Time Domain Sensors for Radiated Impulsive EMI Measurements

Abstract

The purpose of t h i s paper i s to review various sensors and ra d ia to rs commonly used for time domain antenna measurements. For e l e c t r i c f i e l d s trength measurements, l in e a r antennas loaded nonuniformly and c ont inuously with r e s i s t a n c e , or both r e s i s t a n c e and capac i tan ce are d i sc u s se d . Also a conica l antenna and an asymptotic conica l antenna are d i scus sed from the standpoin t o f an improved c h a r a c t e r i s t i c . For an improved d i r e c t i v i t y , var ious types o f TEM horns are d i s c u s s e d , e . g . , a conducting TEM horn, a CALSPAN antenna, and a r e s i s t i v e l y loaded TEM horn. 1. In trod uc t ion . Elec tromagnetic in t e r f e r e n c e (EMI) measurements are among the most complicated and s o p h i s t i c a t e d th a t c h a l le n g e tech nology today s in c e some dev ices and systems are much more s u s c e p t ib l e to sh o rt -d ura t io n impulse i n t e r f e r e n c e than to common cw in t e r f e r e n c e . Therefore, a strong need e x i s t s to develop a small broadband sensor to d e t e c t and record impuls ive e lec trom agnet ic f i e l d s and to g ive alarm when th e se f i e l d s exceed predetermined hazardous l e v e l s . The purpose of t h i s paper i s to review various sensors and r ad ia tors commonly used for time domain antenna measurements. The sensors and ra d ia to rs discus sed here are p a s s iv e and analog dev ice s which convert the e lec trom agnet ic quant i ty o f i n t e r e s t to a v o l ta g e or current a t t h e i r terminal p o r t s . Moreover, they are primary standards in the sense tha t t h e i r t r a n s f e r fun c t io n s can be c a l c u l a t e d from t h e i r geometr ies and are f l a t ( cons tan t ) across a wide frequency range. One o f the major requirements for these sensors and r a d ia to rs i s tha t the e lec trom agnet ic far f i e l d , transm itted or r ec e iv e d , i s an accurate r e p l i c a or high f i d e l i t y d e r iv a t i v e o f the o r ig in a l p u l se . For e l e c t r i c f i e l d s trength measurements, l in e a r antennas loaded non-uniformly and cont inuous ly with r e s i s t a n c e , or both r e s i s t a n c e and capac itance are d isc u s se d . Also a con ica l antenna and an asymptot ic conica l antenna a r e .d i s c u s s e d from the standpoin t o f an improved c h a r a c t e r i s t i c . For an improved d i r e c t i v i t y , var ious types o f TEM horns are d i s c u s s e d , e . g . , a conducting TEM horn, a CALSPAN antenna, and a r e s i s t i v e l y loaded TEM horn. 2 . Transmitt ing and Receiving Transfer Function [ 1 ] . I t i s rather well known t h a t the tr a nsm it t ing t r a n s fe r funct ion o f an antenna i s proport ional to the t ime d e r iv a t i v e o f the r e c e iv in g t r a n s f e r funct ion o f the same antenna. For example, c o ns ide r an antenna designed to r ec e iv e an impulse with l i t t l e d i s t o r t i o n . Using t h i s same antenna to rad ia te with an impulse input , the radiated waveform i s a d i f f e r e n t i a t e d impulse, i . e . , a dou b le t . This property can be exp la in ed n e a t ly by use o f the plane-wave s c a t t e r i n g theory [ 2 ] . The tr a nsm it t ing t r a n s f e r funct ion o f an antenna St ( f ) i s given as St ( f ) = jkS 10 ( 0 ) f 0j k r