Quad-band bandpass filter with controllable frequencies using asymmetric stub-loaded resonators

In this paper, we present a compact quad-band bandpass filter (BPF) using asymmetric stub-loaded resonators. The resonator consists of a main transmission line with a short stub and two pairs of open stubs with different lengths. The whole resonator can be regarded as two different half-wavelength resonators and a simple stub-loaded resonator. Therefore, the resonator consists of four controllable modes, which are utilized to realize a quad-band response. Meanwhile, by using hook shape feed lines and source-load coupling structure, transmission zeros are generated near the passband, resulting in high selectivity. For demonstration, a compact quad-band BPF with operating frequencies of 1.57/2.45/3.5/5.15 GHz is implemented. Good agreement between the measured and simulated results is presented to verify the theoretical prediction.