Compact Branch-Line Coupler Based on an Inductor and a U-Shaped Capacitor

This paper describes the analysis of a compact branch-line coupler design. The miniature structure is obtained by substituting the quarter-wave sections with equivalent circuits consisting of a microstrip line and a U-shaped capacitor connected to it. Such structure possesses similar characteristics with those of a substituted segment at the central frequency and its surroundings. It is worth noticing that the compact layout has technically feasible dimensions and characteristics, comparable to those of standard layout. The standard and compact branch-line coupler layouts were modeled using the AWR DE electrodynamic simulation software. Using this PCB etching method a prototype was obtained, operating at the central frequency of 900 MHz. Afterwards the frequency dependencies of the compact branch-line coupler were obtained using R&S vector network analyzer. Thus, using the suggested miniaturization method it was possible to reduce the conventional branch-line coupler surface area by 77.6%. Meanwhile the working band got reduced by only 3%.