Lots of Hams still use this tried-and-true system for tuning
antennas for efficient operation on a variety of bands. There
are plenty of multi-band designs that rely on traps to reactively
isolate portions of the antenna that properly resonate at the
desired frequency, but there is usually a price to be paid in
VSWR. Poor VSWR; i.e., higher mismatch loss, can be overcome
with higher transmitter output power, but the real sacrifice
for poor matching is loss of receiving range. The utter simplicity
of using an insulated cord to vary the physical length of the
antenna element(s) for tuning is hard to beat. It could be impractical
on a setup where access to the antenna mount is difficult, but
my guess is most people can make good use of it.
Tune Your Antenna with a String
By Major H. Ulyat, W4JPW
By using the extended ends of a twin lead feeder, an antenna
may be remotely tuned to its resonant frequency or any harmonic.
With the development of plastic covered wire in the past
few years, a number of hams have found the 75-ohm twin conductor
line makes a good antenna feeder and that it is plenty strong
enough for the antenna itself. It has been found waterproof,
weatherproof, and tough enough to stand up over long periods
of use. Once started, it will separate easily when the ends
are pulled apart. This wire should not be confused with certain
rubber-covered varieties which have high losses when exposed
to wet weather.
The ideal setup, therefore, would be to use one solid piece
of this wire for the whole antenna system, with no joints to
solder and tape and no uninsulated wires to get wet. Even the
ends may be treated with waterproofing dope.
If, in addition to this, the actual length of the antenna
can be changed to secure maximum loading for any operating frequency
within anyone particular band, as well as the bands that lie
within the odd harmonic of the fundamental, it stands to reason
that we will need no antenna tuning unit. The feeders can be
connected directly to the link in the final tank. The result
is a minimum amount of loss and a maximum amount of radiation.
Loading can then be regulated by adjusting the link in the final
The secret of this accomplishment lies in the mechanical
setup of the antenna itself. The system to be described avoids
all cutting and splicing and all the other headaches connected
with pruning and splicing a doublet six inches at a time, trying
to find the right length to make it load properly. No test instruments
are required, other than the plate current meter on the transmitter
itself. Once the system is erected, you can tell your helpers
to go on home; you are able to sit by your transmitter in the
shack and make your antenna adjustments on the nose from your
The following instructions give dimensions for a 75- to 80-meter
doublet which will load satisfactorily on 20 meters. A similar
40-meter doublet, which would serve for 10-meters, could be
made by reducing the antenna length by one-half.
Fig. 1 - The pulley system used at the center
of the tunable antenna system.
A suitable mast or tree at each end will be needed to support
the antenna. These should be at least 140 feet apart. Midway
between them, we shall have to erect another pole to support
the center, which need not, however, be as strong as the end
supports, as its job is mainly to support the center. There
will not be much side pull on this mast if the weights on the
ends are the same, and the pulleys turn easily.
On the center mast, securely fasten two paraffin-soaked,
hardwood pulleys as shown in the illustration. If pulleys made
of better insulating material can be found of the right size,
so much the better, but wooden spools can easily be turned on
a lathe and grooved to the right depth. The groove should taper
Fig. 2 - The antenna supporting system. Counterweights
take up slack in the antenna ends.
When properly set up, these pulleys should touch each other
and roll freely. It should be possible for the wire to work
back and forth in them without jamming, and yet the separated
wire sections should not have any appreciable space between
them beneath the pulley. It might be advisable to have several
similar sets of pulleys along the split section of the feeder
to keep the two wires of the feeder close together at all points.
A small piece of wire can be used to make the experiment before
raising the mast. This precaution is very necessary.
The length of the line to begin with will be about 70 feet,
plus the distance from the pulleys on the center mast to the
transmitter. If in doubt, use a little extra so no splicing
will be needed. The wire is fed from the bottom, up through
the opening made by the grooves in the pulleys on the center
mast. It is then separated and pulled apart, with enough coming
through so that the ends can be reached when the pole is erected.
The center mast is then set up, using whatever guy wires or
Now, fasten the ends of the antenna to strong, paraffin-soaked
cords and run them through pulleys secured to the tops of the
end masts. These cords are pulled at the same time until the
antenna stretches out to about 110 feet in length. Do not pull
more than this for the present.
When the ends are pulled, the feeder will come up through
the pulleys on the center mast, where it will separate by itself.
Next, tie another strong, paraffin-soaked cord securely to the
feeder. (Tape should be used here to prevent the cord cutting
into the insulation.) It should be tied to the feeder about
20 feet below the pulleys on the center mast. The exact location
where it is tied on the feeder may have to be changed later
to suit your particular set-up.
If the cord begins to get too near the pulleys when making
initial adjustments, it should be tied lower down. Fasten it
securely to some firm support, preferably inside the shack.
It should extend straight down from the pulleys to avoid any
side pull on the center mast. Running the cord through a pulley
near the base of the mast would accomplish this. Fastening the
bottom end keeps the feeder from being drawn further than desired
through the pulleys.
Weights are now attached to the cords by the masts at the
ends of the antenna; use a step ladder to tie them up about
10 feet or more from the ground. These weights should be heavy
enough to keep the antenna from sagging, but not so heavy that
they will interfere with the proper working of the mechanism.
Window weights are ideal, as their slender shape gives them
a neater appearance.
Next step is to connect the bottom end of the feeder to the
transmitter tank link. Set the transmitter frequency near the
low end of the 80-meter band. Untie the bottom end of the cord
that is fastened to the feeder, and, with the transmitter on,
slowly let it out while you keep an eye on the plate current
to the final. As the weights now pull the antenna, making it
longer and pulling the feeder up through the pulleys, separating
it there to form the antenna, the plate current will rise excessively
at one point, indicating maximum load. This will be the maximum
length of the antenna on this particular band. Tie the cord
Allow several more feet of feeder before cutting off the
surplus length and connecting it permanently to the transmitter,
since the harmonic on 20 meters will require a longer antenna
wire to load to resonance.
If, in this process, the weights have reached the ground,
they will, of course, have to be tied up again, preferably about
10 feet from the ground.
Tune the transmitter near the high end of the 75-meter band.
While it is on, pull the cord, drawing the weights up and shortening
the antenna until it loads to maximum again. This will be the
shortest position. For convenience in tuning, the cord could
be wrapped around a l/2-inch shaft, with a crank attached to
When the antenna is shortened, it may be drawn back through
the pulleys in the center and become part of the feeder. This
section that is pulled back through the pulleys has been separated,
but if the grooves in the pulleys are not cut too deep, the
wires will come down practically together because of the tension
Loading on the third harmonic, 20 meters, now becomes an
easy matter. The lower end of the 20-meter band should load
with the antenna slightly longer than it was when loading on
the low end of 80.
Now there will be no more need to say, "Well, OM, I usually
work on this frequency because my antenna seems to load up better
here than anywhere else." For you have now constructed an antenna
without any of the "cut and try" methods and will always be
ready to load it to any frequency in the band by a mere "pull
of the string!"
Posted September 23, 2015