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 tank circuit.
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 "easy chair."
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 slightly.
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 supports necessary.
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 securely again.
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 it.
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 on them.
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