COMPACT PLANAR TRIPLE-MODE BANDPASS FILTER WITH ENHANCED PARASITIC COUPLING

This paper presents a novel approach for designing planar triple-mode bandpass filters with good selectivity. The proposed triple-mode resonator is developed from traditional dual-mode stub-loaded resonator, and it is composed of a section of half-wavelength (lambda/2) open-ended microstrip line and a pair of open-ended stubs. The field patterns of the three modes associated with theoretical analysis are investigated. By modifying the proposed triple-mode filter, a compact triple-mode bandpass with several transmission zeros in stopband is realized. The controllable transmission zeros provide a good selectivity to the filter. An experimental filter is fabricated, and both the simulated and measured results are presented.