A HIGH-PERFORMANCE 1-WIRE DX ANTENNA
By Gary Huff, K9AUB
We all would love to have a very
high tower with an elaborate array of Yagi antennas to assist us in our pursuit
of DX. However, many of us have limited
funds, and we can’t afford such elaborate equipment. Indeed, some of us live in areas where towers
are prohibited. However, if you have a
tall tree or other high support in your yard, don’t think that you can’t work
DX with a simple wire antenna. You can
put that tree to work for you!
It is often said that the best
simple antenna for DX is a vertical antenna.
This is true, but often a vertical is disappointing because it really
doesn’t perform all that well. However, there
are verticals, and then there are VERTICALS!
Vertical antennas can be improved
upon by making them higher (longer), and by installing a decent field of ground
radials under the antenna.
The “standard” vertical for HF
work is a ¼ wave vertical. When placed
over a decent set of ground radials, it will perform well for DX work (over
1000 miles). It may not seem to be much
of an improvement on shorter DX paths, out to 2000 – 4000 miles. Beyond about 5000 miles, the ¼ wave vertical
does begin to outperform a dipole. By
the time you get out to 6000-8000 miles, a ¼ wave vertical has a noticeable
advantage over dipoles. Still, they
aren’t always the panacea for working DX that they are often described to be.
However, there are more verticals
than just a simple ¼ wave vertical. You
can also lengthen them to a height of ½ wave, or 5/8 wave! When you do this, their performance does
improve by a noticeable amount, because their angle of radiation is
lowered. (There isn’t a lot of
improvement to go beyond 5/8 wave.)
A ¼ wave vertical has
approximately a 30 degree angle of radiation, which is an improvement over a
dipole, because a dipole wastes so much radiation at higher angles. For short-range communications, a higher
angle of radiation is an advantage, which is why dipoles are superior for
short-range work. Still, a 30 degree angle
of radiation is not ideal for DX work.
To achieve maximum performance over a long distance, we need to lower
that angle of attack to an even lower angle.
Ideally, the best angle of attack is approximately 16 degrees. Can we achieve that with a simple vertical
antenna? Well, in a word, yes.
A ½ wave vertical has a lowered
angle of radiation of about 20 degrees, and displays slight gain over a ¼ wave
vertical. This is an improvement over a
¼ wave vertical! However, the ½ wave
vertical presents some significant problems with feeding it. The bottom of a ½ wave vertical is at a very
high impedance, high voltage point, and requires some carefully engineered
matching networks to feed it. Capacitors
and inductors must be tuned to match the high impedance, and they must have
high voltage ratings, since there are at least several thousand RF volts at the
end of a ½ wave vertical. This problem
can be overcome by feeding the ½ wave vertical at the center, where it becomes
simply a dipole antenna hung straight up and down. If you can run the feedline at a 90 degree
angle from the vertical wire, this may be a solution for you.
Can we improve on this vertical
antenna? Yes, we can! If we lengthen the vertical antenna to 5/8
wave in length, the angle of radiation lowers to an ideal 16 degrees, making it
perfect for DX performance! And, the
base of the 5/8 wave vertical can be fed with ordinary 52 ohm coax. There is a slight amount of capacitive
reactance at the base, and the “perfect” 5/8 wave vertical has a small amount
of inductance in series with the base of the antenna, to tune out this
reactance. Fortunately, this isn’t a
critical coil, and can be simply 6 or 8 turns of wire, with a diameter of about
2 “. If you demand a perfect 1:1 match,
you might want to wind a longer coil and then tap it 1 turn at a time until you
find the perfect inductance to exactly match your antenna. But, for practical applications, you can
simply feed the antenna with coaxial cable, and you’ll get good
performance. If you use an antenna
tuner, you’ll be just fine with this simple antenna.
Take a look at this graph, which
shows the gain and angle of radiation for each type of vertical:
This 5/8 wave antenna also
displays about 3 dB of gain over a ¼ wave vertical. So, you get an effective increase in radiated
power, AND it’s at the more ideal 16 degrees of radiation! The effective performance can often be an
actual improvement over a ¼ wave vertical by several S-units on long-distance
paths!
How long should the antenna
be? Well, here’s where it gets
interesting. The formula for a 5/8 wave
antenna is 585 / F (mHz). This compares
to the formula for a ¼ wave vertical, 234 / F (mHz), and the ½ wave vertical
formula is 468 / F (mHz).
Let’s see… if we install a ¼ wave
vertical for 40 meters CW, that’s approximately 33.4 feet. If we want to install a 5/8 wave antenna for
17 meters (18.1 mHz), that length is 32.3 feet.
WAIT A MINUTE…. Those numbers are VERY close! That means that a ¼ wave vertical for 40
meters can also be used unmodified for a 5/8 wave vertical for 17 meters! You get two for one! Nice!
But, we’re not finished. Let’s look at what else such an antenna can
do. We all know that a 40 meter antenna
can be used on its 3rd harmonic, or 15 meters. In actual practice, we usually discover that
a 40 meter antenna isn’t really ideal for 15 meters. For various reasons, the 40 meter antenna
resonates very high in the 15 meter band, up at the top of – or outside - the
phone end of the band.
Well, can we play with this
antenna a bit and make it a better performer?
Well, of course we can! Turns out
that by making the 40 meter vertical slightly long so that it resonates at the
very bottom of the band (7.000 mHz), the resonant spot on 15 meters drops down
to the middle of the 15 meter phone band, about 21.3 mHz. If we make the 40 meter vertical just
slightly longer, making it match about 6.985 mHz, we still have an almost
perfect 1:1 SWR on the very bottom of 40 meters CW, and that number rises to
about 1.7:1 SWR at 7.300 mHz, the top of the 40 meter phone band. Most modern transmitters with pi networks can
easily match this antenna across the entire 40 meter band. Or, we can use an antenna tuner. This would call for a length of about 33.5
feet.
A 33.5 foot vertical will have a
perfect 1:1 SWR on 15 meters with a resonance of close to 1:1 around 21.250
mHz. It becomes a 3/2 wave length
antenna, which means it will have a much lower angle of radiation,
approximately 18 degrees, and it will have about 3 dB of gain on 15
meters. This means it will be a “hot”
performer on 15 meters, and can be matched across the 15 meter band with ease.
Now, can we squeeze more
performance out of this 40 meter vertical?
Well, it’s only slightly longer than a 5/8 wave vertical for 17 meters,
which ideally needs a 32.3 foot length.
The difference is small enough that, again, this antenna can be matched
with ease on 17 meters!
So, summarizing, if we install a
¼ wave vertical for 40 meters, resonant at the bottom of the 40 meter CW band,
we end up also with an excellent performing 5/8 wave vertical for 17 meters,
AND a very decent performing ¾ wave vertical on 15 meters. 1 antenna, 3 bands! And all it takes is one single vertical
radiator of about 33’4” in height.
What shall we construct this
vertical radiator from? Well, we can
install aluminum or steel tubing, install it over a base insulator (a champagne
bottle works nicely here!), and add suitable guy wires. That’s what you’ll need if you have no trees
on your property. But, if we have any
mature trees on our property, then we almost certainly have a limb at least 34
feet or higher from the ground. With a
sling shot or casting rod, a rope can be shot over one of these tall limbs, and
an ordinary piece of wire can be pulled up to vertical position. A vertical wire will work just as well as a
length of tubing. Install an insulator
at each end, and a short ground stake or ground anchor at the base to hold the
bottom of the wire in place, and you’ve got your vertical radiator.
Now, the next important part of a
vertical antenna is ground radials. The
books try to intimidate you into thinking that you need a ground field of 120
radials, or at least 60 radials if you want the antenna to work at all. But is that true? What if you only have room for 8 or 10
radials? Will the antenna still
perform? In a word, yes. The difference in performance between a vertical
antenna with only 4 ground-mounted radials will be improved by about 1 S-unit
if you proceed with the full 120 radials.
Hardly worth the effort for all but the most heroic installations. In real life, 8 or 16 radials is perfectly
adequate. The difference between 16
radials and 120 radials is only a fraction of an S-unit.
How long should the radials
be? Well, ¼ wave on the lowest frequency
you plan to use the antenna on is adequate, about 33 feet each. These radials should be made of copper wire,
if possible, but you can use galvanized steel (electric fence wire) if you’re
on a budget. The heavier the wire, the
longer it will stand up to the elements.
Thin steel wire will rust out and be gone in a few years. Thick copper wire (10 AWG) will last for many
years. Let your budget be your guide.
Whatever you use, the radials
should all be connected at the base of the antenna to a ground ring, which can
be as simple as a loop of heavy copper wire encircling the base of the
antenna. Solder each radial to the
loop. Use a heavy soldering gun to make
the connection.
Is the length of the radials
critical? Well, actually not. If your property line prohibits running a
full ¼ wave radial out in each direction, don’t worry if you can only achieve
part of this length. Or, you can run the
radial out to your property line, and then bend it to complete the length of
the run.
Can the radials be longer than ¼
wave? Yes, they can. In fact, longer radials tend to give improved
performance. Feel free to run your radials
out to the limits of your property line.
A 50 foot radial works better than a 33 foot radial, and a 100 foot
radial works even better. The reason
that is true is that the radial is forming a capacitive coupling to the earth,
and the longer wire improves that coupling out to a longer distance around the
base of the antenna.
The radials may be buried a few inches
below the surface, to keep them out of the reach of the lawn mower. However, if you lay the radials out in late
Fall, the snows and rain will bury your radials for you. By Spring, they will have sunk slightly below
the surface of the ground, and they will be safe. Inspect the radials in the Spring: if there are any high spots, go ahead and
push them slightly into the ground while it’s still soft and muddy. After a couple of years, you’ll never know
the radials are there.
What kind of wire can we use for
the vertical? It can be anything you
have on hand, but for the vertical portion, it should be strong enough to stand
up to Illinois ice, wind and snow storms.
Use a quality wire if you can, 14 gauge copper clad steel is
perfect. But, you can also use 12 or 14
gauge ordinary single-conductor house wire.
Don’t use 16 gauge hookup wire; it’s not sturdy enough to last more than
about 1 season. If it’s 16 gauge
copperweld, that will work fine.
Can the wire be insulated? Yes, but be aware that there is a slight
Velocity Factor to insulated wire, which will make your antenna be very
slightly shorter than bare copper wire.
There is no deterioration in performance with insulated wire, but you’ll
probably have to trim your vertical radiator slightly to bring the antenna to
resonance.
For the radials, use any wire you
have on hand. It can be bare or
insulated. For a cheap source of wire
for radials, check with a contractor friend who is remodeling an old house or
store. Often, they have a lot of wire
they’ve pulled out of the building, and if it’s not long enough, simply solder
it end to end and use it for your radials.
If all else fails, buy some 14 or 16 gauge ignition wire from a farm
store, where it is sold in 100 foot rolls very cheaply.
What if your tree limb is just
slightly too low, and you can’t stretch out the antenna as you’d like? Well, you could add a loop of wire at the top
of the antenna, which acts as a capacitive hat.
The larger the loop, the shorter the physical length of the antenna can
be. You’ll need to trim your antenna to
its final length. It’s wise to start out
with your vertical radiator being slightly long, and then trim it slightly to
bring it to resonance. After you’ve got
your length worked out, then solder everything in place.
Remember, the 585 / F (mHz) works
for all other bands as well. If you
wanted a very high performance 40 meter vertical, you COULD erect a 5/8 wave
vertical on 40 meters. However, be aware
that the length would be 585 / 7 = 83.5 feet.
That’s a pretty heroic vertical radiator in anyone’s book, but if you
really want a super performing 40 meter vertical, you might investigate whether
this would be a possibility for you.
And, if you can’t get all 83.5 feet to stand vertically, feel free to run
it as an Inverted L, with as much of the wire as possible running vertically,
then bend over the top and run the remaining wire over to another tree. 20 meters becomes much more practical: 41.7 feet is a practical length of wire if
you have a tall tree with a high limb. A
5/8 wave vertical on 20 meters will perform very similar to a 2-element
beam. For 10 meters, you would only need
a vertical of 20.8 feet.
Give the 5/8 wave vertical a try
if you want to work DX, but you just can’t afford a tower and beam. You’ll be very pleasantly surprised at its
performance.
