This paper presents the comprehensive design and analysis of a tri-band bandpass filter (BPF) based on a novel quad-mode defected ground structure resonator (QMDGSR) fed by two 50 Ω microstrip lines under the source-load coupling condition. Four transmission zeros (TZs) are produced in the proposed tri-band bandpass structure with two TZs beside each passband. All the four TZs are thoroughly analysed using equivalent circuit models based on the even-/odd-mode theory, and the corresponding equation for extracting the frequency of each TZ is developed and verified. The bandwidths (BWs) of the 1st and 3rd operating bands are broadened by incorporating the proposed triband bandpass structure with a traditional microstrip stepped impedance resonator (MSIR). Also, two additional TZs are generated due to the coupling between the feeding lines and the newly incorporated MSIR, which significantly result in the passband selectivity improvement. The lower and upper stopband rejections of the fabricated prototype are as high as 83.3 and 43.9 dB, respectively.
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