Analysis on the Frequency Characteristics of Coupling Windows in Tunable Cavity Filters

This paper focuses on the design and research of recent tunable cavity filters, analyzing the frequency variation characteristics of common coupling windows. Based on the coupling structures, we discussed an in-depth analysis and comparison of their coupling characteristics and explained the reasons about why the value of coupling windows changes when tunning the center frequency. Ultimately, we identified several optimum coupling windows types in a specific frequency range (19-21 GHz) that exhibits minimum variation in the coupling coefficient, demonstrating stable performance verified by full-wave EM simulations. This coupling window can be better applied to the design of tunable filters with excellent return loss and insertion loss, high quality factor, and essentially stable bandwidth for the whole tuning range. This finding holds significant engineering importance for the future design and implementation of high-frequency flexible tunable filters.