----- Original Message -----
From: "Guy Olinger, K2AV" <olinger@bellsouth.net>
To: <towertalk@contesting.com>; "W2RU - Bud Hippisley"
<W2RU@frontiernet.net>
Cc: <antennaware@contesting.com>
Sent: Wednesday, December 15, 2004 7:59 PM
Subject: Re: [TowerTalk] Re: radials for shunt fed tower
>
> For Bud's issue I would lay a halfwave dipole for the frequency on the
> ground in question, using insulated wire with known insulation
> thickness and characteristics. Measure its resonance frequency and
> impedance. Trim the dipole until the resonance is at the desired
> frequency. Be sure to stay away from conductors in or above the ground
This is an approach that has been used a fair amount back in the 50's and
60's (obviously not with NEC4, but with analytical expressions for dipoles
on the boundary between half planes.. lots of pubs by J.R. Wait, etc.). It
has some problems because it's very, very sensitive to the spacing between
wire and boundary.
A better approach is the Open Wire Line technique developed by George Hagn.
Basically, it's two 1/2" diameter rods stuck into the ground 6" apart to
form a balanced transmission line. You do it with several lengths of rods.
There's actually a drawing in the ARRL Antenna Book (or maybe ON4UN's book).
The practical problem is that the Z is typically a few hundred ohms, so you
either need a really good transformer, or an impedance bridge that works
with decent accuracy at 200-300 ohms.
Hagn claims that if you make measurements at a couple or three frequencies,
that is sufficient to determine the properties at all frequencies, because
of some fundamental properties of the medium.
>
> Stick the trimmed length in a model with Norton-Sommerfeld ground
> calculations and insulation specification available (EZNEC v4 or ...)
> and adjust the ground characteristics until the model gives you the
> same Fres and Zres. Use those ground numbers in the model to decide
> whether the radial wire is worth it.
I've been experimenting with another technique where you use a multi turn
loop, with a tuning capacitor, and you tune it to resonance "far" away from
the ground) measure the Z, then put it on the ground and measure the Z. (or
in several orientations). There's fairly good analytical backup for it, but
I'm still working out all the details.
>
> I have sometimes also had to drop the level of the ground in the
> model, down from "zero" to match the measured characteristics. This
> may be an indication of the non-monolithic, or "partially transparent"
> RF ground structure that is sometimes discussed, giving the appearance
> of an RF ground "below ground."
>
This is because of the high sensitivity of the measurement to spacing
between wire and ground. Most "ground" also has a somewhat drier (or
wetter) layer on the surface. You really want a technique that uses more of
a bulk measurement (probes stuck a foot or two into the ground, or a fairly
large diameter loop), so you can sample the top meter or so (corresponding
to the skin depth).
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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