Design Parameters of Three Coupled Microstriplines

The parameters of the general configuration of three coupled microstriplines are derived in terms of self and mutual static capacitances of the system in the filled and empty structures. These capacitances were computed by using the network analogue method. The mode impedances of the centre conductor for equal width lines are found to be always larger than the mode impedances of the outer conductors. For equal mode impedances of the three lines, the width of the centre line should be increased relative to that of the outer lines. A set of curves and a corresponding formula are obtained to determine this ratio as a function of the geometric parameters. Curves for the mode impedances according to the new geometric configuration are also given. A three-line coupler was manufactured in order to verify the theory presented in this paper.