Single-Fed Triple-Mode Wideband Circularly Polarized Microstrip Antennas Using Characteristic Mode Analysis

A method of triple-mode operation by capacitive slot loading is proposed for bandwidth enhancement of single-fed circularly polarized (CP) patch antennas. Instead of using even-numbered linearly polarized (LP) modes with quadrature phase, three orthogonal LP modes are used for CP bandwidth enhancement, where the middle mode is shared by two cross-polarized modes with the same polarization. The advantages include reduced constraints, lower complexity, and a higher degree of freedom for antenna design. Guided by the method, a U-slot antenna and an E-shaped antenna are proposed and designed with characteristic mode analysis (CMA). Both antennas work with a TM
₁₀
-like mode and a TM
₀₁
-like mode. Differently, the U-slot antenna works with an additional slot mode and the E-shaped antenna works with an additional TM
₁₁
-like mode. The operating modes are manipulated by the slot loadings for creating phase difference. As a result, wideband CP radiation is achieved with a single feeding. CMA-based empirical formulas are derived for fast design. The proposed method and antennas are experimentally validated. Both antennas measure a bandwidth exceeding 21% for 10 dB return loss and 3 dB axial-ratio (AR), a significant improvement compared with conventional corner-truncated U-slot patch antennas of similar thickness or volume.