Miniature UWB Filter Using Three-Quarters Wavelength Resonator with Direct-Connected Feed Method

Abstract

A miniature ultra-wideband (UWB) bandpass filter using three-quarters wavelength resonators is presented in this paper. Direct-connected feed method is employed between the input/output ports and the resonators in order to overcome the shortcomings due to the gap-coupled feed method and produce two transmission zeros in the lower and upper stopbands. On the other hand, two quarter-wavelength matching transmission lines are introduced to the input/output ports to improve the reflection loss characteristic in the passband of the filter. In addition, the resonators are folded to be open ring structures, which are more miniaturized than the conventional linear structure. As a consequence, the filter is compact in size and exhibits good performance. The filter is successfully realized in theory and verified by full wave EM simulation, and simulated frequency response results show that the fabricated filter has an insertion loss of better than 1dB in the passband and two rejections of greater than 25dB in most of the stopbands.