Design Features of Fully Shielded Stripline Bandpass Filters Affecting Their Frequency Responses

The article examines the design features of fully shielded stripline bandpass filters (BPFs) that affect their frequency responses. These BPFs are formed by two dielectric substrates between which a conductive metal layer is presented. This layer reproduces the topological pattern of resonators and coupling elements. It was established that thickness of the conducting layer t has a different effect on the magnetic ${K} _{\text {m}} \gt 0$ and electrical ${K} _{\mathrm {e}} \lt 0$ components of the mixed coupling coefficient K between resonators. The values of ${K} _{\mathrm {m}}$ and $\vert {K}_{\mathrm {e}}\vert $ are increased and decreased, respectively, when the value of t grows. We obtain a significant degradation of the frequency responses of the filter if the prescribed thickness ${t} _{0}$ , which is determined by the technological process, is not observed. A stripline BPF containing two substrates with different dielectric constants $\varepsilon _{\mathrm {r1}}$ and $\varepsilon _{\mathrm {r2}}$ is first considered in this article. A quasi-TEM wave propagates in this design and magnetic cross-couplings occur between nonadjacent resonators in the inline BPF. This allows to implement mixed couplings between nonadjacent resonators in the fully shielded design and obtain more diverse frequency responses. Three experimental stripline BPFs are investigated. One of them with simple couplings, and the other two have mixed couplings.