IMAGING OF LINEAR SOURCE DISTRIBUTIONS EK ... - CiteSeerX

IMAGING OF LINEAR SOURCE DISTRIBUTIONS

E. K. Miller, Electronics Engineering Depanment, Lawrence Livermore National Laboratory, Livermore, California 94550

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Abstract An approach based on P r o n y ' s method i s used t o f i n d t h e spat i a l p o l e s o f s t r a i g h t w i r e s of v a r y i n g l e n g t h and r a d i u s from t h e f a r - f i e l d p a t t e r n s . For t h e a n t e n n a c a s e , p o l e s h a v i n g t h e l a r g e s t r e s i d u e s a r e found n e a r t h e feed r e g i o n and antenna e n d s ; s m a l l e r o n e s a r e found o u t s i d e t h e s e r e g i o n s . The p o l e s a r e a t t r i b u t e d t o r a d i a t i o n from t h e e x c i t i n g s o u r c e and w i r e e n d s , due t o charge a c c e l e r a t i o n , and from a s l i g h t l y damped t r a v e l i n g wave. In t h e s c a t t e r i n g case, the poles with t h e l a r g e s t residues a r e l o c a t e d a t t h e w i r e ' s ends f o r n e a r - a x i a l i n c i d e n c e and a r e a g a i n due t o end r a d i a t i o n . By examining t h e r e s i d u e magnitudes a s a f u n c t i o n o f w i r e r a d i u s , a c o r r e l a t i o n i s found w i t h r a d i a t i o n from t h e f e e d r e g i o n of an i n f i n i t e a n t e n n a and from t h e ends of a s e m i - i n f i n i t e antenna.

1.

INTRODUCTION

One v e r s i o n o f t h e i n v e r s e problem i s d e t e r m i n i n g t h e s o u r c e d i s t r i b u t i o n t h a t produces a given f i e l d . When t h e s o u r c e s l i e on a p e r f e c t l y cond u c t i n g o b j e c t , and when t h e f i e l d s c a n be o b t a i n e d a r b i t r a r i l y c l o s e t o i t , then t h e i n v e r s i o n is t r i v i a l . Of more p r a c t i c a l i n t e r e s t i s t h e s i t u a t i o n where t h e f i e l d s a r e o b s e r v a b l e o n l y a t a l a r g e d i s t a n c e from t h e o b j e c t . I n t h i s c a s e , i t might be expected t h a t o n l y t h o s e s o u r c e s t h a t produce f a r f i e l d r a d i a t i o n w i l l be d i r e c t l y d e r i v a b l e . (By a n a l y t i c c o n t i n u a t i o n o f t h e f i e l d , i t i s possible i n p r i n c i p l e t o d e r i v e a l s o the near-Field sources, i f t h e boundary c o n d i t i o n s a p p r o p r i a t e t o t h e s o u r c e s a r e known.) This r e p o r t d i s c u s s e s t h e use o f t h e Far f i e l d s t o l o c a t e t h e r a d i a t i n g s o u r c e s , an e x e r c i s e t h a t may be regarded a s imaging. We p r e v i o u s l y demonstrated t h a t P r o n y ' s method can be used t o image n s t r a i g h t - l i n e d i s t r i b u t i o n of d i s c r e t e s o u r c e s from i t s f a r f i e l d , and t o s y n t h e s i z e d i s c r e t e s o u r c e d i s t r i b u t i o n s t h a t produce t h e f i e l d s o f c o n t i n u o u s d i s t r i b u t i 0 n s . l Here we examine t h e p o s s i b i l i t y o f u s i n g t h e same procedure t o image t h e s o u r c e d i s t r i b u t i o n o f a s t r a i g h t wire from i t s f a r f i e l d . The e f f e c t s of v a r y i n g t h e w i r e ' s l e n g t h , r a d i u s , and e x c i t a t i o n a r e d i s c u s s e d . As we found p r e v i o u s l y , a r e s o l u t i o n l i m i t o f about two s o u r c e s p e r wavelength seems t o hold. Sources a r e c o n s i s t e n t l y o b t a i n e d whose locat i o n s s u g g e s t t h a t they a r e due t o r a d i a t i o n from t h e w i r e ' s ends and e x c i t a t i o n p o i n t , when o p e r a t e d a s an antenna. O t h e r s o u r c e s a r e a l s o found which may be i n t e r p r e t e d a s due t o t r a v e l i n g wave r a d i a t i o n from an a n t e n n a , and t o t h e c o n t i n u o u s e x c i t a t i o n a s s o c i a t e d w i t h a s c a t t e r e r . The i m p l i c a t i o n s of t h e s e f i n d i n g s w i t h r e s p e c t t o imaging s i m p l e o b j e c t s a r e d i s c u s s e d . Work performed under the auspices of rhs US. Dcpsrrment of Energy by the Lawrsnm L i ~ r m o r sNational Laboratory under contram number W.7405-ENG4.

Electrornagnetics 3:21-40. 1983 0272~6343/83/010021-20S2.25 Copyright 0 1983 Hemisphere Publishing Corparation

E. K. MILLER

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2.

BACKGROUND

Motivation for the present discussion was provided by the experimental results o f ~ z u k a ,who ~ found that a long monopole antenna exhibits two sources o f radiation (see Fig. l), based o n its reconstructed image using microwave holography. These sources can be identified a s the excitation o r feed point o f the antenna and its end. This finding confirms that radiation is caused by charge acceleration (due to the excitjng field) o r deceleration (due to end reflection), as shown by the equations for the fields of a point charge q moving with (nonrelativistic) velocity v and acceleration v:

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and

where

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the magnetic field vector at time t and vector coordinate ;, the electric field vector, the medium permeability, the medium permittivity, she velocity o f light, r r'(T), the vector difference between the observation point and source point coordinate vectors, R = IRI, T = t r - r /c, the retarded time.

H(r,t) = ~ ( r , t )= !Jo EO = c = =

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R

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As a n alternative to holography, Prony's method can be used to determine radiation sources from their far-field patterns. The far field o f a straightline distribution o f S isotropic point sources is given by

where % and da are the strength and location o f the g t h source. substitution x = cos 8, Eq. ( 2 ) becomes

f(x)

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S

S

Raexp (ikdax) = a=1

Raexp (sax)

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for -1 < x < 1

With the

,

(3)

a=1

which c a n be processed by applying Prony's method to samples o f f(x) to It should be noted that, determine the set o f S values o f & and sa. in using Prony's method, the samples o f f(x) are required to be evenly spaced in x, which means that rather than being distributed uniformly in 0 , they are distributed in equal steps of cos 8 .

IMAGING OF LINEAR SOURCE DISTRIBUTIONS (a1 Monopole antenna

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Near-field scanner

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. Experiment

Optical reconstruction

.'

FIG. 1. Schematic of Iruka's experiment Hologram generated by scanning field of long monopole antenna (a) is optically constructed to give two point These points are associated with radiation from the feed point images (b). and end of the antenna. The pattern poles produced by this exercise can have the general form

From Eq. ( 3 ) it can be seen that

and that

I

= 0 for isotropic sources

Thus, a complex pole implies source directivity. For isotropic sources, a nonzero value for s a , ~can occur because of numerical inaccuracy. The poles of actual sources will usually be such that s a , ~