Stub-Loaded Theory and Its Application to Balanced Dual-Band Bandpass Filter Design

Novel stub-loaded theory is presented and applied to design balanced dual-band bandpass filter with independently controlled passband frequencies and bandwidths. According to the presented stub-loaded theory, the first differential-mode resonance can be independently designed by the loaded stubs, while not affecting the second one. Besides, except for the main coupling path, one more coupling path between the loaded stubs can be used to independently control the coupling coefficient of the first passband. Therefore, both the passband frequencies and bandwidths of each differential passband can be easily designed. To validate the proposed idea, the balanced dual-band BPF is designed, simulated and measured. Simulated and measured results are well agreed.