Miniature two-mode filters

Significant miniaturization of band filters for such applications as digital radio relaying has been achieved by use of waveguide structures beyond cutoff with dielectric filler. Further miniaturization is possible by use of two-mode resonators based on such structures. The resonator is a segment of a waveguide with square cross section and a dielectric insert placed at the center equidistant from both ends. The dimensions of this insert and the dielectric characteristics of its material ensure propagation through it. The insert can cover the center cross section completely or partially, but it must be symmetric with respect to the plane at a 45 deg angle to the polarization plane of the excited mode. This will ensure identical condition for excitation of two mutually orthogonal modes within the space of one resonator. As a mathematical model for performance analysis, the filter can be treated as a pair of waveguide-dielectric resonators connected in series. The design is based on attaining maximum Q-factor, minimum insertion loss, and optimum frequency characteristics of the voltage standing wave ratio.