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I .INPRODUCTIONThe e f f e c t of mechanical support structure on.gain and sidelobes plexity involved in the multiple scattering phenmena from the of a prime focus reflector antenna is a c l a s s i c problem. The canvarious forms of support structure has defied a rigorous and export structure was treated as an o p t i c a l shadow i n the r e f l e c t o r act solution.The most crude analysis was suggested that the supdicting the main beam and f i r s t few sidelobes. Since then many aperture. This approach produces reasonably good r e s u l t s i n preauthors have attempted t o *rove either the accuracy of the ana l y t i c a l model and/or considered more precise effect f r a n the geometry of the support structure.However, the interaction between the supporting structures and the reflector was not taken into consideration.As the requirements of sidelobe levels are agencies, the predicted results will not be accurate without in-specified a t very low level by various ccrmmunication policy making cluding the effect of these interactions. The purpose of t h i s paper is to present an accurate and systematic analysis based mainly on the Geometrical Theory of Diffraction (GTD) to predict the low radiation energy from the complex s c a t t e r i n g f i e l d of the support structure. This enables a designer to predict the l e v e l of each scattering component and control the magnitude of each component by adjusting the configuration of the scattering play a very similar form i n sidelobe structure as compared t o ex-structure. It w i l l be shown that the analytical predictions disperimental results. I I . METBOD OF ANALYSIS As i l l u s t r a t e d i n F i g u r e I, the magnitude of sidelobe energy radiated by a reflector antenna i s controlled by several different contributors. Based on t h e c h a r a c t e r i s t i c s of energy radi a t e d by each contributor, different methods a r e proposed to pred i c t t h e r a d i a t e d energy level. For example, the aperture field and the feed blockage can be calculated by using the Aperture Field method. The Gemetrical Theory of Diffraction is then used to analyze the rest of the scattering cwponents. A computer R. J. Marhefka a t the Electro Science Laboratory, Ohio S t a t e Univ-program developed by R. C. Rudduck, W. D. Burnside, N. Wang and e r s i t y , is employed t o compute the radiation level of each scatt e r i n g component. The total radiated energy i s the vector sum of the energy generated by the various mechanisms described above.It i s proposed that the whole radiation sphere should be divided into three spatial regions: the main beam and f i r s t few sidelobes

This paper presents a survey of the state-of-theart in multibeam antennas, as particularly applicable to data communications satellites, and analyzes a new configuration to meet such specific needs oyer the U.S. during the 1980's. Three basic forms of multibeam antennas are descrlbed--those providing space diversity, polarization diversity and variable-shaped beams. The use of lenses, reflectors and phased arrays in various multiheam configurations is described. Hardware developments in the field which are surveyed include both waveguide and TEM lenses, and an offset fed reflector for INTELSAT V.

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