A Half-mode Substrate-Integrated Filter With Tunable Center Frequency and Reconfigurable Bandwidth

This letter presents a half-mode substrate integrated cavity bandpass filter with both continuous center frequency and 3 dB bandwidth tuning capabilities. The filter adopts a superposed two layers PCB with lumped varactors loading the central resonator to tune the center frequency. The first-invented semicircular metal sheet on the center rod not only reduces the filter size given a certain resonant frequency significantly by increasing the capacitance between the semicircular metal and the upper ground metal but also facilitates fine frequency tuning by using appropriate varactor Ctune. Surface mount varactors between two resonators change the inter-resonator coupling to adjust the 3 dB BW. Another varactor on the feed line can be used to adjust the external coupling. This filter demonstrates a BW range of 75-130 MHz around 1.5 GHz, a 3 dB BW range of 46-130 MHz around 1.3 GHz and a 3 dB BW range of 55-90 MHz around 1.11 GHz with 10 dB minimum return loss. Insertion loss ranging from 2.262 dB to 7 dB have been obtained.