Balanced bandpass filter with tunable center frequency based on microstrip patch resonator

This paper presents a novel balanced bandpass patch filter with tunable center frequency. The filter uses three right angle isosceles triangle patch resonators (RAITPRs) with the TM
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mode. By introducing varactor-loaded short stubs on the two sides of the hypotenuse of the three RAIPRs, a balanced bandpass filter (BBPF) with wide tuning range of center frequency from 1.48 to 2.25 GHz is realized. The verified results show that insertion loss is 1.2–1.38 dB, and the return loss is better than 15 dB over the whole tuning range.