Highly Selective Circularly Polarized Filtering Slot Antenna Based on Stub-Loaded Resonators

This letter presents a novel approach to designing a highly selective circularly polarized (CP) filtering antenna. It is based on rectangular waveguide cavity resonators that are directly coupled to each other. Capacitive stubs are loaded onto the cavity ends without increasing the circuit size to create adjustable transmission nulls. Two orthogonal radiating slots are opened onto the side-wall of the last coupled resonator to achieve end-fire CP radiation. An equivalent circuit model is presented to analyze the numerical results. The approach is eventually demonstrated by designing, simulating, and fabricating a fourth-order CP filtering antenna operating at X-band. The measured and simulated results are in excellent agreement, showing a quasi-elliptic response with a gain value of 7.0 ± 0.1 dBic over the passband. The 10-dB reflection coefficient (S₁₁) and 3-dB axial ratio (AR) bandwidths are 7.5% and 5.5%, respectively. The competitive performance and compactness of the proposed device make it a promising candidate for stringent frequency selectivity applications.