A NEW HELICAL GROUND PLANE ANTENNA FOR 30 TO 50 MC. L. H. Hansen

ANDREW C o r p o r a t i o n , P. 0. Box 807, Ch icago 42, I l l i n o i s

ABSTRACT

A u n i t y g a i n g r o u n d p l a n e a n t e n n a o f g r e a t l y r e d u c e d s i z e i s p r e s e n t e d . T h e g r o u n d p l a n e c o n s i s t s o f f o u r e v e n l y s p a c e d s l o p i n g h e l i c e s . T h e f e d e l e m e n t i s c o n s t r u c t e d o f d o u b l e h e l i c a l w i n d i n g s . All t h e h e l i c e s a r e e n c l o s e d i n F i b e r g l a s tubes .

I NTRODUCT I ON

T h e p r i m a r y s t a t i o n a n t e n n a f o r f r e - q u e n c i e s 30 t o 50 MC has been the UNIPOLE a n t e n n a , F i g u r e 1. T h i s u n i t y g a i n a n t e n - n a u s u a l l y m e e t s t h e r e q u i r e m e n t s o f 2-way systems i n t h i s band, and when d i - r e c t i v i t y i s d e s i r e d , a v e r s i o n o f t h e b a s i c UNIPOLE antenna such as, a c a r d i o d o r t h e b i d i r e c t i o n a l a n t e n n a i s used. T h i s g e n e r a l c l a s s o f g r o u n d p l a n e a n t e n - na has a q u a r t e r w a v e l e n g t h w h i p o r r a d i - a t o r a n d a g r o u n d p l a n e .4h t o .5h a c r o s s . F o r u s e a t 30 t o 50 MC, UNIPOLE an tenna i s a v e r y l a r g e a n d c u m b e r s o m e u n i t , t h e h e i g h t a b o v e t h e m o u n t i n g m a s t s i s 9 t o 11-1/2 f e e t a n d t h e g r o u n d p l a n e i s 14 t o 16 f e e t a c r o s s . W i t h e l e m e n t s o f t h i s l e n g t h , t h e w i n d l o a d i n g i s severe and the e lements themse lves must be made r e l a t i v e l y l a r g e i n c r o s s s e c t i o n s t o ach ieve any deg ree o f s t r e n g t h a n d r e - l i a b i l i t y f o r t h e a n t e n n a . A l u m i n u m i s g e n e r a l l y e m p l o y e d t o k e e p t h e w e i g h t a s l o w a s p o s s i b l e , b u t m a k i n g t h e u n i t s m a l l a n d e a s y t o h a n d l e i s s t i l l t h e most des i r a b l e goa 1.

W i t h t h e i d e a o f m a k i n g a s m a l l e r u n i t y g a i n a n t e n n a f o r 30 t o 50 MC, an a n t e n n a c a l l e d t h e HELIPOLE has been de- ve loped , F ig . 2 and 3. E f f o r t s w e r e made t o s h o r t e n a l l p o s s i b l e d i m e n s i o n s o f t h e UNIPOLE antenna and s t i l l m a i n t a i n i t s d e s i r a b l e e l e c t r i c a l f e a t u r e s . The p a t t e r n o f s u c h a n a n t e n n a m u s t b e o m n i - d i r e c t i o n a l a n d h a v e u n i t y g a i n , l o w m a s t c o u p l i n g i s r e q u i r e d , a n d VSWR s h o u l d be l e s s t h a n 1 . 5 0 w i t h a b a n d w i d t h com- p a r a b l e t o t h a t o f t h e UNIPOLE antenna,

SHORT R A D I AT I NG ELEMENTS

T h e s h o r t e n i n g o f r a d i a t i n g e l e m e n t s t o s o m e t h i n g much s h o r t e r t h a n a q u a r t e r w a v e l e n g t h r e s u l t s i n a n e l e m e n t h a v i n g a r a d i a t i o n p a t t e r n t h e same a s t h a t o f a r e s o n a n t d i p o l e . T h e d i s a d v a n t a g e o f t h e s e e l e m e n t s a r e t h a t t h e y a r e n o n - r e s o n a n t u n l e s s some t y p e o f l o a d i n g i s

u s e d , T h e d e v i c e s u s e d f o r l o a d i n g o f t e n c o m p l i c a t e s a n d w e a k e n s t h e s t r u c - t u r e m e c h a n i c a l l y a n d may a l s o c o n t r i b u t e c o n s i d e r a b l e l o s s e s t o t h e a n t e n n a c i r - c u i t . T h i s p o i n t s u p t h e need f o r t h e d e s i g n e r t o m a k e ~ e v e r y e f f o r t t o a c h i e v e a s e l f r e s o n a n t r a d i a t i n g s t r u c t u r e so t h a t t h e l o s s e s may b e k e p t a t a m i n i - mum.

I n v e h i c u l a r a p p l i c a t i o n s , s h o r t w h i p an tennas a re common ly used w i th a load- i n g d e v i c e s u c h a s s m a l l c o i l s . T h e s e l o a d i n g c o i l s a r e u s u a l l y s h o r t a n d v e r y l i t t l e e n e r g y i s r a d i a t e d b y them; they m e r e l y a c t a s t u n i n g c o i l s w i t h o u t a f f e c t i n g a n y o f t h e o t h e r r a d i a t i o n c h a r a c t e r i s t i c s when c o n n e c t e d a t t h e a n t e n n a b a s e . T h e s e l o a d i n g c o i l s may be moved up t h e a n t e n n a t o some p o i n t be - t w e e n t h e a n t e n n a b a s e a n d t o p t o a c h i e v e a m o d i f i e d c u r r e n t d i s t r i b u t i o n , T h i s r e s u l t s i n a n i n c r e a s e i n r a d i a t i o n r e - s i s t a n c e a n d t h e r e f o r e a n i n c r e a s e i n r a d i a t i o n e f f i c i e n c y b u t w i t h o b v i o u s mechan ica l d i sadvan tages .

K n o w i n g t h e d i s a d v a n t a g e s o f t h e s h o r t l i n e a r r a d i a t i n g e l e m e n t , h e l i c a l d i p o l e s appear t o be a g o o d c h o i c e f o r s h o r t r a d i a t i n g e l e m e n t s . T h e h e l i c a l d i p o l e may be p i c t u r e d a s a c o n t i n u o u s l y l o a d e d antenna and can be made s e l f r e s o n a n t . The r a d i a t i o n r e s i s t a n c e - i s h i g h e r t h a n t h a t o f a l i n e a r a n t e n n a o f t h e same l e n g t h , a n d t h e p i t c h a n d d i a m e t e r o f t h e h e l i x c a n b e a d j u s t e d t o g i v e e s s e n - t i a l l y l i n e a r p o l a r i z a t i o n .

DESIGN OF GROUND RODS

To make t h e UNIPOLE a n t e n n a s m a l l e r and l ess cumbersome, t h e l a r g e s t a n d most t roub lesome i tem, the g round rods must be reduced i n s i z e . The 4 ground r o d s u s e d t o make the g round p lane were l i n e a r r e s o n a n t e l e m e n t s a n d w e r e d i - r e c t l y r e p l a c e d b y h e l i c a l r e 2 o n a n t e l e m e n t s . H e l i c e s wound on 2 d i a m e t e r F i b e r g l a s t u b e s w e r e made a n d t r i m m e d t o r e s o n a t e w i t h t h e s t a n d a r d UNIPOLE f o l d e d d i p o l e r a d i a t o r . T e s t s w e r e made of i m - p e d a n c e , p a t t e r n , a n d m a s t c o u p l i n g o f a n a n t e n n a c o n s i s t i n g o f a f o l d e d d i p o l e w i t h a g r o u n d p l a n e o f h e l i c a l e l e m e n t s . The p a t t e r n a n d m a s t c o u p l i n g w e r e e s s e n t i a l l y t h e same a s w i t h t h e UNIPOLE an tenna . The impedance o f t h i s an tenna

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was 50 ohms, w i th an impedance bandw id th somewhat nar rower than the UNIPOLE'S bandw id th .

DES I G N OF R A D IATOR ELEMENT

T o c o m p l e t e t h e a n t e n n a d e s i g n , a h e l i c a l e l e m e n t was d e s i g n e d t o r e p l a c e t h e f o l d e d d i p o l e . T h i s h e l i c a l e l e m e n t had a t w i n o r b i f i l a r w i n d i n g t o g i v e a h i g h e r i n p u t i m p e d a n c e . T h e b i f i l a r r a d i a t i n g e l e m e n t i s a n a l o g o u s t o t h e f o l d e d d i p o l e , i n t h a t b o t h t h e f o l d e d d i p o l e a n d t h e b i f i l a r h e l i x o p e r a t e i n t h e d i p o l e mode a n d t h e t r a n s m i s s i o n l i n e mode. The c u r r e n t s i n t h e t w o c o n d u c t o r s a r e i n p h a s e i n t h e r a d i a t i n g mode, and o u t o f p h a s e i n t h e t r a n s m i s s i o n l i n e mode. I n t h e b i f i l a r , t h e h e l i x p i t c h a n d t h e v e l o c i t y o f t h e t r a n s m i s s i o n l i n e mode a r e a d j u s t e d so a s t o g i v e a h e l i c a l r a d i a t i n g e l e m e n t 1/4 wave length long , and a t r a n s m i s s i o n l i n e e l e m e n t 1/2 wave- l e n g t h l o n g a n d o p e n c i r c u i t e d , F i g u r e 4. T h i s r e s o n a n t h e l i c a l r a d i a t o r s h u n t e d w i t h a 1/2 w a v e l e n g t h o p e n c i r c u i t e l - ement g ives a s tepped up input impedance s i m i l a r t o t h a t a c h i e v e d w i t h t h e f o l d e d d i p o l e w h i c h i s a q u a r t e r wave r a d i a t i n g e lemen t shun ted by a q u a r t e r w a v e l e n g t h s h o r t c i r c u i t e l e m e n t .

TESTS AND PERFORMANCE

The r a d i a t i o n c h a r a c t e r i s t i c s o f t h i s a n t e n n a a r e t h e same a s t h o s e o f t h e UNIPOLE, o m n i d i r e c t i o n a l h o r i z o n t a l p a t t e r n , a n d a f i g u r e 8 v e r t i c a l p a t t e r n w i t h a beam o n t h e h o r i z o n , F i g u r e 5. M a s t c o u p l i n g i s a s l o w a s t h a t o f t h e U N I POLE.

The impedance o f t h e a n t e n n a i s 5 0 ohms w i t h a VSWR l e s s t h a n 1.50, a S m i t h C h a r t p l o t o f a 50 MC u n i t i s shown i n F i g u r e 6.

A c u r v e was made f o r t r i m m i n g t h e h e l i c e s f o r o p e r a t i o n a t a n y f r e q u e n c y f o r 30 t o 5 0 MC. S e v e r a l u n i t s w e r e t e s t e d a n d f o u n d t o r e p e a t w i t h e x - c e l l e n t c o r r e l a t i o n .

MECHANICAL ASPECTS

The h e l i c e s h a v e b e e n c o n s t r u c t e d o f s o l i d w i r e , f l a t b r a i t , a n d f l a t c o p p e r s t r a p , a l l wound on 2 d i a m e t e r F i b e r g l a s t u b i n g . A n y o f t h e s e t e c h n i q u e s will p r o d u c e l i g h t a n d s t r o n g e l e m e n t s . E a c h o f t h e s e a r e e a s y t o h a n d l e a n d trim so t h e c h o i c e o f c o n d u c t o r was based on a c o m b i n a t i o n o f m a t e r i a l c o s t a n d e a s e o f w i n d i n g o n a F i b e r g l a s t u b e . T h e t u b e s have a m e t a l p l u g i n t h e e n d w h i c h m o u n t s t o t h e p e d e s t a l , w h i c h i s t h e m a i n s e c t i o n o f t h e a n t e n n a . T h i s c e n t r a l p o r t i o n i s

a n a l u m i n u m c a s t i n g a n d h a s t h e f o l l o w i n g f e a t u r e s : 5 mount ing pads to accommodate t h e h e l i c a l e l e m e n t s , r e c e p t a c l e f o r t h e feed cab le , and a m o u n t i n g c l a m p w h i c h i s a n i n t e g r a l p a r t o f t h e c a s t i n g . T h e moun t ing c lamp will accommodate a s t a n d a r d 1-1/2 i n . o r 2 i n c h p i p e . T h e t o t a l w e i g h t o f a 30 MC u n i t i s o n l y 13 pounds, w i t h a h e i g h t o f 6-1/2 f e e t a n d a w i d t h o f 4 f e e t . T h e a n t e n n a i s d e s i g n e d t o w i t h s t a n d 3 0 p s f w i n d a g e w i t h 1/2 i n c h r a d i a l i c e l o a d .

CONCLUSIONS

T h e d e s i g n o f a g r o u n d p l a n e a n t e n n a o f h e l i c a l e l e m e n t s h a s b e e n p r e s e n t e d . T h i s d e s i g n i s a r e s u l t o f t h e d e s i r e t o r e d u c e t h e w e i g h t a n d s i z e o f g round p l a n e a n t e n n a s a t 30 t o 5 0 MC. The use o f h e l i c a l e l e m e n t s a n d u n i q u e c o n - s t r u c t i o n m e t h o d s g i v e a u n i t w h i c h i s e a s y t o h a n d l e , l i g h t w e i g h t , a n d s t r o n g . T e s t s show t h e HELIPOLE t o be equ iva- l e n t i n e l e c t r i c a l p e r f o r m a n c e t o a UN I POLE.

ACKNOWLEDGEMENTS

T h e a u t h o r w i s h e s t o a c k n o w l e d g e t h a t i n i t i a l c o n c e p t i o n a n d d e v e l o p m e n t w o r k on t h i s a n t e n n a was done by M. W. S c h e l d o r f , f o r m e r l y o f ANDREW C o r p o r a t i o n .

TYPICAL UNIPOLE INSTALLATION

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COAXIAL INPUT

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