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April 24th 06, 09:59 PM posted to rec.radio.amateur.antenna

wrote:
The MFJ-962C "1.5kW" Versa Tuner III description claims to contain a
4:1 current balun. I'm using that "balanced" output on my HF
almost-doublet (slightly unequal leg lengths) with fairly decent
results.

The balun only has a single core. From what I understand, these "4:1
current baluns" aren't.

I don't know of any way to make a 4:1 current balun with a single
one-hole core. I think it's possible with a two-hole (binocular) core if
done right.

Furthermore, my antenna presents both higher and lower impedances than
50 ohms depending on band.

The "4:1" transformer probably has a 4:1 impedance transformation only
when the load Z is near 200 + j0 ohms. Elsewhere, it'll have a different
transformation ratio and also will add series and/or shunt impedance.

For convenience, I'd like to have a single port that I connect my
antenna to at all times. I've remoted this tuner and I don't want to
make more complicated switching arrangements. Is it worthwhile to
rewind the "4:1 current balun" as a 1:1 choke balun?

Maybe somebody else can take a crack at this, but I think it's
impossible to say. It depends on both the differential and common mode
impedances seen at the feedline input as well as the tuner's matching
range and efficiency.

It seems that it might be more appropriate for my application. I
should expect the core to work well for any HF transmission line
transformer I'd like to wind, as long as I don't saturate the thing,
right?

I've seen a lot of tuner baluns made with powdered iron cores which
don't give enough impedance for a decent balun of any kind. And the flux
density will be much greater in a 4:1 voltage balun like you have now
than in a 1:1 current balun. Type 43 ferrite is a good compromise choice
for a 1:1 current balun. If you want to try using the core, put a bunch
of turns on it and measure the impedance with an antenna analyzer if you
have one. You'll need about 500 - 1000 ohms to make an effective balun
for an approximately matched antenna. It might not be possible to get
enough impedance for it to work well on all bands with a multiband antenna.

Roy Lewallen, W7EL

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