Tunable narrow bandwidth of a bandpass filter combined microstrip and suspended substrate stripline

This paper introduces a narrow bandwidth tunable filter. The filter has been tuned electronically by using varactor diodes. A set of mechanically tuning screws have been utilized to overcome the manufacturing tolerance of the filter's resonators. In contrast to most the combline filters, which depend on the cross-coupling approach to generate the transmission poles, the proposed filter has a pair of transmission zeros located on both sides of the passband, by employing two uncoupled microstrip resonators that have been electronically tuned independently. The proposed filter bandwidth tuning range is 59-88MHz with 3dB fractional bandwidth 1.94-2.76 respectively. A detailed demonstration of the suggested tunable filter is introduced based on a mathematical approach.