A Miniaturized Unequal Multi-Line Coupled-Line Microstrip Bandpass Filter with Widely Tunable Center Frequency

In this paper, a miniaturized unequal multi-line coupled-line microstrip bandpass filter with widely tunable center frequency is proposed. The filter is based on a combline structure, but the feed structures at the input and output ports have narrower linewidths compared to the resonators, thus achieving a reduction in overall size. In addition, some of the resonators are bent into open-loop structure, which is different from the traditional combline coupled filter. The center frequency of the filter can be tuned between 0.625 GHz and 0.745 GHz, and the 3 dB bandwidth can be tuned between 45 MHz and 62 MHz. The measured results show excellent agreement with the simulation, validating the practical applicability of the proposed design.