Compact Microstrip Ring Branch-Line Coupler with Harmonic Suppression

Abstract

A novel ring branch-line coupler is presented in this paper. To minimize the physical size of the coupler, four high-low impedance resonator cells are loaded within the free area of the conventional coupler. A design methodology calling for united optimization is used to optimize the structure parameters of the main transmission lines and loading high-low impedance resonators. Furthermore, an equivalent circuit of one loaded cell is proposed to model the operation of the coupler. The proposed new coupler not only effectively reduces the occupied size to 22% of the conventional one at the operating frequency of 1.39 GHz, but also has high third harmonic suppression.