Most transmitters are optimized to work into
a 50-ohm non-reactive load. Most homebrew antennas contain some reactance
and have an impedance other than 50 ohms. A device called an antenna tuner
(also known as an "ATU" or "transmatch") will
transform your antenna system into a non-reactive load
of the proper impedance.
A properly matched antenna will radiate more efficiently and,
Most transmitters are optimized to work into
a 50-ohm non-reactive load. Most homebrew antennas contain some reactance
and have an impedance other than 50 ohms. A device called an antenna tuner
(also known as an "ATU" or "transmatch") will
transform your antenna system into a non-reactive load
of the proper impedance.
A properly matched antenna will radiate more efficiently and,
in some cases, will produce a better-sounding AM signal.
Loading Coils
Instead of a complete and complex antenna tuner, many
LPAM stations use a variable inductance located at the transmitter
output jack or at the feedpoint of the antenna. This approach is
popular with Part 15 experimenters using simple wire antennas or
shortened verticals. First you insert a tiny "grain of
wheat" light bulb, a flashlight bulb, or a similar small
lightbulb between the inductor and the antenna. (Stations using 100
milliwatts might need a bulb rated for 1.5 or 3 volts; operators using
up to 10 watts may need a 6-volt or 12-volt bulb.) Then you try
various inductances while the transmitter is on, and see which setting
makes the bulb glow most brightly. Then remove the light bulb from
the circuit and transmit.
The variable inductor can be, for example, a coil
made of 100 turns of 22-guage (#22 AWG) enameled wire that
you wind around a piece of PVC pipe 2 or 3 inches in diameter, with
taps (connection-points) built into the coil every 10 turns; or any
similar inductor.
The various taps in your coil can be wired
to a rotary switch, or you can use a small alligator clip to
select one at a time.
The diagram below shows a more elaborate
scheme that can be used with a vertical antenna.
One end of the coil is grounded, in other
words, connected to earth-ground. That lower third of the
coil is made with several taps 3 or 4 turns apart, and the
transmitter can be connected to any of these taps on the
ground end of the coil. From the middle to the top end of
the coil are several taps about 10 turns apart, and the
antenna can be connected to any of these upper taps.
For this application, try the following coil
dimensions: 145 turns of #20 AWG enameled wire, closewound (no
spacing between turns), on PVC pipe having an outside diameter of 3.5
inches. The total inductance of this coil is about 1 millihenry,
enough to base-load an 8-foot whip antenna at frequencies of 1000 kHz
and above. (Data source: Jon Smick’s info-packed messages on the
Community Radio USA message board.)
Use a field strength meter or the light bulb
trick to find the best taps for both the antenna and the
transmitter.
This system of tapped inductors only provides crude
tuning and is not likely to take you to the point of exact resonance.
To get a fine tuning control, some experimenters insert a variable
capacitor between the bottom of the coil and the ground system.
Others build a "variometer" inside their coil. A variometer is a
small coil that is able to turn within the larger coil. When the
variometer is parallel to the outside coil, inductance is maximum;
as you turn it away from the parallel position, the overall inductance
is reduced. To see a diagram of a variometer,
click here.
To see pictures of a giant variometer designed for the
160-190 kHz frequency range,
follow this link.
Something considerably smaller could be built for the AM broadcast band.
Important notes: Some
transmitters’ output circuits already contain a big inductor which
is designed to match the impedance of a short wire antenna. If you are
building your own antenna tuner, you can often get the best results by
bypassing or downsizing the loading inductor that was built into your
transmitter, if any.
If you are building your coil on PVC pipe, use white
PVC pipe (sched. 40), not gray electrical conduit material or black
sewage pipe material. These different types of plastic have different
dielectric properties and the white stuff is best for radio work.
After building the coil, you can apply a coat of clear lacquer to help
hold the wire in place.
And yes, all that wire in the loading coil does introduce
some resistive losses and a significant amount of RF power is lost
along the way. However, the improved radiation efficiency of a properly
matched antenna more than makes up for it! Delivering one milliwatt
of power to a properly matched antenna will produce a much stronger
signal than delivering 10 milliwatts to a severely mismatched antenna.
Antenna Tuners
An antenna tuner, also called a
"transmatch," is a combination of variable inductors and
capacitors in a single box that goes between your transmitter and
antenna. A factory-made antenna tuner might also include an SWR
meter. Basically you fiddle with the knobs until you find the
combination of settings that gives you the lowest SWR or the highest
field strength reading. Antenna tuners can be home-made or bought
from an amateur radio supply dealer.