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I have been publishing this technical article since January of 2013. For the past 38 years I have manufactured the Isotron Antennas. This also involved helping many with their very difficult installations. Some of the information will be from what we learned solve these problems. Some information will be right from a text book or credible article.
You are welcome to contact me at wd0eja@isotronantennas.com with question you may have.
PLEASE FORWARD THIS ARTICLE TO THOSE YOU FEEL WOULD BE INTERESTED.
THE AFFECTS OF GROUND IN THE REACTIVE NEAR FIELD FOR VERTICALS
Last article dealt with dipoles and the affects of the ground under the antenna. A vertical is affected more by the earth underneath
it. Why?
Using a 1/4 wave mono-band vertical as an example, you have only 1/2 of your antenna. Where as the dipole has both halves. With half
the antenna gone, the vertical by itself is not resonant on the band it is intended for. So what do you do?
Mount it on the ground and use the earth as the other half. However, can you picture a 1/2 wave dipole with one side of fine
copper wire and the other side made of dirt? The dirt will conduct, but not with near the conductivity as the copper. Apply this to your vertical. If it is mounted on the ground with no radials, the RF current is being conducted through dirt. At best it is a poor conductor. So, how can it be improved? Radials.
There are a variety of radial designs. For ground mounted verticals the most affective radial system is a metal mesh. How big do you
make it?
For a mesh, it does not necessarily need to be 1/2 wave out like a broadcast radial system. It will vary with installation, but a
much shorter radii will work. Laying down a mesh that fits your real estate will no doubt improve your vertical system. You will
want to concentrate on being sure the antenna is resonant in the band you are operating. This will be indicated by a minimum dip in
SWR as you scan the band. You most likely will find the minimum is not 1:1. This is due to the nature of a vertical where the
Radiation Resistance is below 50 ohms. This however, is a minor mis-match on the HF spectrum and can be matched with a tuner.
Wire radials can be done in a variety of ways. Lots of 1/4 wave radials on the ground or just below ground reduces ground losses
drastically.
If you are elevating the vertical, 4 radials do a good job. However, now you need enough height to reduce the ground losses
much like a dipole.
Do you need a full 1/4 wave radial? It would be nice, but not always practical.
How about 16 radials at a .1 wavelength with a full 1/4 wave vertical? Radials are spaced at 22.5 degrees. Power loss is
estimated at 3db. This is like 1/2 S unit from full efficiency. It will also come close to 52 ohms.
This information is to help get you started. Now you can look at your lone vertical, hold up your thumb and devise a ground system
that may improve your performance.
73,
Ralph WD0EJA
AUGUST 2018
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