Circularly polarized co-designed filtering annular slot antenna

In this paper, a compact circularly polarized co-designed filtering annular slot antenna (ASA) is proposed. The antenna comprises a distributed capacitor-loaded annular slot radiator to produce circular polarization (CP) radiation, which is seamlessly integrated with a microstripline bandpass filter (BPF) to enhance out-of-CP-band suppression. Two tightly coupled arc branches protruding into the annular slot are used to provide the required perturbation capacitance. The filter is designed by employing three concentric arc-shaped coupled microstripline resonators. The ground plane is shared by the annular slot radiator and the filter to reduce size occupation. It is found that the proposed antenna can not only yield high frequency selectivity, but also obtain widened CP bandwidth under the assistance of the filtering circuit. An antenna prototype has been fabricated and measured, showing that the antenna achieves a 10 dB return loss impedance bandwidth of around 13.7%, enclosing a 3 dB axial ratio (AR) bandwidth of more than 10.6%. Details of design considerations and experimental results are presented and discussed.