Millimeter-Wave Quarter-Wave Plate for Diffusion Bonded Slot Array Antennas

A single-layered quarter-wave plate (QWP) is proposed and demonstrated for linear-to-circular polarization (LP-to-CP) conversion in the near-field of a slot array antenna. The QWP is based on a square unit cell with five elliptically shaped air holes - four on the edges and one in the center. The central ellipse is rotated by 45° which decomposes the incoming linear polarization into two orthogonally polarized components with 90° phase difference. The LP-to-CP conversion is demonstrated by simulating the proposed QWP in the near-field of an LP slot array antenna with a 3 dB axial ratio and −10 dB matching bandwidths of 5.1% and 8.9%, respectively, around 60 GHz.