This paper discusses the development of a compact light-weight planar spiral antenna for use in amplitude comparison and interferometer direction finding (DF) applications covering 1 to 18 GHz. The antenna uses a slowwave (zigzag) spiral radiator which results in a cavity diameter which is only 68% of the wavelength at 1 GHz. The two-arm spiral is fed by a Marchand balun and achieves a VSWR of less than 1.5:1 over almost the entire 1 to 18 GHz frequency range. The spiral antennas are supplied in phase and amplitude tracking sets for wideband DF applications. Amplitude and phase tracking performance for a fully integrated linear interferometer DF panel which includes the radome and feed cable effects are presented. Phase DF performance for the panel with pulse receivers as measured in an anechoic chamber is discussed.
P 0 BOX 395, PRETORIA, SOUTH A F R I C A , 0001 INTRODUCPION : The s p a t i a l volume a v a i l a b l e f o r t h e i n s t a l l a t i o n of broadband a n t e t y s is often limited. Extremely broadband double ridged pyramidal horns have been developed [ll, [21 however, t h e s e horns have appreciable lateral dimensions. When the sipewalls of a double ridged horn are remved, the antenna structure becomes a gradually diverging two conductor balanced transmission line. A l i n e of t h i s t y p e a c t s as a broadband r a d i a t o r when t h e s e p a r a t i o n between the conductors i s several wavelengths [31. In this paper, a novel balanced transmission line antenna, derived from double-ridged waveguide, is described where t h e maximum wavelength a t t h e lowest operating frequency. The poor VSWR s e p a r a t i o n between t h e two conductors is only a f r a c t i o n o f a performance and t h e back radiation caused by r e f l e c t i o n s from t h e t i p s o f t h e r a d i a t o r s a t t h e lower end of the frequency range are microwave absorbing material. Measured r a d i a t i o n p a t t e r n , g a i n improved by a combination of "aperture matching"[41 and tine use of and VSWR d a t a a r e p r e s e n t e d f o r t h i s compact,aperture-matched antenna over the 2 t o 11 GRz frequency range. DESCRIPTION : The compact antenna (Fig. 1) c o n s i s t s of a short is used t o launch a wave on the exponential extensions of t h e r i d g e s s e c t i o n of double-ridged waveguide, with a 50 R c o a x i a l i n p u t , which (see (11 for design procedure for launcher). With t h e o r i g i n a t t h e end of the double-ridged waveouide section, the ridge extensions millimeters. The r i d g e e x t e n s i o n s a r e 66 mm long, yielding an were constructed according t o the expression y = 2 0.9 exp ( 0 . 0 4 2~) a p e r t u r e of about 2 9 m m . The 9 mm wide ridge extensions are terminated with 2 4 m diameter cylinders 9 mm high. Ideally these operating frequency [41 ; a t 3 GHz t h i s would be 100 rn which is cylinders should be about a wavelength in diameter a t t h e l o w e s t l a r g e r t h a n t h e a n t e n n a i t s e l f . I t has been found experimentally t h a t c y l i n d e r s o n l y a f r a c t i o n of a wavelength in diameter reduce t h e d i f f r a c t i o n from the tips considerably. Finally, the outside e d g e s o f t h e r i d g e s a r e c o v e r e d w i t h s t r i p s o f microwave absorbing material (eg Eccosorb MF117). of large aperture was constructed according t o t h e e x p r e s s i o n For comparison a second antenna y = 0.9 exp (0.031~) millimeters.Here the ridge extensions were 140 m long yielding a 140 mm a p e r t u r e which is more than a wavelength a t 3 GHz.
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