A compact quad-band bandpass filter with high skirt selectivity based on stub-loaded resonators and λ/4 resonators

In this paper, a quad-band bandpass filter (BPF) with compact size and high skirt selectivity is proposed by using j both stub-loaded resonators (SLRs) and λ/4 resonators. One set of SLRs is utilized not only to generate the lower passbands but also to feed the λ/4 resonators. Three extra transmission zeros (TZs) are created in the stopband by using a zero feed structure. For demonstration, a exemplary filter has been designed and fabricated. Good agreement is observed between the simulated and measured results.