Tunable Dual-Band Filter Based on Stub-Capacitor-Loaded Half-Mode Substrate Integrated Waveguide

This paper presents an original and simple approach to the design of dual-band bandpass filters (BPFs) using the proposed stub-capacitor-loaded half-mode substrate-integrated waveguide (HMSIW) structure. The stub-capacitor-loaded HMSIW structure is investigated and a set of analytical equations is provided for the determination of center frequency, transmission zero, coupling coefficient, and external quality factor. Dual-band BPFs can be achieved by loading stub capacitor on the two sides of the HMSIW structure. The two passbands can be designed separately and controlled independently. To validate this method, a class of dual-band filters with a wide range of fractional bandwidth ratios of the two passbands is designed. Finally, a dual-band filter, with each passband tuned only by one control voltage, is fabricated and measured. The results show that the first passband can be tuned in a frequency range from 1.85 to 2.67 GHz (36.4%) without any effect on the second passband. In the meantime, the second passband varies from 3.84 to 5.34 GHz (32.7%) without any effect on the first passband. Good return loss is observed within a wide tuning range.