Flat-Gain, Single-Layer Annular Cavity Filtenna Based on TM and Quasi-TM Modes

This letter proposes a novel, low-cost, single-layer filtering antenna (filtenna) for wireless communications. The design consists of a compact circular substrate-integrated waveguide (SIW) cavity, a centrally offset metal post for feeding, and a slot radiator for efficient radiation. It is based on an SIW coaxial line and utilizes multiple TM and quasi-TM modes. Unlike filtennas that rely on inline topologies, the proposed design employs a four-mode transversal topology to shorten signal propagation path, thereby reducing losses, particularly at millimeter-wave frequencies and beyond. The antenna is implemented on a single-layer PCB to minimize size, complexity and cost. The proposed filtenna achieves a 10 dB impedance bandwidth of 24.73% (16.83-21.58 GHz) in simulation and 26.54% (17.29-22.58 GHz) in measurement. It exhibits a stable radiation pattern with an average gain of 5.0 dBi and sideband suppression levels exceeding 50 dB on the lower sideband and 20 dB on the upper sideband. With its single-layer structure, low cost, and wideband capability, the proposed design is well-suited for emerging radio frequency applications.