A tunable bandpass patch filter with varactors

This paper presents a miniaturized tunable bandpass patch filter using varactors. The filter is conceived with a triple-mode circular patch resonator with four slots, where the varactors were connected. The extracted equivalent model, the varactors assembly configuration and their biasing are presented. A DC bias from 0 to 20 V was applied to vary the filter´s center frequency from 2.35 GHz to 1.8 GHz and its 3-dB fractional bandwidth from 31.5% to 8.5%. Within these variations, all measurements showed insertion loss lower than 2 dB and return loss better than 10 dB over the passband, ensuring a good quality factor.