The effects of re-radiation from high-rise buildings and transmission lines upon the radiation pattern of MF broadcasting antenna arrays

Results of experimental measurements employing antenna modeling techniques on the re-radiation from model high rise buildings and power lines, are presented. It is found that high rise buildings are resonant in the middle of the MF broadcast band and that buildings located ten or more wavelengths (3 kilometers) from the MF broadcasting array could affect the far field pattern. It is shown that high voltage power transmission line towers, the connecting sky wires, and their image in the ground form loops which are resonant within the braodcast band. A long power line is a system of over coupled, staggered tuned loops, and if the resonant frequency of the power line is that of a nearby broadcast station, the effects of re-radiation on the pattern of the MF broadcast antenna is large. Numerical modeling by the method of moments is in agreement with experiment although it is difficult to calculate the re-radiation from extended structures like power lines. The experimental and numerical modeling represent worst case situations, since the 'earth' is perfectly conducting and the structures employed are either insulated from or connected to it.