Novel solutions to low-frequency problems with geometrically designed beam-waveguide systems

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 foci, resulting in substantial spillover and defocusing loss. Two novel solutions are proposed: (1) reposition the mirrors to focus low frequencies and redesign the high frequencies to utilize the new mirror positions and (2) redesign the input feed system to provide an optimum solution for the low frequency. A novel use of the conjugate phase-matching technique is utilized to design the optimum low-frequency feed system and the new feed system has been implemented in the Jet Propulsion Laboratory (JPL) Research and Development beam-waveguide (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.