[GCARC] Measuring the efficiency of radials

[Kenneth Bozarth]

What I do is not worry about a remote receiver's signal such as WSPR, as
propagation and polarization may be greatly affecting the transmission to
that site, but rather to measure impedance at the base of the antenna. This
could be in the design stage, where you are trying to optimize performance
of a permanent antenna. For a completed design that can't be changed
(portable antenna that's quickly deployed), you can roughly calculate the
efficiency. If we're talking about a vertical such as 1/4-wave, theoretical
radiation resistance is about 35 ohms or so. If your SWR is 1:1 (base
impedance of 50 ohms resistive, looks great on the meter), then you have
ground losses of about 15 ohms, not bad at about 70% efficiency (35/50). It
takes more than 4 radials to reach this value in my experience. If your SWR
is not 1:1, then you need to measure with an impedance meter or good
analyzer to see if impedance is high or low. With a very poor ground, you
may have a 2:1 at 100 ohms, which means that you have about 65 ohms of
ground loss, or about 35% efficiency.

Note that even with 50% efficiency, that's only 3dB loss, or half of an
S-unit.

Now, if you want to improve things, keep adding radials until SWR is 1:1,
then add more and you'll see it get higher again, reaching about 1.4 to
1.5. This usually takes many radials to reach. Also, more short radials,
even if shorter than 1/4-wave is better than fewer longer radials. Try to
saturate the ground with lots of short radials out to about 1/8-wave.
Making them 1/4-wave long won't help much in terms of having them resonant,
as the ground will destroy the resonance. When SWR stops rising, you've
reached the point of diminishing returns. Then if you want a perfect match,
you can add a matching network at the base. At home on a 40m 1/4-wave, I
reached this point with about 24 radials. It depends on your ground
conditions under the radials. You will read that professional radio
stations use upwards of 100 radials, to get that last few percent of
efficiency out of the antenna.

As an aside, a set of only four raised radials, if you can manage to get
them and the feedpoint about 8' off ground or higher is extremely efficient
but they have to be resonant, or 1/4-wave long.

Sorry if this was too much info.
Ken

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On Tue, Jun 29, 2021 at 3:40 PM Christopher Wawak <kc2ieb at wawak.org> wrote:

> How do you measure the effectiveness of your radial system? Do you use
> WSPR, and account for changes in propagation during time of day? Do you
> have a specific net you try to contact? Do you look at physical or
> electrical properties of the antenna system?
>
> Just doing some idle thinking this afternoon...
>
> 73 Chris KC2IEB
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