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This paper proposes new microstrip directional couplers with high directivity using current path detour to compensate for unequal even-and odd-mode phase velocities in the U-NII upper band (5.725～5.850 GHz). The proposed method involves matching the phase velocities of even and odd modes by designing a longer current path for the odd mode, whose phase velocity is faster than that of the even mode. The proposed idea is demonstrated by the design of two directional couplers: one is a directional coupler with triangular edges on each side of the coupling area, and the other is a coupler with circular patterns in the coupling gap. The measurement results of the proposed two directional couplers exhibited an isolation of more than 38.6 dB and 46.0 dB, respectively, and a directivity of more than 25.7 dB and 29.7 dB, respectively. Compared to the conventional structures, the directivity of the proposed structures was enhanced by more than 10.8 dB and 14.8 dB, respectively.

This paper proposes a compact bandpass filter using a folded ring resonator, which occupies an area, one-quarter the size of a conventional ring resonator. For the filter design, circuit modeling is used to derive the coupling coefficients between the even-and odd-mode resonances of the proposed resonator with the perturbation stub. The validity of the proposed filter is demonstrated by designing and fabricating a two-pole bandpass filter with a 100 MHz bandwidth at the central frequency of 2.45 GHz.