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Re: [TowerTalk] Questions about radials

To: Tom Rauch <w8ji@contesting.com>,"Jim Idelson" <k1ir@designet.com>,"TowerTalk Post" <towertalk@contesting.com>
Subject: Re: [TowerTalk] Questions about radials
From: Jim Lux <jimlux@earthlink.net>
Date: Tue, 14 Sep 2004 11:18:47 -0700
List-post: <mailto:towertalk@contesting.com>
At 12:21 PM 9/14/2004 -0400, Tom Rauch wrote:


> The March/April 2004 issue of NCJ has a great article by
Al Christman, K3LC, on
> optimizing the number and length of radials given the
amount of wire available
> and the type of soil in which the radial field will be
placed. I have a few
> questions.

Hi Jim,

I don't want to rain on anyone's parade, but absolutely the
least accurate way to use models is to use them to predict
losses with wires near or in the soil. I equate using a
model to predict the number and length of radials or the
performance of a low dipole something about as useful as
using dial calipers to measure rubber bands.

I don't know if I'd say totally useless, but it would probably take more time and effort to do a decent evaluation with modeling than with just going out and doing it. If you were looking for a project while snowed in with lots of time on your hands, it might not be too bad. You'd have to use NEC4, and probably that might not even been appropriate for some of the reasons Tom mentions below.



There are a large number of reasons for this:

First, you'd never have any idea what your average ground
characteristics are. There isn't even a practical way to
measure localized characteristics that has any accuracy.

The Hagn Open Wire Line (OWL) approach can measure ground charactistics at a point (well, on the scale of a meter or so). There's some work using small loops to do the same. Measuring the "fine scale" variation of soil EM properties is widely used in the archaeology and prospecting areas, as well in toxic site remediation. This is the kind of measurement that is well within the typical amateur radio capability, and, to boot, you can make your own measurement equipment and get measurements just as good as the commercial products at substantially reduced cost (in dollars... you're going to spend MUCH time)




Second, virtually all installations are not over homogeneous
media. Not only does moisture vary, the layers of soil are
stratified. What are you going to do, design each radial for
optimum placement and length??

This is actually where modeling could help, in the form of a sensitivity analysis (something that the various QST articles haven't done much of). You could run models that span the range of soil conditions you expect (and/or measure), and optimize the situation accordingly.



Third, the amount of change in performance and the possible
savings are so small it isn't even worth talking about.

This is probably the best argument for the: "Just get the spool of wire, lay the radials until you run out of wire and stop worrying about it" approach.



Fourth, the models have never been verified.

I would agree with the models featured in QST. More sophisticated models such as those in NEC-4 or, more particularly, in the work derived from J.R.Wait's work have had extensive validation. One would need to do some serious analysis to establish whether the amateur antenna radial situation is within the "region of validity" of the modeling code. (none too trivial a challenge in itself, by the way, given the inhomogenous nature of most amateur installation (except maybe Rick Karlquist's array in the San Joaquin Valley, where he probably comes pretty darn close to a homogenous ground)



Jim, W6RMK


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