A novel perturbation arrangement for dual-mode resonators and its dual-band bandpass filter applications

In this paper, by using microstrip resonators having dual-mode characteristics, a novel dual-band bandpass filter is presented. Designed resonator structures have been formed by placing a couple of open-circuited stubs to the corner points of a square loop resonator. By means of movement of the gaps between the open ends to obtain a perturbation effect, two degenerate modes are excited, mode frequencies and transmission zeros can be controlled. A novel perturbation arrangement without using an extra perturbation element is formed. On the other hand, location of these gaps allows for creating either an elliptical or a linear filter characteristic in the related passband without shifting the center frequency and changing the bandwidth. Two resonators which represent two different passbands are fed by only one feed-line. Designed dual- band microstrip filters have totally 14.2mm × 14.2mm surface area and so, such a configuration has been satisfied a compact structure. Four dual-band filters using dual-mode resonators are demonstrated with their simulation and measurement results.