A novel microstrip loop-type resonator with four resonant modes is proposed in this letter. The resonator is formed by a loop-type microstrip line loaded with four shorted stubs. It has a symmetrical structure, thus the odd-even-mode method is adopted to implement the resonant analysis. The novelty of the proposed resonator lies in two aspects. One is that its resonant frequencies can be adjusted in a more flexible way. The other is that its resonant modes have a uniform electromagnetic field distribution, which is beneficial for the excitation of resonant modes. For the purpose of demonstration, based on the novel resonator, a single-band bandpass filter with four transmission poles and a dual-band bandpass filter with two transmission poles in each passband are constructed. Additionally, source-load cross coupling is introduced, and several transmission zeros are generated in the stopband, which improves the out-of-band performance greatly. The designed single-band filter has the central frequency of 2.4 GHz and fractional bandwidth (FBW) of 4.5%, and the dual-band filter has the central frequency of 1.8/2.4 GHz and fractional bandwidth of 2.0%/2.5%. The two bandpass filters are designed, fabricated, and measured. Agreement between the simulated and measured results verifies the effectiveness of the proposed resonator and filters.
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