Penta-Mode Ultrawideband Circularly Polarized Stacked Patch Antennas Using Characteristic Mode Analysis

A penta-mode concept is proposed for the design of single-fed ultrawideband circularly polarized (CP) antennas. The concept involves five linearly polarized (LP) modes in a way that each two adjacent LP modes are designed with orthogonal polarizations of equal amplitude and quadrature phase difference. In this manner, each LP mode is shared by two LP modes nearby except for the first and the last mode in the band. As a result, the CP bandwidth is significantly improved. For proof of concept, a stacked U-slot patch antenna is proposed and designed with a characteristic mode analysis (CMA). The five modes are designed by optimizing the U-slot and the coupling between stacked patches. With the single-fed CP antenna element, a
$2\times $
2 CP array is further designed and prototyped. The measured CP bandwidths of antenna element and array are 52.7% [within 2:1 voltage standing wave ratio (VSWR) and 3 dB axial ratio] and 74% (within 2:1 active VSWR and 3 dB gain), respectively, a significant improvement compared with the conventional CP stacked structures of similar thickness or volume.