Small Wideband Coupled-Line Ring Hybrids With No Restriction on Coupling Power

To realize any coupled-line ring hybrid without any restriction on coupling power, a set of coupled-line sections with two shorts was synthesized using one- and two-port equivalent circuits, and design equations were derived to yield perfect matching regardless of the coupling power. Based on the design equations, a new modified Pi-type transmission-line equivalent circuit is suggested. It consists mainly of a set of coupled-line sections and can be used to reduce a transmission-line section, especially when the electrical length is greater than 180deg. Therefore, the 270deg transmission-line section of a conventional ring hybrid can be reduced to less than 90deg. To verify the performance of the modified one, two kinds of simulations were performed: in the first, the electrical length of the coupled-line sections was held constant, in the second, the coupling coefficient was held constant. Simulated bandwidths of the resulting small transmission lines depended strongly on coupling power. Using modified and conventional Pi-type transmission-line equivalent circuits, a small wideband coupled-line ring hybrid (SWCLRH) was designed. Compared to the conventional ring hybrid, the SWCLRH has much wider bandwidth, but is less than one-third as large. To test the method, a microstrip SWCLRH with a total transmission-line length of 220deg was fabricated and tested. Measured S ₂₁, S ₄₁, S ₂₃, and S ₄₃ were -2.78, -3.34, -2.8, and -3.2 dB, respectively, at a design center frequency of 2 GHz. Matching and isolation with less than -20 dB were achieved in more than 20% fractional bandwidth.