Suspended Substrate Stripline Bandpass Filters with both Broad Passband and Wide Upper Stopband

This paper presents broadband filters with broad bandwidth (up to 70%), very wide upper stopband (nearest spurious passband occurs up to five times of passband center frequency (f₀)), good stopband rejection performance (better than -30--40 dB in the whole stopband region), and easy reproduced by the conventional low cost printed circuit board (PCB) process with low dielectric constant substrates. The proposed filters using suspended substrate stripline stepped impedance resonators (SIRs) with ground aperture technique to realize the extremely tightly coupled low impedance coupled-line sections as well as the extremely high-impedance uncoupled-line sections. The extremely tightly coupled lines enable the proposed filters to have very wide passband and the extremely high- impedance lines lead to large low-to-high impedance ratio that push the nearest spurious passband up to 5f₀. Furthermore, it has been shown that the equivalent circuit model accompanied with the design charts could accelerate the design procedures but keep the accuracy of the results. The design procedures have been described in detail and have applied to designed examples. A good agreement between simulation and measurement has been observed for the given examples.