Yes, the literature about radials can be confusing unless you keep in
mind an important distinction. In the case of an elevated 1/4 wave
vertical, the main purpose of radials is to make up the other 1/4 wavelength
so the antenna becomes like a 1/2 wave vertical dipole. The radials make
the antenna resonate and gives the feedpoint something to work against. So,
if you ignore the losses you can use full length elements for the vertical
and the radials, or use loaded shortened ones, like hamsticks.
When you use a 1/2 wave vertical against the ground, however, the ground
forms the other half of the dipole, but not because it is resonant, but
because it is for all practical purposes an infinite ground plane. In this
case the main purpose for radials is to increase the ground conductivity.
As such, consideration for the size and number of radials in the elevated
vertical case is very different than for the ground mounted vertical case.
In the ground mounted case, it's the amount of covereage of the wire in
the ground in the near field of the antenna that makes the difference. So,
where hamsticks are practical (although lossy) radials in an elevated
vertical, they would add no value to a ground mounted vertical. Since its
surface area coverage you need, the more long wires going out from the base
of the antenna the better. How many and how long is determined by some
simple rules of thumb and usually limited by what you think is a good cost
(or hassle) for a given benefit. Typically 20 to 100+ radials that vary in
length from 1/8 to 1/4 wavelength is the range most hams work in. By
measuring the feedpoint resistance of the antenna as you add radials, you
can watch it drop from 60+ (full 1/4 wavelength vertical) ohms with no
radials towards the ideal of around 32 ohms. However, the curve shows
diminishing returns, so at some point you see that the next added set of
radials produces a small enough gain so that you declare it Miller Time.
The lengths of each radial is very non-critical so if some have to be
shorter than others due to real estate considerations, its perfectly ok.
One caveat, though. The scenario above, of bringing 60 ohms down to 32
ohms applies to a vertical element that is physically close to 1/4 wave. In
that case, the approx. 32 radiation resistance of the vertical element
radiates half the power and the rest goes into heating up the 30+ ohms of
ground resistance. As you add radials, you decrease the ground resistance
to a few ohms so that a small percentage of the energy goes into the ground
and the rest goes into space. However, a shortened loaded vertical like a
hamstick has a very low radiation resistance in the area of a few ohms or
so. That means that you have to have a ground system whose resistance is in
the tenths of ohms to get good efficiency. Now you are talking about a very
ambitious project that is much more work than putting up a longer vertical
I would recommend against the hamstick vertical on the ground approach,
unless you feel you are very limited in what you can deploy. On the other
hand, I had a lot of fun doing 40m CW mobile using a hamstick on a long
commute for a number of years. A good mobile hamstick installation is about
10 - 15% efficient, but is sure a lot of fun. Anyway if that is your
intent, you might want to rethink it. Regardless of the answer, the one
thing I can say for sure is that hamsticks would add no value as on-ground
or underground radials.
Dudley - WA1X
Date: Fri, 22 Oct 2004 01:30:49 -0500
From: "Scott Fike" <firstname.lastname@example.org>
Subject: [TowerTalk] Ground radials- the long and short of it
Content-Type: text/plain; format=flowed
Pertaining to a ground-mounted vertical antenna with buried radials a few
inches below the surface of the grass:
Do ground radials have to be electrically the same length as the main
vertical radiator element?
Example: Take a 1/4 wave 80 meter hamstick stuck vertically in the ground.
An 80 meter hamstick is physically about 6' tall but has about 60' of wire
wound hellicaly around its 6' core insulator rod. Would it be better to make
my ground radials using hamsticks (with their 60' of wire but physically
still 6' long overall) or can I use 6' pieces of wire to match the hamsticks
physical overall length or should one string out 60' radials?
Some books I've read say: ".... make the ground radials 1/4 wavelength
long to match the main 1/4 wavelength long vertical element". Other books
and sources say: "..... just make the ground radials as long as you can to
fit in the availible space in your yard".
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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