Chapter 10 Practical

8/12/2019 Chapter 10 Practical

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10. Test Facilities

The first difficulty encountered while EMI measurement performing is theisolation of electromagnetic radiated energy of interest in the tests from theelectromagnetic environment. Thus we can perform EMI measurements in acontrolled test site or in situ measurements (e.g. testing systems or installations intheir places of use) .The physical controlled environments for performing EMI tests are:

Open Area Test Site - OATS ; Underground allery! Screened "ooms or Shielded #ham$er! "% Anechoic Screened #ham$er or A$sor$er &ined #ham$er ' A! Trans erse Electromagnetic #ell - TEM ; igahert Trans erse Electromagnetic #ell - TEM ; Stirred Mode #ham$er ; Three &oop Antenna System - T&AS .

10.1. Open area test site - OATS

The normalised open area test site is shown in Fig 10.1 where the distance ! "etweenthe E#T and the receiving antenna depends on fre$uency f . %ecause on this area we

perform only far field measurements it results:

&> * '10.1(

where

) c*f is the wavelength of radiated field. +enerally the distance * can "e , 10 or,0 m "ut some standards specify distances of 100 or ,00 m.

Focusesof ellipse

-eceivingantenna Transmission line

'screened ca"le(

rea within ellipse which must "e

free of all -F reflecting o"/ects

E#T

&!

!

, *

Measuring e$uipment 'receiverspectrum analy er oscilloscope(

Fig. 10.1. pen area test site.


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The site should "e placed well away from sources of electromagnetic noise or "roadcast transmitters into a $uiet am"ient environment.

The E#T is laid on a turnta"le situated in one focus and the receiving antennais situated in the other focus of an ellipse. 2ithin this area 'ellipse and even asurrounding one( the terrain should "e flat and free of any o"structions such as

"ushes trees and metal o"/ects which could reflect radiated energy. The measuringe$uipment 'receiver spectrum analyser( and antenna positioning e$uipment areusually located close to the receiving antenna "ut under the ground plane. lso themain power supply and support e$uipment for the E#T are located close to E#T "utunder the ground plane.

Even though the ellipse area and its surrounding 'at least eight times themeasurement distance ! away from the ellipse( is free of reflecting o"/ects thefollowing problems exist, and must be managed and sol ed!a) "eflection of the earth.

The signal measured at the receiving antenna is a vector com"ination of thecontri"utions from direct path and reflected path from the ground. Thus the amplitudeand phase changes introduced into direct and reflected paths "y the increased pathlengths will produce a systematic variation of received signal as the height of thereceiving antenna is changed. %ecause of this the test antenna is varied in height

"etween two selected ranges '1 m to 3 m for ! ) , m or ! ) 10 m and & m to 4 m for! 5 10 m(.

lso the ground at an open area test site is covered with a large metal plate ormesh 'ground plane( to provide a 6nown constant conductivity and dielectric constantfor o"taining predicta"le and invaria"le -F reflections. nonmagnetic metal'aluminium( or a galvanised steel 'hardware cloth( having calculated si e andsmoothness is placed directly on the surface of the earth or raised slightly a"ove. Theground plane acts only as a reflector for incident wave its purpose is to sta"ilise thereflected wave.

The com"ined effect of ground plane and height scan results in a measuredvalue that is 784 d% higher than the direct wave alone.

For ! 5 ,0 m the metal plate is not needed if the earth conductivity is goodand there is no "uried metal or other de"ris that could distur" its homogeneity.$) Attenuation of the test site.

9ite attenuation is defined as the ratio of the input power reaching thetransmitting antenna to the output measured power of the identical receiving antennaoperating with a given polarisation when "oth are located at 6now positions on thesite 'focuses of the ellipse(

'ma (log10

OUT

I+ ,

, A = '10.&(

where: is site attenuation;I< is the transmitting dipole8input power;

#T 'ma ( is the ma imum receiving dipole output power selected frommeasurements with receiving antenna at height = r ) 1 3 m from ground plane.

The site attenuation may "e theoretically calculated 'generally for simpleantennas e.g. tuned dipole( and*or determined "y preliminary measurements.

In Fig. 10.& a is given the set8up for measuring the site attenuation.The two identical dipole antennas are located at ellipse focuses and the receivingantenna is scanned '= r ) 1 3 m( until one o"tains the ma imum power indication to


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EMI receiver at each fre$uency for hori ontal and vertical polarisation at typicallymeasurement distances ', m 10 m ,0 m(. >ertical polarisation displays an inherentgreater sensitivity to ground plane anomalies "uried metal and surrounding o"/ectsthat can affect EMI measurements. If is possi"le the signal generator measuringe$uipment and au iliary devices are placed under ground plane.

9ite characterisation may "e done "y normalised site attenuation ? - ? F T ? F - ? 'B T 8 B- ( 8 FT T @d%A '10.,(where:

> T is the voltage into the transmit antenna;> - 8 voltage delivered into the EMI receiver "y the receiver antenna;F - ? antenna factor of the receiving antenna;F T ? antenna factor of the transmitting antenna;

BT and B - ? transmit and receive ca"le losses; FT T 8 a correction for mutual coupling of the antennas.In order to eliminate B T B- and even > T a su"stitution procedure is used.

The first reading of > - 'called > direct ( is ta6en with the two coa ial ca"lesdisconnected from the two antennas and connected to each other via an adapter ? Fig10.&". In this case the relation '10.,( "ecomes:

0 ) > T ? > direct ? 'B T ? B - ( '10.3(The second reading of > - 'called > site( is ta6en as the ma imum o"tained with

the ca"le reconnected to their respective antennas and the receiving antenna scanned

9ignal generator or trac6ing generator

= t

Transmitantenna

I< ; > TMeasuringe$uipment 'EMIreceiver(

= r ad/usta"le

#T 'ma:(; > - with = r )1 3 m

-eceiveantenna

"eflected

path

*irect path

+round plane ' ( !

dapter 9ignalgenerator

EMIreceiver C

> - ) > direct> T

"(

a(

Boa:ial ca"les

Fig. 10.". pen area test site cali"ration set up; a( and "( su"stitution procedure fordetermining


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over the prescri"ed range of height ? Fig 10.&a. In this case the relation '10.,( "ecomes:

T ? > site ? F T ? F - ? 'B T 8 B- ( 8 FT T '10.7(In "oth cases the two readings '> direct and > site( are ta6en for the same signal sourcevoltage > T.From relations '10.3( and '10.7( results:

direct ? > site ? F T ? F - 8 FT T @d%A '10.4(c) The a$sence of any protection against electromagnetic en ironment (am$ientelectromagnetic fields from other intentional or unintentional sources)Emissions standards in general re$uire that the test site environment should "e "elowthe limits "y at least 4 d%.Ideally the test site should "e located in a $uiet electromagnetic region 'away fromindustrial electromagnetic noise and "roadcast transmitters(.

It is not always economically via"le to set up an T9 at some distance from city tota6e advantage of the low electromagnetic am"ient. Thus the radiated emissionmeasurements must "e made in presence of high level and random changing am"ientsignals. lthough difficult and time consuming there are some method used in thiscase:

Fre$uency EUT

emission Am$ient

noise


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a&( The operating mode of the E#T is then changed and repeat the tests '"( c( d( e((.

The main disad antages of open area test site are!8 The reproduci"ility of the EMI measurement result for the same E#T '"ut made ontwo different open area test sites( is wea6er than the o"tained one in two +igahert

Transverse Electromagnetic cell ? +TEM for e ample.8 There are a lot of factors affecting the measurement accuracy on an open site @10.GA: reflection from o"/ects; non8conductive "ad8weather cover structure; antenna impedance and >oltage 9tanding 2ave -atio 8 >92- changes with

height; greater antenna si e affects the resolution "y field averaging; % H#< >92- and losses; differences in commercial antenna; ca"le and connector losses and >92-; position of E#T ca"les and peripherals.8 !ealing with measurement in far field region at fre$uency under 70 M= thedistance "etween E#T and antenna '!( must "e greater. The strength ofelectromagnetic field generated "y E#T in the measurement point is small thus theelectromagnetic am"ient signals may distur" the measurement.8 The tests made in open area test site as well as those made in TEM cells and +TEMcells re$uire complicate rotations of E#T or of the receiving antenna to detect thema imum emission thus they are time consuming.

The last two disadvantages might "y removed "y performing vectorialmeasurements in close field region.

10." %easurements in screened rooms

In EMB emission and suscepti"ility tests it is desira"le to isolate the test space fromthe outside electromagnetic environment.This could "e done using shielded enclosures which have two "asic purposes: to protect sensitive EMI emission test set8ups against high level am"ient

electromagnetic fields; to protect the am"ient electromagnetic environment against high level emissions

generated while testing EMI suscepti"ility.

&onstruction and operation

The screened rooms are constructed from galvanised sheet steel or copper 'sheets orfine mesh screening( and wood for supporting. There are "oth demounta$leconstructions 'structures of varia"le si e which can "e dismantled and re8sited to ananother location( and permanent installation at a given location. For "oth situationsthere are many types of construction: single shield! dou$le shield - cell type! dou$led

shield ' isolated . In Fig. 10.3 are presented a demounta"le 'modular( single shieldroom and a permanent installation room dou"le shield insulated option.


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The dou"le shield rooms are useful for e ample to prevent very powerful magneticfields from saturating the high permea"ility steel. In this case the layer of copperfacing the high field source reduce the field reaching the steel to a lower level

preventing the saturation.For modular installation the separate panels are clamped together with special

pressed metal strip or with #8tensioners to produce a -F*EMI tight /oint '"utt /ointand corner /oint 8 Fig.10.3 a((.The permanent installation constructions are superior as the seams are welded theelectromagnetic lea6s are decreased and periodic maintenance for various /oints issimplified.

The apertures and penetrations necessary for outside8inside connecting and

room facilities are designed to preserve shielding integrity. They should "e oftenchec6ed to confirm their mechanical integrity.%esides the doors and the various /oint and seams many other discontinuities

or aperture can compromise the performance of screened rooms and complicate themaintenance.

The prominent apertures in a screened room are those for: the entry of power service and its associated filtering; the entry of E#T support e$uipment; the lighting heating and air conditioning systems; the gas water and compressed air services.

The filter for power line or for E#T support e$uipment 'signal filters( must provide a certain minimum amount of attenuation usually approaching the d% levelof shielding effectiveness for the enclosure itself. They are placed in shielded "o andare usually installed outside the screened room with the filtered line entering theenclosure through pipe nipples.For outside8inside room signal lines are used tria ial or $uadria ial ca"les.

The lighting system must avoid the -F noise associated with ionisation. irvent may "e of honeycom" type.

If is necessary gas water and compressed air must "e furnished through steelor copper pipes which are dimensioned to act as a wave8guide. wave8guide "elowcut8off fre$uency 'f c( "ecomes an attenuator.

For a round wave8guide the cut8off fre$uency 'f c( and the magnetic field shieldingeffectiveness '9 = ( for operating fre$uency f 'much less than f c ( are:

ir vent

Filter panel

ir vent

!oor

ir vent

Filter panel

ir vent

Electricalinsulation !ual

shields

Insulatedsliding door

a( "(

%uttoint

Borner oint

Metallicwaveguide

Fig. 10.3 a( !emounta"le single shield room; "( ermanent installation dou"le shieldroom


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d f c

D10G71 = = '10.G(

d l

S / ,&= d% '10.D(where d is the diameter and l is the length of the wave8guide "oth e pressed inmeters.

The si'e of a screened room depends on si e of e$uipment under test 'E#T(and the space necessary for EMB testing.There are very large screened rooms '100 D0 &0 m( for military EMB testing of

"allistic missiles and aircraft or generally E#T having very large dimensions 'carsfor instance(.


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%esides the main room where the test is set8up there are one or twoanterooms for house EMI receiver E#T support e$uipment and e citation e$uipment'power amplifiers signal sources(. In Fig.10.4a it is shown the location of aerials forradiated suscepti"ility testing in accordance with MIH 9T! 34& test method -9 0,.!ue to reflection there is a large field strength variation inside the room. The

varia"ility in the field strength at the E#T is at lest a factor of 10 for constantemitted field controlled with - field measuring antenna @10.GA. This uncertainty is

present in radiated suscepti"ility and emission test made inside a simple undamped8screened room. There are attempts to e ploit the standing wave proprieties of a simpleshielded room for generate high field strengths. Thus with a special suscepti"ilityantenna 'Bavitenna( it is possi"le to e cite the entire room "y using the structure as aground plane to increase the radiatorJs effective electrical si e. In this mode it is

possi"le to generate high field strength with limited input power and small dimensionantenna 'e. g. a 1.& meters long antenna can produce the same field as a 7 meters log

periodic at a fre$uency of ,0 M= (. The minimum si e of a screened room is a"out34D m in height width and length and its cost is a"out 30.000 #9!. If the screenedroom is larger with a higher ceiling and the "ench is located away from the roomwalls the errors owing to reflections may "e reduced.

Shielding effecti enessThe degree of shielding for an enclosed volume is e pressed "y the shieldingeffectiveness.9hielding effectiveness '9( is defined "oth for electric field and for magnetic field.

i E E S 0log&0= d% '10. (

%enchtop ground plane

E#T

1 m

1 m

1 m

1 m

51 m

-eceiver system

T :

- :

ain room

nteroom 0 (e1uipment room)

E#T supporte$uipment

Emissionsystem

nteroom 2

- : or T :

1 m

1 m

#T

- : or T :

nteroom

1 m51 m

a( "(

Fig. 10. 4 9creened room dimension and arrangement: a( suscepti"ility testing; "(suscepti"ility or emission testing with "ench located in the middle of room


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i / /

S 0log&0= d% '10.10(

where E i '= i( is the incident field strength and E 0 '= 0( is the field strength of thetransmitted wave as it emerges from the shield.The total shielding effectiveness is e$ual to the sum of the reflection loss '-( plus thea"sorption loss ' ( plus the internal loss '%(.

9 ) - K K % @d%A '10.11(The reflection loss is dependent on the type of material and the fre$uency. For a planewave source

f

" log1014D = @d%A '10.1&(where f is fre$uency in M= ? material conductivity 8 material permea"ilityFor magnetic field source or electric field source - depends not only of f "utalso of the distance from the source to the shield.The a"sorption loss is dependent on the type of material and its thic6ness thefre$uency :

f t A ,13.1= @d%A '10.1,(where t is material thic6ness in cmf ? fre$uency in = ? material conductivity 8 material permea"ility.The a"sorption loss is independent regarding the type of wave 'electric or magneticsource(.

The internal loss factor % 'a correction term caused "y multiple reflection "etween the plate surfaces( is neglected if 5 17 d%.The previous e pressions of shielding effectiveness assumed a solid shield with noseams or holes.

Hea6age through panel seams /oints access doors and penetration panelsmainly determine the practical shielding effectiveness.

The shielding performance of screened rooms may "e measured according tothe method specified in MIH 9T! &D7. The techni$ues descri"ed in this standard areused for determining screened room attenuation versus magnetic and electric fieldfre$uency in the &0 = &4 += fre$uency range. The emission antenna T isoutside the room and generates the field incident 'E incident or = incident (. The receiving

antenna - 'identical with T ( is localised inside of the room and measures theattenuated field 'E attenuated or = attenuated (.The measured shielding effectiveness conforming '10. ( is:

attenuated

incident

E

E S lg&0= '10.13(

The operating fre$uency of the screened room is typically from 10 6= to tens of+= and the necessary attenuation is a"out 100 d%.

n improved low fre$uency shielding of 48mm thic6 mild steel all weldedconstruction "y comparison with typical modular screened room is shown in [email protected]


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(esonance

The accuracy of radiated measurements within a screened room is affected "y: resonance within the room; the pro imity of sensors to the walls; reflections of energy from its metal walls.The resonance is the ma/or draw"ac6 in the use of shielded enclosures.

shielded room with the walls from high conductive material and rectangular prismform "ecomes a high L multifre$uency resonator. The resonant fre$uencies of thecavity are:

++=

+

+

= &

&

&

&

&

&4

&&&

10170&

1

c

p

$

n

a

mc

p$

na

m f pnm

'10.17(

where a3 $3 c are the dimensions of the rectangular prism 'interior width length andheight in meters( and m3 n p are integer num"ers corresponding to the resonancemodes 0 1 & ... 'not more than one of the three may "e e$ual to ero for anyresonance mode(.

10.). (adio fre*uenc+ anechoic screened rooms

%ecause the electromagnetic field in a screened room has a great varia"ilityand its configuration depends on E#T and transducers position inside the room themeasurement errors in an emission or suscepti"ility testing could reach 30 d%.

In order to suppress the standing waves inside a screened room the reflecting

surfaces 'conductive walls( must "e covered with radio a"sor"er material forattenuating the reflected waves.

0 d%

30 d%

40 d%

D0 d%

100 d%

1&0 d%

130 d%

&0 d%

1001 = 10 1 6= 10 100 1 M= 10 100 1 += 10

4 mm welded steel plate

Typical modular screened room

Fre$uency

9creened room attenuation

Fig.10.G. ttenuation of cylindrical ' ) 13 m h ) 1& m( welded room and typicalmodular screened room '888(


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The "asic categories of a"sor"ing material utilised in anechoic cham"ers arelossy dielectric and ferrite. To achieve ma imum e traction of electromagnetic energyfrom an impinging wave8front the a"sor"er impedance must gradually decrease fromthe air8contacting surface to the mounting one. For implementing this two designtechni$ues are used:

the material is shaped into pyramids or cones so to decrease the effectiveimpedance per unit depth of a"sor"er;

tiles or sheets of materials are manufactured in three or more layers of differentcomposition 'from ma imum impedance at the impinging surface to minimum atthe mounting surfaces(.

For the a"sor"er to wor6 efficiently the distance "etween the source of field

'E#T or emission antenna( and the a"sor"er must "e greater than

&ma and

minimum depth of dielectric material with resistive loading used as a"sor"er to "e

3

ma @10.4A. Thus for decreasing the fre$uency operation in tens of M= domain an

a"sor"er depth of appro imately & meters is necessary. These a"sor"ers would imposea 3 meters increase in hori ontal dimensions of room and & meters increase in verticaldimension. In the last years the ferrite "ased plane wave a"sor"ers with a depth ofonly 1D mm have good performance down to ,0 M= "ut their cost and weight areconsidera"le.

%ecause full anechoic rooms are very e pensive they are used mainly forantenna measurements.

The semianechoic cham$ers (SA#) which reduce standing wave to accepta"le proportions are used in EMB measurements. These rooms '9 B( are "ased on thefollowing solutions @10.&A: using the performing a"sor"ers only in intensive reflection areas ?Fig.10.Da; using the adapted shaped enclosures e.g. pyramid followed "y cu" and an

ade$uate placement of the e$uipment ? Fig.10.D"; using the mo"ile a"sor"er panels Fig.10.Dc.


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10. . %easurements using the T % cell

In EMB measurements a compactnessN and greater versatility '"etter planewave simulation uniform field in test area "etter efficiency wide fre$uency "and(could "e o"tained if the E#T is introduced in a TEM line 'open tri8plate transmissionline which allow the transversal electric and magnetic propagation mode( or in aTEM cell 'completely closed device(.

TEM cell 'Fig. 10. ( is a rectangular coa ial transmission line that graduallyincreases the si e in "oth ends 'from source and from load( for housing small tests

9ource

E#T

."sor"er

E#T

9ourceantenna

&D

E#T .ntenna

Mo"ile panel

a( "(

c(

Fig. 10. D. 9emianechoic cham"ers

.

g

Hoad impedance70 'coa:ial

mount(

- uter conductor 'shield( Inner

conductor

!ielectricsupports ccess door

9eptum 'inner conductor(

E$uipmentunder test

g

&t&$

&p

-F

source

Fig. 10./ TEM cell a( view of a TEM cell for esta"lishing standard EM fields; "(cross section dimensions


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e$uipment "eing used for esta"lishing standard uniform electromagnetic fields in ashielded environment.

The design of the TEM cell is done imposing the characteristic impedance 'O 0(of the rectangular transmission line 'calculated with appro imate e$uation '10.14((to "e 70 @10.DA.

0

0

&sinhln

&3

,GG

#

1 g

1 p

4

'10.14(

where p3 g3 1 t are TEM cell dimensions 'Fig. 10. ( and # 0

is related to the

fringing capacitance "etween the edges of the septum and the side walls 'for g5p 5

0 & results0

# 0(.

The upper fre$uency for a TEM cell is limited "y the test field distortions caused "yresonance. Increasing the cell dimensions means reduction of the upper fre$uency.In ta"le 10.1 are shown some cut8off fre$uency values function of the celldimensions @10.,A @10.GA @10.DA.

Ta$le 06.0&ut-off fre*uencies of T % cell for some dimensions.

"* "p "p-"g "t &ut-offfre*uenc+

@cmA @cmA @cmA @M= A170 1&3 0 17G 10070 31 0 17G ,00

,0 &7 0 17G 700

t fre$uencies "elow the cell cut8off fre$uency the strength of the electric field inwor6ing space of the TEM cell may "e calculated with the simple relation:

E U 1

, 4

1= = 0 '10.1G(

where:8 # is the rms voltage value on the septum 'inner conductor(;8 $ ? distance "etween the septum and lower or upper walls;8 ? net power flowing the cell;

8 O0 ? real part of the cellPs characteristic impedances 'O 0 70(.The magnetic field is calculated considering the free space impedance e$ualwith ,GG as for the far field plane wave propagating in free8space.

/ E

=

,GG '10.1D(

"viously at lower fre$uencies this approach is progressively unsatisfactory.The detailed electric field distri"ution inside the cell can "e calculated e.g. "y usingthe $uasistatic appro imation.

-eferring the field value in the 6ernel of the wor6ing space '10.1G( we sensean increasing while going to the septum and a decreasing reaching the up and downwalls. lso considering the field distortion due to the E#T is advisa"le to have E#Tdimensions lower than 1*, of the distance "etween septum and e ternal conductor.


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The TEM cell is capa"le to generate calcula"le E and = fields. !ue to its versatilitygood measurement uncertainty '0 7d% &d%( and operation simplicity it hasmultiple applications: cali"ration of the electrical small antennas and of radio fre$uency pro"es; electromagnetic illumination of the electronic devices in suscepti"ility testing and

also of the shield or gas6et for determining their shielding effectiveness; e posure in electromagnetic field 'plane wave( of the "iological tissue and "eings

for evaluating the "iological effects of fields. 10. . %easurements in 2T % cell

igahert Trans erse Electromagnetic Mode #ell ( TEM) is an improvedversion of TEM cell which e tends fre$uency range to += and increases wor6ingvolume. The +TEM 'Fig.10.10( might "e viewed as a hy"rid "etween TEM cell 'good

"ehaviour at low fre$uency( and anechoic cham"er 'good "ehaviour at highfre$uency(.Bomparing with TEM cell for +TEM we can identify: the increasing of the wor6ing space 'the septum not pass through the centre of the

cell( the septum plate or the central conductor is ended "y a distri"uted noninductive 70

resistive load across the large end of the cell and provides a return path for thecurrent via the conducting outer case;

the field generated inside +TEM is not reflected in its terminal space "eingattenuated "y -F a"sor"ers 'li6e in a shielded anechoic room(.

Inside the +TEM we have not standing waves and the main advantages are thefollowing:

a compara"le cost to an T9; a"sence of am"ient electromagnetic fields; time savings while tests performing '"etter than D:1 comparative to open

area test site and &:1 comparative to anechoic cham"ers(; receiving all the E#T emissions; very good overall "ehaviour in emission and suscepti"ility tests; possi"ility to create transient fields with a rise time "elow 1 ns and

unlimited fall time or generally possi"ility to measure in time domain.

The TEM cell is very versatile and wide use e$uipment in modern suscepti"ility andemission EMB testing.


16/21

For suscepti"ility testing is used an ad/usta"le signal generator at +TEM cellinput and the e$uipment under test is positioned at region where the field uniformityis ma imum 'centre "etween inner conductor and outer conductor(.

For emission testing the e$uipment under test is positioned as forsuscepti"ility testing and at input in stead of the signal generator is connected anEMI receiver.

wing to its flat fre$uency response '!B 8 += ( +TEM is used for ma6ingfast pulse measurements or generally time domain measurements.

There are many up8to8date attempts for comparison and lin6s amongmeasurements performed "y +TEM anechoic cham"ers and T9.

10 .3. %easurements in mode stirred chambers

The enclosures of this type 'rever"erating enclosure mode8tuned enclosure(have large dimensions comparing to the wave8length of the EM field many other

propagating modes of higher order "eing allowed. The usual fre$uency domain forthese measurements is from 100 M= till &0 += .

paddle wheel stirrer modifies the created field inside the enclosure "y anemission antenna in a random manner. wing to multiple reflections the field has aninner uniform distri"ution as many plane waves coming from various directions.#nder these conditions the E#T or antenna movement 'for achieving the worst case(is not necessary. Bonse$uently the time test is reduced.

!ue to the "etter statistic uniformity ' "ellow 1 d%( of such an electromagneticfield these enclosures are used for immunity tests and shielding effectiveness'gas6ets coa ial ca"les(.

10.4. %easurements using 5Three 6oop Antenna S+stem7 - T6AS

TH 9 is a system of three loop antennas perpendicular one to the other. Eachof them has two diametral8opposed gaps where the load impedances are connected.The system is a"le to determine radiation characteristic of an electrically small source

placed in its centre.

8rinciple of the method

C

Boa:ialinput

-F a"sor"er Efield termination

70 distri"uted resistivetermination 'load(

=ighest accuracy testvolume '1*, of the septum

height and cell width(

Ma:imum testvolume

-eference point 0 d%

Bontour Q&d%

Bontour Q1d%

9eptum 'centralconductor(

EMImeter

ST

ET

Blosed metalend wall

E#T

a( "(

Fig. 10.10 +TEM cell a( 9chematic diagram for suscepti"ility or emission testing; "(Bross sectional view.


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2e shall consider a circular loop antenna of " radius placed on y plane in6 propagation constant area ? Fig. 10.11.

The antenna fulfils small electrically loop condition '6" R 1(. It is made ofconducting material and it has a O H load impedance in each of the 0= and = gaps.In loopPs centre there is an electrical small radiation source. This field source whichradiation characteristic has to "e determined may "e represented "y an electricaldipole moment em ' ' 9im ++= e) eye7e mmm ; where i, 9, ' are y unityvectors( and a magnetic dipole moment m m 'm i 9 'm m7 my m) m m m= + + (.

The following steps are to "e successfully passed:a( !etermining of tangential electric field component on loop surface E i ' (

"ecause only this induces current in the loop. "( !etermining the current "y the two loads I ' (0 and I ' ( .c( !etermining one of m e component and one of m m component.

d( -epeat a( "( and c( for the loop in y plane and then in plane representinga complete determination of m e and m m .e( !etermining the magnetic induction : and electric field as a function ofmoments m e and m m and the distance r "etween source and measuring point whichmeans determining of the radiated field in each point.

a( Bonsidering the electromagnetic fields generated "y the two elemental dipoles theincident electric field tangent to the loop E i ' ( , may "e written as:

E i ' ( ) E i ' ( m m K E i ' ( m eyK E i ' ( m e '10.1 ( "ecause only m m me and m ey components contri"ute to E

i ' ( .

-eplacing the values of the fields generated "y the two dipoles 'm e mm( andconsidering the orientation of these moments towards the receiving loop we o"tain:

sin11

3cos

1133

('

,&0

,&0

&

&0

++

++

=

e7 89$

ey

89$m)

89$i

me$9 8$$

9 8m

e$9 8$$

9 8me

$

9 8$

9 E

'10.&0(

or

y

Ei' (

me:

mey

OH

I' (OH

mm

em

mm

Fig. 10.11 E$uivalent electric and magnetic dipole of the source situated in the middleof the dou"le loaded loop antenna


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For all the here presented test facilities the E#T supporting e$uipment andmeasuring instruments themselves will "e placed in shielded rooms 'special care forthe connecting system(.

%efore starting the test we have to evaluate the test facility performances.n overview over these facilities accompanying the measurement errors and

corresponding fre$uency range is presented in Ta"le 10.&.Ta$le 06.2

8erformances of measurement sites

Test site Fre*uenc+ range rrors ;n estment

pen area testsite ' T9(

,0 8 1000 M='10 6= 8 1000

M= (

'& 8 D( d% ,00000

nechoiccham"ers' nechoic

cham"ers inmicrowaves(

,0 8 1000 M=

'0 & 8 1D += (

', 8 10( d% 5 &000000

+TEM 1700 !B 8 1D00 M= "etter of 3 d% 1000008170000 TH 9 'ol. &4


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@10.DA ol. ,G

Chapter 10 Practical

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