Numerical Computation of the Reradiation from Power Lines at MF Frequencies

Abstract

The encroachment of civilization on the site of a medium wave broadcasting array can give rise to distortions of its radiation pattern resulting in inadequate coverage of the service area, or even in the violation of licensing provisions. The radiation pattern distorts because RF currents are induced by the broadcasting antenna's field on any large metallic structures such as a highrise building or a high-tension power line. If the structure is resonant, these currents can be large and the reradiated field can be strong. This paper uses modern digital computer techniques to calculate the field reradiated from a power line near an AM broadcasting tower. The current distribution on the towers and skywires for a uniform power line is shown to exhibit distinct off-resonance and behavior. The frequency dependence of the reradiated field is found to be simple loop resonance for a uniformly spaced power line, and so is a super-position of the resonant behavior of loops of many sizes for a complex site. The computer model is applied to analyze the azimuth pattern for a complex site near Hornby, Ontario where two power lines come close to an omnidirectional antenna. The computations are compared with scale model measurements. Since a perfectly conducting ground was used in the computations, the method may help to estimate the upper bound on the distortions of the azimuth pattern which could arise from the construction of a proposed power line near an existing broadcasting antenna.