Perturbation-Free Circularly Polarized Annular Sector Patch Antenna Under Resonance of Multiple Nondegenerate Modes

Circularly polarized (CP) microstrip annular sector patch antennas (MASPAs) with adjustable 3 dB axial ratio beamwidth (ARBW) under resonance of nondegenerate modes are proposed. Within an annular sector patch radiator with both inner and outer circumferences to be short-circuited, the odd-symmetric, circumferentially resonant TM_π/α,1 mode and the even-symmetric, radially resonant TM₀₁ mode are simultaneously excited to yield dual-mode resonant MASPAs without additional perturbations in an asymmetrically fed manner. The equal amplitude and self-phase-shift conditions for circularly polarized operation are theoretically deduced and employed to design different dual-mode resonant MASPAs on air substrate for validations. The squint CP angle θ_t and adjustable ARBW are distinctive to conventional microstrip patch antennas under resonance of a pair of degenerate modes. In this sense, the fabricated MASPA successfully exhibits 3 dB AR bandwidths of 4.5% and maximum 3 dB ARBW of 110°, while keeping the simplest single-layer, single-fed configuration without incorporating additional perturbed stubs, slits or pins.