Compact narrowband and wideband circularly-polarized filtering antennas

In this paper, we present the design, fabrication, and experimental evaluation of compact circularly-polarized (CP) filtering antennas with both narrow and wide operational bandwidths. For the narrowband design, the radiator consists of a complex load on the filter, while for the wideband design, the radiator serves as the last resonator of the filter. Coupled stripline open-loop resonators were used to realize the filter circuits. Two examples are presented targeting the 2.4-2.48 GHz and 3.75-4.25 GHz bands, respectively. Prototypes are fabricated and characterized, showing strong agreement between simulated and measured results. Both filtering antennas achieve S₁₁ below -13.5 dB and axial ratios less than 3 dB in the targeted band. The gain response exhibits a profile similar to the transmission of a BPF. The integrated filtering and radiating modules with a compact form factor and improved performance metric are expected to find potential applications in a variety of wireless communication systems.