On-Demand Beam-Widening Reflector Antenna for a J-Band Imaging Radar With Micro-Doppler Measurement Capability

In this paper, a novel technique is proposed to widen the beamwidth of a millimeter-wave (MMW) imaging radar system that employs a mechanically scanning elliptic toroidal reflector antenna. For applications requiring Doppler measurements, increased beamwidth enhances Doppler resolution while maintaining scanning speed, enabling improved extraction of micro-Doppler features. The reflector antenna’s beamwidth is widened by employing a feed horn with an adjustable phase center. Relying on the fact that the horn’s phase center is near its aperture, the design incorporates a hollow cylindrical sleeve that extends the aperture forward, thereby shifting the phase center. This phase center displacement de-focuses the reflector and results in a wider radar beam. The design of the feed horn and its electrically actuated sleeve is presented, along with fabrication details. The phase center position of the horn is determined using two experimental techniques. In the first, the horn is placed behind a dielectric lens illuminated by a plane wave, and the phase center is localized by identifying the position that maximizes the received signal power. In the second method, coherent signal measurements on the plane perpendicular to the horn’s boresight are collected using a network analyzer and processed to localize its phase center. In both techniques, the phase center is shown to shift when the sleeve is extended. Beam widening is verified by measuring the reflector’s radiation pattern with the sleeve both extended and retracted, showing good agreement with simulations. A beamwidth increase by a factor of four is demonstrated using a 20 mm sleeve extension. The technique’s effectiveness is further validated through Doppler measurements of a walking person, where Doppler spread is observed using the de-focused beam at scanning speeds of up to 5 cycles per second.