Wideband 1-bit Reconfigurable Circularly Polarized Transmitarrays in Ka-Band

This study details the design of an active Transmitarray (TA) operating in the Ka-band made of a reconfigurable linear-to-circular-polarization (LP-CP) unit-cell. The unit-cells incorporate a true-time-delay (TTD) polarization conversion mechanism and a stacked patch configuration to achieve wide transmission and axial ratio bandwidths. Each cell integrates a pair of PIN diodes for electronically controllable 180° phase shifts. This is the first demonstration of a wide-band reconfigurable and circularly polarized unit-cell in millimeter wave bands, without the use of an external polarizer. A 20 × 20 cells Ka-band circularly polarized TA was designed and fabricated. A random symmetric distribution of rotations applied to the inner patch antenna enhances CP purity and increases the scan range. Measurement results demonstrate a broadside gain of 20.55 dB, with both -3 dB gain bandwidth and 3 dB axial ratio bandwidth exceeding 22%, which surpasses all previous millimeter-wave reconfigurable design. In the case of the 20 × 20 cell array manufactured the scan range extends to ±60° in both E and H planes, with a maximum scan loss of 4.2 dB and axial ratio values under 2 dB for elevation angles up to ±50°. Additional full wave simulations of a larger 60 × 60 cell array demonstrate a better maximum scan loss of 2.4 dB for elevation angles reaching ±50°, 3.8 dB for an elevation angle of ±60° and 6 dB for an elevation angle of ±70° with axial ratio values consistently under 3 dB.