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Abstract
In this paper, a novel triple-band microstrip filter with improved out-of-band suppression is presented. The theoretical design of triple-band filter is based on frequency transformation method, which is then realized by a group of triple-band resonators as the building block of the filter. Two open stubs with different lengths are loaded directly in the middle of two resonators to introduce a pair of transmission zeros, which achieves high out-of-band suppression filtering response with no need of cross-coupling. To validate the method, a practical triple-band filter with pass-bands at 3.3–3.4 GHz, 3.5–3.6 GHz, 3.7–3.8 GHz and transmission zeros at 3.1 GHz, 4 GHz is designed and fabricated.