Miniaturized and Large-Division-Ratio Ring Coupler Using Novel Transmission-Line Elements

In this letter, a novel transmission-line element that is able to exhibit a high-impedance passband is proposed and applied for coupler realization with a large power division ratio. The analysis of the transmission-line element was conducted by its equivalent circuit model and the closed-form equations for the pole and zero frequencies of the effective impedance were derived to help control the passband impedance and phase responses by the lumped parameters. The high-impedance feature of the presented artificial line was exploited to experimentally develop a 12 dB ring coupler, which occupies only 29% footprint of the conventional coupler at 3.5 GHz. Experimental results are in good agreement with the simulation data.