Design of an Ultra-Wideband Multimode Monopulse Coaxial Quad-Ridged Horn Feed

A novel ultra-wideband multimode monopulse coaxial quad-ridged horn (CQRH) feed operating over a 4:1 bandwidth is proposed. The feed utilizes the coaxial waveguide TE11mode and TEM mode to generate the sum and difference beams for communication and monopulse tracking applications, respectively. The TE11single-mode operating bandwidth is extended by inserting a quad-ridged structure into the coaxial horn, while TEM mode inherently has excellent wideband characteristics. Therefore, a TE11/TEM dual-mode CQRH and a coaxial quad-ridged orthomode transducer (CQROMT) are designed, fabricated, and measured for an ultra-wideband circularly-polarized monopulse tracking feed operating at 3-12 GHz. A good agreement is obtained between the simulated and measured results. The measured sum and difference reflection coefficients are better than -10.4 dB and -5.1 dB, respectively, and the isolation between two orthogonal circular polarization ports is better than 22.1 dB. By feeding a f/D=0.54 parabolic reflector antenna, the antenna aperture efficiency exceeds 65.6%, the peak gain difference between the sum and difference beams is less than 10.3 dB, and the null depth of the difference beam is better than -30 dB.