Tech-Talk
Part 13
Welcome to Part 13 of our discussion.
We've been looking at one of the very basic antennas -- the
1/2 wave horizontal dipole. This time we'll look at its cousin, the 1/4
wave vertical. You've no doubt seen a picture very similar to this
one:
It has been said, partly in jest, that a vertical antenna
radiates equally poorly in all directions. While it's true that a
typical vertical has an omnidirectional radiation pattern, I disagree
with the "poorly" characterization. In fact, in many cases a vertical
will outperform a horizontal antenna at the same height. We'll be
talking about that in a coming edition, but first we'll cover some
general ideas about vertical antennas.
At its simplest, a vertical antenna consists of a (nominal)
1/4 wavelength element with some type of ground beneath it. The
"ground" can consist of a series of radial wires; a solid metal surface;
water; or real earth. In many cases it can be a combination of some or
all of these. Why do we need a ground? Because we're essentially
taking our 1/2 wave horizontal dipole and standing it on end. The
ground makes up for the missing half of the dipole. So all we need to
do is drive a little ground stake, cut 1/4 wave of wire, hook up our
coax, and we're all set? Well, not exactly.
Because soil is conductive, an antenna can induce some
current to flow in it. That power is dissipated as heat in the soil.
It may make the worms happy, but it does nothing for your signal.
That's what we mean by ground loss. And minimizing those ground losses
means that our antenna becomes more efficient. We do that by putting
conductive material underneath the antenna. Salt water makes a great
ground plane. If you're fortunate enough to live on a salt water marsh
(preferably on top of a rotatable mountain) you've got it made. If not,
read on.
Classic antenna theory holds that you need 120 buried
radials, each ~~1/4 wavelength long, for a vertical antenna. That's
about a mile and a half of wire on 80M, 3/4 mile on 40M and 3/8 mile, or
about 2000 feet of wire on 20M. Have you priced copper wire lately?
Ouch, my wallet! Double Ouch, my back!! Well, it turns out that you
may not need all of that wire. Rudy Severns, N6LF, has done extensive
testing on ground systems for verticals. He summarized his results in
an article in the March, 2010 issue of QST. ARRL members can read it
on-line. It's actually a condensed version of a six-part series by Rudy
that was published in QEX magazine. You can order reprints of the
whole series from the League if you want all the details. (On a side
note -- QEX is the "Experimenters Journal", also published by the ARRL.
It comes out 6 times a year. The articles can be quite technical, and
are quite honestly way over my head in many cases. But it seems like
every issue has at least something I can understand and use. For about
24 bucks a year I think it's well worth subscribing to it.)
I'm going to grossly oversimplify things here, but in
essence: If you can elevate the radials, you need far fewer than if
they are buried. The first few wires make a great deal of difference.
After about 8, the benefits really start to tail off, and become almost
pointless after about 16. Another advantage comes into play if you can
slope the radials down from the feedpoint at about 45 degrees. The
impedance of a 1/4 wave vertical over a good horizontal ground plane is
about 36 Ohms, or about a 1.4:1 SWR. That's not bad at all, but by
sloping the wires you'll raise the impedance much closer to 50 Ohms.
Since the ends of the radial wires are high voltage points, you'll want
to keep them at least 8 feet or so off the ground to avoid contact with
people or animals. To figure the height of the feedpoint, divide the
length of the radial wires by the square root of 2 (1.414) and add the
desired height above ground. Using our example of 8 feet clearance
above ground, the feedpoint of a 40M vertical will be about (33/1.414) +
8, or just a bit more than 33 feet.
That's it for this month. Next time we'll talk some more about vertical antennas.
73 for now
John Bee, N1GNV
Quicksilver Radio Products
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