Millimeter-Wave 2D Retrodirective Beamformer Based on Waveguide Rotman Lens

Millimeter-wave (mmWave) technologies offer high directivity and narrow beamwidth with compact size, making them suitable for wireless power transfer (WPT). To support efficient beam steering, retrodirective operation is advantageous. While prior Rotman lens-based retrodirective systems have focused on one-dimensional (1D) beamforming, their applicability is limited. To enable more adaptive and efficient beamforming, two-dimensional (2D) beamforming is essential which can be achieved with Rotman lens as it features fully passive, true-time delay operation. Therefore, this paper proposes a waveguide-based Rotman lens beamformer capable of two-dimensional (2D) retrodirective beamforming implemented using a two-stage stacked lens structure that independently handles azimuth and elevation planes. Operating at 33 GHz in the Ka-band, the beamformer enables beam steering at 10° intervals within ±20° (azimuth) and ±10° (elevation) using 15 input and 16 output ports. Simulation and measurement validate the feasibility of the proposed design for high-performance mmWaves WPT applications.