A study of parallel coupled microstrip transmission lines with dielectric overlay

The purpose of this study was to investigate the effects of placing a layer of dielectric material over a pair of edge coupled microstrip transmission lines.

Basic coupled microstrip lines exhibit unequal even and odd mode propagation velocities, resulting in a degredation of the coupled circuit parameters of directivity and isolation, The object of the dielectric overlay is to decrease the difference between the even and odd mode propagation velocities, hence improving the overall circuit parameters of directivity and isolation.

To analyze the coupled microstrip pair, a TEM analysis is performed, solving Poisson's equation in the Fourier transform domain. Each strip is divided up into a finite number of substrips and the moment method with Galerkins constraint is applied. This analysis yields the modal microstrip line capacitances, from which the even and odd mode effective dielectric constants, propagation velocities and impedances are determined. A Fortran computer program has been developed to perform this.

To analyze the coupled pair as a circuit element, odd and even mode equivalent transmission line circuits are solved, yielding the parameters of transmission, coupling, directivity and isolation as a function of frequency. A Fortran program has been developed to do this.

Results have shown that the effect of placing dielectric overlay above the coupled pair was more significant in altering the odd mode components than the even mode components. As the overlay to substrate ratios of height and relative dielectric constants are increased, the modal velocities decrease, the effective dielectric constants increase and the impedances decrease.

Parameters of transmission, coupling, directivity and isolation were compared for typical microstrip couplers, with and without overlay. The change in transmission and coupling was found to be not as significant as the change in directivity and isolation. The directivity of the coupled circuit improved from -12dB to -23dB, and isolation improved from -25dB to -33dB.