On Elementary Cell Selection for Miniaturized Microstrip Rat-Race Coupler Design

The purpose of this paper is to provide means for systematic selection of the best slow-wave structure candidates for miniaturization of rat-race couplers. Our focus is on minimization of the coupler footprint area. The proposed approach allows for identifying the smallest attainable circuit size using a given slow-wave structure as its elementary building block. This permits discrimination between available slow-wave cells with respect to miniaturization rate they offer for a particular design specification imposed on the coupler circuit. At the same time, we introduce a class of slow-wave structures with unified (topology-independent) parameterization which facilitates handling of various slow-wave structure geometries. Our considerations are illustrated using a rectangular-like rat-race coupler with two different design specifications and a six-element set of candidate slow-wave building blocks. Numerical results clearly indicate fundamental importance of appropriate slow-wave cell selection and dependence of the optimum choice on coupler topology and given performance requirements.