A novel design technique for beam-waveguide antennas

The poor low-frequency performance of geometrically designed beam-waveguide antennas is shown to be caused by the diffraction phase centers being far from the geometrical optics mirror focus, resulting in substantial spillover and defocusing loss. To generate a new solution by a straightforward analytical design would prove cumbersome because of the large number of scattering surfaces required for the computation. Rather, a unique application was made of the conjugate phase-matching techniques to obtain the desired solution. A plane wave was used to illuminate the main reflector and the fields from the currents induced on the subreflector propagated through the BWG to a plane centered on the input focal point. By taking the complex-conjugate of the currents induced on this plane and applying the radiation integral, the far-field pattern was obtained for a theoretical horn that maximizes the antenna gain. To synthesize a horn quickly and inexpensively, the theoretical horn was matched as well as possible by an appropriately sized circular corrugated horn and the new feed system has been implemented in the JPL Research and Development BWG as part of a dual S/X-band feed system. The new S-band feed system is shown to perform significantly better than the original geometrically designed system.