Parallel‐coupled Microstrip Lines

Microstrip filters employing the parallel‐coupled structure are usually of the bandpass or bandstop types. This chapter presents the theory behind coupled microstrip lines. The theory and concepts are applicable to nearly all coupled transmission lines. Two sets of formulas for characteristic impedance are presented which were developed by Bryant and Weiss, and Hammerstad and Jensen, respectively. Simple, semi‐empirical analysis formulas are available for the determination of characteristic impedances to an accuracy of within 3%. These could be useful to the designer, who might start with an approximate synthesis and employ the analysis formulas to provide an improvement in accuracy using an iterative technique. The appropriate shape ratios are determined directly from a graph or from closed‐form expressions which are readily available. The approximate static‐TEM formulas presented in the chapter can be used for more comprehensive CAD.