Contactless Tuning Plunger and its Application to K-Band Frequency-Tunable Cavity Filter

In this paper, we present a frequency-tunable constant-bandwidth waveguide filter structure with contactless tuning plungers. For the purpose of adjusting the center frequency of the filter, we, for the first time, developed the contactless plungers of waveguide corrugation structure. The new plungers do not require any electrical and mechanical contacts with the waveguide circular resonators, and this allows the plungers to move freely. Although each plunger does not have electrical and mechanical contacts with each resonator, its ${Q}$ -factor is comparable to a conventional resonator with no openings. We have also developed a design method for a frequency-tunable waveguide filter capable of having a constant absolute bandwidth. The tuning plungers are used only for the resonators, and the coupling structures are designed to be static in order to minimize the number of tuning devices. For demonstration, a fourth-order waveguide filter has been designed, fabricated, and measured. The measurement shows that it can be tuned from 19.00 to 19.48 GHz having the absolute bandwidth of 54.5 ± 2.5 MHz.