Microwave directional coupler design using spectral domain technique

A systematic algorithm for selecting the basis function for microwave directional couplers is presented. It is shown that microwave directional couplers can be grouped under four major categories: simple configurations, such as broadside-coupled suspended substrate stripline structure, broadside-coupled strip with overlay, and coupled suspended lines with septum; more complex configurations, such as edge-coupled strips with overlay; structures involving anisotropic material such as coupled microstrip on double anisotropic substrate layers; and asymmetric structures such as broadside-coupled striplines of unequal width. For each category, there exists an optimal basis function: one-term quadratic function; two-term quadratic function; logarithmic function; and Chebyschev function weighted by an edge condition.<>