Re: NEC-LIST: Undesired mutual coupling

Neil,

Yeh Gods yes!! That is the value of the program. Some antennas can be
analytically analyzed, but only numerical modelling (e.g. by NEC) will give
you the performance of antennas in their operating environment.

I have a 2-part paper to be published in QEX addressing the issue of GP
loops and full wave loops supported by towers, and towers with a yagi on
top --- discussing currents induced in the support structure, and the
effect on pattern --- which can be significant --- even dominating the
radiation pattern.

But recall that NEC-2 cannot have wires connecting to ground. To simulate
a grounded structure, grounded by earth rods, use 4-resonant elevated
radials --- for HF frequencies I use radials at a height of 5 mm --- but
before you connect them use NEC-2 to determine the resonant length. If you
want to simulate a *better* connection to ground, use more radials, or lift
the radials off the ground --- for HF a radial height of 1 metre will do.
Four resonant radials will give you closely the same gain as 120-radials
burried.

I discuss the subject of elevated radials in detail in the IEE Tenth
International Conference on Antennas and Propagation, Conference
Publication No. 436, pp. 1.190 to1.195, April 1997.

Jack, VE2CV

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>We would like to use NEC2 to show how some structure alters the antenna
>pattern of an antenna (for the sake of argument, a helix). If the
>structure is clearly in the near-field, will NEC2 yield a composite
>pattern with accuracy? The external structure can be a wire, so as to
>keep things simple. Thanks in advance.
>
>Neil

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 Fri May 09 1997 - 21:28:00 EDT