[OKDXA] Balun

[Nelson Derks]

Gents:

Don't forget the power rating of a balun is based on a fairly stable and
non-reactive impedance on both sides... If you're working the ragged edge of
the antenna bandwidth and trying to compensate the mismatch through a tuner
in the shack, the balun may be seeing some truly weird RF at the feed point.
TVI is one of the first signs of a balun having a bad day. Push it harder
and you'll risk insulation breakdown and permanent damage to the cores, no
matter what the nominal power rating might be.

Robust baluns don't come cheap, but, if you do a little research you'll find
the better 4:1 baluns use a dual-core Guanella design wound with silver
teflon aircraft type wire. You can build one for less than a Franklin that
should handle the power... Start with four T-240 (or larger) toroid cores in
a Mix 61 or 43 material and tape the cores together in two stacks of two
cores each. Wind them with 14 gauge silver teflon and mount the whole mess
in a good weatherproof plastic electrical box.

Here's an example of a medium power design using one core: (I've had much
better luck using two cores. Above 14 MHz or so the windings start to talk
to each other and efficiency suffers. The phasing and polarities remain the
same, just put the windings on separate cores and you'll be a Happier
Camper)

http://home.earthlink.net/~n0ss/qrp_4-1_guanella-type_balun.pdf

Here's an example of a dual-core design, but the builder used enameled wire
instead of insulated. Not a good idea, as the impedance of the windings will
be a bit too low. When you're transforming 50 to 200 ohms, the internal
impedance of the windings should be near the mid-point of 125 ohms.
Insulated wire will add a little spacing for a better broadband match.

http://www.werdau.net/qrpproject/images/BalunOffenKlein.JPG

Since I know you want to learn as much as you can and aren't afraid of
plowing through an engineering paper, here's a source I respect:

http://standards.ieee.org/reading/ieee/ept/trans.pdf

Pay particular attention to Figure 3 on the right half of the drawing...
That's a 4:1 Guanella Balun. Four windings, two per core (bifilar),
connected in parallel at the input and in series at the output. Notice the
phasing. This is what I use at the feedpoint of The Wonder Wire using a pair
of Mix 43 cores and I've had zero problems with TVI or RF in the shack, plus
the broadband receive is outstanding for BC-AM / SWL use. You can buy Sevick
design baluns through Amidon & Associates if you're not in the mood for a
home-brew. Poke around here for more:

http://www.amidon-inductive.com/associates_prodselection.htm

Think big cores and beefy silver teflon wire inside a weathertight
non-reactive enclosure and you should be FB. W2FMI likes to use Scotch
fiberglas tape on the cores for added insulation resistance, but I've had no
problems with Scotch 88 PVC tape at 100 watts. I suspect that would be
adequate at higher power when the antenna is near resonance.

- AC5UP