Coupling Coefficients Between Resonators in Stripline Combline and Pseudocombline Bandpass Filters

This article discusses the patterns of coupling coefficients between stripline resonators in combline and pseudocombline structures. It was established that there is an electromagnetic (EM) coupling between λ/4 (λ/2) resonators in such structures, and they are bandpass filters (BPFs). Between stripline stepped-impedance resonators (SIRs), both positive and negative mixed couplings can be realized. This coupling can widely vary by changing the shape of resonators and the gap between them. Moreover, the tuning of coupling is carried out without the use of a conducting pin, as in the case of microstrip resonators. The patterns of changes in the coupling coefficients between stripline SIRs at higher resonant frequencies were studied. These changes have a wave-like (alternating) character. The effect of transitioning the coupling coefficient through zero can be used to expand the rejection band of BPF by suppressing the nearest spurious bandwidth. It was found that the coupling coefficients between stripline resonators, all of whose side surfaces are metallized, depend only on the geometric parameters and are not dependent on dielectric constant ε_r. The dielectric constant only moves the coupling frequencies of an insulated stripline structure, while maintaining the ratio between these frequencies. The measurement data for some combline and pseudocombline stripline BPFs are presented.