ITU-R REPORT BS.1063 PDF

Introduction

Re-radiation

When the radiated field from a broadcast transmitting installation intercepts a metallic structure, currents are induced in that structure and it radiates a second field which adds vectorially to the original. This is known as re·radiation.

Re-radiating structures

Sound broadcasting at MF normally involves vertical radiators, usually insulated at the base, over an extensive conductive earth screen composed of many radial wires. Vertical metallic structures of appreciable height (about 15% or more of a wavelength) in the vicinity of such transmitting antennas may cause re-radiation problems. Typical structures include masts, towers, tall chimneys or heavy industry complexes. High-rise buildings may also be troublesome sources. However, the most common re-radiation hazard is high tension power transmission lines which use steel towers. It is normal on such lines to attach one or two "sky wires" to each tower, running parallel to and above the power conductors, so as to afford protection from lightning and to disperse any destructive currents present from either a lightning stroke or a fault involving the power conductors. The towers and sky wires, together with their images in the earth, form loops which tend to be one-wavelength resonant somewhere in the lower part of the MF band and two-wavelength resonant in the upper part. The power conductors themselves have little effect on the re-radiation.

Survey results

In Canada, about three-quarters of MF stations employ directional arrays.

A survey conducted in 1976 indicated that 28% of the respondents had suffered re-radiation problems and a further 25% were anticipating them. Steel-tower power lines were the chief offenders, with masts, tall buildings, smoke stacks and industrial complexes also mentioned. Costs of correction had varied from $1000 to $500 000 and in three instances the transmitter site had to be abandoned. One station anticipating problems for a new critical array required measures which involved the following: broadcast consultant - 5 days; station staff - 30 days; power utility engineers - 6 days; and riggers and linemen - 40 days [DOC, 1985].