Re: NEC-LIST: How to get actual far field value??

Ed Troy wrote:
> ...if I use a 60" whip at 30 MHz, I would suspect that the field
> strength would be at least 10 or 20 dB lower than if I fed a quarter
> wave vertical with a 50 ohm source, but I don't get that with NEC.

You may be modeling the _antenna_ OK with NEC, but not (1) the feed
and matching system; and (2) the ground. A good quantitative article
about the practical problems of getting good efficiency from a short
vertical whip antenna mounted on the rear bumper of a car appeared
either in last month's (September) QST or in the latest (4th?) edition
of the ARRL's Antenna Compendium -- I forget which, but someone else
reading this List may recall.

In your modeling, include a realistic model of the matching network
that you'd have to put between the transmitter and the feedpoint of
the antenna, to present, say, a 50-ohm resistive load to the
transmitter. This network will include a big inductor with a finite Q
-- which you can model in NEC by an L in series with an R. What
you'll find, of course, is that a lot of power is dissipated in the R.

A significant parasitic susceptance (i.e., capacitive admittance) mat
also appear in parallel with the feed terminal-pair of the antenna;
see the article for a discussion.

You must also account for ground loss. Use realistic ground
conductivity and permittivity (or dielectric constant) values in NEC.
Received on Thu Sep 25 1997 - 10:38:05 EDT