A dual-band bandpass filter using parallel doubly coupled structure with loading capacitance

A dual-band bandpass filter using parallel doubly coupled structure with loading capacitance is designed for dual-band applications at 17/35 GHz. New coupling structure is proposed to replace the normal counterparts. It can provide two passband respectively in Ku-band and Ka-band. By using the capacity-load structure, the slow-wave effect occurs in this filter, which can reduce the circuit size and cause the bandstop effect to suppress the harmonic wave. Besides, two open-circuited stubs are used to produce transmission zeros in the stopbands, which can get sharp out-of-band attenuation. The isolation between these two passbands is better than 20 dB from 20 to 33 GHz. This filter is analyzed and designed by CST and HFSS software, and the simulated results of which show good agreement with the theoretical analysis.