A new method for designing tunable bandpass filters

In this work, a tunable microwave bandpass filter is designed based on a Fabry-Pérot interference (FPI) cavity, which consists of two nonequivalent FSSs. The upper and lower FSS layers are composed of periodic cross-type units of different sizes, thus having two different central frequencies. The transmission peak of the double-layer structure lies in between these two frequencies, having a narrow bandwidth (FWHM∼ 0.2 GHz) and is tunable by changing the cavity length. The two FSS layers were fabricated by laser machining on metal sheets. Single FSS layers and the double-layer structure were tested using a vector network analyzer in a microwave anechoic chamber, and their characteristics are consistent with CST simulation results and FPI theoretical calculations. Our method can be utilized to design tunable bandpass filters operating in other bands. These filters may be used for 5G and 6G communication systems.