Early History in using NEC to study Reradiation Problems

Early History in using NEC to study Reradiation Problems
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This is further to my e-mail late Friday afternoon, concerning the use of
NEC to study re-radiation effects of support towers on the radiation
patterns for wire antennas, and other metal conductors in the near field of
antennas. It seems that my note might leave the impression that we have
just recently discovered this ourselves, since I referred to a study about
to be published.

In fact we have been using NEC to study re-radiation problems for more than
two decades. Our early experience was concerned with re-radiation from EHV
power lines and high rise buildings on the directional patterns of MF
broadcast antennas. Such structures can change the antenna's pattern
significantly, not just the directional pattern of critical arrays (null
filling for example), but even the pattern of omni-direction monopoles,
since re-radiating structures can be resonant in the MF broadcast band.

EHV power lines consist of a long series of fairly well grounded towers
joined by a skywire for lightning purposes. It just so happens that the
height of the towers, the length of the connecting skywire, the height of
an adjacent tower, and the image of this half-loop in the ground, can be
2-wavelength resonant in the middle of the MF broadcast band.

High rise building are also effective re-radiators. Why? If you look at a
building you see lots of concrete, bricks, windows, etc. But a high rise
building is in fact a wire grid structure, formed by the ground wire of the
electrical wiring system --- and high rise buildings can be resonant in the
middle of the MF broadcast band.

Early work on this subject (experimental modelling, numerical modelling
using NEC and measurements in full scale) is overviewed in my chapter in
The (IEE) Handbook of Antenna Design, Volume 2 (editors Rudge, Milne, Olver
and Knight), 1983, pp. 615-624. Since then much work has been published by
my colleagues on this subject.

Our paper on the effect of supporting structures on HF wire antennas (QST
December 1982, Feedback July 1983) is perhaps the first such study, showing
clearly by experimental modelling and numerical modelling using NEC the
importance of these effects.

John S. (Jack) Belrose, VE2CV

John S. (Jack) Belrose, VE2CV
Director, Radio Sciences
PO Box 11490 Stn. H
OTTAWA ON K2H 8S2
CANADA
TEL 613-998-2308
FAX 613-998-4077
Received on Wed May 14 1997 - 09:26:10 EDT