The Asymmetrical Split Phenomenon of the Odd and Even Modes and Its Application to UWB HTS Filters

This paper analyzes the asymmetrical split phenomenon of the odd and even modes in microstrip for closely coupled resonators. In general, when two resonators are coupled, the odd- and even-mode split is asymmetrical. This paper finds that, for two closely coupled resonators, odd mode always changes faster than even mode. This asymmetrical phenomenon is first predicted by using the effective capacitances of the even and odd mode. Then, the electric, magnetic, and mixed couplings are all discussed to demonstrate this phenomenon. Numerical results help us to control the odd mode in a wide range. This paper presents an ultra-wideband filter by using the interdigital coupled structure for the first time. A high-temperature superconducting filter operating at 7 GHz with -3 dB FBWs of 103% is designed and fabricated. Measured results show good performance and agree well with the simulations.