Novel Combined Coaxial and Dual-Mode Dielectric Resonator BPFs With Symmetric and Asymmetric Responses Using Virtual Negative Couplings

In this article, bandpass filters (BPFs) with symmetrical or asymmetrical responses are precisely designed by using novel combined coaxial TEM-mode and HE11 dual-mode dielectric resonators (DRs). Novel tuning structures of the coupling coefficients among the TEM-mode and dual HE11 modes are proposed with full descriptions of the coupling mechanisms. Both positive and virtual negative couplings in the triplet coupling topologies are achieved with flexible choices of the tuning parameters, and transmission zeros (TZs) can be precisely designed at prescribed frequencies. Individual controls of all the design parameters of the BPFs, including all resonant frequencies and all interresonance coupling coefficients, are accomplished, and the designed responses of three BPF examples agree well with the theoretically synthesized ones. Finally, a six-pole BPF with two TZs on both sides of its passband is designed and fabricated. The measured response of the BPF agrees well with the simulated one and shows a center frequency ( $f_{0}$ ) of 2 GHz and a bandwidth of 40 MHz. The insertion loss at $f_{0}$ is 0.95 dB. The stopband with rejections larger than 48.2 dB is extended up to 5.6 GHz (about $2.8~f_{0}$ ).