|4:1 balun for 160 to 10
By Clay Wynn
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
Two T-80-2 Iron Powder Toroids,
Red. (Oak Hills Research, 10 for $3.60)
inches of blue 30 gauge wire wrap wire. (Radio
40 inches of white 30
gauge wire wrap wire. (RS-278-502)
jack, or substitute your favorite
Two binding posts, or
substitute your favorite, Pkg. of 4
Enclosure 3 1/4" x 2 1/8"
x 1 1/8", plenty of room (RS-270-230)
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.
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
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.
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
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.