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AUTOMATED SYSTEM FOR ELECTROMAGNETI C FI ELD GENERATI ON AND I MMUNI TY TESTI NGEr i c J . VanzuraNat i onal Bureau of StandardsEl ect romagneti c Fi el ds Di vi si on

325 Br oadwayBoul der , CO 80303

Abst ractAn i nt eract i ve comput er- cont r ol l ed system has beenconst r ucted f or r adi at ed i mmuni t y measur ement s. I tcan set up a desi r ed unper t urbed fi el d st r ength at apoi nt i n space and si mul t aneousl y measur e f i el dst rengt hs and pol ari zat i ons at up t o ten di f f erentposi t i ons. Fi el d mappi ng experi ment s have beenper f ormed wi t h t he syst em i n an anechoi c chamber, apar t i al l y l oaded shi el ded room and an unl oadedshi el ded r oom Resul t s conf i r m dramati c i mprovementi n spati al f i el d uni f orm t y as more absorber i s used.I f the unpert ur bed f i el ds i n a speci f i ed t est vol umedo not devi at e mor e t han a desi r ed amount f or anyf r equency of i nter est , meani ngf ul i mmuni t y t ests canbe perf or med. An equi pment under t est ( EUT) i s pl acedi nto t he test vol ume and i t s response to radi ati on i smeasur ed usi ng curr ent pr obes and a spectr umanal yzer .Such an exper i ment r equi r es many m d- exper i mentcal cul ati ons maki ng automati on hi ghl y desi r abl ebecause of s i gni f i cant ti me savi ngs.

Our f r equency r ange of i nter est i s 50 t o 200 MHzbecause th i s i s a part i cul ar l y di f f i cul t f requencyband i n whi ch to per f orm r el i abl e i mmuni t y test s. Bymeasuri ng the f i el d var i ati ons i n a t est zone, we canmore accur atel y esti mate systemati c uncert ai nty l i m t sdue t o spat i a l f i el d devi at i ons. Thi s al l ows test i ngi n an anechoi c chamber bel ow what i s normal l yconsi dered i ts l owest usabl e f requency. I f ananechoi c chamber i s not avai l abl e, a part i al l y l oadedshi el ded r oom can be used, w t h the necessaryrequi rement t hat speci f i cat i ons on spati al f i el duni f ormt y be r e laxed.Thi s measur ement syst emcan be used i n conj unct i onw th many other f ac i l i t i es , such as a TEM ce l l , anopen f i el d si t e (gr ound screen) or a r everber ati ngchamber, and can be used to t est at f r equenci es fr omt he l ow kHz up to 2 GHz.

I ntr oduct i onThe automat ed r adi o f r equency ( RF) f i el dgener ati on and i mmuni t y t est syst em descr i bed her esati sf i es a need to measure r adi at ed- t o- conducted RFr esponses i n the 50 t o 200 MHz f r equency r ange. Thi si s an i mport ant f r equency range because manycommerci al t r ansm t t ers, i ncl udi ng t el evi si on and FMr adi o, operat e i n t hi s fr equency band. However ,common t est f aci l i t i es such as t he TEM cel l , anechoi cchamber , open f i el d s i t e , shi el ded room andr everberati ng chamber are not wel l sui t ed f orgener ati ng a uni f orm f i el d thr oughout a l arge enought est vol ume at t hese f r equenci es. As f r equency

i ncreases, TEM cel l s have to become smal l er i n si ze t oavoi d mul t i modi ng. The upper f r equency l i m t of ath ree- met er TEM cel l i s about 50 MHz [ I ] TheNati onal Bureau of Standar ds ( NBS) anechoi c chamberi s l i ned w t h one- met er absorber and i s not normal l yused bel ow 200 MHz because of absor ber i nef f i ci ency[ 2 ] . At l ower f r equenci es, ref l ect i ons f rom thewa l l s , f l oor and cei l i ng [ 3 ] w l l make the cal cul ati on

o f f i el ds f roma t r ansmt t i ng antenna di f f i cul t 4 , 5 ] .An open- f i e l d s i t e can generat e f a i r l y uni f orm f i e ldst hroughout a l arge t est vol ume, but onl y aut hor i zedf r equenci es may be t est ed. Shi el ded r ooms aredi f f i cul t t o use i n any f r equency r ange becauser ef l ect i ons and r esonant modes cause l arge andunpredi ctabl e f i el d var i at i ons [ 6 , 7 ] . Rever ber ati ngchamber s are used pr i mari l y at hi gher f r equenci es andrel y on adequate st i r r i ng of r esonant modes. Thel ower- f r equency l i m t of the NBS 2 . 7 4 x 3 . 0 5 x 4 . 5 7 mr ever ber ati ng chamber i s 200 MHz [ E ] . Even at 200MHz spat i al var i at i ons of the sta t i s t i cal l ydeterm ned E-f i el d are no bett er t han +8 dB f or t hi sr ever ber at i ng chamber .Because i t i s di f f i cul t to predi c t spat i al f i el ddi str i but i ons i n the 50 t o 200 MHz f r equency r ange, amore empi r i cal appr oach i s needed. By measur i ng t he

el ectr omagnet i c f i el d di str i but i on i n a test vol ume,we can def i ne t he syst emati c uncer t ai nty of a t estf i e l d. I f the measured devi at i ons i n f i el d str engthand polar i zat i on are wi t h i n acceptabl e l i m t s,i mmuni t y t est i ng of an EUT may be perf or med. Fi el dmappi ng r esul t s of a 4 x 40 cm test a rea arepresented f or experi ment s perf ormed i n an anechoi cchamber , a part i al l y l oaded shi el ded room and anunl oaded shi el ded room Large fi el d devi ati ons i n t heunl oaded shi el ded r oom prevent proper i mmuni t ytest i ng. I n the part i a l l y l oaded shi e lded r oom f i e lddevi at i ons are l essened, but st i l l requi re l argeuncer t ai nty l i mt s . F rom 50 t o 200 MHz t he mostsui tabl e faci l i ty f o r genera t i ng a re l at i vel y uni f ormt est f i el d i s t he anechoi c chamber , even t hough i t i snot nor mal l y desi gned f or such a l ow f r equency r ange.Thi s i s because i t s absorber st i l l adequatel yatt enuates ref l ect i ons to g i ve a fa i r l y uni f ormel ect r omagneti c f i el d generated by a t ransmt t i ngant enna.

Measurement Obi ect i vesThe over al l probl em to be sol ved i s how toaccur at el y measur e t he conducted r esponse of an EUT t oa radi ated f i el d i n the 50 t o 200 MHz f r equency range.The pr i mary d i f f i cul ty i s i n generat i ng a rel at i vel yuni f orm f i el d throughout a t est vol ume l arge enough tocontai n an EUT. Pol ari zati on and f i el d str engt hs mustnot devi ate si gni f i cantl y at any poi nt i n the testvol ume to ass ure good cor r el at i on bet ween EUT r esponseand f i e ld str engt h. Theref ore the f i rst obj ect i ve i sto def i ne err or bounds due to f i el d devi ati ons i n theanechoi c chamber , t he part i al l y l oaded shi el ded roomand t he unl oaded shi el ded r oomThe second obj ecti ve i s t o obser ve per t urbat i onscaused by the pres ence of an EUT. The EUT i nt r oducesa mutual coupl i ng w t h t he t r ansm t ant enna, causi ngaddi t i onal var i at i ons i n f i el d str engt h surr oundi ngthe EUT and possi bl y modi f i ed pol ari zati ons.Observat i on of t hese phenomena coul d yi el d desi gni nf ormat i on f or EUT hardeni ng and show t est engi neersdesi r abl e EUT ori ent ati ons f or f ut ure exper i ment s.

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The thi rd obj ect i ve f or t hese tests i s to val i datethe use of cur r ent probes f or measuri ng r adi at ed-t o-conduct ed cur r ent s on w r es i nsi de the EUT. Curr entprobes and associ at ed cabl i ng must be i mmune t or adi at ed pi ckup and have a l i near response. Tomeasure absol ute cur r ent, t he probes must becal i bra ted i n l i near uni t s o f AD . Cal i bra t i on f i l esf or t he curr ent pr obes are l oaded by t he comput erdur i ng the exper i ment al conf i gur ati on process.Measur ement Syst em

The test confi gur ati on f or part i al l y l oadedshi el ded r oom measurements i s shown i n f i gure 1. Theshi el ded room measures 7 . 1 8 m l ong, 3 . 3 5 m deep, and2. 7 6 m hi gh. 28 pi eces of 0. 66- meter (26- i nch)absor ber were pl aced on opposi t e wal l s as s hown. Thi sroom i s convert ed t o an unl oaded shi el ded r oom byr emovi ng t he absorber . The si gnal generat or out putdri ves an RF ampl i f i er connect ed to a bi coni calt r ansm t t i ng ant enna l ocat ed 1 . 1 3 m above the f l oor.I nci dent and r efl ected powers at t he bi coni cal ant ennaare measured usi ng a bi di r ecti onal coupl er wi t h RFpower met er s. EUT responses are measured usi ngcur r ent probes connect ed to a spect r umanal yzer. Thesi gnal gener ator , RF power mzt ers , and spect r umanal yzer are cont r ol l ed by comput er t hrough an I EEE488 (GPI B) i nterf ace. The el ectr i c f i e ld str engthprobe syst em i s cont r ol l ed usi ng a General Pur poseI nput Output ( GPI O) i nter f ace. The same syst em wasused f or anechoi c chamber measur ement s, except t hat a50 W ampl i f i er was used i nstead of a 3 W ampl i f i er toachi eve hi gher f i el d st r engths. The desi red f i el dst r engt h i n shi el ded r oommeasurements was 6 V/ m Thedesi r ed f i el d st r ength f or most anechoi c chambermeasur ement s was 15 V/m

An NBS- devel oped i sot r opi c pr obe syst em measuresel ectr i c f i el d str engths [9]. Ni ne i sotr opi c probeswere used to measure el ectr i c f i el d st r engths at ni nedi f f erent l ocati ons on a 4 0 by 4 0 cm test area. Thi ssquare t est ar ea is l ocated 1. 25 m above the f l oor .Each pr obe l ocat i on i s numbered one t hrough ni ne asshown i n f i gure 2. Each i sotr opi c probe has t hreeort hogonal 5 cm di pol es whi ch respond to or t hogonalcomponent s of t he radi ated el ect ri c f i el d. A smal l

di ode at t he center of each di pol e generat es a DCvol t age whi ch i s measur ed by t he pr obe s yst em s A/ Dconvert ers vi a RF- t r anspar ent hi gh- i mpedance l i nes.The di gi t al out put f r omeach di pol e channel i s r ead bythe comput er and cal i brati ons ar e then appl i ed. I nth is way, f i el d strength f or each pol ar i zat i on i scal cul ated. Net f i el d st r ength i s then det erm ned byt aki ng the square root of the sum of t he squares oft he t hree ort hogonal r esponses.The experi ment i s perf ormed i n a f our s t epprocess. Fi r st , t he operator answers pr ompts by t hecomput er t o set t he f r equenci es to be t est ed and toset the desi red unpert urbed f i el d st rengt h whi ch w l lbe gener ated at t he r ef erence posi t i on. The probe atposi t i on 5 i n t he cent er of the test area isconsi der ed t he r ef erence probe. For the experi ment aldata presented, t he bi coni cal t r ansm t t i ng ant enna wasi n the vert i cal or i ent at i on, 70 cm f romt he ref erenceprobe posi t i on. Exper i ment s al so were perf ormed w t hthe bi coni cal ant enna i n the hor i zont al and endf i r eor i enta t i ons , and yi el ded si m l ar resul t s .Next , t he comput er makes a r ef erence r un to f i ndt he si gnal gener at or power out put l evel s t hat pr oducethe desi red fi el d str engt h at the ref erence posi t i onf or each f r equency. To do s o t he comput er set s t hesi gnal gener ator to t he f i r st f r equency and rai ses RFpower unt i l t he measured f i el d str ength at r efer enceposi t i on 5 i s w t hi n 0 .1 dB of the desi red f i el dst rengt h. The si gnal generator out put l evel i s st oredby t he comput er and t he process i s r epeat ed f or each

f r equency.The thi rd st ep i s t o repeat t he r efer ence si gnalgener ator l evel s and measure unper t urbed f i el dst r engths i n t he test area. Thi s st ep i s perf ormed toobserve the r epeat abl i t y of t he ref erence fi el d and t omeasure f i el d str engt hs and pol ari zat i ons at al l ni neprobe posi t i ons. Af t er measuri ng spat i al devi ati onsof the unpert urbed fi el d i n the t est ar ea, testengi neers can set el ectr omagnet i c f i el d uncert ai ntybounds.

Fi gure 1. T o p vi ew of automat ed syst emand absorber l oaded shi el ded room

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2 0 c mILast l y, t he operat or pl aces an EUT i n the testzone and measur es EUT r esponses and f i el dper t urbat i ons. The EUT used is an al umnumcase w t ht hree t ype N connect ors as s hown i n f i gure 3 . A 45 cmmonopol e was at t ached t o t he cent er conduct or of t het op connect or and a 60 cm monopol e was at t ached t o t hecent er pi n of t he connect or on t he l ongest si de of t heEUT. Two EUT r esponses ar e measur ed. Cur r ent sexci t ed by r adi ated pi ck up f l ow i nt o t he EUT, wher e acur r ent probe detect s RF curr ents f l ow ng to t he th i r dt ype N connect or. The r esponse of t he curr ent probei s measured at one i nput to t he spectr um anal yzer.

Thi s curr ent probe i s cal i brat ed before t heexperi ment , and the cal i brati on dat a ar e l oaded duri ngt he exper i ment al conf i gur ati on process. Thecal i brat i on f i l e i s i n uni ts of amperes per vol t as af unct i on of f r equency. The second EUT r esponse i ssi mpl y a coaxi al cabl e connect ed di r ectl y to t he thi rdt ype N connect or . The si gnal i s measur ed at a secondspectr um anal yzer i nput . The per t ur bed f i el ds aroundt he EUT al so may be measur ed at t hi s t i me.

Fi gure 2 Test area show ng ni ne probe posi t i ons.

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ExDeri ment al Resul t sFi gures 4 5 and show unper t urbed f i el d st r engthr esul t s measur ed i n t he anechoi c chamber, t hepart i al l y l oaded shi el ded room and t he unl oadedshi el ded r oom respect i vel y. I n an anechoi c chamber ,f i el d str engt hs and pol ari zat i ons behave predi ctabl ybecause ref l ect i ons fr omt he wal l s ar e att enuat ed. Ateach f r equency, t he fi el d st rengt h at t he ref erenceposi t i on repeat s to wi t hi n 0 . 2 dB and fi el d uni f orm t yover t he test area i s wi t hi n 3 dB of t he desi r edf i el d. Fr o m 50 t o 200 MHz, the test z one i s i n t henear f i el d of the tr ansmt ant enna, so the el ectr i c

f i el d i s st r onger at probe posi t i ons near er t heant enna due to 1/R2 and l / R3 t erms of t he f i el dequat i ons. I f t he t est zone were l ocat ed f art her awayf rom the tr ansm t ant enna, f i el d str ength uni f orm tycoul d be i mpr oved because t he 1/R component of t heradi ated f i e ld woul d dom nate. El ectr i c f i el dpol ari zati on i n the anechoi c chamber l i ned up w t h thevert i cal t r ansmt t i ng antenna at al l f requenci es.I f an anechoi c chamber i s not avai l abl e, thepart i al l y l oaded shi el ded room coul d be used f ori mmuni t y t est s, provi ded t hat uncert ai nty bounds arer el axed. Al t hough t he f i el d mappi ng r esul t s of f i gur e5 f or the part i al l y l oaded shi el ded room do notencour age i t s use f or pr eci se i mmuni t y t est i ng, use ofthe part i al l y l oaded shi el ded r oom i s much morepref erabl e t han t he unl oaded shi el ded r oom I n t hepart i al l y l oaded shi el ded room f i el d strengths i n thetest area di f f er by up to 20 dB. I n t he unl oaded

shi el ded room f i el d str engths i n the rel at i vel y smal l40 x 40 cm t est area di f f er by up to 40 dB. I n bot hthe unl oaded and part i al l y l oaded shi el ded room f i el dmappi ng experi ment s, t he el ect ri c f i el d pol ari zati onal i gned vert i cal l y w t h the tr ansm t t i ng ant enna bel ow90 MHz. However , at t he hi gher f r equenci es,pol ari zati ons and f i el d str engths were domnat ed byr esonant modes i nsi de t he r oom and vari ed w t h

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f requency. Usi ng a di f f erent absorber conf i gurat i oni n the part i al l y l oaded shi el ded room coul d gi vebett er f i el d uni f orm t y by r educi ng some of t heser esonant modes. The l arge spati al f i el d var i ati onsmake the unl oaded shi el ded r oom undesi r abl e f ori mmuni t y test i ng because t her e woul d be pract i cal l y nor el at i on bet ween EUT r esponse and a wel l known f i el dstr ength.Fi gure 7 shows t hat net power t o the tr ansm t t i ngant enna can change si gni f i cant l y when t he EUT per t urbsthe t est zone. I n the part i al l y l oaded shi el ded r oomthe di f f erence i n per t urbed m nus unper t urbed net

power var i es by up to 4 dB. I n t he anechoi c chamber ,net power t o t he tr ansm t antenna r emai ns w t hi n 0. 1dB f or most f r equenci es. Thi s demonst r ates t hatant enna r adi ati on ef f i ci ency is more af f ected byref l ect i ons fr om wal l s t han by l oadi ng due to thepresence of t he EUT i n t he near f i el d of t hetr ansm tt i ng antenna.Pert urbed fi el d st r engths around t he EUT weremeasured i n the anechoi c chamber . Fi gur e 8 shows t hepert urbed f i el d st rengt hs at probe posi t i ons 1, 3 4

4 , 7 and 9 . Compar ed to t he unper t urbed case, f i el dst r engths were gener al l y l ower by about 2 dB. Near -f i el d ef f ects st i l l can be seen by t he di f f erence i nf i el d st rengths at probe l ocati ons cl oser t o thet r ansm t ant enna as compar ed to t hose posi t i onsf art her away.Anechoi c chamber r esul t s whi ch demonst r ate syst em

l i neari t y ar e shown i n f i gure 9 . EUT r esponses wer emeasured usi ng r eference fi el ds of 3. 1, 10 and 31. 6V/m 10, 20 and 30 dBV/ m r espect i vel y). Both t hecurr ent probe r esponses and the di r ect l y measur ed EUTresponses were l i near to w t hi n 0 . 4 dB. Thi sl i neari t y measur ed i n the anechoi c chamber i ncl udest he fi el d generat i on and response det ecti on syst ems.

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Err or Anal ysi sUsed i n conj unct i on wi t h an anechoi c chamber , t hi si mmuni t y t est syst emhas a measur ement uncert ai nty ofapproxi mat el y 0 . 4 dB and a systemati c uncert ai nty ofapproxi matel y 3 dB. Measur ement uncert ai nty i sdeterm ned pr i mar i l y by system l i near i ty andrepeat abi l i ty . F i el d str ength repeatabi l i ty i sapproxi mat el y 0. 2 dB. Repeat abi l i t y of t he r esponsedetect i on system i s about 0. 2 dB. System l i near i tyhas been shown t o be bet t er t han 0 4 dB over a 20 dB

r ange. The system l i near i t y t est combi ned bot h f i el dst r engt h and response detecti on r epeat abi l i t i es i ntoone t est t o show t he overal l measur ement uncert ai ntyof t he syst emThe systemati c uncert ai nty i s det erm ned pri mari l yby f i el d st rengt h var i ati ons i n t he t est zone. Thi swas appr oxi mat el y 3 dB f or t he ver t i cal ant ennaori ent ati on. Bett er uncer t ai nty coul d be achi eved bymovi ng t he t est z one i nto t he f ar f i el d of t he ant ennawhere t he f i el ds are more uni f orm A syst emati c err orwhi ch is di f f i cul t t o quant i f y i s the uncert ai nty i nt he EUT r esponse measur i ng syst emdue to cabl e l ossesand m smatches. The curr ent pr obe i s cal i brated wi t hthe cabl e i t uses dur i ng t he experi ment t o account f orm smat ches, st andi ng waves and cabl e l osses. Thesyst emati c uncert ai nty i n the cur r ent probecal i brati on i s appr oxi matel y the same as t hemeasurement r epeat abi l i t y of t he cal i brated spectr um

anal yzer , about 0. 1 dB. However, t he EUT r esponse,measur ed by a coaxi al cabl e connect ed di r ectl y to t heEUT pot ent i al l y has l arge m smatches at the EUTconnect or and uncompensated cabl e l osses. Thi s f actcoul d expl ai n why t he cur r ent probe and EUT r esponsesi gnatures were not i dent i cal .

Reference f i el d str engt h repeat abi l i t y i mprovesdramati cal l y as more absor ber i s i ntr oduced t o t heroom I n t he unl oaded shi el ded room ref erence fi el dst r engt h repeat abi l i t y was wi t hi n 3 dB at al lf requenci es. Repeat abi l i t y of the ref erence f i el d i nthe part i al l y l oaded shi el ded room i mpr oved t o w t hi n2 dB. I n t he anechoi c chamber , f i el d st r engthrepeatabi l i t y at t he r eference posi t i on was w t hi n 0 . 2dB.Summar y

The capabi l i t y of an aut omat ed f i e1. d gener ati onand i mmuni t y t est sys t em has been demonst r at ed. Adesi r ed f i el d st r ength can be qui ckl y generat ed at aspeci f i ed poi nt i n space f or many fr equenci es. Fi el dmappi ng of a desi r ed t est zone hel ps est i mate t heuncert ai nty val ues of unper t urbed fi el d st r ength andpol ari zat i on uni f orm t y. Fi el d mappi ng al so al l owsmeasurement of per t urbed f i el d di st r i but i ons.Measur i ng net power t o the tr ansm t t i ng ant enna hel psto check f or ant enna l oadi ng due t o t he presence of anEUT. Two r adi at ed respo: i ses of an EUT may be measur edusi ng curr ent probes or ot her methods.Automati ng r adi at ed i mmuni t y test s saves a gr eatdeal of t i me because t he bookkeepi ng necessar y toassur e pr oper t esti ng becomes easy. I f such a testwere perf ormed manual l y, t he process of set t i ng t hef r equency and power l evel , measuri ng t he pr obe and EUT

out put s and appl yi ng cal i brati on corr ecti ons woul dt ake many hours and be prone to err ors . Fi f t yf r equenci es can be t est ed i n about t went y m nut esusi ng comput er contr ol .

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Use of t he anechoi c chamber al l ows generat i on ofmore- uni f ormfi el ds f or i mmuni t y t ests t han ei t her t heunl oaded or t he part i al l y l oaded shi el ded r oomAl t hough t he 50 t o 200 MHz f r equency range t est ed herewas wel l bel ow t he speci f i ed l i m t f or normal use oft he RF absorber , adequat e at t enuat i on was achi eved i nt he anechoi c chamber t o gi ve wel l behaved f i el dsuni f ormt o wi t hi n 3 dB. Bett er f i el d uni f ormt y coul dbe achi eved i n the anechoi c chamber by movi ng t he t estzone out of the near f i el d of t he tr ansmt t i ngant enna. I n the unl oaded shi el ded r oom f i el dstrengths on a re l at i vel y smal l 40 x 40 cm test areavari ed up to 40 dB. I n part i al l y l oaded shi el ded roomexperi ment s, f i el d str engt h vari abi l i t y was reduced to20 dB usi ng 28 pi eces of 0. 66- met er absor ber pl aced onopposi t e wal l s. Thi s may not be t he opt i mumpl acementf or the absor ber , and bett er f i el d uni f orm t y m ght beachi eved w t h a di f f erent absorber conf i gurat i on.

Two i mpor t ant concl usi ons can be made. The f i r sti s t hat t he l owest usabl e fr equency f or radi atedi mmuni t y t est s i n an anechoi c chamber can be muchl ower t han t he recommended absorber cut of f f r equency.Thi s perm t s eff ecti ve i mmuni t y test i ng i n thef r equency gap bet ween TEM cel l s at t he l owerf r equenci es, and anechoi c and r ever ber ati ng chamber sat t he hi gher f r equenci es. The second concl usi on i st hat t he part i al l y l oaded shi el ded room may be avi abl e i mmuni t y t est f aci l i t y, as l ong as enoughabsorber i s used to bri ng fi el d vari ati ons w t hi nto l erabl e l i m ts. Obvi ousl y , the f i el ds i n thepart i al l y l oaded shi el ded room w l l not be as uni f ormas i n an anechoi c chamber , so t ol erances i n syst emati cuncert ai nty woul d have t o be r el axed f or measurementsmade i n the part i al l y l oaded shi el ded r oom

Acknow edgement sThe author woul d l i ke to t hank J ohn W Adams f ort he oppor t uni t y to work on th is pr o j ect ; Mark T. Maand Per r y F . W l son fo r i l l uc idat i ng anal ysi s ,di scussi on and gui dance; Gal en H. Koepke f or t hesof t ware used t o operat e t he aut omated probe syst emandMot ohi sa Kanda f or hi s cr i t i cal comment s and support .

Ref erencesCr awf ord, M. L. and Workman, J . L. Usi ng a TEMCel l f or EMC Measurement s of El ectr oni cEqui pment , Nat' l . Bur. Stds. ( U. S. ) NBS TechNote 1013; J ul y 1981.Tsal i ovi ch, A. Eval uat i on of RF Anechoi c RoomAbsorbers i n the 30- 1000 M z Range, I EEE I nt ' lSymp. El ect r omagn. Compat . , Sept ember 1986.F i t zGerre l l , R. G. Usi ng Free- Space Transmssi onLoss f or Eval uati ng Anechoi c ChamberPerf ormance, I EEE Trans. El ectr omagn. Compat . ,Vol . EMC- 24, NO. 3, pp 356- 358, August 1982.Kanda, M. and Wyss, J . C. Eval uati on of Of f - Axi sMeasur ement s Per f or med i n an Anechoi c Chamber ,Nat' l . Bur. St ds. ( U. S. ) NBS Tech Note 1305;Oct ober 1986.Yaghj an, A. D. Appr oxi mat e Formul as f or t he FarFi el ds and Gai n of Open-ended Wavegui de, Nat' l .Bur. Stds. ( U. S. ) NBSI R 83- 1689, 34 pages; May1983.St u ckey , C . W , F r e e, W R. and Rober t son, D. W ,Prel i m nary I nterpretat i on of Near- f i e l d Ef f ectson Measurement Accuracy i n Shi el ded Encl osur es, 'I EEE- EMC Symp. Recor d, pp 119- 127, J une 1969,Asbury Par k, NJ .Krst ansky, J . J . and St andl ey, R. D. Theor y of RFIMeasur ement s i n Shi el ded Encl osur es, 8t h AnnualI EEE Symp. on EMC, 5 pages, J anuar y 1966, SanFranci sco, CA.Cr awf ord, M. L. and Koepke, G. H. Desi gn,Eval uati on and Use of a Reverber at i ng Chamber f orPer f orm ng El ect romagneti c Suscept i bi l i t y/Vul -nerabi l i t y Measurement s, Nat' l . Bur. St ds.( U. S. ) NBS Tech Note 1092; Apri l 1986.Bensema, W D. , Koepke, G. H. , and Medl ey, H. WHandbook f or Oper at i on and Mai nt enance of an NBSMul t i sensor Automated EM Fi el d MeasurementSyst em Nat ' l . Bur . St ds. ( U. S. ) NBSI R 83-3056;Oct ober 1986.

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