Compact and Wideband Parallel-Strip 180° Hybrid Coupler with Arbitrary Power Division Ratios

This paper presents a class of wideband 180° hybrid (rat race) couplers implemented by parallel-strip line. By replacing the 270° arm of a conventional 180° hybrid coupler by a 90° arm with phase inverter, the bandwidth of the coupler is greatly enhanced and the total circuit size is reduced by almost half. Simple design formulas relating the characteristic impedance of the arms and power division ration are derived. To demonstrate the concept, four couplers with different power division ratios of 1, 2, 4, and 8 were designed, fabricated, and tested. -parameters of the coupler are simulated and measured with good agreement. All working prototypes operate more than 112% impedance bandwidth with more than 25？dB port-to-port isolation and less than 5° absolute phase imbalance. The proposed 180° hybrid couplers can be employed as a wideband in-phase/differential power divider/combiner, which are essential for many RF and microwave subsystem designs. 1. Introduction Nowadays the demands for compactness, light weight, and broadband have a great impact on design issues of a high performance RF and microwave front-ends at both component and system levels. Power dividers and combiners play critical roles in RF and microwave circuit designs as they are the fundamental and indispensable components in the wireless communications systems. The Wilkinson power divider, 90° hybrid coupler, and 180° hybrid coupler have unique features in both dividing and combining RF signals with port-to-port isolation in 0°, 90°, and 180° phase difference, respectively [1]. For applications such as push-pull amplifiers, balanced mixers, frequency multipliers, and antenna arrays, the 180° hybrid coupler is preferred. It is because not only the load impedance has a small impact on the isolation and input/output impedance matching of the 180° hybrid coupler, but also its operation bandwidth is wider than that of the 90° hybrid coupler [2]. However, circuit area of the 180° hybrid coupler is larger compared with both the Wilkinson power divider and the 90° hybrid coupler. In addition, demands from many widebands sparked the exploration of bandwidth enhancement techniques of various RF and microwave components, and the 180° hybrid coupler is no exception. Parallel-strip line [3, 4] belongs to the family of balanced transmission lines. It is a simple structure of a piece of dielectric substrate sandwiched by two strip conductors. Signals flowing on the upper and lower strip conductors are always equal in magnitude but 180° out of phase. This nature was applied to realize a 180° Wilkinson