Build it Well
Rick Borken KØXB
køxb@arrl.net
You’d think after
being a ham radio operator for more than forty years, a person would know
something about antennas. I am not sure if that’s a good assumption or
not, but what is true is you have a lot of experience with antennas. You
can’t help it. Just like speakers in a top-of-the-line audio system, or
the lens in a good camera, the antenna is the weakest link in a ham radio
station. And it is also the area with the most opportunity to improve your
setup. Between all the different QTHs I’ve had and combinations of
equipment I have used since I was licensed in 1961, the single most interesting
and sometimes frustrating thing has been the antenna. Antennas for DX-ing on
the HF bands are my focus, but many of the same points carry over to the other
bands as well.
The selection, design and
installation of an antenna involve five steps:
1. Understand the physics
2. Be creative
3. Build it well
4. Use good quality materials, and
5. Be patient
Understand the Physics. When a beam antenna is out of the question, my
favorite antenna is some type of vertical. This is because vertical antennas
radiate most of the RF energy at low elevation angles and thus are effective
for long-distance communications. But the very first antenna I put up as a
newly licensed novice was a vertical, and it was probably the worst antenna I
ever had. It wasn’t the antenna’s fault, it way my fault. I
didn’t understand the physics.
Most vertical antennas used at
HF frequencies are ¼ wave or shorter in length and are fed at the base. They
rely upon a good electrical ground at the base of the antenna in order to
provide a “mirror” image which can be thought of as the second half
of a ½ wave dipole. A ground rod at the base of the antenna is important to
provide good lightening protection, and you should never put up a vertical
antenna without this. But it’s usually worthless as an electrical ground
for RF purposes. You need radials, lots of them. Radials are lengths of wire,
attached to the base of the antenna, that extend radially on the ground in all
directions. Any kind of wire will work, and the length is not critical as long
as it’s, say, 1/8 wave or more. Try for twenty, one-hundred is better.
Radials are only one
example, but if you don’t understand the basic theory behind a particular
type of antenna you will likely make mistakes. And don’t rely on the
manufacturer either. I have seen many ads for miracle antennas that simply
don’t make sense.
Be creative. Like many newly married couples, my wife and I first
lived in a small apartment, with no opportunity to put up a conventional
outside antenna. But I had a windowsill next to my operating position. Using a
flagpole mount, a section of thin PVC tubing, some wire and some scrap aluminum
stock, I built helically-wound vertical antennas for 15 and 20 meters. We also
had a small outside deck, with a metal railing. That railing loaded up quite
well on 15 meters. That was just luck, but you never know what will work if you
don’t try. Another time, in a basement apartment, I strung a wire ¼
wavelength long on 20 meters from outside the window up to a tree. It was an
effective, and almost invisible vertical. These antennas weren’t terrific,
but I was able to make contacts and I had fun.
Build it well. Take care when you build an antenna. Whether
it’s a commercial unit or purely homebrew, assemble it paying strict
attention to the dimensions. Make up your mind that all dimensions are
critical. If it says to set something at, say, 4 ¾ inches, measure it
precisely, tighten the coupling and re-measure. If it’s not perfect, do
it again. When you solder, solder cleanly and carefully and use enough heat to
flow the solder and wet the joint. Work at it until it looks exactly like the
pictures in the book. If you need to adjust the antenna in order to tune for a
low SWR, keep trying until you get the lowest possible SWR. Different
adjustments often interact, but keep at it until everything is right. If two
parts are supposed to be symmetric, make them as symmetric as possible. Use
anti-oxide compound on all joints, and weatherproof all connectors. A
well-built antenna works better. It really does! With luck, the antenna will be
up for a long time. A few extra hours at first are well worth it.
Use good quality
materials. I have a ground-mounted
vertical antenna that I use for 40 and 80 meters. It is mounted in the woods,
so I can only get to it easily at this time of year, once the leaves are off
the trees and before the snow flies. It had been loading-up poorly recently, so
I thought I better get out there and see what was going on. As I usually do, I
checked all the dimensions, tightened all the fittings, etc. But it still
wasn’t right. As I pried open the weatherseal on the coax connector which
was attached to the antenna, the connector came apart in my hand. All of a
sudden, I remembered I had been anxious to put the antenna up when we first
moved to this QTH, and I had purchased a length of coax cable with connectors
attached which never looked too good to me. There are certainly pre-assembled
lengths of coax with good-quality connectors, but this wasn’t one of
them. I should never have used it.
In addition to good-quality
connectors, be sure to use the best coax you can find and use stainless steel
hardware as much as possible. Mount it securely. If you think you may need
guys, put them in right away. If something looks like it’s made poorly,
it probably is. Don’t use it.
Be patient. I don’t have a scientific survey to back it
up, but I believe there is a correlation between when you first put up a new
antenna and poor propagation conditions. After I got my 40 and 80 meter vertical
put back together and tuned-up, I was anxious to check it out on the air. That
was the day of the big solar “mega flare.” There was basically a
complete radio blackout, and I couldn’t have worked anyone on 40 or 80
meters if I had a full sized Yagi. But the bands came back to normal, and the
antenna works better than ever before. To truly evaluate how well a new antenna
works, use it for at least several months, maybe a year. Propagation conditions
vary a lot and can easily fool you into thinking a good antenna is poor or vice
versa.
There are many good references
about antennas. Some of the books I use all the time are: The ARRL Handbook, The ARRL
Antenna Book, and Antennas and
Techniques for Low-band DXing.
Good luck, and go out and
work some new ones!
Copyright 2003. All rights reserved.