Aspects of UHF communications on overhead earth-wires in power transmission networks
The motivation for this research is a proposed UHF surface wave communication system in which the waveguides are the stranded, overhead earth wires of Power System transmission lines. Attention is confined largely to an investigation of certain aspects which affect the overall surfaces wave transmission loss, a full-scale system having been set up in the laboratory for experimental purposes.
For the prediction of transmission loss the stranded conductor is assumed to be equivalent to a solid conductor of the same diameter but with surface anisotropy in the form of two mutually orthogonal surface impedances the major reactive component of which is attributed to the effects of the helical stranding. This reactance is determined from a consideration of the fields which are assumed to exist within the cavities between the strands, and externally. From a comparison between experimental and theoretical loss characteristics there is sufficient inducement to accept the anisotropic model of the stranded conductor for practical design purposes.
Approximate equations are developed to simplify the calculation of transmission loss and the notion of ‘capture cross-section’ is employed for the estimation of the efficiency of conical horn launchers. It is deduced from ‘sensitivity’ relationships that the horn loss is relatively insensitive to small changes in the fictitious surface reactance representing the effects of helical stranding, which tends to justify the assumptions upon which the anisotropic model is based. On the other hand, variations in the helix angle are shown to have a marked effect upon the calculated horn loss. This influences the choice of the stranded conductor used as the waveguide for the experimental verification of the model.
The Author’s experimental research is described at length, the principal objective being to establish the anisotropic model as an acceptable theoretical substitute for the stranded conductor. To reduce the horn loss, dielectric sheaths are ted to the waveguide in the vicinity of the horn apertures. The discrepancies which then appear between theory and experiment are attributed both to the scattering of the surface wave by the boundary discontinuities at the ends of the sheaths and to the anomalous behaviour of commercial-grade PVC dielectric. Considering the increase in the transmission efficiency which may be realised by fitting dielectric sheaths to the conductor near the horn apertures it is concluded that a theoretical investigation of the scattering properties of the discontinuities s in order.
Thus, the remainder of the Thesis is devoted, to this scattering effect as it may be encountered in the proposed scheme, the theoretical analysis following the lines of earlier documented research. A short-cut method is applied for the determination of certain ‘half-plane’ functions which appear in the expressions for the scattered power. Theoretical results are presented together with a discussion of some experimental measurements and a brief theoretical examination of the effects on the horn loss of varying the thickness of the dielectric sheaths. It is argued that the horn loss may be reduced if the dielectric thickness is graded in steps to a value at the horn apertures consistent with the desired ‘power capture’.
The Thesis is concluded with an Addendum which outlines a number of topics suggested by the Author for future research.