This paper reports on a tunable bandstop filter (BSF) with both flexible center frequency and bandwidth. The tunable BSF is based on half-mode substrate integrated waveguide (HMSIW), of which the broadside is coupled with reconfigurable resonators. Using liquid crystal (LC) material as the substrate, a tunable stopband could be obtained by adjusting the bias voltage of the interdigitated microstrip resonator on LC. In order to increase the tunable range of stopband bandwidth, two additional open resonators are implemented at the broadside of HMSIW cavity. For practical validation, the bandstop HMSIW filter using LC is designed and fabricated. Numerical results demonstrate that,by adjusting the voltages of three resonators on LC, either the lower or the higher cutoff frequency could be controlled independently, leading to the flexible and independent tuning ranges of both center frequency from 4.9 to 5.4 GHz and 3-dB bandwidth of stopband from 1.2 to 1.8 GHz (@5.1 GHz). INDEX TERMS Bandstop filter (BSF), half-mode substrate integrated waveguide (HMSIW), liquid crystal (LC), tunable filter.

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