A Dual Circularly Polarized Phased Array Antenna with Enhanced Polarization Purity for Multi-Azimuth Plane Scanning

This paper presents an 8×8 dual circularly polarized (DCP) phased array antenna and an associated analysis framework for achieving high polarization purity during wide-angle, multi-azimuth-plane circularly polarized (CP) beam scanning. First, polarization-incorporated array factors are formulated for 2×2 sequential-rotation (SR) subarrays and extended to large arrays, enabling explicit prediction of the right-handed and left-handed CP radiation patterns. The analysis shows that SR yields ideally pure circular polarization in the main beam direction under the simplifying assumption that, for a fixed elevation angle, the phase of the element RHCP component is invariant with respect to the azimuth angle φ (and analogously for the LHCP component) but inevitably generates cross-polarized (Cross-pol.) lobes in off-boresight directions, whose angular locations depend on the beam steering angle and inter-element spacing, whereas their peak magnitude is fundamentally bounded by the axial ratio (AR) of the array elements. Guided by this analysis, a compact self-phase-shifting DCP element operating at 5.7-5.9 GHz is developed, exhibiting port isolation exceeding 15 dB. An 8×8 DCP phased array composed of 2×2 SR minimum periodic subarrays is then designed, fabricated, and measured. Simulation results show that, during ±60° beam scanning in the φ = 0°, 45°, 90°, and 135° planes, the active VSWR remains below 2.0 within ±45° and below 3.0 within ±60°, while the AR in the main-beam direction is kept below 1.18 dB and Cross-pol. levels in the entire upper hemisphere are at least 15 dB lower than the peak co-polarized gain. Measurements confirm a slight (0.05 GHz) upward shift of the operating band and validate the predicted wide-angle, high-polarization-purity DCP performance.