By Matt Erickson KK5DR
First, read the R.L. Measures article. That article will give you the technical background to start construction.
Next, gather parts…
Here is what you will need;
(Two) identical roller inductors (must be exactly the same).
(One) variable capacitor, with a voltage rating of at least 3kV. The capacitance needed is dependent on the used frequency of operations. For use on 160 meters, you'll need a max of about 800pF, for use on 20 meters, that large of capacitance will be to much, so a max cap of 200pF will be better. In order to operate the tuner on high frequency bands like 15-10 meters, a max cap of about 150pF will be needed, and a reconfiguration of the tuner will be needed, as per the measures article.
My tuner is designed for optimal operations and efficiency on 160 & 80 meters, but it will function with reduced abilities on 40 & 30 meters.
(one) turns counter for the inductors.
(one) vernier drive for the capacitor, 6:1 ratio is preferred.
(three) shaft insulators, one for each inductor, and one for the tuning cap.
(one) wood board, stain and vanish to your desire (mine is 18" x 18", 3/4" thk.)
(one) 18" long, 5" dia PVC pipe.
(30ft) of RG-213 coax.
Misc hardware for mounts, and wiring.
Geardrives and or belt drives. (users choice)
Fig.1
Figure 1. shows the typical circuit layout.
Something I learned about this type of tuner is, DO NOT place the circuit inside a metal cabinet! The metal cabinet will severely distort and unbalance the RF current and cause the feed-line to radiate RF. The best ways to mount the circuit is on a wood board, or plexi-glass cabinet. If the circuit is built and tuned properly, there will be little or no radiation. I measured the radiation from my tuner, @ 1500 watts, and little RF field was detected beyond 3" from the feed-line or tuner components.
A convenient place to install the tuner unit was on my wall, very close to the entry of the feed-line. Orientation of the controls for mounted on a wall would be necessary. Keeping the amount of balanced feed-line in the shack to a minimum should be the goal, in this you reduce the possibility of RF feed-back from the feed-line.
Most builders will find that mechanical linkage of the roller-inductors will be the most difficult part of building this tuner. If you can't find any gears to drive the inductors, you can drive them in an end-to-end line up, making sure that proper spacing between the two inductors of one inductor length.
Fig.2
Fig.3
In figure 2. you see the right-angle gear drives I used on my tuner (I found them inside a tuner at a hamfest).
In Fig. 3, you see a set of timing/cog belt and pulleys, which I bought from a local industrial bearing shop. They are available from Martin or Dodge bearing manufacturers. Check with your local bearing shop, look in their catalog to find the size you need. The belts have matching pitch pulleys and bore size can be selected to match the shafts you wish to mount them on. They are not very expensive, I think I paid about $6 per set.
Fig. 4
Fig.4, shows the coaxial/choke type balun, which is placed on the input end of the tuner.
Wrap the RG-213 coax on the PVC tube/pipe, each winding should be tight, and at each end of the winding, use a tie-wrap to secure the coax in place.
This unit acts as a fixed impedance input balun. It MUST be used! Not using this balun, will cause severe RF feed-back on the exciter feed-line.
Fig.5
Fig 5, is of the tuning controls, bottom is the turns-counter for the roller-inductors, above is the vernier drive knob for the balancing capacitor.
The tuning of the unit is extremely sharp, so high resolution controls are needed to make it easy to reset them quickly, using your "cheat-sheet". More about that later…
OK, now you have collected all your parts…. Right? You need to lay them all out on the board you plan to use, marking the positions of each. A lot of this part of the construction will be trial-and-error, but should be fairly straight forward. Since all the parts are mounted in wood, there is no real need for insulators.
Test your work at each stage of the job, then move to the next area.
One area I can't stress enough, is that the unit must be electrically symmetrical, meaning that wire must be all the same lengths, components mounted equally spaced, and symmetrical. The wires that go from the inductors to the tuning cap, must be the same length, as well was the jumpers that go from the input end of the inductors, and the rollers. Symmetry will help balance the unit, which is the goal of the system.
The same symmetry used in the tuner unit, should be repeated in the feed-line and antenna.
Be sure to tie down or otherwise restrain the feed-line, keeping it away from all metal objects, and not allowing it to bounce around in the wind. Each leg of the antenna should be exactly the same length, unless the antenna is a loop, then, it should be kept at a uniform height above the ground, and away from any metal roofs of buildings. I used small PVC pipe clamped to the side of my tower to restrain the 450 ohm feed-line I use. The pipes are spaced at about 10' apart, about 3' away from the tower. This keeps the line tight, and minimizes movement. Less movement also means less wire breakage too.
The antenna elements themselves should be cut in lengths that are greater than 5/8th of wave length for a given band of operation, which allows the feed-point impedance to closely match that of the feed-line.
Tuning; Do yourself a big favor, and use an MFJ-259 SWR analyzer or something like it to set up the tuning of the system. Once the antenna has been installed, feed-line run and attached to the tuner, place the analyzer at the input, set it to the operating frequency and adjust the tuner for minimum SWR, adjusting the inductors first, then fine tuning it with the capacitor. Write down all the setting for each frequency set, this will be your "cheat-sheet", keep it near the tuner, for quick resetting.
Now, you have built the "balanced-balanced" tuner system, enjoy!
73 de Matt KK5DR
Copyright © 2006 M.A. Erickson, KK5DR. All rights reserved.