Parallel‐Coupled Lines and Directional Couplers

Good coupler performance is achievable if special modifications are made to the basically simple structure, consisting of two parallel microstrips. Analysis formulas provide an improvement in accuracy using an iterative technique. The synthesis technique developed by Akhtarzad et al. can be modified to obtain greater accuracy. This chapter shows how to design a 10DB microstrip coupler by using Bryant and Weiss curves, and using Akhtarzad synthesis curves, and shows the calculation of appropriate coupled-region physical length. In several applications a poor frequency response is intolerable, and a number and of techniques have been developed to ‘flatten’︁ the response. The chapter discusses the concept of coupler directivity, and describes special coupler designs. The last sections of the chapter deal with thickness effects, power losses, fabrication tolerances, and crosstalk between microstrips used in high-speed digital systems.