A Tunable Coaxial Filter Based on Mode-Switching Using a Single Tuning Element

This article presents a novel design for a tunable coaxial filter with the ability to switch between two resonant modes over the tuning range. The use of two resonant modes makes it possible to increase the tuning range while maintaining a constant absolute bandwidth. Another feature of the proposed resonator is that it is tuned using a single tuning rotating element regardless of the filter order. The cavity structure is designed such that when the coaxial resonator is tuned, it switches from a lambda/4 resonator to a lambda/2 resonator, as a result, the filter operates over two adjacent tuning ranges, yielding effectively a wider tuning range. To validate this concept, two prototype tunable filters, a two-pole filter, and a three-pole have been designed. The simulated tuning range of both filters spans from 1.8 to 3.8 GHz yielding a tuning range over 2:1 with less than +/-%10 variation in BW and 0.05 dB for insertion loss. The two-pole filter made of aluminum housing was fabricated and measured demonstrating tunability from 1.92 to 3.78 GHz. To the best of our knowledge, this is the first design to simultaneously utilize the characteristics of a lambda/4 resonator and a lambda/2 resonator to achieve such a wide tuning range with almost constant absolute bandwidth.