Rapid dimension scaling of compact microwave couplers with power split correction

In this paper, a technique for rapid re-design of compact microwave couplers with respect to operating frequency is discussed. Our methodology involves an inverse surrogate model setup using several reference designs optimized (at the level of equivalent circuit representation of the coupler) for a set of operating frequencies within a range of interest. The surrogate establishes the relationship between the operating frequency and geometry parameter values. Upon suitable correction, it can be utilized to re-design the coupler at the EM simulation model level. Further correction - derived from sensitivity analysis of the equivalent network model - is introduced to ensure an equal power split of scaled structure. The overall computational cost of the redesign process, including both correction stages, corresponds to only two EM analyses of the circuit. Our considerations are illustrated using a compact microstrip rat-race coupler scaled for the operating frequency range of 0.5 GHz to 2.5 GHz.