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4:1 balun for 160 to 10 meters

By Clay Wynn

Two T-80-2 Iron Powder Toroids, Red. (Oak Hills Research, 10 for $3.60)
40 inches of blue 30 gauge wire wrap wire. (Radio Shack-278-503)
40 inches of white 30 gauge wire wrap wire. (RS-278-502)
BNC panel jack, or substitute your favorite (RS-278-105)
Two binding posts, or substitute your favorite, Pkg. of 4 (RS-274-661)
Enclosure 3 1/4" x 2 1/8" x 1 1/8", plenty of room (RS-270-230)

Stack two T-80-2 powdered iron toroid cores together. Use super glue, or some other means to hold the two cores together. The powdered iron cores should be tumbled smooth during manufacture so no protective wrapping is necessary. You may want to wrap the cores with tape to stabilize the wire during the winding operation. I did not use tape but took extra care and time on the winding.
Using two colors of wire helps keep track of the connections after winding. Wind 23 evenly spaced bifilar turns, white wire parallel and adjacent to blue wire. That's 23 white turns, 23 blue turns, white and blue wound simultaneously. Leave an approx. 4 inch tail at the start and finish of the windings. Space the turns so that the distance on the core between the start turn and the end turn is approx. 1/4 inch. Drill the holes and mount the jack and binding posts on the enclosure.

Cut short and strip the white wire on the starting pair tail and the blue wire on the ending pair tail. Leave enough to twist together, and solder to a common ground strap. I used a 3 inch long piece of solder wick braid for the ground strap. Any kind of hook up wire will do. The wire wrap insulation is tough, so use an Exacto knife or razor blade to strip the wire before soldering. Trim to fit and solder the blue tail of the start turn to one of the binding posts, likewise the white tail of the ending turn to the other binding post. Take care not to nick the copper when stripping the insulation.
Using a piece of insulated hook up wire, solder one end to the center conductor of the BNC Jack, the other end to one of the binding posts. Now one of the binding posts will share a connection with the wire from the BNC center conductor and a wire from the balun winding. The other end of the ground strap is soldered to the shell of the BNC panel jack. Some ASCII art is attached that shows the wiring connections. If the wiring arrangement is still unclear, consult the literature on baluns.

After dressing the leads, the balun is mechanically attached to the side of the plastic box. I used a hot melt glue gun and the clear elastomer sticks. I carefully placed the nozzle of the gun in the center hole of the balun and built up an inverted mushroom column of clear elastomer, slowly extracting the gun and feeding the melt until flush with the top of the balun. Choose any technique that best suits you. Simply packing the enclosure with scraps of polyurethane foam sponge would probably work.

The size of toroid core was chosen to allow the balun to be housed in a pill bottle or a plastic 35mm film cannister, although this involves a little more "ship-in-the-bottle" construction planning. The toroid winding is self-shielding, so practically any material for the enclosure will do. Powdered iron vs. ferrite is a little more forgiving if you forget and dump 100 watts through the QRP balun like I have been known to do.
If you use an all metal enclosure, additional care must be taken to insulate the binding posts from the metal. Insulating washers are provided with the RS parts. One problem with plastic material is that parts mounted solely with a threaded shell and one nut, like the BNC jack, eventually will loosen. This may be an important consideration for field rugged equipment.

A number of experiments were done with different wire size, insulation thickness, number of turns, and number of cores to arrive at the above configuration. 23 turns are not that critical. Somewhere between 21 - 24 turns will suffice.
Performance of the balun is good from 160 to 10 meters. I've used it up to 10 watts carrier power with good power transfer and no detectable core heating. Computed ohmic losses are less than 50 milliwatts at 5 watts input. The balun and enclosure assembly specified will fit in a shirt pocket and weighs 2 oz.

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