A compact microstrip resonator based on the interdigital structure is proposed. Theresonator has several times higher unloaded quality factor compared to similar resonators presentedpreviously and can even reach the Q-factor of a regular λ/4 resonator. The size of the resonator canbe significantly reduced with a substantial increase in quality factor by incrementing the number offingers in the interdigital structure. In addition, for each gap between the fingers, there exist an optimalnumber of fingers that correspond to the maximum Q-factor. An extension of the upper stopband fora bandpass filter designed using the resonator can be achieved by the interconnection of the fingers ineach of the comb structures. The simulation results are proven by fabricated resonators and a fourpolebandpass filter. For the central frequency of 2000 MHz and 16.2% fractional bandwidth, the lateralsize of the filter is only 11.5mm × 3.8mm for alumina substrate (eps = 9.8). The filter has an upperstopband up to 5.8f0 at the level −40 dB.
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