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Design of a microstrip quad‐band bandpass filter with controllable bandwidth and band spacing for multifunctional applications

A design methodology for a small-size multifunctional quad-band bandpass filter (BPF) exhibiting high selectivity is proposed in this study. In this design, the resulting BPF would comprise a dual-band split-type BPF, wideband BPF and narrowband BPF, and it can execute narrowband/wideband operation and provide close/wideband spacing. Since each passband can be formed by respective filters, the design methodology has high flexibility in that it can achieve different filter functions and can be compatible with various communication systems. In addition, because of the source-load coupling, four additional transmission zeros could be created over the stopband, thus enhancing the selectivity. A quad-band BPF operating at 1.45/1.65/4/6GHz with fractional bandwidths of 6.6/5.8/35/3% was implemented to verify the design methodology. The fabricated BPF was determined to have a compact circuit size of 0.3 λ
 g
 × 0.44 λ
 g
 (
 λ
 g
 is the guided wavelength at the central frequency of the first passband) and it exhibited an in-band insertion loss of no more than 2.8dB. The experimental and simulation results were determined to be in good agreement.