Controlled Excitation of Waveguide High-Order Modes for a Simple and Accurate Monopulse Tracking System Test Bench

Monopulse tracking systems require a calibration procedure, which is typically performed using a calibration tower or a known satellite, leading to high costs, antenna downtime, and potential inaccuracies. This work proposes a simple circular waveguide component, based on a step junction, able to excite high-order modes (HOMs) in a controlled way, so as to mimic the modes generated by the tracking antenna. This allows testing in laboratory the tracking system, avoiding the drawbacks of the traditional techniques. The theoretical working principle of the step-waveguide junction is discussed and an analytical model for the quick calculation of the offset between the two waveguides to achieve a desired HOM excitation coefficient is derived. Then, a Ku-band mode exciter is designed and its effectiveness is proved in conjunction with a designed TE21 tracking coupler. Finally, the fabrication and experimental validation of a prototype is reported, demonstrating the claimed performance.