwith the bare statement that the resistanceand inter -electrode capacity of valve circuits,particularly H.F. valve circuits, should beas small as possible.

What may be termed the " static inertia "of circuits is a rather different-thoughclosely allied-matter, which, consideringits extreme importance where actua lsensi-tivity is concerned, is usually insufficiently

December; 1923. 162 EXPERIMENTAL WIRELESS.

considered. It should be appreciated that,whilst the current required by the gridto operate a valve is very small, and iscommonly dismissed as " negligible," it is,m reality, by no means negligible whenconsidered in relation to that available forits operation, e.g., the currents due to theincoming signals, which are also minute.Thus, since P.D. is always proportional tocharge for a given capacitance and inverselyproportional to capacitance for a givencharge, it follows that the smaller thecapacitance of a circuit the greater thepotentials .set up therein by given currents,';o that, valves being potential -operateddevices, the inference is obvious that allvalve circuits, and in particular grid circuits,should have as small a capacity as possibleif sensitivity to weak signals is desired.Since the valve electrodes themselves, aswell as external parts of the circuit-suchas wiring, terminals, condensers, etc.-willmanifestly enter into this matter of capacity,it follows that not only should the latterbe kept as small as possible consistent withefficiency in their respective functions, butthat the electrodes of valves intended todeal with weak signals-such, fcr instance,as H.F. valves-should also be small ; andthis, of course, applies especially to the firststage in a receiver, where a special high -frequency valve should always be used.

Now comes the all-important question ofdetector efficiency, upon which the successfulreception of weak signals above all depends.

It being essential in this case that thesystem adopted be specifically suitable tothe purpose intended, and not merely forgeneral use, crystal detectors, owing to theircomparative unreliability, can be ruled outand some form of valve detector regardedas essential ; and amongst these it will beobserved that no form of anode rectificationwill be suitable, since the characteristics ofsuch are sensitivity to strong and relativeinsensitivity to weak signals, which is thereverse of what we here require ; so thatsome form of grid rectification is indicated.This may be either of the grid current orcumulative variety, the latter, being some-what the more sensitive to very feebleimpulses, being the more suitable ; and eitherof these, moreover, permit of the detectorbeing utilised as the local oscillator in anautodyne circuit without loss of efficiency.

Our receiver should, therefore, employ leaky -grid -condenser rectification, and a valvespecially designed for this purpose shouldpreferably be used.

The next item of importance is amplifica-tion, i.e., valve amplification cs distinctfrom heterodyne amplification, which wehave already discussed ; and the first effectof this-assuming it to be on the radio -frequency side-will naturally be to increasethe amplitude of the incoming oscillationsso that stronger beats may be produced bythe heterodyne action.

Now, since signals received from distancesof 3,000 miles may be expected to be attenu-ated cut of all comparison with the localoscillations produced by even the smallestvalve, and since it is known that the rectifiedor telephone current output of a detectcris proportional to the square of the impulsesapplied to it, it will be evident that pre -heterodyne amplification of the signals will beof very great advantage in enabling reasonablystrong beats to be produced for rectification,and may, in this case, be regarded as essential.

A further advantage accompanying theuse of H.F. stages is the fact that it enablesa non -radiating receiver-even of the auto -dyne type-to be readily designed.

Since, however, radio -frequency valvecircuits are somewhat tricky and liable tocause trouble if over multiplied, H.F..amplification has certain limits imposed bythe need for practicability in a receiverand should not be overdone, and the con-dition to be aimed at is maximum ampli-fication efficiency per valve, so that thenumber of stages necessary to produce agiven result may be as few as possible-which, of course, also holds good from thestandpoint of economy.

Since, moreover, this condition may befulfilled by efficiency of circuital arrange-ments only up to a certain point (vide" practicability ' below), it must be fulfilledas far as possible by the valves themselves-i.e., the amplification factor of valves usedfor radio -frequency amplification should behigh, and their characteristics made as steepas possible by suitable values of plate voltageand filament temperature.

This will, in the present case, apply alsoto audio -frequency stages, since many suchwill be undesirable owing to their tendencyto noisiness ; in fact, in the interests of self-

163 December, 1923.

tuned circuits throughout the radio -frequencyside of a receiver, for example, but since thelatter puts this out of the question wheremore than one-or at most two-H.F. stagesare concerned, we must endeavour to findthe best possible compromise.

Since the tendency to instability increaseswith each stage and becomes dispropor-tionately greater with each tuned stage, andinasmuch as the complexity of tuningincreases as the square of the number oftuned circuits employed, it will be evidentthat the latter must have a very strictlimit, so that a receiver intended for receptionof intermittent signals of indeterminate fre-quency should have as few tuned circuits as


silence of the receiver, which is here soimportant a matter, it is recommended thatnot more than one L.F. stage, and preferablynone at all, be used.

Now comes the essential factor of practic-ability, i.e., ease of searching tuning, etc.,and we are faced with the necessity ofreconciling this with efficiency as far aspossible, since we cannot afford to lose eitherto any great extent.

Fortunately, both efficiency and selectivityare inherently greater in heterodyne than 'intelephonic reception, so that less difficultyexists in arranging a practical receiver whichshall at the same time be satisfactory inthese respects.

La

.0001 -

O

Fig. 2.-An " Interdyne " reaction circuit or tour -stage autodyne receiver.

This is due to the absence of " spreading "-of the carrier due to modulation, to the highdegree of regeneration present in the circuitsunder working conditions, to the polarisationof the detector by the local oscillations, tothe heterodyne amplification effect, and tothe natural increase in selectivity due to theheterodyne action, which latter is so greatas to make almost any heterodyne receiverinherently selective in such degree that,for short-wave work at all events, no specialconsideration need be given the matter sofar as interference due to undesired signalsis concerned. That due to atmosphericsand to arc " hash " is a different matter,and will be discussed in due course.

We thus see that it is not in the matter ofselectivity, but in the reconciliation ofefficiency with practicability that the diffi-culty chiefly lies.

The former admittedly demands fully -

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may be consistent with reasonable efficiency,and experience shows the maximum numbergenerally permissible in practice to be three.

Now, since two-viz., the aerial and theheterodyning circuits will in any casebe a sine qua non, this will leave only onemore available irrespective of the number ofH.F. stages adopted, so that, if more thanone stage be used, these will have to beuntuned, i.e., semi-aperiodic.

In practice, however, it is found thateven three tuned circuits in a receiver makeanything like rapid and reliable searchingand tuning difficult, and cause the chancesof missing signals to be considerable-infact, no less than 9 to r, which odds arerather long, and may have a serious effectupon the percentage and total number ofstations logged in a given period.

This rather cramps us, as it leaves onlytwo tuned circuits with which to produce

December, 1923. 164

" efficiency " ; but, since it is in practicepossible to dispense with one such circuitout of three without serious loss of efficiency,and as the advantage in practicability gainedby doing so will usually outweigh any suchloss, it is suggested that a limit of two tunedcircuits may be adopted with advantage inreceivers to be used in the Transatlantic Tests-" tuned " circuits, of course, here meaningsuch as actually require variation in searchingand readjustment for any change of wave-length.

The desirability of avoiding interferencewith other stations that may be workingbeing manifest, it is also suggested thata non -radiating arrangement be in all casesadopted-e.g., that reaction be not applieddirectly to the aerial circuit, either open orclosed ; that at least one H.F. valve stagebe interposed between the aerial circuit andthat to which the heterodyne is applied ;and that signals be in no circumstancesheterodyned in the aerial circuit itself orin any circuit directly coupled thereto.

Three types of receiver designed in accord-ance with the various considerations enu-merated are now shown.

Fig. 2 shows a four -stage autodyne circuitin which the amplified signals are hetero-dyned by a detector -oscillator V4, precededby three H.F. stages. Here the audio beatsare produced in the grid circuit of V4, threevalves thus separating the aerial and oscilla-tion circuits and energisation of the aerialbeing completely avoided. It will be ob-served that two tuned circuits only-Li-CIand L6-are used, inclusive of the oscillator,CI being merely a vernier for fine tuning,and C2 a static coupler. (The iron -corechoke L7 is merely an audio -frequencyrejector of the beat -oscillations, so that thesemay be effectively built up in the grid circuitof V4 and passed on for rectification in theirentirety-a point usually neglected, butwhich will be found repeated in each of thereceivers here illustrated. It is shownshunted by a small fixed condenser so asto constitute an acceptor of the higher

frequencies and not interfere with the actionof L5 as a high -frequency circuit. It hasno effect when the receiver is used for telephony, spark, etc., other than slightly toincrease the efficiency of the detector as anaudio -frequency amplifier. Both it and the


impedance choke L8 should have a resistanceof at least 4,000 w.)

It has already been stated by the writerelsewhere in these pages (Vol. I, No. 1)that an autodyne receiver may be used forshort wave work as efficiently as a separateheterodyne, and, as this may not be thegenerally accepted view, the reasons for itare here given.

The first is that, whilst it is manifest thata certain amount of de -tuning of one orother of the circuits concerned will benecessary for the production of an audiobeat -note, this de -tuning in the case of shortwaves will only be very small, and, moreover,if the receiver be scientifically designed forthe purpose, need not occur in the actualreceiving circuit at all, so that the incomingsignals need by no means be inefficientlytuned. Thus in Fig. 2 the de -tuning, orheterodyne frequency, may be and shouldbe in L6 only, and not in Li, so that, L6being post-rectificational, and its tuningwithout very material effect upon precedingcircuits, the loss of signal strength will benegligible.

The second is that, since the receivedsignals will pass through all the valves,and each will, therefore, be directly operativeas an amplifier (which is not the case in aseparate heterodyne), any small loss instrength due to de -tuning of L6 will bemore than compensated for by the extraamplification due to V4 itself.

The third reason is that searching andtuning are generally easier in an autodynethan in a separate heterodyne, as the receiv-ing and heterodyne frequencies are variedto a certain extent simultaneously, so thatadjustment of the beat -note is less criticaland signals are less likely to be missed. Thisis an actual advantage of the autodyne,and it is suggested that, for wave -lengthsbelow, say, Zoo metres, the latter is actuallythe superior arrangement, although onmedium -short waves there is probably littleto choose between them.

Fig. 3 shows a five -stage receiver withseparate heterodyne, the amplified signalsbeing here heterodyned in the anode circuitof V3, preceded by three H.F. stages, andfollowed by the detector, two tuned circuitsonly being still used. The local oscillatormay here be simply a heterodyne wavemeter.


The advantage of this type of receiver isthat the required beat -note is producedby variation of the local oscillations atL5 -V5 only and not in the actual receiver,the signals thus not losing strength throughde -tuning of the receiver itself. More-over, the majority of headphone instru-ments are designed to have a natural periodin the neighbourhood of i,000, whichcorresponds to that of most acute audibility,and such a frequency may here readily begiven to the beat -note without other con-siderations becoming involved, whereas inthe case of an autodyne receiver the amountof de -tuning of the circuits necessary toproduce such a note may appreciably affectsignal strength, so that, in practice, a lower

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December, 1923.

valve is not itself directly useful as anamplifier, and is, therefore, partly wasted.

Fig. 4 shows an entirely different andconsiderably superior arrangement, whichhas been specially designed for the purposein view, and operates on the Supersonicprinciple.

Space will not permit of any enlargementupon the subject of super -heterodynereceivers here, but the circuit itself is givenowing to its special value in this case. Sufficeit to say that it operates in the main uponexactly the same principles and is subjectto the same considerations as have alreadybeen discussed, with the difference that thebeat -note produced, instead of being givenan audible frequency, is given a supersonic

12.0043

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Fig. 3.-An " Exterdyne " reaction circuit or receiver with seperate heterodyne.

note of the order of 500-600 usually hasto be adopted.

In the case of a separate heterodynereceiver the note will, of course, be adjustableby variation of the heterodyne frequency(at L5) only, as variation of the receivertuning cannot alter the frequency of theincoming signals themselves, or, therefore,the beat -note.

The disadvantages of this type of circuitare, on the other hand, the criticalness ofadjustment of the heterodyne frequency onshort waves, and the fact that the oscillating

frequency, i.e., one above the limits ofaudibility and corresponding to a fairlylow radio frequency, and is subsequentlytreated as such.

Thus, in Fig. 4, the incoming signals areamplified at their original frequency by Viand V2 in the usual way, and are thenheterodyned by the local oscillator V3 toproduce a beat -frequency, also as usual,except that the frequency chosen in thiscase will be of the order of, say, roo,o00instead of r,000, corresponding to a wave-length of about 3,000 metres instead of

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an audible note. This is conveyed to V4and V5, where, being still a radio -frequency,it is amplified as such before rectificationby V6, which also acts as a second oscillatorto re -heterodyne it for the production of afinal audible note in quite the usual way.

V3 is an ordinary separate heterodyne,which is, in this case, desirable as the de -tuning necessary to produce a beat -frequencyof anything like roo,000 would cause con-siderable inefficiency in an autodyne arrange-ment. It may in this case, as it will form anessential and permanent part of the receiver,be " built in as shown in the diagram.Similarly, though only an audio -frequencyis required at the final stage V6, consider-able de -tuning will be necessary to pro-duce it when the origin'ating frequenciesare as low as ioo,000, so that, Although aself -heterodyne arrangement will here answerquite well, and is actually adopted in Fig. 3with a view to suiting the receiver to perma-nent general use, a separate oscillator isreally to be preferred.

L6 in the anode circuit of V5 is semi-aperiodic at the first beat-frequency-say,3,000 or 4,000 metres-and constitutes anacceptor of the no longer required originalfrequencies and of any interfering currents-such as arc -hash, spark, and,. to some extent,static-which may accompany them, andwhich usually force us to work at fourack-emma, these being thus filtered outto a great extent. It may be tuned ifdesired, but this is by no means necessary.

They are further suppressed by the useof resistance coupling between V4 and V5,which, whilst quite efficient for amplificationof relatively low frequencies such as thatof our beat, is, very inefficient at the highfrequencies in which the " mush " demonlives, so that any of these that may escapethe filters L6 and L7 remain unamplifiedand comparatively pure signals at the newfrequency to which they have now beenconverted are passed on to the lastvalve V6, which is the oscillator -detector,the grid circuit L7 of which is tuned to thenew frequency and the anode L8-05 to aslightly different one, such as to produce anaudible rectified note in the telephones(except, of course, in the reception of tele-phony, when V6 will be used as a detectoronly and not as an oscillator).

It would appear from the diagram that a

December, 1923.

considerable number of tuned circuits areinvolved, and that the arrangement wouldbe rather awkward to operate, but this isby no means the case. It must be appre-ciated that, whatever the original wave-lengths received, the beat -frequency towhich they are converted will always bewhat we choose to make it, so that a suitablebeat -frequency may be adopted once for alland used for all receptions, the long -wavepart of our receiver being set *thereto in thefirst instance and permanently left so. Like-wise, when a suitable audio note has oncebeen found there will be no need to alterit again, and this may be permanently set,too. In fact, the variable capacity C4,which is merely for the purpose of settingthe long -wave circuit in the first instance,may be dispensed with altogether if theinductances L6 -L7 are carefully calibrated.Thus in practice the number of tuned circuitsactually used in searching and tuning -inis still two only-Li-Ci and L4-C3-inclusive of the oscillators, notwithstandingthat adequate amplification of the funda-mental-an important matter too commonlyneglected-is duly provided. The rejectorL5 in the anode circuit of Va is unusual,and its purpose is to improve the circuitin its function of long wave oscillator, whilstit, is shown shunted by a small condenserto improve it as an acceptor of the higherfrequencies so as not to impair the circuitas a short wave oscillator. It should beresonant at the lower frequency-say 3,000-4,000 metres-and L6 -L7 made the same.

The coupling between L3 and L4, by meansof which the local oscillations are introducedinto the receiver, is shown variable in thefigure, but in practice once a suitable settinghas been found any adjustment in thestrength of the oscillations that may subse-quently be necessary can be obtained withinsufficient limits by variation of the anodevoltage to V3 or by adjustment of itsfilament temperature, or both, without anydisturbance of the tuning of any of thecircuits, the same applying to V6 and itsstatic coupler C6 (which is made variable toallow of the oscillation here being stoppedwhen the receiver is used for telephony,spark, etc.), and, indeed, to the heterodynecouplings in each of the figures ; and itshould here be pointed out that, whilstone form of oscillation circuit is shown in

s*


these diagrams, any other appropriate formmay, of course, be used according to choice.

The outstanding advantages of a super-heteordyne receiver are :-

(1) Great sensitivity and amplification,particularly of C.W. signals, due, in thelatter case, to the double heterodyning.

(2) Extreme selectivity from the samecause, without increased criticalness oftuning.

(3) Ability to eliminate arc -hash, etc.-which is amongst the most serious difficultiesin the way of short-wave long-distancereception-to a considerable extent.

(4) Ability to use a long -wave amplifier,relatively insusceptible to stray capacities,for the greater part of the receiver, withconsequent reduction of body -capacity andother disturbing effects, resulting in far lessinstability and criticalness of adjustmentand a more easily operated receiver for shortwaves.

The use of such a receiver, whilst perhapsa little more troublesome to set up andbecome accustomed to in the first instance,is likely to make a material difference tothe success of receptions and is stronglyto be recommended.

With regard to details, all inductancesshown in the diagrams, particularly thoseforming the anode coils of valves, which areintended to be semi-aperiodic, should bewound to have the least possible self -capacity.Whilst any efficient form of tuner, such asshown in the preceding figures for example,may of course be used for the aerial circuit,variometer tuning will be found very prac-tical and efficient and may safely be usedwith either of the receivers shown.

The aerial itself should preferably be aninverted L or T of the single or twin wiretype, as high and long as possible, and maybe used with either direct earth or counter-poise as lower capacity. Too many wiresin the antenna tend to increase the inter-ference due to mush, static, etc., and areinadvisable.

" R " valves may be used quite wellthroughout either of the receivers at thefrequencies concerned, but those recom-mended are V.24 or ORA B. for the H.F.stages, and R.4B. for rectification. A " Q "valve will give particularly good ' ampli-fication at V4 in Fig. 3 if a high-tensionvoltage of about 15o is used with it.

The H.T. battery should be of largecapacity, whilst the filament accumulatorshould have a capacity of at least 6o amp. -hours (actual), unless of course dull -emittervalves are used, as is sound practice owingnot only to current economy, but to the verydesirable silence in operation of valves ofthis type. In this case suitable valveswill be D.E.V. for the H.F. stages, and B.5for rectification, the filament voltages ofthese coinciding very closely.

It will be noticed in each of the circuitsshown that the last valve-e.g., the detector-is made to function as an L.F. amplifieralso, partly by the action of the iron -corechoke in its grid circuit, and partly by theuse of a static transformer, operating onthe impedance or resistance -capacity prin-ciple, in its,plate circuit, with high -resistancetelephones, and this arrangement will befound very efficient. Low -resistance 'phoneswith ordinary transformer may, of course,be used if desired, and instruments of theadjustable -reed type may in either case beused with advantage.

Suitable constants are given in the figures,and, it being appreciated that few will wantto build special receivers for the short periodof the tests which may subsequently be oflittle use to them, an effort has been madeto suggest circuits which shall olso be ofall-round use and permanent value to theexperimenter afterwards.

4:4

The Institution of Electrical Engineers.Mr. E. H. Shaughnessy, O.B.E., Chairman

of the Wireless Section of the Institution ofElectrical Engineers, delivered his InauguralAddress at, the Institution Biilding, London,on November 7. He commenced by pointingout that the meetings of the Wireless Sectionare open to all members of the Institution ;they are not, as seems to have been theimpression, limited to members of the Sectiononly. The address was of an interestingcharacter ; it reviewed the inception anddevelopment of broadcasting, mentioned thedifficulties which had occurred in the UnitedStates, and predicted the eventual establish-ment of a sound national order of broadcast-ing in this country, Mr. Shaughnessy referredto the broadcasting of music as a wirelesstriumph.


Radio 2UV A.R.R.A.By W. E. F. CORSHAM.

December, 1923.

Amateur transmission stations usually show a marked dissimilarity in circuits, systems,and apparatus employed. This is due no doubt to the fact that many experimentersbuild their sets as a result of their own investigations. In order that experimenters maybecome acquainted with the work of others, details of stations embodying novel methodsand circuits would be welcomed in these pages.

EXPERIMENTAL station 2UV, betterknown as " Two Uncle Vic," thestation's 'phone call, is located in the

valley at the bottom of Harlesden Gardens,London, N.W.io, and came into actionat the beginning of 192o, the twin aerialbeing hoisted almost immediately after mydemobilisation from the R.E.S..S. The first

Fig. 1.-Showing the general appearance of 2UV.

difficulty consisted of getting gear together,and, due to this trouble, a crystal set was thefirst set in use. I wonder if the broadcastingpeople know what a wonderful amount ofinterest can be got out of a crystal set ?Most of the Mediterranean stations werecopied off this set, and it was in use for quite

a long period, giving entire satisfaction, andon some nights some remarkable perform-ances.

When the first amateur transmittersbegan to get into operation on I,000 metresdid anyone ever hear such a large amount ofQ.R.M. that immediately began to springup on that wave -length ; BYK and otherNavy sparks using about 2 to 5 kw. seemedto work most of the day and nearly allnight, and the " Sorry, 0.M., Q.R.M. ! " gotso frequent that it was obvious that a newwave -length would have to be found,especially as Croydon was occasionallybeing Q.R.M. by a slight miscalculation of

Fig. 2.-The simple modulation system consists of an absorptioncoil coupled to the aerial circuit.

wave -length, and PCGG had commencedmusic tests. The non -transmitting amateursbegan to voice their disapproval of theheterodyning effect of carrier waves on thisstation, so that the poor transmittingamateur, already well bound up by restric-tions, began to look for some other wave torelease his experiments on, and the firststragglers began to appear on 36o metres,where, in course of time, new emigrants fromi,000 appeared every evening, until in time36o began to get as bad as i,000 metres forcongestion. Then the Cross Channel com-mercial sets began to get busy, and shipping

December, 1923. 170

to increase their use of the 300 -metre wave.D.F. stations on 400 to 44o also got busy, anddown we went again to 200. Finally thePost Office authorities sanctioned the use

Fig. 8.-Here the microphone replaces the normal grid leak.

of 200 and 44D metres and the i,000 -metrewave -length was closed for amateur work.

The first transmitter at 2UV consisted ofa tonic train set, radiating .o6 on 1,000metres, 0.1 on 36o metres, and 0.13 on 200metres. This was, I believe, the first tonic

EXPERIMENTAL WIRELESS

train set of its kind to be used in London_after the war, the feed consisting of 4 volts.on a I" spark coil, taking about 2 amps. inprimary coil, and supplying heavens knowshow many volts on the plate, but very fewmilliamps., hence the low radiation. Verysuccessful results were obtained with thisset, quite good distances being worked,an example being the tests worked with2 JZ of Huntly, Aberdeenshire, in February,1922, when he received my signals andreplied to me on a set whose input was lessthan so watts. I got his speech very well ona three -valve L.F. set. That's 50o miles on aset radiating o1 amps., whose actual wattagemust have been in the region of 2 to 3 wattsoutput, and good clear speech on 5o watts.

The set used can be seen in Fig. 1. A smallpower 'phone set, also radiating 0.1, cameinto being at this time, with about 120 voltson plate ; good speech at 20 to 3o miles wasobtained on this input, and on some veryspecial tests arranged with 20D and 2SX,they received my speech Q.R.Z., but clearlywhen my input consisted of the six -volt

Fig. 4.-The apparatus employed by 21:IV during the last Transatlantic Tests.


Fig. 5.-A near view of the apparatus now in use at 2UV.

accumulator supplying the valve filaments,and a four -volt dry cell from the H.T., verymuch the worse for wear ; in other words, adoubtful ten volts. How did I modulate it ?Perfectly simple-a coil coupled to the aerialcircuit on the earth side will do this verywell, but a better and much easier system isas in Fig. 3, where the microphone, shuntedby a condenser, takes the place of the gridleak. This system works excellently onradiations up to o2, but heavy saturationbegins to set in here, and ordinary gridcontrol is far more reliable then, but wonder-fully clear results are obtainable on thesesystems at low inputs, and a good deal ofunnecessary trouble and fuss can be saved.So much for the transmitter.

The receiver at 2UV is possibly one of themost efficient of its kind in London. Thefirst record put up was the first reception ofthe American amateurs in Great Britain,when 2UV successfully logged 'AFT ofSalem, Mass., on a three -valve set (L.F.), a

December, 1923.

most astounding piece of work, consideringthat it was the first time the receiver had

Tuned earth..

H.T.

TunedNate

eoutiterizoise.Fig. IL-Illustrating the circuit employed for transmission.

got down to 200 metres, and that the firstthree hours it was at work saw the code word


YLPMF safely in the log book. Since thennumerous experiments have taken place, toolong to elaborate upon in this article, but thefirst French tests were arranged on October31 with 8LBC, and very good Q.S.A. resultsobtained. Before the 1922 transatlanticswere commenced a rather interesting inci-dent, worthy of mention here, occurred, andpossibly the first of its kind in this country.On November 26, 1922, at 11.59 p.m., whentesting out the set I was going to use for the

2SH remember those early morning chatsthat passed away the hours of waiting forthe period, the station at 2UV in use thenbeing pictured in Fig. 4.

The present station will be seen in Fig. 5,and consists of a tonic train set capable ofradiating 1 to 2 amps. ; a C.W. and 'phoneset capable of radiating o 7, and a five -valvereceiver one to five valves at will. The usualnumber in use being two. Most of the U.S.A.broadcasters have been logged on this,

Fig. 7.-The receiver employs a single valve regenerative circuit with tuned plate coil, followed by a special note magnifier.

coming tests, and working with 20D, ofGerrard's Cross, to my annoyance a C.W.station came dead on top of him, makingreception exceedingly hard. I couldn't getone tuned out without getting the other, andaccordingly I took steps to listen to theQ.R.M. man with a view to asking him tochange his wave -length, when, to my surprise,he turned out to be American amateur2AWF, working 1XM. Needless to say, Idid not call him, but that reception certainlyaugured well for the success of the comingTransatlantic, and, despite the awful Q.R.M.from Northolt GKB, I logged nearly 200stations from periods 3 a.m. to 6 a.m.,GKB making reception hopeless up to 3 a.m.,I am so near him. I first heard W JZ's howlin February, 1922, and oNX I logged onDecember 1o, 1922, together with oNY.I took part in the transmission period testsfrom this side, and had some good reportsfrom various places on the Continent, tonictrain being used. No doubt 2KF, 20M, and

together with a good number of Americanamateurs.

The circuit of the transmitter is shown inFig. 6, and that of the receiver in Fig. 7.I always use L.F. because it is my practicalexperience that much better results areobtainable on distance work if great care isexercised to clear distortion for speech, andit is quite possible to get rid of that bugbearto the low -frequency worker. The only goodword I have for H.F., and I have at timesused it fairly extensively, is for its betterselectivity, and I use it occasionally whenGKB is Q.R.M., but a good tuned circuit isjust as good.

This concludes a rough outline of 2UV,and the work accomplished since its in-auguration. A i-kw. license has been grantedto the station by the P.M.G., and if all goeswell, I hope for good DX work with the U.S.amateurs in the near future, together withmy brother amateurs of the A.R.R.A." Best 73's C.O. ! "


The Month's " DX."Recorded by HUGH N. RYAN (5BV).

December, 1923.

The increasing efficiency of amateur transmitters and receivers is resulting in thecreation of many new long-distance records which are undoubtedly worthy of mention.It is proposed to record month by month work in this direction, and the Editor will bepleased to receive details for inclusion in these pages.

THE outstanding feature of the pastmonth has, of course, been the extra-ordinary spell of bad conditions,

lasting about five weeks, and including thewhole of October, during which reception ofAmerican amateurs was practically impos-sible. The conditions on the early morningof September 23 were perfect, and Americanswere coming in nearly as fast as they couldbe logged. It was a few days after thisthat last month's notes were written.Apparently I was crowing about the splendidconditions a little too soon, as I heard not asingle American between that date andNovember io, neither have I heard a reportof any other listener doing so. 2JF, 2ZS,2KF and other well-known men all reportto the same effect.

The Transatlantic aspect of these noteswas in imminent danger of disappearingthis month when quite suddenly normalconditions returned. They may or may notlast, but at the time of writing all is well.Now for the actual results obtained. Reportsfrom the North are not yet to hand, butcertainly London has been getting someexcellent logs.

On the morning of October II 5NN loggedtwelve Americans on one valve. 2WYreceived the very fine total of twenty-threeon the same morning, using three valves.About twelve Americans were received onone valve by 2AAH (British, not American !),who has one of our latest series of call -signs,but is nevertheless an " old-timer " in DXwork. I only kept a short watch that nightand received ten Americans on one valve.

I heard 2JF and 5K0 calling ICMPseveral times, so it may be assumed thatthey were also on the war -path.

The two strongest Americans were ICMPand 2BY, both of whom produced very loudsignals indeed. An interesting item was anA.R.R.L. broadcast message from IFD.This was rather badly jammed in London,

but parts were quite readable, including areport of a cable just received from Englandin connection with Transatlantic work. Themessage was signed " Schnell," the A.R.R.L.traffic manager. That practically concludesthe American news, since the bad weatherspell only broke a few days before writing.

Canadian 'AR has written asking for theco-operation of British amateurs in Trans-atlantic work to show our American " Radio -Cousins " how DX really can be conducted.He is using transmitting apparatus of Britishmanufacture, and works with an aerialcurrent of 7i amps. He would appreciatereports of reception in this country, havingbeen received here a record number of timesin August and September.

A curious feature of the spell of bad con-ditions was that it only seemed seriouslyto affect Transatlantic work. European DXhas not been seriously hindered by it. TheDutch stations have been coming in verywell, but nothing of great interest hasoccurred in connection with any of themexcept PCII of Leiden, who reports havingeffected two-way working with 7ACM onthe morning of Sunday, October 4, between3 and 4 a.m., Amsterdam time. The wholeof the working was overheard and confirmedby oMX of Amsterdam.

The Dutch " Radio-Expres " states thatPCII was using zoo watts, and radiating3 amperes, using a Mullard valve with1,500-2,000 volts on the plate. The workingremains to be confirmed, but if it turns outto be authentic it is a very fine piece of work,as the seventh district is the one mostremote from Europe, and 7ACM is at Cam-bridge, in Washington State, right over onthe Northern Pacific coast of America ! Wewill leave our congratulations until it isconfirmed.

The only other notable feature of Dutchwork this month is the sudden and con-siderable increase in the strength of oDV.


The French stations are now awaking fromtheir summer sleep, and have been verymuch in evidence during the last month.I suggested in last month's notes that 8AQ'ssilence was due to the non -arrival of his newalternator. It appears that the reverse isthe case. The alternator turned up, full ofbeans, and 8AQ is now having his valvesrepaired ! 8BF, of Orleans, has just installeda new transmitter, using 25 cycles A.C. Thesignals from this set are extremely strong,and the note is exactly like the once -familiarnote of 8AB. 8BF tells me that the powerof this set is one kilowatt, and that duringthe Transatlantic tests he is going to use italternately with his old zoo -watt pure C.W.set. In spite of the power of the 25 -cycleset, I think I would put my money on thepure C.W.

There are now several more British stations" on the air " with their potential Trans-atlantic transmitters. 5NN has been suffer-ing from valve trouble, but is now workingagain. 2SH is again using big power, undera special licence for zoo watts. He is puttingabout 4 amps. into the aerial. Severalother stations have been granted thesetemporary licences for increased powerduring the Transatlantic season, and maysoon be expected to establish some interestingrecords. An important point in connectionwith these licences is that, under theirterms a British station calling an Americanshould prefix the American call sign withthe letter " n " and his own call sign withthe letter " g," e.g., " n ICMP de g 2JF."This will help to avoid confusion betweenBritish call signs and their American dupli-cates. It is to be hoped that all Britishtransmitters using sufficient. power to renderdistant reception possible will, in future,use this prefix. It is much less clumsythan the present " British " or " Brit."prefixes.

In connection with the issuing of speciallicences for Transatlantic work, it is interest-ing to note that the Dutch authorities havesanctioned the erection of a station for thispurpose, the licence allowing the station tooperate until May, 1924.

The station is to use C.W., power soo watts,and wave -length 200 metres. It is to besituated at Delft, and its call sign is PA9.

It will be remembered that last monthI suggested that the fault of the non -reception

of our signals in America did not, perhaps,lie entirely with the receivers on the otherside, as is rather commonly supposed by ourmen. I think that my view is confirmed bythe results obtained by several Americanamateurs in working with the MacmillanExpedition Station WNP. The Macmillanship is at present frozen in the Arctic at anearer point to England than to an averagepoint in the States, but yet he has carriedout two-way working with several Americanamateurs, while he has never yet beenreceived in England, neither has he receivedany British signals. That, I think, shoulddispel the idea that American amateurscannot receive. This should give us morehope of getting over so long as we send outgood stuff, and it should also encourage usto try to receive WNP. I know that manyof our stations are trying to do so, and it isdifficult to explain their lack of success.

With regard to the well-known Americansof last year upon whose absence this year Icommented in the last notes, the list stillholds good with the exception of IBM,who has now been logged this year by5NN (November ii).

Apart from the chronicling of DX, I shouldlike this month to put forward a suggestionfor the better reporting of signal strength inDX and other work. The present " R "code of signal strengths has become useless,firstly because it has far too many differentdegrees of audibility, and secondly becauseeverybody applies a different meaning to it.Nobody can really say what is the difference,for instance, between R5 and R6, nor isthat difference of sufficient importance tobe worth worrying about.

I suggest a new code, which should becalled by a different letter to distinguishit from the old " R " code, which shouldhave only four degrees of audibility, andwhich should take into account the receiverused. Let us call it the " A " code, then" A21 " would mean strength 2 on onevalve.

The figures for strength would be-I-just readable (with difficulty).2-comfortably readable, but not very

strong.3-good strong signals (the best strength

for good consistent work).4-very strong.

The second figure should indicate the


number of valves used for reception. Eventhis code is far from perfect, but it is animprovement on the practically meaningless" R " code. If you like the code you canuse it without much fear of the other mannot understanding, as I think that thispaper is read by nearly all transmitting men.

Just before going to press the log from theNorth has come to hand. 2JF has received

175 December, 1923.

eighteen Americans so far this month. Histransmitter has also been heard in Christiania(indoor aerial and two valves) and Toulouse(two valves). 8CWR has reported receiving2JF, but the time is not yet confirmed,though it appears to check O.K.

2PC, 2GW and 2KW are all rebuildingtheir stations for the tests, and may soonbe expected on the air."

The Trend of Invention.We summarise below the more important wireless inventions which have beendisclosed during the month, special reference being made to those of immediateinterest to the experimenter.

Elimination of Interference.A great deal of experimental work has been

done, and a great many patents have beentaken out, on the subject of the eliminationof interference. Still, however, much remainsto be done in this direction. British PatentNo. 205,117 (of German origin) shows amethod of attacking this problem in a some-what novel way. In order to increase theselectivity of resonant circuits the inductancethereof is made very large and the capacityvery small. In order to produce coils ofvery large inductance and very low self -capacity these are wound with fine wireand may have iron cores and be of multi -layer form. The losses so occasioned areneutralised by using a retroactively -coupledtriode. A wireless receiving circuit is shownin Fig. r, in which a separate triode (R1)is used to neutralise the aerial losses and theclosed circuit (S., C2) of the receiving set isinductively coupled to the aerial tuningcoil (Sr). S, is the coupling coil of a separatehetercdyne. The telephdne may be insertedin the anode circuits of either R1 or R2,thus making possible the use of either adirect -coupled or loose -coupled receiver. Itdoes not appear that the arrangementdescribed would overcome the difficulty ofheavy atmospherics setting the aerial intooscillation at its own natural frequency.However, the arrangement produces a veryfeebly -damped aerial circuit for a loose -coupled tuner and thereby enhances thebenefit of loose coupling. A possible im-

provement might be found in tuning thereaction coil S, for example.

Relay Arrangements.A certain amount of experimental work

has been done on the lines of causing anincoming signal to set a local valve inoscillation at a frequency independent of the

NH.450

incoming signal. This, as will be remem-bered, is the principal of the Turner valverelay. British Patent No. 183,130 (of Ger-man origin) deals with arrangements of thistype. Fig. 2 shows the simplest arrange-ment. Owing to the rectifying action of thetwo diodes 3 the negative charge on thegrid of the triode 6 is reduced in proportionto the strength of the oscillations in theclosed circuit 2. This effects the raisingof the conductivity of the filament -anode

December, 1923. 176

space of the triode 6. The anode circuit ofthis triode is in series with the anode circuitof a second triode 7, which is retroactivelycoupled. Thus the triode 7 will oscillate,when the potential on the grid of 6 becomessufficiently positive, at the frequency towhich it is tuned, which would generally

be an audible frequency. Other arrange-ments are shown in the patent specificationusing triodes only, and circuits may bearranged for the relaying of received signalsat a different frequency.

Another relay device is described in BritishPatent No. 186,305 (of French origin). Thisarrangement depends upon the effect ofultra - violet light upon metals in a low-pressure atmosphere. In the arrangementparticularly described the beam from anarc lamp is focussed upon one electrodein a two -electrode tube, and the beam iscontrolled by perforated slip. The current

in the tube circuit is said to be proportionalto the intensity of light falling upon theelectrode. The beam may be controlledby a mirror or mirrors, or, for example,by a pair of grids, of which one is connectedto a sound -collecting diaphragm. It is


possible that in this way a very sensitivemicrophone arrangement might be made,since a grid of fine wires, to cover slits in ascreen, may be made very light indeed.Controlling Decrement of Circuits.

A most interesting principle and means forcarrying this into effect is shown in BritishPatent No. 204,482 (E. Y. Robinson, British).According to this patent the decrement ofreceiving circuits is caused to decrease witha weak and to increase with a strong signal,and remain constant when the signal isconstant or zero. Preferably, the incomingsignal itself effects these changes, and insuch a manner that the change of decrementis proportional to the rate of change ofsignal strength. This can be effected inseveral ways, all dependent upon changingthe normal grid potential of a triode inproportion to the rate of change of the signalstrength. An extra transformer is includedin the anode circuit of the detector triode,and its secondary winding is connected in

series with the grid and filament of anothertriode. An inductance in the anode circuitof this latter triode is coupled to the aerialinductance so that some energy is absorbedtherefrom, the amount of which will dependupon the anode circuit conductivity, andhence grid potential of this triode. Reactioninto the aerial circuit is used in this case.A circuit arrangement employing this prin-ciple is shown in Fig. 3. A similar effectmay be produced by changing the amountof regeneration due to a retroactively -coupledtube by varying the grid potential of thistube. A suitable arrangement for thispurpose is illustrated in Fig. 4. It is saidthat by this method the modulation of tele-phony can be improved and circuits of lowerdecrement than usual can be used. Itappears that some precautions are necessary

FXPERIMENTAL WIRELESS. 177

to make the arrangement work well ; forexample, it would seem necessary to designthe H.F. transformer in Fig. 4 so thatsubstantially no audio -frequency energytransference could take place between itswindings. Incidentally, there is a similaritybetween this arrangement and some fairlycommon reflex arrangements. Are some ofthe wonderful results claimed for certainreflex circuits due to the unbeknownstemployment of the principle disclosed inthis patent ?

Y T

-r

.Fis,: .

Crystal Detectors.It is sometimes useful to be able rapidly

to change from a single -valve circuit to acircuit employing a crystal or similar detector.British Patent No. 205,148 (P. G. A. Helmuth,British) describes two arrangements forplugging in to the socket of a detector valve,the first being for crystal detector only, andthe second being for a crystal and triodereflex arrangement. Diagrams of thesearrangements are shown in Figs. 5 and 6.

December, 1923.

The circuits used are somewhat unusual,but appear simple, but the construction of thehigh -frequency transformer in Fig. 6 mightrequire some care.

Reflex Circuits.Much attention has recently been devoted

to reflex circuits, particularly those employ-ing a crystal detector. In reflex circuits ithas been usual to impress the rectified signalupon the grid of the first H.F. triode, andthereafter carry out audio -frequency ampli-fication throughout the series of triodes inthe same order as they are used for radio -frequency amplification. This method hasthe disadvantage that one triode carriesboth weak radio -frequency currents andweak audio -frequency currents, while another

III 11111111 1111111111'

carries strong currents at both frequencies,and is therefore liable the more easily tobecome saturated. British Patent Applica-tion No. 204,301 (of American origin) setsforth a method of overcoming this troubleby carrying out audio -frequency amplifica-tion throughout a series of triodes in an orderdifferent from that in which radio -frequencyamplification is performed, preferably in thereverse order. The diagram of connectionsis shown in Fig. 7. Particular care must betaken in designing and making the radio -frequency transformers for this arrangement,as it is essential that practically no energy ataudio -frequency should pass between theirwindings. Amplifiers working on this systemhave found some popularity in America,and are said to be much more stable than theordinary reflex amplifier. In that countrythese circuits are called " Inverse duplexcircuits."Duplex Telephony.

Many experimenters have during the lastyear been working hard at duplex telephony,

December, 1923. 178

and in this branch of work there is greatscope for experiment. British Patent Appli-cation No. 190,699 (of German origin)describes a method of effecting duplextelephony which appears very simple. The

8.

arrangement is shown in Fig. 8. The oscil-lating valve has four electrodes, a kathode,anode and two grids. The grid nearer theanode is used in conjunction with the anodeand kathode to function as a triode and form


the oscillator. The oscillatory circuit is ofa fairly usual form, except that in the anodeand grid leads respectively a telephone andmicrophone are connected. The extra gridadjacent to the filament is connected to thejunction of a resistance and the anode ofanother triode. The grid of this lattertriode has potentials of the requisite fre-quency impressed upon it by suitable means,shown as a transformer with the primaryshunted by a condenser. The anode currentin the oscillator is controlled by the potentialof the extra grid. When the anode currentis large the set functions as a transmitter,and when the anode current is small the setfunctions as a receiver on the same wave-length. The specification states that ordi-nary triodes may be used, but does not detailhow this may be done. It is possible thatthe magnetic control, described by Mr.Andrewes in the November issue of EXPERI-MENTAL WIRELESS, could be used to replacethe extra grid shown by this specification.

Recent Wireless Publications.Figures after the title of each publication indicate Volume and Number of Publicationcontaining the article. Where only one number is grven, this indicates the serial numberof the publication. The abbreviations used in this bibliography will be found in theprevious issues of "Experimental Wireless."

I.-TRANSMISSION.AN IMPROVED METHOD OF MODULATION IN RADIO

TELEPHONY.-Charles A. Culver (Proc. I.R.E.,2, 5).

UBER PARALLELSCHALTUNG VON Roll RENSEN DERN .-A. Semen (Jahrb. d. drahtl. Telegr., 22, 3).

AN INTERESTING CONTROL DEVICE FOR RADIO-TELEPHONY.-Alan L. M. Douglas (W. World,221).

SIDE -BAND TELEPHONY.-E. H. Robinson (Exp.W 1, 2).

THE PRINCIPLES OF CHOKE CONTROL.-L. E. Owen,(Exp. W., 1, 2).

RADIO ANTENNA DESIGN.-Frank Conrad (IV. Age,11, 1) .

SUR LES CONDITIONS DE REN DEMENT DES LAMPES-VALVES GEN ERATRICES AVANT UNE CARACTERIS-TIQUE D ARC CHANTANT ET SJR LA DEFINITIONDE LEUR PUISSANCE.-Andre Blondel (R. Elec.,4, 14).

A REAL SHORT WAVE TRANSMITTER.-Brown,Dame and Basim (Q.S.T., 7, 3).

I.C.W. WITHOUT MECHANICAL MOTION.-HowardM. Williams (Q.S.T., 7, 3).

.-RECEPTION.DER EMPFANG VON HOC HFREQUENZSCHWINGUNGEN

MIT NIEDERFREQU EN ZMODULATION. - G. Joosand J. Zenneck (Jahrb. d. drahtl. Telegr., 22, 3).

MESSUNGEN DER EMPFANGSINTERSITAT DER ATMOS-PHA RISCHEN IONISATION UND ANDERER METEORO-LOGISCHER ELEMENTE WAHREND SONNEN FIN -STERN IS AN 8 APRIL, 1921.-B. Iliin (Jahrb. d.drahtl. Telegr., 22, 3).

ZUR FRAGE EACH DEN URSACHEN DER SCHW AN-KUNGEN IN DE EMPFANGSINTERSITAT.-B. Iliin(Jahrb. d. drahtl. Telegr., 22, 3).

THE AUTOMATIC RECEPTION OF WIRELESS SIGNALS.-E. R. Batten (W. World, 219).

" BLIND SPOTS " AND " FADING OF SIGNALS."-S. R. Chapman, B.Sc. (W. World, 221).

A NEW DUAL CIRCUIT.-James Strachan, F.Inst.P.(W. World, 221).

THE SUPERSONIC HETERODYNE RECEIVER. -W. S.Barrell (W. World, 222).

NEW TYPES OF VALVES.- ( W. World, 222).VALVE RECEIVERS ON D.C. MAINS.-Alexander

Gayes, M. J.Inst.E. (Exp. W., 1, 2).DULL EMITTER VALVES.-(Exp. W., 1, 2).LES ACCROCHAGES DANS LES AMPLIFICATEURS.-Andre Delvigne (R. Elec., 4, 15).RESONANCE WAVE COILS.-- (Q .S. T., 7, 1).

M.-MEASUREMENT AND CALIBRATION.THE CONSTRUCTION AND MANIPULATION OF WAVE-

METEks.-Leonard J. Sayce, B.Sc. (Exp. W1, 2).


FACTORS OF WAVEMETER DESIGN AND OPERATION.-L. R. Felder (W. Age, 11, 1).

A, METHOD OF MEASURING VERY SHORT RADIOWAVE -LENGTHS AND THEIR USE IN FREQUENCYSTANDARDIZATION.-Francis W. Dunmore andFrancis H. Engel (Proc. I.R.E., 2, 5).

IV.-THEORY AND CALCULATIONS.VACUUM TUBES AS POWER OSCILLATORS.-D. C.

Prince (Proc. I.R.E., 2, 5).THE EFFICIENCY OF THREE -ELECTRODE TUBES

USED FOR THE PRODUCTION OF CONTINUOUSWAVES IN RADIO TELEGRAPHY ; THAT IS, THECONVERSION OF DIRECT INTO ALTERNATINGCURRENT.-Marius Latour and H. Chireix (Proc.I.R.E., 2, 5).

LONGEUR D' ON DE OPTIMUM.-Leon Bouthillon(R. Elec., 4, 14).

V.-GENERAL.OBSERVATIONS ON LAFAYETTE AND NAUEN STATIONS

IN WASHINGTON, MARCH 1, 1922, TO FEBRUARY28, 1923.-L. W. Austin (Proc. I.R.E., 2, 5).

RADIO FREQUENCY TESTS ON ANTENNA INSU-LATORS.-W. W. Brown (Proc. I.R.E., 2, 5).

DIGESTS OF UNITED STATES PATENTS RELATINGTO RADIO TELEGRAPHY AND TELEPHONY ; ISSUEDJUNE 26, 1923-Auousx 21, 1923.-John B.Brady (Proc. I.R.E., 2, 5).

UNTERSUCHUNG EINES ELEKTRONRELAIS AUF GRUNDELEKTROSTATISCHER ABLENKUNG DES ELEK-TRONENBUNDELS DURCH EIN QUERFELD.-A.Gebbert (Jahrb. d. drahtl. Telegr., 22, 3).

L'INFLUENCE DE TRACES DE GAZ DANS LES LAMPESA TROIS ELECTRODES.-(L'Onde Electrique No. 14,1923.)

A NOVEL METHOD OF RECTIFICATION.-F. L. Hogg(W. World, 219).

DISTORTION IN RADIO-TELEPHONY.-H. A.Thomas, M.Sc. (W. World, 222).

MAGNETICALLY -CONTROLLED VALVES.-H. An-drewes B.Sc., A.C.G.I., D.I.C. (Exp. W., 1, 2).

179 December, 1923.

AMATEUR RADIO WORK IN HOLLAND.-J. Wester-houd (Exp. W., 1, 2).

A PRIMARY CELL H.T. BATTERY.-N. K. Jackson(Exp. W., 1, 2).

IONIZATION IN VACUUM TUBES.-W. A. Dickson(W. Age, 11, 1).

NOTES ON INSULATION PHENOMENA.-A. Reisner(W. Age, 11, 1).

SINGLE -LAYER INDUCTANCE COILS SUITABLE FORRADIO FREQUENCY STANDARDS.-(Mod.W., 2, 2.)

SOME CAUSES OF POOR RECEPTION.-R. W. Hallows,M.A. (Mod. W., 2, 2).

THE CONSTRUCTION OF A NOVEL FOLDING FRAMEAERIAL. (Mod. W., 2, 2).

ANTICIPATIONS SUR LA TRANSMISSION DE L'ENERGIEA rusTANcE.-Leon Bouthillon (R. Elec., 4, 14).

SUR LES ORIGINES DE LA T.S.F.-Prof. A. Turpain(R. Elec., 4, 15).

LA T.S.F. EN YOUGOSLAVIE.-R. Belmere (R. Elec.,4, 15).

LE SERVICE D'ECOUTE PENDANT LA GUERRE.-Gen.Cartier (R. Elec., 4, 16).

A PROPOS DES ORIGINES DE LA T.S.F.-(R. Elec.,4, 16.)

LA RADIOTELEGRAPHIE EN TCHECOSLOVAQUIE.-R. Belmere (R. Elec., 4, 16).

LE PHONOGRAPHE DE L'AVENIR.-E. Pepinster(R. Elec., 4, 16).

TRANSATLANTIC RADIO TELEPHONY.-H. D. Arnoldand Lloyd Espenschied (Electn., 2,372).

THE PARIS RADIO CENTRE.-(Electn., 2,373.)ELECTRIC FILTERS. PART II.-F. S. Dettenbaugh

(Q.S.T., 7, 1).FINAL REPORT ON THE FADING TESTS.-(Q.S.T.,

7, 1).HARD RUBBER IN RADIO INSTRUMENTS.-(Q.S.T.,

7, 1).A NEW RADIO SYSTEM.-Howard J. Tyzzer

(Q.S.T., 7, 3).ELECTROSTATIC VOLT M ETERS .-R. R. Ramsey

(Q.S.T., 7, 3).

Correspondence.Variable Condensers.

To the Editor of EXPERIMENTAL WIRELESSSIR,-As an experimenter in wireless telegraphy

of long and pre-war standing, I am wishful toassist the present experimenter, and incidentallythe broad -catcher, who intends constructing hisown set, by pointing out to him a possible causefor disappointment in the results obtained whenendeavouring to follow too literally the valuableinstructions for constructing a set as given by thevarious undeniably expert writers who contributearticles to the several wireless journals.

I write with regard to the capacities of thecondensers therein stated to be necessary, and therisk when purchasing many of the condensers nowon the market, of being supplied with one of muchless capacity than the value at which it is rated.

I have recently come up against a most glaringinstance of the gross inaccuracy of one of the types,for although I possess a number of Mark III con-densers, and others made up to various capacities,

I was led to purchase a condenser purely becauseit possessed an extra Vernier plate built therein.

It was sold to me as a condenser of .0005 micro -farad capacity, and I had no reason to doubt itsapproximate accuracy, as, so far as my memoryserves me, it had about 31 closely -spaced plates.

Being found to be mechanically defective, it wasreturned to the dealer, and by him to the makers,and another make of condenser supplied in sub-stitution, with an apology from the makers forthe same being of less capacity, namely, .0003micro -farads, as they were out of stock of .0005condensers for the moment.

Upon their stock being replenished, the makersforwarded to the dealer several condensers, andthen stated that the condenser sent as a substitutewas of .0005 and not of .0003 capacity.

Upon it being measured, and its capacity cal-culated, it was found to be round about .0003micro -farads, and therefore representations weremade to the makers, who now reply that " there

December, 1923. 18o EXPERIMENTAL WIRELESS.

is a standard number of plates usual in the tradefor the various capacities" (apparently withoutregard to the diameter and thickness of the movingplates, and thickness of the spacing washers, thatis, to the actual active air space between the plates).

Now, this cannot be a correct statement, as onecannot believe that trade firms would knowinglymisrepresent the capacities of the condensers theysell. Moreover, a reference to the list of one ofthe most prominent makers of condensers showsthat his .0005 micro -farads condenser has 29 plates,and a .0003 19 plates, whereas the so-called .0005I purchased had 11 moving plates, equal to 23plates all told, and the .0003's they sell have sevenmoving plates, equal to 15 plates in all, so thatpresumably, if there is a standard number of plates,the latter types do not comply with the standard.

Had the condenser been sold to me as havinga certain number of plates, I could not have objected,excepting that the one sent in substitution had amuch smaller number of plates.

I have also compared the said .0005 condenser,having 11 moving plates, with a cheap Americancondenser, containing 28 moving plates, with amuch closer air space, and stated to be of .001capacity, and this confirms my view that therecan be no standard custom in the trade, as to thenumber of plates.

The point which affects the amateur is that heis advised that with a certain coil and a certaincapacity of condenser he can cover, say, the broad-casting range of wave -lengths, and finds uponassembling his set that it falls short of this range,and wonders why.

Summing up for the guidance of the investigator,there can only be one correct method of calculatingcondenser capacities. One square inch of active

1surface, with an air space of -1000of an inch,

gives a capacity of .0002246 micro -farads, andeach moving plate of 2* in. diameter if faced oneach side with a fixed plate (having a centre pieceof 1 in. radius cut out for clearance, and having

1an air space of Two of an inch) will give a capacity

of approximately .0008 micro -farads, and in thecase of a 2* in. moving plate, .001 micro -farads.

25If the spacing is -00 of an inch, then the capacity

per moving plate will be one twenty-fifth of theabove respectively, and if the plates are 23 S.W.G.,and the spacing washers * in. thick, the capacitywill be about one -fiftieth of the above.

Finally, it would be interesting to know thelegal position of a dealer who knowingly sells analleged .0005 micro -farad condenser (actually ofabout .0003 capacity) as a condenser having acapacity of .0005 micro -farads.

GEO. H. STRONG.

To the Editor of EXPERIMENTAL WIRELESS.DEAR SIR,-With reference to the article entitled

" Efficient Transmission," by Mr. F. L. Hogg, inthe October issue, there appear to be certain pointswhich, so far as my limited technical knowledgegoes, do not fit in with each other, and as to whichI would much appreciate an explanation-even ifit only shows up my ignorance.

From particulars given, it appears that Mr.Hogg's aerial is 70 ft. above the counterpoise levelat the lead-in end. The height of the other endis not stated, but I think it safe to assume that theaverage height is 50 ft., or say, 16 metres. Mr.Hogg further states that he has reduced the totalresistance of his aerial system to 5 ohms, and onthis basis shows how 1.26 amps can be obtainedwith an input of 10 watts to the valve at 80 percent. efficiency.

What puzzles me is this : How does Mr. Hoggmake his aerial system have a total resistance of5 ohms when the radiator resistance alone worksout at over 10 ohms (calculation attached).

If Mr. Hogg actually gets 1.26 amps. in hisaerial on 10 watts at 80 per cent. efficiency, it isobvious that the total resistance of his aerial cannotbe more than about 5 ohms, else he would havemore than 10 watts in his aerial, which is ridiculous.

Awaiting your reply.-I remain, yours faithfully,A. B. RICHARDSON (6FQ)

CALCULATION OF RADIATION RESISTANCE OF MR.F. L. HOGG'S AERIAL ON 200 METRES.

Formula 1580 (171)2k

Where H=effective height of aerial.X Wave -length .

See Wireless World, page 652, issue of 17-2-1923.( 16 Vk200)

10.112 ohms.

To the Editor of EXPERIMENTAL WIRELESS.DEAR SIR,-With reference to Mr. Richardson's

remarks re my aerial, I can only say that I wishmy aerial was 70 ft. above the counterpoise ! Ipurposely split up this length into parts to empha-sise the actual length of wire. The lead-in comesthrough the roof and across the room. The counter-poise lead goes back across the room a greaterdistance and across a short roof before droppingvertically on to counterpoise. Actually the aerialis 45 ft. above ground at lead-in, and 35 ft. at freeend. Its effective height is only 28 ft., as thecounterpoise is 12 ft. high. This figure gives aradiation resistance on 200 metres of 3.2 ohmstrue radiation resistance. Actually owing to Rgand Rd the total resistance is 5.8 ohms, and theaerial current is 1.20 amps. at 85 per cent. efficiency.I hope this clears up Mr. Richardson's difficulty.-Yours faithfully,

FREDERIC L. HOGG.

To the Editor of EXPERIMENTAL WIRELESS.DEAR SIR,-There seems to be a prevailing

impression that when D.C. mains have theirpositive lead earthed they can only be used forhigh tension by putting a condenser in series withthe earth lead (which still leaves the aerial at highpotential), or using loosely -coupled circuits. This,however, is not the case ; all that is necessary isto connect the bottom end of the inductance tothe H.T. positive instead of the L.T. negative.All the tuning instruments, as well as the 'phones,are now at earth potential. Of course, if the firstvalve is a high -frequency amplifier, the grid con-denser and leak have to be specially introduced.-Yours faithfully, W. LAWRENCE.

EXPERIMENTAL WIRELESS. z8z December, 1023.

Some Impressions of the WirelessExhibition.

That the sea oand All -British Wireless Exhibitionand Convention has been a gigantic success is no.secret. Some idea of its popularity may be gatheredwhen it is realised that at one time the queue ofinterested sightseers waiting to gain admissionreacted a length of some three-quarters of a mile.We offer our heartiest congratulations to theorganisers who paid attention even to the mostminute details with such gratifying results.

It is only natural to imagine that an annualexhibition of this nature should reflect the advanceand progress which the industry has made duringthe preceding year. If we adopt this attitude,then we must certainly express some disappoint-ment. Taking the exhibits as a whole, we cannothonestly say that we found signs of any startling

emitter valves, several new types making theirfirst appearance. Amongst theae were the Cossor" Wuncell," constructed on the lines of the familiarCossor valve, and the small thoriated filamentvalve, such as the D.E.3, A.R.O.6. and B.4, whichresemble the U.V.199 described in the Novembernumber of EXPERIMENTAL WIRELESS. Two othernewcomers were the B.T.H. B.5, somewhat similarto the U.V. 202A and the Cossor dull emitter poweramplifier. The construction of the latter is ofinterest, as it will be observed that the grid is veryrigidly connected to the anode by means of a sup-porting glass lug. So far as power valves areconcerned there is little to report, except, perhaps,the new Cossor low -impedance high-tension rectifier,in which the drop is only of the order of some

The Frenopione, a frictional loud speaker ; the Burndept ' Vase " and the " Crystavox," Incorporating a microphone relay.improvements or innovations. True, the manu-facturer has had to devote the whole of his time tomeeting the demand for broadcast receivingapparatus, and perhaps thereby he has been unableto give as much time to research as he would haveliked. Nevertheless we were able to notice somereally excellently designed receivers. The exhi-bition was essentially a " broadcast show," and itwould be unnatural to imagine that the experi-mental side of amateur wireless work shouldpredominate

Perhaps the most striking feature amongstmodern developments was to be found in valveconstruction and loud -speaker design.One could not glance even casually around theshow without noticing the predominance of dull

30 volts. This should be of special interest toamateur transmitters. We were interested to notethat the Mullard 50 -watt valve is now capable ofbeing re-tilamente1.

Progress in loud speakers has been in threedirections. Better quality, better appearance andnew principles have been the chief considerations.The well-known Amplion is now fitted with awooden horn, a welded and pressed " throat,"and an improved movement with a view to reducingresonance effects, resulting in an even more pleasingtone. In appe trance the Burndept vase loudspeaker is certainly the most novel. The Freno-phone frictional loud speaker made its first appear-ance before the general public. A small Brownloud speaker, fitted with a microphone relay to

6


operate from a crystal set, proved an attractiveexhibit. Other attempts to eliminate resonancewere to be found in the T.M.C. " copper -lead -copper " horn and the Hart Collins' instrumentdesigned on the lines of the human throat.

While dealing with the subject of distortion, onemay mention several loud -speaker filters such asthe Beldam, Peronet and Fuller tone compensator.These components, of course, are connected to theoutput circuit, and are adjusted to give the desiredspeech quality.

So far as batteries are concerned, the mostinteresting exhibit was the Darimont primary cell,which is too complicate -1 to deal with here. It

In the new Ampllon the metal Is welded and pressed and a

wooden horn is used to prevent resonance.

may be said, however, that it has a remarkableperform fit e, and we hope to discuss it at some laterdate. The Alkeum cell, shown by Radio Acoustics,Ltd., is a form of Edison battery having a voltagesuitable for many of the dull emitter valves.

Coils of all types were very prolific, and one wasapt to wonder what material advantages some ofthe specimens possessed. Perhaps the most interest-ing coil development was the " Cosmos Strip,"consisting of several spaced wires fixed to a longroll of insulating paper. A multi -layer coil is

wound with a strip and the two ends are subse-quently cross -connected. Of course there are

many possible permutations and combinations ofconnections, and many applications suggestedthemselves. It would seem that the strip could beof great use in the manufacture of high -frequencytransformers. We were interested to note a newskeleton gimbal -mounted Igranic duolateral coil

and coil -holder incorporating several novel feature',

Rough coupling is obtained in the ordinary way,final adjustment being secured by means of a longfrictionally -geared anti -capacity handle. AnotherIgranic innovation is a type of high -frequencytransformer composed of two concentrically -mounted duolateral rolls. While mentioning the

A new Cossor low -impedance rectifier and a dull -emitter poweramplifier.

subject of tuning, we noticed one or two rejectorcircuits or wave traps, such as the Radio Instru-ments and Peto Scott eliminators.

It was encouraging to see a greater selection ofmechanically and electrically sound variable con-densers, two examples being the Bumdept low losscondenser and a square low brass plate condenserby the Sterling Telephone Co., Ltd. Doublecondensers for tuning two identical circuits simul-taneously were also shown by the Dubilier Con-denser Co., Ltd., Fallons, and A. W. Gamage, Ltd.

Rheostats and potentiometers were present inabundance, some very excellent American typesbeing shown by the Ashley Wireless Telephone

" Cosmos Strip," used for winding multi -layer coils.


An Igranic duolateral H.F. transformer

Co., Ltd., and Igranic Electric Co., Ltd. A glass-enclosed filament rheostat by Houghton was newto us. Practically every stand must have been agold mine to the " gadget " hunter, as gadgets werevery fully represented, so fully, in fact, that it isimpossible to deal with them in such a limitedspace. However, we must certainly mention" Clix," a universal connector, and also " PolarBlox," the latest constructional outfit. Crystalsof the " -ite " variety were to be found on manystands, and we were interested to see " YAI." areal synthetic specimen which functioned ex-cellently.

A useful apparatus for comparing signal strength.

So far as complete receivers are concerned,' therewas a marked tendency to mount them in cabinetsdesigned to tone in with existing furniture schemes.Here we may mention the Jacobean " Efesca-phone," the beautiful lacquer finish of the SterlingTelephone sets, the Roger Foster & Howell set inan imitation piano, the beautiful cabinet model ofthe General Radio Co., Ltd., and the combinedgramophone and wireless set by Abbey Industries,Ltd. The actual circuit which seems to have foundmost favour in the majority of sets is the tunedanode, but several transformer -coupled models wereto be found, and also one or two reflex circuits suchas the smaller Marconiphone, the Radiax set, andthe Climax monovalve.

December, 1923.

and their new coils and coil holder.

In addition to actual wireless apparatus itself,we noticed an audibility meter and a neon resist-ance bridge. Both these instruments are made bythe Bowyer Lowe Co., Ltd.

Last, but not least, amongst the varied attrac-tions, we must mention the 'demonstration of

A neon resistance bridge in which measurements are takenby the flash interval.

broadcasting by the B.B.C. Although we wereinformed on very good authority that some mostexcellent reproduction was obtained, our ownexperience was that on more than one occasionthe speech was of somewhat poor quality, due,no doubt, to the somewhat trying conditions.

One of our visitors described it as resembling an" elephant making a trurdf call " I l

December, 1923.- 184 . EXPERIMENTAL WIRELESS.,

Business Brevities.In our note last month describing the new

instructional model of a two -valve set introducedby Messrs. George Philip & Son, Ltd., a printer'serror made us refer to this as a 10 -valve set. Thiswas an obvious misprint ; but we hope our readerswill observe that this excellent model reallydescribes a two -valve set.

*

A new book which will interest many experi-menters has just been issued under the title ofThe Radio Time Table. This gives a full list of thetransmissions from all the principal stations of theworld during the 24 hours, so that an experimentercan see at a glance what to listen -in for at anyhour of the day or night. The price is 6d., or postfree 70. Any agent for EXPERIMENTAL WIRELESS,or from Percival Marshall & Co., 66, FarringdonStreet, London, E.C.4.

A new list issued by Messrs. Burndept, Ltd.,Aldine House, Bedford Street, Strand, London,W.C.2, covers 88 pages. It is divided into fivesections and deals with " Ethophones " and otherreceiving equipment bearing the B.B.C. seal, homeconstructional receiving sets, auxiliary apparatusand components, and transmitting apparatus. Itis fully illustrated, and is priced at Is.

Mr. H. Saville, Delamere Works, Stretford,Lancs. sends us his H.S. Booklet. It givesillustrations of a number of wireless and engineer-ing specialities supplied by Mr. Saville, and also adirectory of Amateur Transmitting Stations in theLancashire district and their call letters.

Some samples of excellently -made basket coilshave been received from Mr. F. Adcock, 39, ForeStreet, Ipswich. These are known as the " Magna "coils, and are supplied in sizes suitable for wave-

lengths ranging from 100 to 10,800 metres. Thespecial features of the coils are silk insulatioa, nowax, and rigid and regular windings. A list ofsizes and prices may be obtained on applicationto Mr. Adcock.

Messrs. Lionel Robinson & Co., 3, Staple Inn,London, W.C.1, send us a leaflet describing the" Ella " battery charge for A.C. circuits. It is asimple form of vibrating rectifier of the full -wavepattern, and works from any lamp holder.

Two new lists have been issued by Messrs.Fuller's United Electric Works, Ltd., ChadwellHeath, Essex. No. 315 deals with wireless acces-sories, such as transformers, tone selectors, filamentresistances, lead-in insulators, valve holders,potentiometers, fixed condensers, and coil holders.No. 250 B. covers the well-known " Fuller "accumulators for motor -car ignition and lighting,wireless and general purposes. Both standardplate type and block type cells are described.

A wide range of good -class components aredescribed and illustrated in a new 42 -page listissued by The Sterling Telephone and ElectricCo., Ltd., 210-212, Tottenham Court Road, London,W.1. These include 'phones of various patterns,variometers, condensers, valves, transformers,switches, keys, aerial equipment, and other usefulaccessories. The list is No. 368.

Portable " Dade " Batteries are dealt with inthe fourth edition of Catalogue P issued by TheChloride Electrical Storage Co., Ltd., CliftonJunction, near Manchester. A variety of types ofcells are illustrated and described, including specialhigh-tension batteries for wireless sets.

Experimental Notes and News.The Postmaster -General, speaking at the Wireless

Exhibition, said that 492,000 licences had beenissued as the result of the recent campaign, or justfour times as many as there were in March last.He also said that he was about to appoint a Broad-casting Committee, composed of representativesof the industry, the public and the Press, so thatfuture Postmasters -General would not have tocarry the whole burden on their shoulders.

Election results are to be broadcast between10 p.m. and midnight on December 6 and December7. The results will be issued simultaneously fromall B.B.C. stations.

Dr. A. M. Low, writing in the South Wales News,says :-" I have already used a television machineof a crude sort. Placed looking' at a field,and with the twin apparatus one mile distant,the machine, piercing all obstacles, revealed thefigures of men walking in that field. The vision,

however, was so dim and vague that one could notdistinguish such details as features or whether acap or hat was worn by the individual."

It is reported that Mr. H. Knight, a Hull amateur,has recently received a complete church servicebroadcast from New York between 12.30 and 1.45a.m. The congregational hymns, an anthem, thesermon and prayers, and the sound of the peopleleaving the church, were perfectly audible.

Hearing a football match being played 150 milesaway is a stepping -stone towards seeing it. Studentsof Princetown University, New Jersey, recentlylistened -in to their team playing a U.S. Navy teamat Baltimore, some 150 miles distant. The studentsgathered round on the lawn, where four big projec-tors were in place on the roof of a motor car, andthe various happenings in the game were describedby an observer on the spot.

Wireless... · EXperi ent Wireless A JOURNAL OF RADIO RESEARCH AND PROGRESS VOL. I, No. 3....

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Transcript of Wireless... · EXperi ent Wireless A JOURNAL OF RADIO RESEARCH AND PROGRESS VOL. I, No. 3....