A spatially decoupled biasing circuit is proposed for the biasing of varactor diodes in a tunable staircase X‐band suspended substrate filter. The biasing lines are vertically aligned with lines of zero electric field which are created by the symmetric capacitive loading of half‐wave planar resonators. A prototype filter is designed and a tunable centre frequency of between 11.75 and 12.15 GHz is measured.

This work presents a study on the tunability of popular coupled-line filter topologies. The study considers a Chebychev prototype and transforms it to commonly used coupled-line topologies, namely the staircase, interdigital, hairpin, and combline topologies. The filters are designed for Xband frequencies with a 10 GHz centre frequency and a 5 % bandwidth, and are modelled using ideal transmission lines in a circuit simulator. The study is focused on centre frequency tunability while maintaining a constant bandwidth and out-ofband attenuation. It is shown that the classical staircase topology offers the best balance between good centre frequency tunability and out-of-band attenuation in comparison with the hairpin, interdigital and combline filters.