Quasi-Reflectionless Double-Sided Parallel-Strip Line Bandpass Filter with Enhanced Selectivity

A novel quasi-reflectionless bandpass filter (BPF) using double-sided parallel-strip line (DSPSL) is proposed in this letter. It is composed of a BPF and two symmetric absorptive bandstop filters (ABSFs) terminated by a pair of resistors that can dissipate the input untransmitted signal energy. Through detailed analysis, the high characteristic impedance is required for the open-circuited stubs of the ABSFs to enhance the selectivity. This cannot be realized in traditional microstrip designs, but it can be obtained easily by offsetting the DSPSL. Meanwhile, the BPF with T-shaped structure consists of a half-wavelength (λ/2) DSPSL with a centrally-loaded λ/2 stub, which answers for generating two transmission zeros (TZs) to achieve better out-of-band suppression. For demonstration, a prototype is designed, fabricated and measured, and the simulated and measured results with good agreement are presented.