Miniaturized bandpass filter with improved stopband characteristics using microstrip interdigital-loop resonator

A four-pole quasi-elliptic function bandpass filter is designed and fabricated using a miniaturized microstrip interdigital-loop resonator. The novel microstrip interdigital-loop resonator is able to control spurious responses in a compact size. A bandpass filter using this resonator shows good microwave characteristics with harmonic suppression of about -15 dB at 6.52 GHz, which is 3.34 f₀, where f₀ is the midband of frequency of a bandpass filter. The results of measurements are almost similar to those of simulation.