The Truth and Nothing but the Truth: Truth Telling in the ...
Truth and Untruth
-
Upload
juan-manuel-rodrigues -
Category
Documents
-
view
224 -
download
0
Transcript of Truth and Untruth
8/8/2019 Truth and Untruth
http://slidepdf.com/reader/full/truth-and-untruth 1/8
The Truth and Untruth about
Electrically Small Antennas
John S. (Jack) Belrose, VE2CV, VY9CRC
The title of my talk suggests that there are controversies
concerning the performance of electrically smallantennas, and indeed there are concerning in particular
two very different types of antennas: the so-called
crossed field antenna (CFA) and a follow on version theEH antenna, and compact transmitting loop antennas.
But there is a wider range of controversy: concerning
the basic concepts of transfer of power generated by ahigh frequency tuned power amplifier to the
propagation media. I have been involved in this debate
for about 15-years, and so I will begin with a brief
discussion of this topic.
But first concerning controversies there is also acontroversy concerning the use or non-use of baluns,and which balun to use. Since many antenna types are
balanced, and present day HFTPA’s are unbalanced,
and, since the preferred method of feeding antennas is
to use coaxial cable, rather than 2-conductor balanced
transmission line, a balun should, often must be used inthe overall antenna system design.
I have during the past 50-years published about eighty
papers, technical articles and technical correspondence
on antennas (and an equal number concerned with radio
propagation). The basic theme of my work has been
concerned with providing an understanding of thecharacteristics of various antennas, and in particular
addressing the topic: performance of antennas in their
operating environments. Most of what I have written
has not generated controversy, at least prior to about
1991.
CONTROVERSY BEGINS--- WITH JERRY
SEVICK, W2FMI CONCERNING BALUNS
In 1991 QST published an article I wrote entitled
“Transforming the Balun”. The QST Technical
Editor added the byline:
“In this QST breakthrough, W2DU’s peerlers 1:1
current balun design serves as the basis for excellent
ferrite-bead-choke current baluns capable of 4:1 and
9:1 impedance transformation”
-----------------------------------------------------------------
Presented at the QCWA 2004 International Convention,
Amateur Radio Technical Session, Friday, October 15,
held at the Lord Elgin Hotel, Ottawa, ON, Canada
The word peerless caught in the throat of JerrySevick, W2FMI who had written a book entitled “Transmission Line Transformers”, 2nd
Edition published by the ARRL in 1990. Jerry
considered that his transformer type, a bifilar wound
choke type balun on a toroidal core, was superior ---
and perhaps only he knew how to design them.
This resulted in a controversy that went on for a year or
two. Jerry wanted QST to publish a rebuttal article. I
never did see his draft manuscript, which was never
published by the ARRL. But his views were published
under the collective title “Baluns Revisited” in
Communications Quarterly in 1992, four articles, and in
CQ Magazine in 1994.
There is nothing wrong with bifilar wound choke baluns, so called transmission line baluns, but provided
the VSWR is not too high --- in some cases for example
at MF this type is the preferred balun. For example seethe Figure below.
For the amateur in radio the W2DU ferrite bead over coax is a good balun (Figure) --- itexhibits slightly lower loss (Figure).
8/8/2019 Truth and Untruth
http://slidepdf.com/reader/full/truth-and-untruth 2/8
Jerry send me several of his best baluns, which
unfortunately we destroyed during testing --- blue flamearcing and smoke when subject to high VSWR and high
power (1 kilowatt). The W2DU ferrite bead over coax balun did not destruct, excepting loss and heating
increases with increasing VSWR.
Whatever type of balun is used a current balun should be used, and the balun should be on the input (tuned)
side of ther ASTU --- see my QST article in the
October 2004 issue of QST.
A CONTINUING SAGA
But the most controversial topic, I have folders andfolders full of correspondence, arose from the article byWarren Bruene, W5OLY entitled “RF Power
Amplifier and the Conjugate Match”, published in
QST November 1991 issue. Warren wanted to discredit
Walter Maxwell, W2DU’s book “Reflections-
Transmission Lines and Antennas”, published by theARRL in 1990.
Bruene showed measurements (a curious set of
measurements) that in his view showed that the outputsource impedance (referred to by him to be the source
resistance) of a tuned RF power amplifier was 5-timesthe load impedance. This of course is wrong, but
Bruene has stuck to his guns to the present date. He
still thinks Walt Maxwell and I are wrong!!
Walter Maxwell, W2DU, Tom Rauch, W8JI, John
Fakan, KB8MU and I corresponded with WB for
several years, in an attempt to publish a QST article that
addressed our differing views --- but with no success.
In 1997 we gave up and published an extended articlein Communications Quarterly Fall 1997 issue,
expressing our view that for maximum power transfer
the HFTPA should be conjugately matched, and that the
effective output impedance of the amplifier under such
conditions was equal to the load impedance. We
further stated that this impedance was non-dissipative.
We presented the results of seven experiments, and oneof the experiments was the set-up initially exactly like
that used by Bruene, that supported our view (not h is).
The following issue of Communications Quarterly,
Spring 1998 published Bruene’s rebuttal, a detailedrebuttal claiming that every one of our experiments
were flawed, flawed, flawed.
Let me begin at the beginning by quoting from the
IEEE Dictionary:
“Resistance is also defined as the real part of an
impedance. Because the impedance of a network
deals with energy transfer, it has nothing to do with
where the energy came from, or what became of it.
The real part of the impedance of a network
therefore does not dissipate energy of itself. Only
that portion of the real part of an impedance that is
in fact a dissipative resistance will dissipate energy”.
If we measure Vout/Iout, measured at the outputterminals of a properly adjusted PA, tuned for
maximum design power transfer, which in the practicalcase is an antenna system, we can infer an impedance
Zout = Vout/Iout, which is equal to the loadimpedance. But to measure Zout we have, as my long
time colleague Jim Wait, now deceased, told me, we
have to dissipate power. In other words operate the
power amplifier into a 50 ohm resistive load. To
measure Zout change the load a little bit, 5-10 percent,
and observe the change in Vout and Iout (rms values).We have to make a small change in the dissipative load
resistance, because we do not want to change the
operating characteristics of the PA tube(s).
This means we have to accurately measure very small
changes in current and voltage. I show in the next
Figure a comparison between what we (Walter
Maxwell) measured compared with the controversial
curve presently by Bruene.
8/8/2019 Truth and Untruth
http://slidepdf.com/reader/full/truth-and-untruth 3/8
Now let me return to transfer of power to an antennasystem. Zout inferred from Eout and Iout is a
nondissipative impedance --- we do not dissipate power there --- this is the impedance associated with the
generation of power. We want to transfer that power to
the antenna system. Therefore the antenna system must
be tuned to provide a conjugate match to the PA. The
feeder coax is associated with an impedance ---
typically Zo = 50 ohms. Zo is the characteristic
impedance of the transmission line --- it also is a
nondissipative impedance.
The matched antenna system presents an impedance
looking in Zas = Rr + Rloss. For an efficient antennasystem Rr (the radiation resistance) >> Rloss. Rr is a
non dissipative resistance, since power is not dissipated
in this resistance. Rr is an impedance associated with
the power that is transferred to the propagation
medium.
Now before I run out of time let me discuss the topic of
this talk, The Truth and Untruth about Electrically
Small Antennas.
COMPACT LOOPS
I have operations used, evaluated by experimentalmeasurement and by simulation (numerical modelling)
and written on small compact transmitting loops,
dating back to the mid-eighties --- in the amateur literature let me refer to my November 1993 QST
article “An Up-date on Compact Transmitting Loops”.
I considered that the performance of such antennas
(perimeter/wavelength as small as 0.03 to 0.06) wasabout what one would expect, a few percent,
comparable to a mobile whip --- for example a 1.7 mdiameter loop at 1.8 MHz and at 3.75 MHz, and that
traditional formula for radiation resistance, developed
about 60 years ago was correct.
And present day simulation agreed with experiment.
Mike Underhill, G3LHZ for whatever reason disagrees
with most of what has been written on compact loops,
beginning with his attention grabbing paper “Magnetic
Loop or Small Folded Dipole”, published in an IEE
Conference Proceedings in 1997. And since that date
he has written five or six papers/articles, each morecontroversial than earlier papers. In an article entitled
“The Truth about Loops”, published in the RSGB
International Antenna Collection, 2003 he states:
“the very low efficiencies (the few percent for the
very small loop size that I spoke about above),
predicted by simulation and existing theory are,
shown (by his measurements (??)) to be quitefrankly wrong by up to 1000 times (30 dB). How
can such measurements have been overlooked for so
long? It is a bit of a mystery and arguably a bit of a
scandal.”
G3LHZ considers the efficiency of such small loops
to be 80-90 percent, not a few percent.
Pat Hawker, G3VA, in his Technical Topics column in
RadCom, December 2002, I have contributed his TT
Column, discusses this difference of opinion, and he
challenged the antenna establishment to comment. My
response was a 2-part article published in the June/July2004 issues of RadCom.
But the controversy has not ended. In the
August/September issues of RadCom G3LHZ has
published an even more controversial article entitled
“New truths about small tuned loops in a real
environment”.
But my loops were in their operating environment.
Mike seems to have gone completely bananas, but since
he writes under the title of a Professor in the School of
Electronics and Physical Sciences, University of Surrey, I suppose there are some who believe that he
knows what he is talking about??
He claims that his inferred intrinsic efficiencies, 80-90
percent, inferred not from measured field strengths butfrom Q-factor based on measured VSWR, to be
confirmed by his proposed extensions of EM theory ---
Maxwell’s EM theory is not quite right, according to
Underhill,
8/8/2019 Truth and Untruth
http://slidepdf.com/reader/full/truth-and-untruth 4/8
and the Somerfeld-Norton ground wave propagation
theory needs revision. He also disagrees with the Chu-
Wheeler Q criterion. And field strength measurements
over ground need to re-evaluated.
I could go on --- but I will stop there. My
recommendation is that we should stop reading what
G3LHZ has written --- since this will avoid further
confusion.
I cannot believe that such nonsense is published. We
do read nonsense published in some amateur literature, but papers published by the IEE??
Let me tell you what I measure, and what I infer from
our numerical simulation studies ---- Figures.
Since the loop is a high_Q inductive reactance antenna,
it can be tuned by means of a capacitor, and power
couped into it by means of a small auxiliary loop, thesize of which is adjusted so as to realized a 50-ohm
input impedance.
The measured bandwidths of AMA loops, VSWR < 2:1is shown below.
I have numerically modeled various loop antenna using
NEC-4D, as a simple 1-turn loop, tuning capacitor at
the top (open squares), source on conductor, circles at
the bottom of the loop.
The Figure below shows the vertical radiation pattern
for a vertical 3.4m diameter loop, frequency 3.75 MHz, base height 2m, compared with a half-wave dipole at
10m. Compare this figure with measured NVIS
performance, see below.
The following Figure shows measured performance for reception of a near vertical incidence skywave signal
(D2 for the dipole, and A2 for the loop). Notice that the
narrow bandwidth of the loop results in improved
reception of the monitored signal, in spite of the fact
8/8/2019 Truth and Untruth
http://slidepdf.com/reader/full/truth-and-untruth 5/8
that the received signal is about a S-unit less than for the dipole.
In the figure below we show measured gain (dBi) for
a commercial loop (vacuum variable capacitor usedto tune the loop), for a NVIS path (100 km length),
compared with theory (numerical modeling using
NEC).
For the amateur in radio, he wants to know the
space wave gain for distance communication links.
The Figure below shows the calculated gain in free
space for three AMA Loops.
The Figure below shows the vertical radiation patterns
for horizontal loops (diameters 1.7m and 0,8m),frequency 14.15 MHz, compared with a horizontal
dipole, antenna heights10m. A part of the gain
difference, loops compared with dipole, is that thedipole has directional gain, the horizontal loop has an
omni-directional pattern.
THE CROSSED FIELD ANTENNA
Professor Maurice Hately, G3HAT, Brian Stuart, and
Fathi Kabbary, a student of Maurice’s have dreamed upa super controversial antenna. Not only has this
antenna type, and the new EM theory developed by
them to explain how the antenna works, confused theamateurs in radio, but this antenna type has been
patented, and as well presented to a learned audiences,
the IEE in an Antennas and Propagation forum, and the
IEEE Broadcast Technology Society, but it has
attracted wide attention and is being sold.
The original CFA concept was a cylinder over a disk,
both fed, and fed in phase quadrature. The disc wassaid to generate an H-Field, and the cylinder an E-field,
and these fields generated an out-going Pointing vector.
E/H, by adjusting the power fed, was said to be 377ohms in the near field, which is what E/H is in the far
field --- perfect coupling to the propagation medium.
The intrinsic impedance of free space is 377 ohms.
8/8/2019 Truth and Untruth
http://slidepdf.com/reader/full/truth-and-untruth 6/8
Kabbary has sold several of his antennas (costing as
much as $100,000) to MF broadcasters in Italy, Brazil,
Australia, and China. A broadcast consultant group in
Germany erected a CFA, copied in collaboration with
Kabbary, and one of his antennas was erected in the UK
for testing. Performance to be overseen by the Marconi
Research Laboratories, Chelmsford and the BBC.
But none of these antenna systems performed
satisfactorily --- efficiencies very low and bandwidths
too small for MF broadcasting.
The CFA works only in Egypt, and best at only one
location, Tanta, Egypt --- where it is mounted on the
roof of building (Figure), and its ground plane is well
connected to ground --- yet this antenna system is has
attracting wide interest.
CFA antennas,Tanta, Egypt.
Other installations installed at ground level did not
work so well (how well???).
It is a very electrically small antenna system, a few
electrical degrees in height, yet it is said to achieve a
radiation efficiency equivalent to a well ground quarter
wave monople --- and as well it is said to possess other attractive features --- high angle skywave is a minimum--- high angle skywave during nighttime hours limits
the useful range of MF broadcast transmission.
I consider that the basic theory on which the antenna is
said to work is flawed, that the method of feed leads toincreased difficulties, and if the antenna works at all it
is due to current flow on grounding wires (the antenna
is usually elevated) or on current flow on the outer
surface of the feeder coax --- I was in the audience and
said so following the initial presentation of a paper
before a learned audience (the IEE) in 1991.
So finally we decided to numerically model the CFA, in
1996/97, but this only resulted in further controversy --
- the inventors claim that the Numerical EM Code I use
cannot model the CFA --- so we built experimentalmodel(s) of the CFA and measured the characteristics,
and radiation efficiency --- in 1998/99. But it was not
before the autumn of 2000 that we succeeded in tuning
and matching the antenna.
8/8/2019 Truth and Untruth
http://slidepdf.com/reader/full/truth-and-untruth 7/8
I only have time in this presentation to give you a brief overview of our work.
Experimental model.
Numerical model.
Not discussed by the inventors of the CFA, it is merelytwo electrically small antenna system, comprising two
elements mutually coupled. And, when fed in
quadrature this results in power going out, and power
coming back, in fact the return power, returning to thedisc, is almost as large as the outgoing power, from the
cylinder.
This is seen in our numerical modeling (the resistive
component of the disc is negative.
This makes for very difficult tuning, to achieve (sayequal powers to the both elements, and currents in
phase quadrature.
Clearly no one but us have ever achieved quadrature
feed --- the inventors only imagined they did. No one
who has fabricated a CFA has ever observed the
problem of return power.
8/8/2019 Truth and Untruth
http://slidepdf.com/reader/full/truth-and-untruth 8/8
Field Strength (dB microvolts/m) versus distance
meters for the conical extension model of the CFA,
transmitter power 10 watts.
The measured field strength at 200 m for a
transmitter power of 10 watts is 87.3 dB
microvolts/m.
The field strength predicted (according to NEC-4, for
tuner coil Q-factors equal to 75) is 88.7 dB
microvolts/m (difference 1.4 dB).
The measured FS referenced to a short vertical over a perfectly conducting ground reveals that the gain of
our CFA is about
- 14.5 dB assuming no loss in the tuners;
and - 16.5 dB including tuner losses (about 1.5 dB).
REFERENCES
Belrose, J.S., "Scale Modelling and Full Scale MeasurementTechniques with particular reference to antennas in their
operational environments", in AGARD Lecture Series No.131, The Performance of Antennas in their OperationalEnvironment, October, 1983. Available: NTIS Access No.
N84-12367.
Belrose, J.S., G.M. Royer and L.E. Petrie, “HF Wire Antennaover Real Ground: Computer Simulation and Measurement”,
AGARD LS 165, Modern Antenna Design using Computersand Measurement: Application to Antenna Problems of Military Interest, Specialized Printing Services Ltd.,Loughton, Essex, U.K., September 1989 (overview 30 pages).
Belrose, J.S., “Transforming the Balun”, QST, June 1991, pp.30-33.
Belrose, J.S., W. Maxwell and C.T. Rauch, "SourceImpedance of HF Tuned Power Amplifiers and the ConjugateMatch", Communications Quarterly, Fall 1997, pp. 25-40.
Belrose, J.S. and L. Parker, "A tunable all-bands HF
Camp/Mobile Antenna", Communications Quarterly, Fall1998, pp. 47-57.
Belrose, J.S., “Characteristics of the CFA Obtained by Numerical and Experimental Modelling”, CFA Panel Forum,IEEE Broadcast Technology Symposium, Vienna, VA, 27-29September 2000.
Belrose, J.S., “Compact Loops Re-Visited”, AntenneX Online
Magazine, March 2001 (see Archives IV reference No. 70).
Belrose, J.S., “CFAs on the Roof of Buildings”, AntenneXOnline Magazine, June 2001 (see Archives IV reference No.88).
Belrose, J.S., “On the EH Antenna”, Published in the on-line
magazine antenneX April 2003.
Belrose, J.S., “On the CFA and EH Antennas”, TCA – Canada’s Amateur Radio Magazine, pp. 24-26, May/June2003.
Belrose, J.S., “On the Quest for an Ideal Antenna
Tuner”, QST, October 2004.
Belroser, J.S., “A Brief Overview of the Performance of WireAerials in their Operating Environments”, InternationalAntenna Collection (Edited by Dr. George Brown, M5ACN),
Published by The Radio Society of Great Britain, 2003, pp.137-153.
Belrose, J.S., “Performance of Electrically Small TransmittingLoop Antennas: Part I, RadCom, pp. 64-67; Part II, RadCom,
pp. 88-98, June/July, 2004; Technical Feedback, June 2005, p.78; and Technical Note, August 2006.
Belrose, J.S., “Characteristics of the Crossed Field Antennaobtained by Numerical and Experimental Modelling”, IEEE
AP-S Symposium, Washington, 3-8 July 2005.
Belrose, J.S., “Electrically Small Transmitting Loops”, IEEEAP-S Symposium Digest, Washington, 3-8 July 2005.