Designs for an aperture-coupled compact circularly polarised microstrip antenna

Designs of a square-ring microstrip antenna with a cross-strip, and a square microstrip antenna with a cross-slot for circular polarisation (CP) operation, are proposed and experimentally studied. Both the cross-strip and cross-slot are of equal arm lengths, and the two proposed designs can make their respective fundamental-mode patch surface current path much longer than a simple square microstrip antenna of the same size. By adjusting the coupling cross-aperture in the ground plane of the microstrip feed line, the fundamental mode can further be split into two near-degenerate orthogonal modes with equal amplitudes and 90° phase difference. Results show that, for a fixed frequency, a reduction in antenna size of 24~30% can be achieved by using the proposed designs. Details of the proposed antenna design are described, and experimental results for the CP performance are presented and discussed