[OKDXA] Balun

[kd5gho]

Thank you Nelson.

I always lern some useful information when you bring out the technical
stuff. .

It is good to have  Q & A available.

Keep up the help,
TNX OM 73.
Jim


---- Original Message -----
From: "Nelson Derks" <ac5up@vei.net>
To: <okdxa@mailman.qth.net>
Sent: Saturday, October 12, 2002 9:31 AM
Subject: Re: [OKDXA] Balun


> 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
>
>
>
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