LAUSR

A compact tunable cavity filter based on double‐layer suspended stripline resonator (DSSLR) is proposed. The DSSLR consists of a thin substrate suspended in an air‐filled cavity, a stepped impedance resonator loaded with interdigital capacitor on the top layer of the substrate, and a parallel‐plate capacitor series with a varactor at the bottom layer. The multiple capacitors not only achieve compact size by large capacitive loading, but also broaden the tuning range with a small capacitance variation of varactor. The internal coupling of two DSLLRs increases with the height of the inductive stub while decreases with the distance between two resonators. A fifth‐order wideband tunable cavity filter using a common bias voltage is designed, fabricated, and measured, which has a tunable frequency range of 67 MHz, constant bandwidth of 64 ± 7 MHz, low return loss of 15 dB, and high suppression of more than 40 dB.