On the analysis and design of three coupled microstrip lines
Abstract
The general configuration of a system of three coupled microstrip lines is analyzed and its parameters (mode numbers, effective dielectric constants, and modal impedances) are derived in terms of the system capacitances. It is shown that for a system of three equalwidth lines, there are, in general, five normalmode impedances (four of which are independent). This would make the design of a system based on three equalwidth lines, mainly with narrow strips and narrow separations, rather complicated. For a system of three equalimpedance lines, the number of mode impedances reduces to only three, which would facilitate the design. This latter system can be achieved by an appropriate increase of the width of the center line relative to that of the outer lines. Thus, a set of curves and a corresponding formula are obtained to determine this relative increase as a function of the geometric parameters.
 Publication:

IEEE Transactions on Microwave Theory Techniques
 Pub Date:
 November 1985
 DOI:
 10.1109/TMTT.1985.1133197
 Bibcode:
 1985ITMTT..33.1217A
 Keywords:

 Couplers;
 Electrical Impedance;
 Microstrip Transmission Lines;
 Capacitance;
 Network Analysis;
 Permittivity;
 Propagation Modes;
 Electronics and Electrical Engineering