Design of Miniaturized Branch-Line Coupler Based on Novel Spiral-Based Resonators

Abstract

A new type of microstrip branch-line coupler by means of one novel spiral-based resonator, the asymmetrical separated spirals resonator (ASSR), which is initially proposed here, is designed. Due to its intrinsic convoluted geometry, ASSR can be completely embedded in a microstrip line, and a compact transversal dimension is achieved. Through parametric analysis, it is demonstrated that ASSR exhibits bandpass property and that the frequency point with 90-degree phase shift is just right within the given passband. Based on this exotic property, the 90-degree phase shift 30.5 ohm and 50 ohm transmission lines embedded with ASSRs are designed rather effectively. Instead of the corresponding conventional quarter-wavelength transmission lines, the proposed ASSR-based transmission lines are utilized to construct the microstrip branch-line coupler. The comparable performance is successfully achieved without any complicated adjustment, thanks to the given property of ASSR. The corresponding results are confirmed by both fullwave simulation and measurement. Simultaneously, the area of the proposed branch-line coupler is about 44.3% relative to that of conventional one, because the proposed transmission lines are all shorter than the conventional quarter-wavelength transmission lines to some extent. Besides, the proposed prototypes are uniplanar. No via, defected ground structure or lumped element is introduced.