[TowerTalk] Questions about radials

[Jim Lux]

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