Re: NEC-LIST: Undesired mutual coupling

Dante JS Conti,

The referenced paper [1] has nothing to do with the subject of mutual
coupling and guidelines to perform NEC modelling of these effects --- but
has to do with how one can simulate ground connection so that NEC-2 can be
used to model antennas and "grounded" structures that either support the
antenna, or are in the near field.

We have been using NEC to model re-radiation problems for many years, since
the aearly 80s. For example, the subject of distortion of MF broadcast
antenna patterns due to re-radiation is discussed in my overview on VLF, LF
and MF antennas [2], published in 1983.

I have also extensively looked at mutual coupling effects for the case of
HF wire antennas on ships (unpublished); and for the amateur in radio, I
have extensively looked at the effect of the suporting tower (with and
without a yagi antenna on top) on the pattern of full wave and half-wave
ground plane type loops. This paper will appear in a forthcoming issue of
QEX, a publication by the ARRL, Newington, CT.

There is no trick to doing what I have I have done --- that is why I began
my initial response by yeh-gods --- since for me this was obvious. Just
include all conducting towers or unused antennas that are in the near field
of the antenna being modelled in your model. If you include an unused
antenna in the model, you will have to terminate it in a suitable
impedance, since the real antenna will have a transmission line connected.

An interesting comment: Suppose a tower supports an antenna, and the tower
happens to be nearly resonant at the frequency for the antenna. If you
"tune" the antenna for resonance by adjusting its size, you are in effect
also "tuning" the tower. You can see this effect using NEC by changing the
source from the antenna to the base of the tower. If the coupling is very
strong, you will find the tower is also resonant/or more nearly resonant
than it was by itself, because you resonated the antenna in the presence of
the tower. You can also find that the so-called isolated tower (i.e. no
electrical connection to the antenna) can carry more current then the
antenna!!

I do not know which version of NEC you are using. I use EZNEC pro,
available with a NEC-2 or NEC-4 engine (if you are authorized to use NEC-4)
from Roy Lewallen, W7EL, PO Box 6658, Beaverton, OR 97007, USA. This menu
driven program is easy to use, and easy to make changes to the modelled
antenna and the support structure or nearby conductors; and see how this
changes the radiation pattern and the currents on the structures. You can
also add guys to the tower, and see the induced currents on the guys. An
optional automatic wire-grid feature also makes easy modelling simple
solid conducting surfaces or structures.

Regards, Jack, VE2CV

1. Belrose, J.S., "Vertical Monopoles with Elevated Radials: An Up-date",
IEE International Conference on Antennas and Propagation, IEE Conference
Publicatyion No. 436, 14-17 April 1997, pp. 1.190- 1.195.

2. Belrose, J.S., "VLF, LF and MF antennas" in The Handbook of Antenna
Design (editors Rudge, Milne, Olver and Knight), Peter Peregrinus Lts.,
London on behalf of the IEE, 1983, pp. 615-624.

******************************************************************************
>Dear Mr. Belrose,
>
>Very interested about the subject of mutual coupling and guidelines to
>perform NEC modelling of these effects.
>
>May I ask if you can give me directions to have more info, as p.e.
>Conference
> Publication No. 436, pp. 1.190 to1.195, April 1997 as you mentioned in
>this reply mail?
>
>Thanks in advance,
>Dante JS Conti
>phone: + 019 2497328
>fax: + 019 2473545
>transtel_at_embratel.net.br

John S. (Jack) Belrose, PhD Cantab, VE2CV
Director, Radio Sciences
PO Box 11490 Stn. H
OTTAWA ON K2H 8S2
CANADA
TEL 613-998-2308
FAX 613-998-4077
Received on Tue Jun 17 1997 - 16:41:00 EDT