Accompanied with an overcoupled response, a conventional dual-mode rectangular resonator bandpass filter (BPF) inherently has a hump response with a rather wide 3 dB bandwidth and a high insertion loss in the middle of the passband. To improve such a drawback, a miniature dual-mode BPF with a slow-wave modified rectangular resonator is presented in this study. Through a periodic U-shaped inward folding of the resonator, a slow-wave propagation is resulted for two degenerate modes in the guided-wave paths. Having an inter-digital like I/O feed lines for an optimal signal coupling and a perturbation element at the topmost of the slow-wave resonator, the characteristic of fundamental frequency is significantly improved to have sharper rejection skirts with introducing transmission zeros and a single peak response within the bandpass region instead of double peaks response. Due to an utmost use of interior space with an equal amount of 5 inward folding at both sides of the resonator, size reduction could be realized to 41 and 57% against the conventional rectangular ring and square ring, respectively. Besides, the feed lines are also kept along a straight line to favor for a flexible accommodation with the microwave networks.
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