An investigation into some fundamental properties of strip transmission lines with the aid of an electrolytic tank

Abstract

The paper describes a method of deriving numerical values for quantities such as characteristic impedance, attenuation and wavelength with the aid of an electrolytic tank. Numerical results are given for two principal forms of strip transmission line for a range of dimensions for which rigorous solutions by direct analysis are not readily available. This range is nevertheless of considerable practical interest as it encompasses characteristic impedances between 20 and 150 ohms. The recent findings of Black and Higgins in regard to the inaccuracy of the formulae due to Maxwell and Palmer for the strip-above-ground system are confirmed. A new and simple method is derived for calculating approximate values for the pseudo-TEM impedance, wavelength and attenuation of a non-homogeneous dielectric-slab-supported strip-above-ground system (e.g. Microstrip). The numerical results obtained by this method, which depends only on a knowledge of the values for air-spaced lines, are in good agreement with measured results. As a result, it appears that the dominant mode propagating in a Microstrip line is more closely TEM in character than was hitherto supposed. In theory, however there must exist a longitudinal component of the electromagnetic field if the boundary conditions are to be satisfied. Measurements on the electrolytic tank of coupling between adjacent conductors and of field spread are also described. From the analysis as a whole it would appear that the tri-plate (sandwich line) has certain theoretical advantages over the strip-above-ground system. However, these are likely to be offset by a number of important practical disadvantages.