Tunable suspended substrate stripline filters with constant bandwidth

A suspended substrate tunable filter is presented. Meandered input and inter-resonator coupling structures are introduced to reduce bandwidth variation across the tuning range. Simulated and experimental results are presented to verify the validity of the proposed concept. A two-pole filter is fabricated and tested. The presented filters exhibit a bandwidth variation of less than ±7.8% for a tuning range of 4.79GHz±23% with a return loss that is better than 20 dB across the entire tuning range. The meandered line coupling scheme is well suited for wide bandwidth filters. A design and analysis technique for tunable filters based on the coupling matrix is presented to allow extension of the presented method to higher order filters.