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 C
_tune
. 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.