Analog tuning of compact varactor-loaded combline filters in substrate integrated waveguide

The design, fabrication and measurement of continuously tunable filters in single-layer SIW technology is presented in this paper. The filters are based on coupled coaxial resonators embedded into the dielectric and conveniently loaded with GaAs varactors. Firstly, a 2-pole filter demonstrator has been fabricated on a low-cost substrate showing a tunable centre frequency between 2.64 - 2.88 GHz with 1.27 - 3.63 dB insertion losses across the tuning range. Then, the application of this approach to post-fabrication fine tuning of narrow-band multi-pole filters is studied. Thus, a 2.8% FBW 3-pole Chebyshev filter is designed showing a good trade-off between tuning range, additional inband losses and fractional bandwidth variation.