A Dual Circular-Polarized Extremely Thin Monopulse Feeder at W-Band for Prime Focus Reflector Antenna

A thin monopulse feeder is very important in a radar system with strict profile limitation. Besides, to implement polarization-agile radar, a monopulse feeder with two orthogonal polarizations is required to synthesize an arbitrary polarization. To achieve these requirements, a dual circular-polarized extremely thin monopulse feeder at W-band is investigated and realized for a prime focus reflector antenna, which can be used in a tri-mode seeker. To begin with, two kinds of the 2 x 2 substrate integrated waveguide (SIW) slot antenna array topologies are discussed based on the standard of circular polarization efficiency. After that, the better one integrated with a compact and single-layer SIW comparator is used to feed the reflector antenna. The total thickness of the developed feeder is only 0.56 mm. For a prime focus 140 mm diameter reflector antenna with the blockage of a diameter of 40 mm, the measured gain of the entire antenna system is 35.6 dBi at 94 GHz. The DIFF-beam null depth is -20.1 dB in xoz plane and -20.5 dB in yoz plane. The gain ratios of the SUM-and DIFF-pattern are 3.5 dB in xoz plane and 4 dB in yoz plane. The results show that this type of antenna has great prospects in a monopulse tracking system with polarization agile function.