Chapter 2: Shortened Verticals

Most of us don’t have the luxury of building a 5/8 wavelength
vertical antenna. We have to settle for something a little shorter.
(A lot shorter, in the case of people following the
FCC’s Part 15 rules, which limit them to 3 meters in size.) Shorter

Most of us don’t have the luxury of building a 5/8 wavelength
vertical antenna. We have to settle for something a little shorter.
(A lot shorter, in the case of people following the
FCC’s Part 15 rules, which limit them to 3 meters in size.) Shorter
vertical antennas can give acceptable (not spectacular) performance.

frequency selection

If you are using a shortened vertical antenna,
you will get much better results using a frequency on the high
end of the band (above 1500 kHz). For any given combination
of transmitter power and vertical antenna size, your range at 1600
kHz could be up to 10 times greater than your range at 530
kHz.

components of a shortened vertical

As illustrated here, shortened verticals
usually consist of a capacitance hat (C), a loading coil (L), a vertical
radiator (V) standing on an insulator, and a ground system (G). There are many
different ways in which these elements can be built and connected.

construction

ground system

There are two basic choices: a set of ground radials, or a square
ground plane (also called a "ground screen").

square ground plane

A conductive metal screen such as chicken wire is
suspended above the earth or laid on the ground to make a big
counterpoise (artificial ground-plane) for the antenna system. It’s
not pretty but it reportedly works well. Several sections of material
are soldered or welded together to make a large square patch of
conductor, and the vertical radiator is suspended above the middle.

How large should the counterpoise be? Generally, larger is
better. However, there is a point of diminishing returns, and
any ground screen extending beyond a 1/4-wavelength radius from the
base of the antenna will only make a very subtle improvement in
field strength.

ground radials

How many radial wires do you need and how long should they be?
Should they be buried in the ground, laid on the surface of the
ground, elevated a few inches above the ground, or elevated several
meters above the ground? These are the questions that have plagued
users of shortened verticals since the dawn of radio.

There have been many arguments about these questions. It is
important to keep in mind that many of the opinions you will hear
are based on less than perfect information, such as unverified
computer models, experiments done with antennas in different
locations where differences in soil conductivity might have
affected the results, and so forth. A lot of people hold very
strong opinions without a shred of scientific evidence to back up
their beliefs.

Number of radials: More is better, up to a point. In carefully
controlled experiments, it has been proved that increasing the number
of radials from 2 to 15, or from 4 to 16, produces significant increases
in signal strength. Further increasing the number of radials to 60
only produces 1 to 2 dB of increase in field strength.
Follow this link to see some of
the empirical data.

Where to put the radials: For a semi-permanent installation, it is
customary to bury the radials a few inches down in the soil. This makes it
much easier to mow and walk in the area around the antenna. However,
some experimenters have gotten an improvement in performance by raising
the radials and the antenna base a few inches above the soil. Raising
the antenna and ground system several meters above the earth, for
example by installing the base of the antenna on a roof-top, can
improve the antenna’s performance by reducing capacitive earth losses.

Length of radials: In most cases, radials work best when they
are 1/4-wavelength or longer. Considering the large size of a
wavelength at medium wave frequencies, this is not practical for
most experimenters. There are (at least) three ways to cope with
not having enough space for quarter-wave radials:

Some experimenters have installed loading coils in their
radials to increase their electrical length, or have used
helically wound radials.

If the radials have to be relatively short, try connecting
their far ends to ground rods hammered into the soil. Some
experimenters report good results with this technique. However,
some antenna theorists say it’s a bad idea. Take a set of field
strength readings without the ground rods in place, then install
them, re-tune the system, and take another set of readings
quick before environmental conditions change. Then
you can draw your own conclusion.

If the radials are going to be just a few inches above or
below the soil, you can use a combination of radials and a square
ground screen made of chicken wire or similar material. Providing
a low resistance ground in the area closest to the base of the
radiator is most important.

NEC, the famous computer program for modelling antennas, indicates
that 1/4-wavelength is not always the best length for radials if the
vertical radiator is much shorter than 1/4-wave. If the radiator is
1/16-wavelength tall, and if the radials and antenna base are suspended
20 feet above soil of average conductivity, a radial length of 0.15 to
0.18 wavelength would produce peak radiation efficiency, according to
one computer simulation I’ve seen. This is interesting but I won’t
put too much faith in it until people with the proper test equipment
verify it with controlled experiments.

Materials: Bare copper wire is the traditional material for
making radials. Don’t use wire smaller than 18 gauge; you want to keep
resistive losses to a minimum. Some people say it’s okay to use cheap
insulated wire to make the radials, but I think the insulation could
add some capacitive losses to the system.

base insulator

Rubber pads, fiberglass and plexiglass rods, and assorted bits of
plastic have been used successfully as base insulators. High power
broadcasters use porcelain insulators. Don’t use materials like
wood and concrete that can absorb moisture.

vertical radiator

Nearly any low-resistance conductor can be used as the vertical
radiator. Copper wire, copper pipe, and aluminum TV antenna mast
are often used. Larger diameter is better for several technical
reasons. Several pieces of copper wire, connected together at the bottom and then
running parallel up the sides of a large-diameter PVC pipe, could be worth a try.

capacitance hat

The capacitance hat can be a large metal disk, or a small
metal disk with spokes of stiff copper wire radiating out from
it, or several wires in the shape of an upside-down pyramid, or
simply 2 or 3 wires coming out horizontally from the top of the
antenna.

The capacitance hat does not radiate a significant amount
of signal, but it increases the effective height of the
vertical radiator. If there is no capacitance hat, the RF
current in the antenna decreases toward the top and the
upper portion of the radiator puts out very little signal.
Increasing the effective height of the antenna has the
beneficial side-effect of reducing the losses caused by
nearby shrubs and buildings, assuming your antenna is taller
than these objects.

loading coil

Relatively short antennas behave like lossy capacitors and
present a high impedance load to the transmitter due to the
large amount of capacitive reactance that is present. The
loading coil helps to tune out that reactance. Tuning out the
reactance is important because a tuned antenna will accept and
radiate much more power than a mismatched antenna.
We will talk about how to build and optimize a coil later, in the
Antenna Tuners chapter of this Handbook.

When the loading coil is installed at the bottom of the vertical
radiator, we call it a "base loaded" antenna. Base loading
requires the smallest amount of inductance to achieve resonance. However,
if we experiment with inserting the coil into higher locations in the
vertical radiator, we find that the current distribution on the antenna
improves and we get better radiation of the signal. Center loading— putting the coil in the middle of the radiator, or two-thirds of the way
up from the bottom— is generally accepted as a good compromise among
all the factors that have to be considered. Top loading is also possible,
but generally requires a larger loading coil.

Although center loading is often used in CB radio
antennas and other frequency ranges, most LPAM experimenters use base
loading. It is just plain easier to install the coil at the base of
the antenna. The following ideas are offered for anyone who wants
to try center loading.

The drawing above shows how to begin inserting a loading coil into
the middle of a center-loaded antenna. Start by cutting the vertical
radiator in half, if necessary. (Maybe you already have two segments
of material that you’re planning to join together.) Use a hacksaw to
cut some slits into one end of the radiator, so that the clamp will be
able to squeeze it down to a slightly smaller size.

Next insert an insulator that has an outside diameter the
same as, or slightly smaller than, the inside diameter of the
radiator. The insulator has to be long enough and strong enough to hold the
overall antenna structure together during rough weather. This insulator
can be a plexiglass or fiberglass rod, or a piece of wooden dowel. If
you use wood, you must drive out the moisture and then waterproof it
before installing it. You can do this by heating the wood
in a slightly warm oven (180 to 200 degrees Fahrenheit) for an hour, then
removing it from the oven and applying a coat of spar varnish to it. Apply
another coat of varnish after the first has dried, and repeat until you
have 4 coats total. Later on, remember to cap the top end of the
radiator so that rain-water can’t fall into it and de-tune the antenna
by saturating the wood.

R = radiator, I = insulator, C = clamp, L = loading coil, F = coil form.

Insert the insulator into the radiator, then use a hose clamp to
secure it. Put the loading coil over the insulator and secure it with
glue or other non-metallic materials. Now attach the other half of
the radiator to its end of the insulator, then electrically connect
each end of the loading coil to its portion of the radiator. Tighten
the clamps well, but not so tight that the insulator gets cracked or
crushed.

connections

All connections must be weatherproof and must have very low
resistance. Don’t try to make do with a soldering iron if a torch
is required to solder large wire to large conductors. Spray the
solder joints with defluxer or scrub them with alcohol and a
toothbrush to prevent flux from lingering around and speeding up
corrosion of the materials. Some people like to apply a thin layer
of Vaseline or silicon gel to outdoor solder joints, to slow down
the weathering process. Coaxial cable connections can be
weatherproofed with gummy black goop made especially for this
purpose; it is available from ham radio suppliers and other electronics
dealers.

supports

A thin vertical object will not stand up by itself; some
kind of support will be required. Three or four non-conductive
guy lines (perhaps synthetic rope or twine) attached near the top
of the antenna and anchored to some stakes in the ground will
do the trick.

It is true that you can use aluminum or steel guy wires and
electrically connect them to the top of the antenna to add some
capacitance. However, that style of antenna works best with a
feedpoint in the middle of the radiator and a complex antenna tuner.
To keep things simple, use non-conductive guy lines.

location

As with all antennas, shortened verticals work best when
they are "in the clear" — many meters away from any
other objects. Trees, shrubs, power lines, buildings, and
vehicles all have a subtle, negative effect on antenna performance.

A small transmitter can be placed in a weatherproof box at
the base of the antenna. Eliminating the need for a feedline
eliminates the problem of feedline losses. For US citizens who
acknowledge the FCC’s jurisdiction, it also makes it easier to
comply with the FCC’s Part 15 rules which limit the combined
length of the feedline, ground lead and antenna to 3 meters.

bandwidth

There is one possible problem with a shortened and tuned antenna.
If the Q is too high and the bandwidth is too narrow, the sidebands
will be attenuated and the audio quality of the signal will suffer.
However, you probably will not have this problem. It’s almost impossible
to accidentally build a high-Q antenna system.

If you want or need to experiment with antenna bandwidth, the following
actions will lower the Q and increase the bandwidth:

• use larger diameter material for the vertical radiator
  
• re-build the loading coil with smaller diameter and increased length
  

safety

static discharges

During dry windy weather, a large vertical antenna can gather a
static charge that can harm the final amplifier transistors of a
transmitter. If you are in an area where dry windy weather is
common, you can deal with this by installing a 5,000 or 10,000
ohm power resistor between the antenna and ground system. This
allows the DC static energy to be discharged to ground but has
little effect on RF flowing through the system. The resistor must
be rated to handle more power than your transmitter is putting out,
e.g. for a 750 milliwatt transmitter, use a 1 watt resistor. If
you use a wire-wound type of resistor, be advised that it will
add some inductance and change the tuning of the antenna system.

lightning

In areas that have a lot of thunderstorms, a vertical antenna
with a good ground system will be hit by lightning sooner or later.
Licensed radio stations deal with this by installing a pair of metal
balls with a small gap between them. One of the balls is connected to
the bottom of the vertical radiator, and the other is connected to
the ground system. If lightning strikes, most of the energy will
jump across the gap and be discharged into the ground, rather than
flowing into the transmitter.

Most home-made vertical antennas and
low power transmitters will simply be destroyed if hit by lightning.
It may be a good idea to turn off the transmitter and disconnect it
from the antenna system when lightning storms are approaching.

humans

If your transmitter puts out more than a couple of watts of
power, do something to keep people from touching the antenna. If
they touch it, they might get an RF burn, which can be painful.
So put up a wooden fence or a warning sign, or get a watchdog.