Mechanically Tunable Microstrip Bandpass Filter Using a Planar Cycloid-Dipole Resonant Superstrate

This paper presents the design, simulation, and experimental validation of a mechanically tunable microstrip bandpass filter employing a planar cycloid-dipole resonant superstrate. The proposed filter is based on a coupled-line bandpass topology, in which frequency tunability is achieved through electromagnetic field perturbation induced by vertical displacement of the resonant superstrate. By adjusting the air gap between the superstrate and the microstrip structure, continuous frequency tuning is achieved without modifying the underlying filter topology. The simulated results demonstrate an operating passband tuning range from 2.41 to 2.52 GHz for superstrate separations of 0.5–1.0 mm, whereas the fabricated prototype exhibits a measured passband tuning range from 2.32 to 2.49 GHz for separations of 0.5–1.0 mm, corresponding to a measured tuning span of 170 MHz. Throughout the measured tuning range, |S11| remains below −10 dB, while the measured insertion loss is approximately 2.5 dB near the passband center. Furthermore, electromagnetic field distributions and CST-extracted effective propagation parameters are examined to provide qualitative insight into the influence of superstrate displacement on the observed frequency-tuning behavior. The results demonstrate the feasibility of the proposed simple and passive mechanical tuning approach for frequency-reconfigurable filtering applications in the 2.4 GHz ISM band.