EMC behaviour of cable screens

Transcription

1 EMC behaviur f cable screes Alyse.R.Cates*, Alexadrs Gavrilakis*,Mhammed M. Al-Asadi*, Alistair.P.Duffy* Keeth G. Hdge ad Arthur J. Willis *De Mtfrt Uiversity, Leicester, LE1 9BH, UK Brad-Rex Ltd., Glerthes, Fife, KY6 RS, Sctlad Priciple Ctact: alyse@dmu.ac.uk Abstract As distributed electric systems becme mre cmplex, the Electrmagetic Cmpatibility (EMC) behaviur f cables becmes mre sigificat. I rder t ffer prtecti frm electrmagetic iterferece (EMI) may cmmuicati cables are shielded, with ly a small fracti f the ttal curret iduced the shield migratig thrugh it. I rder t describe this screeig perfrmace, the shieldig effectiveess, ad/r trasfer impedace is required, bth are clsely related. This paper describes tw emerget appraches t ivestigatig the EMC behaviur f cables: measuremets usig a reverberat chamber ad simulatis usig the Trasmissi-Lie Matrix (TLM) methd. This paper reviews the use f the reverberat chamber fr the shieldig effectiveess measuremets f cables; three-dimesial umerical mdellig is the used t visualise the effectiveess f the stirrer i prvidig statistically uifrm illumiati f the cable uder test. Results are the preseted t shw hw the psitiig ad legth f the cable effects the shieldig effectiveess results btaied. Trasmissi-Lie Matrix (TLM) Mdellig is used t extract the surface trasfer impedace by hybridisig time dmai mdellig f the cable ad theretical mdels relatig scree primary parameters t its gemetric characteristics. Fially the results btaied frm TLM mdellig are cmpared with thse btaied experimetally. Keywrds Shieldig; Shieldig effectiveess; surface trasfer impedace reverberat chamber; mdelig. 1. Itrducti Electrmagetic cmpatibility (EMC) is ccered with desigig ad peratig equipmet i such a way that it has a acceptable level f immuity frm electrmagetic iterferece (EMI) frm exteral surces ad des t iterfere substatially with ay ther equipmet. Hwever, esurig the EMC f idividual systems is cmprmised if the cectig cables are themselves surces f iterferece r susceptible t iterferece. Als it is essetial that a sigal beig trasmitted is t uduly mdified by a exteral eergy surce r by pr desig (self iductace). Therefre, it is cjectured that ay cectig cables must be rigrusly tested t esure that they are sufficietly shielded t prevet ay electrmagetic fields beig iduced by r iterferig with the sigal i the cable. Further, requiremets f cst, weight ad flexibility prvide a cuter frce t excessive ad uecessary shieldig. The effectiveess f a shield ca be measured i tw ways: either by calculatig the prprti f a electrmagetic field icidet the shield that is trasmitted thrugh it (the shieldig effectiveess SE), r by calculatig the Surface Trasfer Impedace (Z T ) f the cable shield. It is claimed that the shieldig effectiveess ad Surface Trasfer Impedace are prprtial [1-6] s the shieldig effectiveess ca be btaied by first calculatig the Surface Trasfer Impedace. It is, hwever, imprtat t be able t btai SE directly. The shieldig effectiveess f a shield is a rati (i decibels) f the prprti f a electrmagetic field icidet the shield that is trasmitted thrugh it [7]: E t = 0 lg (db) (1) Ei SE 10 Where:E t = electric field trasmitted thrugh the shield. E i = electric field icidet the shield. Figure 1 shws a diagrammatic defiiti f the shieldig effectiveess f a shield. The Surface Trasfer Impedace (Z T ) f a legth L f caxial cable is illustrated i Figure. I the figure I is the curret iduced by the exteral (iterferig) field the shield, ad V is the ptetial differece built up betwee the ier ad uter cductrs due t the curret I. The Surface Trasfer Impedace is usually expressed as a per uit legth parameter f the cable, ad is defied as: Z T V I L = (Ωm -1 ) () Figure 1. Defiiti f shieldig effectiveess 1

2 Figure. Illustrati f Trasfer Impedace. The screeig perfrmace f cables is aalyzed; by measuremets usig a reverberat chamber i sectis -5 ad by the TLM methd i sectis Review f Mde Stirred Chamber Methd It is desirable whe cductig EMI tests t islate the test space frm the exterir electrmagetic evirmet. This is because it is almst impssible t separate the arbitrarily varyig ambiet electrmagetic sigals frm the sigals f iterest prduced by the equipmet uder test, this is e f the mai failigs f a Ope Area Test Site (OATS). T d this the test space ca be set up withi a Faraday Cage, a metal walled rm r chamber which prevets fluctuatis i the electrmagetic field utside the chamber frm affectig the field iside. Susceptibility tests ca be carried ut withi the chamber by ijectig a electrmagetic field usig a atea the havig the equipmet uder test attached t a detectr. Hwever such a chamber ctais strg spatial variatis i the electrmagetic field, ecessitatig the rtati f the device uder test as this decreases the depedece f the results the rietati ad lcati f the device uder test withi the chamber. T elimiate the eed fr rtatig the device uder test a rtatig reflective surface ca be istalled i the chamber. This rtatig reflective surface (r stirrer ) alters the budary cditis withi the chamber its purpse is t cause very large chages i the stadig wave patters. This meas that the stadig waves withi the chamber that add tgether t frm the mdes (r maximums i the electrmagetic field) are altered with stirrer psiti. As the mdes ad ulls mve arud withi the chamber this prduces a field withi a certai vlume that, whe averaged ver e revluti, is statistically uifrm ad hece prvides uifrm illumiati t the device uder test. The mde stirred techique is ideal fr measurig shieldig effectiveess ver a wide frequecy rage ad cables f differet diameters ad sizes ca cveietly be measured i the same chamber [8]. The lwest frequecy at which the mde stirred chamber ca be used is limited by several factrs icludig the stirrer s limited ability t uifrmly excite all the mdes i the chamber [9] ad the umber f mdes i the chamber [10]. The umber f mdes i the chamber is a fucti f chamber gemetry. Due t desig cstraits, the dimesis f the De Mtfrt Uiversity Mde Stirred Reverberati Chamber are 5.00 metres lg,.95 metres wide ad.36 metres high. Therefre the lwest peratig frequecy f the De Mtfrt Uiversity chamber ca be estimated as 18.5 MHz [9-11]. The stirrer has t mve the psiti f the mdes withi the chamber such that whe the electrmagetic field is averaged ver e revluti f the stirrer it is the same fr all pits withi the wrkig vlume. The mde stirrer used here csisted f tw, e metre square, vaes set at 45 degrees t the vertical (see Figure 3). The stirrer was psitied as clse it e crer as pssible thus maximisig the vlume that had equipmet i it, the ucluttered vlume (see Figure 4). Whilst this desig is ulikely t prvide ptimum perfrmace, it is acceptable fr the iitial tests beig udertake ad t prvide a basis fr cmpariss with 3D mdellig. There are tw mdes f perati withi a reverberat chamber. I the Mde Stirred Methd, 00 measuremets are take durig e revluti f the stirrer whilst the stirrer rtates ctiuusly. Hwever, because the stirrer i cstatly mvig crrectig r averagig is pssible. This ctrasts with the Mde Tued Methd where the stirrer is stepped at selected, uifrm icremets (usually 00 per rtati), with the stirrer remaiig statiary i its predetermied psiti whilst the measuremets are take ad averaged. The Mde Tued Methd is mre repeatable ad accurate, hwever it als takes lger t cmplete each test ru. I rder t use the mde stirred chamber t measure the shieldig effectiveess f a cable, the pwer received at firstly the referece atea (P REF ) ad secdly the cable (P CAB ) is measured fr a rage f frequecies fr multiple stirrer steps (usually 00). The the maximum pwer received fr each frequecy durig e rtati f the stirrer is determied ad used i Equati 3. The maximum received pwer is rmally used because it is mre stable, easier t detect ad ctrl [1] ad the result is usually very csistet. PCAB = 10 lg (db) (3) PREF SE 10 Fr the results t be valid the cable uder test must be kept withi the wrkig vlume, the vlume withi the ucluttered vlume i which the electric field is statistically uifrm ver e rtati f the stirrer. I additi, the cable must t be ciled, as this will icrease the self iductace ad adversely affect the results. I rder fr the cable uder test t be lcated i the cetre f the chamber it must be placed a -lssy supprt. Oe ed f the cable was cected t the Netwrk Aalyser whilst the ther was termiated i the cable s characteristic impedace. Figure 5 shws a basic diagram idicatig the cectis betwee the ctrllig cmputer, etwrk aalyser ad the equipmet withi the chamber. I such stadards as [13] the methd fr usig mde stirred chambers fr measurig the shieldig effectiveess f cables is set ut. Hwever, there is a implicit assumpti that the cable is straight ad hriztal, which limits the legth f cable t arud e metre uder rmal circumstaces. The purpse f the wrk reprted here is t ivestigate the electrmagetic susceptibility perfrmace f cmmuicati cables, especially fr practical legths fr distributed systems (i the rder f 10s f metres) which clearly ecessitates a alterative r supplemetary apprach.

3 Figure 3. Illustrati f the Mde Stirrer. Figure 4. Shwig the layut f the chamber. 3. Wrkig Vlume I rder fr the measured shieldig effectiveess t be accurate, all pits alg the cable uder test must be subjected t the same (average) electric field. Therefre a vlume withi the chamber must be defied i which the field is statistically uifrm ver e revluti f the stirrer. This vlume is withi the ucluttered vlume (see Figure 4) ad is kw as the wrkig vlume. The precise lcati ad extet f the wrkig vlume is defied variusly as greater tha 0.5 wavelegth frm the rms budig walls, flr ad ceilig [10], at least λ/3 frm the chamber s walls at the lwest test frequecy [9] ad it is recmmeded that the surfaces budig the wrkig vlume t be lcated clser tha 1 metre frm ay chamber surface, field geeratig atea r tuer assembly [13]. I rder t ivestigate the wrkig vlume f the De Mtfrt Uiversity mde stirred chamber a virtual mdel was created i a cmmercial 3-D TLM slver ad the electric field was btaied fr the verticies f varius ccetric cubids whse surfaces were a csistet distace frm the surfaces f the ucluttered vlume (e such cubid is shw i Figure 6). The electric field utput frm each vertex f each cubid was the examied usig the stadard deviati methd as detailed i [13] t determie whether that cubid was withi the wrkig vlume fr each frequecy uder scrutiy. The stadard deviati respse fr the electric field i the x, y ad z directis ad fr the maximum f all the electric field measuremets f the cubid whse limits were 0.7 metres frm the chamber walls is shw i Figure 7. Fr each cubid ad each stadard deviati the percetage f remaiig data pits t exceedig 3dB was calculated fr each frequecy (Figure 8 shws this fr the cubid examied previusly). T determie the frequecy fr which each cubid ctaied the wrkig vlume a 95% cfidece frequecy was used. This frequecy was that abve which 95% f the stadard deviati frequecy respse was belw 3dB (the permitted tlerace). As ca be see, the miimum frequecy fr which the cubid i Figure 8 ctaied the wrkig vlume is 183 MHz. Based the methd preseted abve the miimum frequecy fr which each cubid ctaied the wrkig vlume ad the distace f that cubid s budig surfaces is shw i Figure 9. Sice the lwest test frequecy at which this chamber will be used is 00 MHz (crrespdig t the EMC testig frequecy rage f MHz) the the cable uder test ca be placed aywhere withi the ucluttered vlume. Figure 5. Blck diagram f the prpsed test set up frm [13]. Figure 6. Picture shwig the psitiig f the utput pits that frmed a cubid. 3

4 Figure 7. Calculated stadard deviati fr cubid whse edges were 0.7 metres frm the chamber wall. Figure 8. The percetage f remaiig data pits t exceedig 3dB fr each frequecy fr the cubid whse edges were 0.7 metres frm the chamber wall 4. Cable psitiig ad rietati T determie whether the cable uder test was subjected t uifrm illumiati ver e revluti f the stirrer, a cable was simulated i a 3-D TLM package. The simulated cable was simply a cylider 1 metre lg with a 0.03 metre radius, with its cetre pit at the cetre f the ucluttered vlume. The cable was rieted i the three rthgal directis (x, y ad z) ad the surface currets bserved fr five idepedet stirrer steps. As ca be see frm Figures 10, 11 ad 1 as the stirrer rtates the ht-spts f higher surface currets (the darker shades i the figures) mve arud s that if it were t be averaged ver e stirrer rtati it wuld be see that the surface currets were statistically uifrm. The cable was the repsitied utside the wrkig vlume ad the simulati repeated. This cable is t subjected t sufficiet surface currets fr ay shieldig effectiveess measuremets t be valid. Accrdig t these results the shieldig effectiveess f the cable shuld be the same irrespective f the cable rietati s lg as it remais withi the wrkig vlume as the cable is subjected t a idetical field i all three rthgal directis T ivestigate whether the rietati f the cable affected the measured shieldig effectiveess a 1 metre legth f cable was placed i the cetre f the wrkig vlume ad the shieldig effectiveess was measured ad calculated usig Equati 3. If the rietati f the cable did t affect the results the the measured shieldig effectiveess wuld be idetical i each case. As ca be see i Figure 13 the rietati f the cable des t affect the results prvided the cable remais i the wrkig vlume. Hwever, whe the cable was placed the chamber flr (see Figure 14) the shieldig effectiveess results are dramatically differet. This differece is prbably because whe the cable is ext t the grud plae it experieces additial resaces, pssibly due t the shield t grud capacitace r trasfer admittace. Hwever, whether this is purely due t the cable beig a grud plae r if it is effected by the cable beig utside the wrkig vlume requires further ivestigati. Figure 10. Cable i Cetre f Wrkig Vlume i Z Orietati Figure 9. The Miimum Wrkig Frequecy Fr Each Cubid 4

5 Figure 11. Cable i Cetre f Wrkig Vlume i Y Orietati Figure 1. Cable i Cetre f Wrkig Vlume i Z Orietati Figure 13. Shwig hw the rietati f the cable affects the shieldig effectiveess 5. Cable Legth I rder fr the mde stirred chamber t be f practical use i idustry it must be able t be used t test practical cable legths (i the rder f 10 metres). Hwever [13] suggests (as previusly stated) that the cable shuld be straight. If this is i fact the case the a rather large mde stirred chamber will be required t perfrm these tests. The cable hwever cat be ciled [] as this wuld frm a iductr. I rder t determie whether a lg cable culd be accmmdated it was essetial t see if the shieldig effectiveess results were the same irrespective f whether the cable is straight r bet. T determie this the 1 metre cable was bet i half ad its shieldig effectiveess was agai measured the cmpared with the straight cable (see Figure 15). These results suggest that a lg cable culd be accmmdated by beig placed i a zigzag patter. T ivestigate whether a lg cable ca be tested withi the mde stirred chamber a 10 metre legth f RG 58 C/U was attached i a zig-zag patter t a pegbard (see Figure 16) ad held i the three rthgal directis durig separate test rus. The shieldig effectiveess results were the cmpared i Figure 17. As the shieldig effectiveess results were the same fr each psiti it was assumed that the cable was subjected t the same electric field ad hece it ca be iferred that lg cables culd be tested usig the mde stirred chamber. The defiiti f shieldig effectiveess fails t take accut f the fact that the receivig atea fr the cable uder test is the cable itself ad hece it chages whe the cable chages. This may cause the shieldig effectiveess f a cable t be legth depedet. This suggesti agrees with the results preseted by Dle ad Kicaid [14] wh tested a 30 metre cable ad cmpared the results with thse btaied usig a 1.5 metre cable. They fud that the shieldig effectiveess results fr their tw cables differed by a factr f 0 db, which wuld be expected if the shieldig effectiveess btaied usig their methd was legth depedat. T ivestigate whether this cclusi applies t cables tested i the mde stirred chamber a 30 metre cable was tested i the De Mtfrt Uiversity chamber. The shieldig effectiveess was the calculated usig Equati 3 ad the shieldig effectiveess results were cmpared with thse btaied usig a 1 metre cable, as shw i Figure 18. The shieldig effectiveess results fr the 1 metre cable ad 30 metre cable are very similar, which is curretly beig ivestigated. Figure 14. Shwig hw the psitiig f the cable ear the grud plae affects the shieldig effectiveess. Figure 15. Straight vs. bet 5

6 Figure 16. Lg Cable cfigurati 6. Theretical Mdels fr the Trasfer Impedace f the Cable. I rder t assess the quality f a shield, the trasfer characteristics, predmiatly the trasfer impedace f the cable have t be tested. A umber f existig theretical mdels that relate the gemetry f the shield t the trasfer impedace have bee prpsed ad these are briefly reviewed i this secti. A relatively straightfrward mdel was prpsed by Tyi [15]. The tw iductaces are the braid ad the leakage iductaces. The braid iductace L b, which arises frm the wve ature f the braid, ad the leakage iductace L a, which is caused by the hles f the braid, are give by, ( 1 ta α ) µ h L = b (4) 4πD m L a π d N b b µ = π csα π D m e (5) Where α is the braid agle, D m is the mea braid diameter, N is the ttal umber f belts (spidles), b is the hle width, d is the braid-wire diameter, h is the radial spidle separati ad µ is the permeability f free space. The trasfer impedace is the apprximated as, Zts jω L b L ) (6) ( a Figure metre Cable pegbard i 3 rthgals The mst sigificat drawback f this methd is that it mits the effect f the shield resistace, which i the lw frequecies (LF) regi ca be quite csiderable. O the ther had the parameters required are relatively easy t btai. Ather mdel examied, was Katakis mdel [16] which is a mdified versi f Tyi s e. Katakis majr mdificati was that he added the effect f the radial spidle separati h, i the calculati f D m, D = D d h (7) m where D is the diameter ver the dielectric. Althugh i mst cases [16] this mdificati imprves Tyi s predicti, it als mits the shield resistace. Ather mdel, prpsed by Heft [17], apprximates the trasfer resistace as, Figure 18. Cmparig the Shieldig Effectiveess results btaied usig a 1 metre cable ad a 30 metre cable R = 1 π D σ T (8) Where σ is the shield cductivity, D is the diameter f the cable ad T is the thickess f the shield. The mutual iductace f the braided shield is give as, 6

7 give M Nµ α 0 m 1 = (9) ( πd ) 4 r 3 α = 3 m (10) Where N is the umber f hles per meter, r is the circular hle radius ad α m is the magetic plarizability. Its mai advatage i cmparis with the previus mdels is that it csiders the effect f the shield resistace but, the ther had sme f the parameters required (e.g. magetic plarizability etc.) are mre difficult t determie, ad subject t apprximati errrs. Furthermre, this mdel is very efficiet whe ly the wrst case sceari fr the trasfer impedace value is required. Vace develped a mdel [18] that relates the trasfer characteristics f the shield t braid parameters such as the fill f the braid ad the vlume f the metal i the braid. The fill f the braid is give as, N d F = (11) 4π a csα Where is the umber f wires per belt, a is the shield radius ad the remaiig symbls have their usual meaigs. The ptical cverage K f the shield is, K = F F (1) The vlume f the braid, U is give as, U = π adf (13) ad the umber f hles per uit legth f the shield is, 4 π a si α csα v = F N d (14) Vace gives the resistace per uit legth f the shield apprximately as, 1 R = (15) π adσf cs α By aalgy t Kade [19], the mutual iductace term M 1 fr v (hles/m) is M 1 µ m = v (16) 4π a where m is the magetic plarizability f the diamd shaped hle determied experimetally usig electrlytic-tak techiques [0]. Additial mdels have bee prduced (e.g. Sali [1], Zhu ad Gg []). As may f these are mdificatis f the abve mdels, they are t discussed further i this paper. The trasfer admittace, ather parameter characterizig the trasfer prperties f the shield, is defied as, Y T 1 di sc = (17) V dz where V is the vltage caused by the exteral field applied the shield ad di sc /dz is the shrt circuit curret per uit legth preset the ier cductr. Similarly t equati (3) the trasfer admittace is usually described by Y = jω (18) T C tr where C tr is the trasfer capacitace. I rder fr the value f the capacitace t be sigificat, the electric field the shield shuld be sigificat as well. This is ly usually sigificat whe the shield is t gruded, but placed clse t a cductig structure Mrever the trasfer admittace is almst egligible fr cables with high ptical cverage [3]. The trasfer admittace is t a itrisic prperty f the shield, as it depeds maily the exteral evirmet f the measuremets. A small umber f measuremets are preset the literature sme f them are reviewed i [4 ]. I the simulatis preseted i this paper, the mdel prpsed by Tyi is applied, as this is bth straightfrward ad reasably accurate fr the upper frequecies beig csidered fr cable testig. 7. TLM Aalysis The Trasmissi-Lie Matrix (TLM) mdelig methd is a time dmai umerical techique ad has bee implemeted successfully fr the simulati f may kid f cables, icludig Ushielded (UTP) ad Shielded (STP) Twisted Pair Cables [5][6]. As described i [7] the TLM apprach csists f may trasmissis lie segmets cected tgether. I this paper a caxial cable is examied by simulatig the effect f the gemetry f the shield the perfrmace f the ier cductr. The mai advatage f the prpsed methd is that it ca be used t examie the behaviur f the system fr ay frm f exteral field surce, either determiistic r radm, lcal r systemic. 7

8 The methd fllwed is similar t the e i [5]. Impedace liks have replaced the cmbiati f per uit legth iductace L ad per uit legth capacitace C. Tw separate circuits have bee csidered fr the shield ad the ier cductr f the caxial as i Figure 19 ad they are itercected by equati 1. Rs k VSL i k VSR i kvs Gs Z s kis Z s VT R k VL i k VR i kv G Figure 19. Trasmissi lie segmets (tp) fr the shield, ad (bttm) the ier cductr Z ki Z The L ad C cmbiati f the ier cductr is replaced by a lik lie f impedace (Z =(L/C) 1/ ). R ad G are time idepedet s they are left uchaged. The crrespdig time step f a wave t prpagate a TLM de is ( t=(l.c) 1/ ). Time sychrizati betwee the TLM des f the shield ad the ier cductr is required, s the shield impedace is calculated as (Z s =(L s / t). The shield capacitace is the btaied as (Cs =( t) /L s ). The TLM mdel f de ad its Thevei equivalet are shw i Figures 0 ad 1, Figure 1. Thevei equivalet circuits f the caxial system Usig the parallel geeratr therem the dal vltages f the abve circuits are give by, k VS i i k VSL kvsr Z R Z Z R Z G s s s = (19) s s s s ad k V i i kvl kvr kvt Z R Z = (0) Z R Z G 0 where VT = Is ZT (1) k k s Figure 0. TLM Mdels fr the caxial system ad k Is is the shield curret f de, ZT s is the trasfer impedace f the shield ad VT is the resultig iduced vltage i the ier cductr. Icidet ad reflected vltages, ad frm, the liks ca be cmputed ad be used i the iterative TLM prcess [7]. The value fr ZT s ca be calculated by usig ay f the theretical mdels (e.g. Tyi, Vace etc.) previusly discussed. 8

9 8. Validati f TLM agaist Measuremets The trasfer impedace was calculated as the rati f the lad vltage f the ier cductr t the curret preset the ear ed f the shield. Iitially, TLM was cmpared with measuremets, btaied by the authrs f this paper, usig the Curret Prbe Methd [8,9] fr the RG-58 cable. Tyi s mdel was used fr the mdelig f the trasfer impedace f the shield. Trasfer impedace (Ohms/m) frequecy (Hz) measured TLM Figure. Validati f TLM agaist measuremets fr 1m f RG-58 caxial cable. Tyi s mdel i cmbiati with the Vace equati fr the trasfer resistace, was used t btai predictis fr the measuremets f URM76 preseted i [3]. Figures ad 3 clearly shw the very gd agreemet betwee the mdels ad the measuremets. The psiti f resaces i the upper frequecies varies with the legth f the cable (as discussed i [30]). The mdellig f these resaces is the mai advatage f the TLM aalysis i relati t the pure theretical aalysis f secti 6, where ly the gemetry f the shield is accuted fr the calculati f the trasfer impedace. Trasfer impedace (Ohms/m) Frequecy (MHz) TLM:.5m TLM: 1m measured (1m) Figure 3. TLM agaist measuremets fr URM76 cable 9. Cclusis The reverberat chamber appears t be a prmisig methd fr directly measurig the shieldig effectiveess f varius cable legths, prvided that several criteria are met. The first f these is that the cable uder test is subjected t uifrm illumiati by the electric field withi the chamber. I rder fr this criteria t be met the cable must be withi the wrkig vlume, the exact psiti f which varies with frequecy, hwever fr the frequecies f iterest i this paper (abve 00MHz) the wrkig vlume starts a miimum f 0.3 metres frm the chamber walls. Whe the cable is placed withi this vlume it is subjected t a electric field that varies with stirrer psiti such that it is uifrmly illumiated, hwever whe the cable is placed utside this vlume it is t subjected t a sufficiet field stregth fr ay measuremets t be valid. S lg as the cable is placed withi the defied wrkig vlume durig the test, its exact psiti ad rietati is irrelevat ad, if ecessary, the cable ca be bet. This fidig permits lg cables t be tested i ay psiti withi the wrkig vlume. The slight differece betwee the shieldig effectiveess btaied with the cable i the x directi ad thse btaied with the cable i the y ad z directis is due t the psitiig f the cectig cable. If the cectig cable betwee the test cable ad the detectr is withi the wrkig vlume this causes the apparet shieldig effectiveess t reduce at high frequecies. T prevet this affectig the results the test cable is usually placed i the z directi whe the shieldig effectiveess is beig measured as i this psiti a abslute miimum f the cectig cable is withi the wrkig vlume. Furthermre, whe cables f the same type but differet legths were tested, they prduced remarkably similar results. Fially, the very gd predicti capabilities f TLM were preseted ad it was verified that the psiti f the resaces depeds the legth f the cable. 10. Ackwledgmet The authrs wuld like t thak Flmerics Ltd., fr prvidig the 3-D Electrmagetic Simulatr used i sectis -5 f this paper. 11. Refereces [1] J.Gedbled, Electrmagetic Cmpatibility,Pretice Hall, (1990). [] Stad.46A/331/CDV, Cable assemblies, cables, cectrs ad passive micrwave cmpets Screeig atteuati measuremet by the reverberati chamber methd,it. Electrtech. Cmissi, (1999). [3] Eurp. Stad. BS EN 6176:000. Cable assemblies, cables, cectrs ad passive micrwave cmpets Screeig atteuati measuremet by the reverberati chamber methd, Eurp. Cmit. fr Electrptech. Stadardizati, (000) [4] J.R. Peel, Simple Relatis Betwee shieldig effectiveess ad Trasfer Impedace/Admittace fr Cables, It. Symp. O EMC, Seattle,WA, ,(Aug. 1988). [5] R.J.Peel, R.Fiey, R.M.Lassise, Relatiship f shieldig effectiveess t Trasfer Impedace/Admittace fr Cable Emissi ad Susceptibility, Nat. Symp. O EMC, Sa Ati,TX, 55-6,(Apr. 1984) [6] G.Dike, R. Walleberg, J.Birki, Electrmagetic Relatiships betwee shieldig effectiveess ad Trasfer 9

10 Impedace, It.Symp. EMC, IEEE, Sa Dieg,CA, ,(Oct.1979). [7] C.R.Paul, Itrducti t Electrmagetic Cmpatibility, Jh Wiley & Ss, (199). [8] R.L. Jesch, Measuremet f shieldig effectiveess f Cable ad Shieldig Cfiguratis by Mde-Stirred Techiques, Tras. EMC, IEEE, 30,-8,(Aug.1998). [9] M.L.Crawfrd, G.H.Kepke, Techical Nte 109: Desig, Evaluati ad Use f a Reverberati Chamber fr Perfrmig Electrmagetic Susceptibility/Vulerability Measuremets,US: Nat. Bureau f Stadards, (1986) [10]N.J.Carter, Mde Stirred Chambers, (Mar 1999), (avail. at stir.htm.) [11] M.Petirsch, I.Striffer, A. Schwab, Mde Stirred Chamber as Test Facility fr Electrmagetic Susceptibility Measuremets, 13 th I, Symp. EMC,Zurich, , (Feb. 1999). [1] N.W. Wehlig, Repeatable Lw-cst Radiated Susceptibility Tests i a Stadard Shielded Eclsure,16-6. [13] Draft Electrmagetic Cmpatibility (EMC)- Part 4 : Testig ad measuremet techiques-secti 1: Reverberati Chamber Test methds, It. Electrtech. Cmmissi,(000) [14] C.W. Dle, ad J.W.Kicaid, Screeig Atteuati f Lg Cables, paper.04-04, IWCS 49 th,(nv. 000) [15] M.Tyi, The trasfer impedace f caxial cables with braided cductrs, Prc. EMC Symp. Wrclaw, Plad, , (September 1976) [16] J.N.Katakis, Trasfer impedace f wire braided caxial cables at radi ad micrwave frequecies, MEg Thesis, Uiv. f Sheffield, UK, (February 1983). [17] L.O.Heft, A Mdel fr Predictig the Surface Trasfer Impedace f Braided Cables, Rec. It. Symp. Electrmag. Cmpat., IEEE,40-404,(1986) [18] E.F.Vace, Shieldig Effectiveess f Braided-Wire Shields, Tras. EMC, 17(), IEEE, 71-77, (May 1975) [19] H.Kade, Wirbelstrme ud Schirmug i der Nachrichte-techik, Spriger-Verlag, Berli, (1959) [0]S.B.Ch, Determiati f aperture parameters by electrlytic tak measuremets IRE(39), , (Nv. 1951) [1] S.Sali A Imprved Mdel fr the Trasfer Impedace calculatis f braided Caxial Cables, Tras. EMC,IEEE, 33(), ,(May 1991) [] G.Zhu ad L.Gg, A imprved Aalytical Mdel fr Braided Cable Shields, Tras. EMC, 3(), IEEE, ,(May 1990) [3] F.A.Bes, P.A.Cudd, J.M.Tealby, Leakage frm caxial cables, Prc. 139(6),85-303,IEE, (Nvember 199). [4] F.Bryde, E.Clavelier, D.Givrd, P.Vallet, Discussi f the Relevace f Trasfer Admittace ad sme Thrugh Elastace Measuremet results, Tras. EMC,IEEE, 35(11),417-4, (Nvember 1993) [5] M.M.Al-Asadi, A.P.Duffy, A.J.Willis, K.G.Hdge, Twisted pair Cable Desig Aalysis ad Simulati, paper , IWCS 49 th, (Nvember 000). [6] M.M.Al-Asadi, A.P.Duffy, K.G.Hdge, A.J.Willis, Retur Lss Predicti fr Cascaded Systems, paper ,IWCS 49 th, (Nvember 000) [7] C.Christpuls, The Trasmissi Lie Mdelig Methd TLM, IEEE Press, New Yrk, (1995) [8] A. Mriell, T. M. Bes, A. P. Duffy, C. F. Cheg: "Surface Trasfer Impedace Measuremet: A Cmparis Betwee Curret Prbe ad Pull- Braid fr Caxial Cables". IEEE Tras - EMC, 40(1), 69-76, (1998) [9] A.R.Cates, The Develpmet f a Simple Methd fr the Evaluati Of The Surface Trasfer Impedace Of Caxial Cables, MEg Thesis, Uiv. f Nttigham, UK, (1999) [30] E.P. Fwler, Screeig f cables ad cectrs, Electrics ad Cmmuicatis Jural,IEE, 93-10, (April 1994) Authrs Bigraphies Miss Alyse Cates studied fr a Master f Egieerig with Hurs Degree i Electrical ad Electric Egieerig at Nttigham Uiversity, Eglad fr fur years ad graduated i July Her third ad fial year prjects ivlved develpig a simple methd fr measurig the surface trasfer impedace f c-axial cables. She jied the Applied Electrmagetics Research Grup at De Mtfrt Uiversity, Leicester, i September She is curretly ivlved i develpig a mde-stirred chamber t measure the Electrmagetic Susceptibility f cmmuicatis cables. Mr Alexadrs Gavrilakis hlds a BEg (Hs) degree frm Staffrdshire Uiversity, UK ad a MSc frm the Uiversity f Bradfrd, UK. He jied the Applied Electrmagetics Grup at De Mtfrt Uiversity, UK i 000 where he is readig fr a PhD. His research iterests ivlve mathematical ad cmputer mdelig f shielded trasmissi lies ad EMC. Fr bigraphies ad phts f Mhammed. Al-Asadi, Alistair.P.Duffy Keeth.K.Hdge, Arthur J.Willis, please refer t paper A Geetic Algrithm Tlkit fr Cable Desig. 10