A New Isolated Antenna System for Electromagnetic Emissions Measurements in Shielded Enclosures

An o p t i c a l l y i s o l a t e d a n t e n n a sy s tem was d e v e lo p e d f o r u se i n s h i e l d e d e n c l o s u r e s t c p r o v i d e mea surement o f e l e c t r i c f i e l d e m i s s io n l e v e l s w i t h i n +3 dB o f t h e i r open a r e a v a l u e s . The uncompensated sy s te m r e s p o n s e to f r e q u e n c i e s be tween 20 MHz and 500 MHz i s f l a t w i t h s u f f i c i e n t s e n s i t i v i t y f o r narrowband and broadband measurem ents t o MIL-STD-461A r a d i a t e d l i m i t s . I n t r o d u c t i o n R e l i a b l e measu remen ts o f e l e c t r o m a g n e t i c f i e l d s t r e n g t h f o r e l e c t r o m a g n e t i c c o m p a t i b i l i t y (EMC) t e s t ­ in g a r e d i f f i c u l t t o a c h i e v e . E l e c t r o m a g n e t i c f i e l d s t r e n g t h measurem ents a r e made w i t h i n s h i e l d e d e n c l o ­ s u r e s o r i n open a r e a t e s t s i t e s . I d e a l l y , t h e mea­ su re m en t s o f f i e l d s t r e n g t h made in s h i e l d e d e n c l o s u r e s s h o u l d c o r r e l a t e w i t h t h o s e made in open a r e a s . E l e c t r o m a g n e t i c measurem ents f o r EMC a r e n o r m a l ly pe rform ed w i t h i n s h i e l d e d e n c l o s u r e s t o keep u n d e s i r e d i n t e r f e r i n g s i g n a l s f rom b e ing i n t r o d u c e d i n t o t h e mea­ s u r i n g eq u ipmen t when making r a d i a t e d e m i s s i o n s mea­ s u r e m e n t s . A d e s i r a b l e f e a t u r e o f such f i e l d s t r e n g t h mea surement s i s t h a t t h e y may be c o r r e l a t e d w i t h s i m i ­ l a r measurem ents i n open a r e a s . Such c o r r e l a t i o n can r e a d i l y be o b t a i n e d in t h e f r e q u e n c y r a n g e below a b o u t 1 2 20 MHz. ’ Above t h i s f r e q u e n c y , s t a n d i n g waves w i t h ­ i n t h e e n c l o s u r e n o r m a l l y p r e v e n t c o r r e l a t i o n w i th i 3 a q is 7 p. q open a r e a m easu rem en t s . ’ ’ ’ ’ ’ ’ ’ T h i s u n d e s i r a b l e s i t u a t i o n has g e n e r a l l y been overcome in t h e f r e q u e n c y r a n g e above 200 MHz by u s in g RF a b s o r b i n g m a t e r i a l a n d / o r hooded a n t e n n a s which a r e c a p a b l e o f r e d u c i n g unwanted r e f ^ e c t i o n s . ' ' ' , '*' ,̂ ''''*', ^ , ^ , 14 Background o f Problem The pr obl ems t h a t a r e f r e q u e n t l y e n c o u n t e r e d d e ­ pend on which o f t h e two c l a s s e s o f measu remen t t e c h ­ n i q u e s a r e u se d . For measu remen ts made in s h i e l d e d e n c l o s u r e s , i n t e r a c t i o n s amongst t h e e qu ipm en t un de r t e s t (EUT), s h i e l d e d e n c l o s u r e , and measu remen t a n t e n ­ na a r e most f r e q u e n t and s e v e r e , w h i l e f o r m ea su re ­ ments made in open a r e a s i t e s , amb ie n t n o i s e , EMI m e t e r s e n s i t i v i t y and p r o c e d u r a l problems a r e most f r e q u e n t . S h i e l d e d e n c l o s u r e s o f f e r many a d v a n t a g e s and some s e r i o u s d i s a d v a n t a g e s f o r making r a d i a t e d EMI m easu rem en t s . The am b ie n t n o i s e i s low and t h e mea­ su re m en t i s made c l o s e t o t h e s o u r c e o f e l e c t r o m a g ­ n e t i c e m i s s i o n s (EME) so t h a t amb ie n t masking o f t h e EME i s p r a c t i c a l l y e l i m i n a t e d and EMI m e t e r s e n s i t i ­ v i t y r e q u i r e m e n t s a r e m o d e r a t e . By f a r , t h e most s e r i o u s problem t o be encoun ­ t e r e d i n making r a d i a t e d EME m easurem ent s i n a s h i e l d e d e n c l o s u r e i s t h e e f f e c t s o f t h e e n c l o s u r e on t h e e l e c ­ t r o m a g n e t i c f i e l d s i n s i d e i t . These e f f e c t s can be c o n s i d e r e d i n two s e p a r a t e f r e q u e n c y r e g i o n s t h a t a r e d e f i n e d by t h e s i z e o f t h e s h i e l d e d e n c l o s u r e . One r e g i o n i s t h e f r e q u e n c y ra n g e below t h e l o w e s t r e s o n a n t f r e q u e n c y o f t h e e n c l o s u r e c o n s i d e r e d as a c a v i t y r e s o ­ n a t o r , t h e o t h e r r e g i o n i s t h e f r e q u e n c y r a n g e a t and above t h i s l o w e s t r e s o n a n t f r e q u e n c y . C o n s i d e r a b l e i n ­ v e s t i g a t i o n o f t h i s problem has been done and i s r e ­ p o r t e d i n t h e l i t e r a t u r e . R e f e r e n c e s 15 , 17 , 19 , 20 , 24, 25 , 27 , 30 , 3 1 , 32 , 33 , 34 , and 35 d i s c u s s t h e s e i n v e s t i g a t i o n s . For f r e q u e n c i e s an o c t a v e o r more below t h e l o w e s t s h i e l d e d e n c l o s u r e r e s o n a n c e , measu remen ts made in t h e e n c l o s u r e can u s u a l l y be c o r r e l a t e d t o t h o s e made in an open a r e a w i t h i n 4 t o 6 dB. However, i n t h e v i c i n i t y o f t h e l o w e s t e n c l o s u r e r e s o n a n c e and h i g h e r , t h e f i e l d p a t t e r n s i n t h e e n c l o s u r e become v e r y complex , and w i th t h e us u a l measu remen t a n t e n n a s , t h e measured f i e l d s t r e n g t h s o f t e n d e p a r t f rom t h o s e t h a t would be measured i n an open a r e a by more th a n +40 dB. F i g u r e s 1 and 2 21 p r e s e n t t h i s probl em f rom two a s p e c t s . F i g u r e 1 shows S H I E L D E D E N C L O S U R E R E S O N A N C E E F F E C T S