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Re: [TowerTalk] radials for shunt fed tower

To: "Rick Bullon" <>, <>
Subject: Re: [TowerTalk] radials for shunt fed tower
From: "Jim Lux" <>
Date: Sun, 5 Dec 2004 16:03:00 -0800
List-post: <>
----- Original Message -----
From: "Rick Bullon" <>
To: <>
> =============================================================
> The question I have about this is how do I tell if my ground is excellent
> good or poor??
> If I just assume that I have poor ground conductivy here and use the max
> length for 160 that is about 16000 ft of wire.
> In another thread on here I believe the price of a 500ft spool of wire at
> the big box stores was about $14 a spool that would make the radial system
> cost about $450 dollars!!! and then there is all the time and work of
> putting down 100+ radials
> 73
> Rick
> kc5ajx

The short answer to your first question: "how do I tell.." is that you
can't, at least not easily. So you're left with some systems engineering
kinds of questions.

1) You could measure your antennas feedpoint impedance over a wide range of
frequencies and compare it to a very high quality EM model that properly
handles wires in a dielectric (i.e. NEC4 and its ilk).  Practically
speaking, you'd probably spend more time modeling than laying the wires, and
if you're buying 3 miles of wire, you can probably get it cheaper than from
Home Depot (if nothing else, you don't want insulated wire, and you probably
don't need AWG12).  On the other hand, maybe the ground is frozen right now,
and you've nothing better to do (or, at least more interesting) than to run
models, so the modeling time is essentially free.

2) You could make a system tradeoff and decide that $450 in wire and tens of
hours in labor might be better invested in buying a high power amplifier.
Say some simple model (like the graphs in the ARRL antenna book) say that
your antenna with the current grounding system might be 25% efficient, and
going the full boat with miles of wire might bring you up to 50%.   That's a
3dB improvement.  Say you're currently running 100W barefoot into the
antenna. You could go out and buy a 500W amplifier for the same general
magnitude of expense as the wire (and a LOT less labor), and get a 6dB
improvement. Sure you can argue about the accuracy of the model, and whether
it's really 25% or 30%, etc., but, when all is said and done, that's arguing
about tenths of a dB, and you can get twice the improvement by another
means..  If you're already running 1500W, of course, then you're stuck with
improving the ground. Even if you're at the 500-600W level now, the next
bump in power costs a bunch more.

3) You could do some sort of incremental improvement strategy.  Look for
some low frequency beacons or AM radio stations in your general vicinity.
They'll propagate by ground wave (during the day), and the EM properties of
the soil and your ground radials won't be much different between those
signals and the 160m band (esp for AM broadcast).  Measure the received
signal level of those stations (who cares if there's an impedance mismatch
or the antenna's not tuned for the signal, you're doing relative
measurements, what you're looking at is ground system losses, and those are
non-resonant (to a first order) ).  LF nav beacons (NDBs) are nice because
they're constant power and not much modulation (during the CW ID). AM
broadcast will vary due to the modulation, but with averaging it should

Add some radials, and see if there's a noticeable improvement.  If there is,
keep going.  If not, you've got enough, and you can quit now.

4) If your ground resistivity is high because it's dry, investing in
sprinklers and water might be a better strategy. In Las Vegas, of course,
this is probably going to incur the wrath of the water police, but in a
wetter climate, or one with cheap (free) water.  Turn on the sprinklers the
week before the topband contest and soak that soil down real well.  I
suppose there's also the possibility of sowing the soil with salt but that
has some other problems, although you could use fertilizer. Any ionic
compound will help reduce resistance.. and fertilizer in ton quantities is
pretty cheap ($10s of dollars/ton)).


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Stations", and lot's more.  Call Toll Free, 1-800-333-9041 with any questions 
and ask for Sherman, W2FLA.

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