Bandwidth Enhancement and Size Reduction of a Low-Profile Polarization-Reconfigurable Antenna by Utilizing Multiple Resonances

A new design method to significantly expand the axial ratio (AR) bandwidth of a single-fed, low-profile circularly polarized (CP) antenna without relying on any complex feed network or occupying a large transverse dimension is presented. First, four small patches arranged in chessboard pattern are utilized as the main radiator, and meanwhile, a cross slot with unequal slot lengths is adopted to excite triple sequentially orthogonal modes, aiming to attain two CP modes (AR1and AR2). Second, in order to further considerably enhance the AR bandwidth and concurrently reduce the overall size, the feeding cross-slot resonance is favorably excited and effectively coupled by the chessboard patches to produce a new slotline-based CP mode (AR3) located before AR1. Finally, an I-shaped top-loaded monopole fence, as a new degree of design freedom, is employed to collaboratively regulate these multiple resonances to form a wide AR bandwidth covering AR1, AR2, and AR3. Results show that, on the premise of keeping the inherent compact-size and low-profile advantages of microstrip antenna, the 3 dB AR bandwidth is remarkably enhanced up to 30.1% by the combination of the three CP modes. Based on the design method, a wideband polarization-reconfigurable antenna with switchable linear polarization (LP), left-handed circular polarization (LHCP), and right-handed circular polarization (RHCP) states is further designed and experimentally validated. The measured results demonstrate that, with quite compact transverse size (0.16λ ₀²) and low profile (0.04λ ₀), a wide overlapped bandwidth (27.6%) of the polarization-reconfigurable antenna is successfully accomplished.