Thanks Sjoerd.

Two questions arise.

I'd thought earlier of using a 1/4 wave transformer, using paralleled
50 ohm coax to give me 25 ohms, which transforms the 12.7ohm antenna
to to 49, say 50 ohms, but isn't the DK7ZB match still an unbalanced
feed to a balanced radiator?

Won't this perturb the directivity?

The same thought came to my mind when I read the DK7ZB pages this
afternoon (it's a neat design idea and is going into my keep-this
notebook!) Seems to me there might be a pretty easy way to deal with
this.

A 1/4-wavelength coax transformer for 2-meter operation, using typical
V=.66C foam-dielectric coax cable is going to be just about a foot
long. I'd think that one could wind the pair of coaxes through a
modest-sized ferrite toroid core a few times and tie-wrap the ends
firmly into place, rather than just running them out along the boom
and back. This should create a reasonably effective choke balun...
something like a Reisert 1:1 style, but acting as an impedance
transformer thanks to the paralleled coaxes. You'd get both the
12.7 : 50 ohm transformation, and the balancing, out of one package.

You'd probably want to wind only part of the impedance transformer
around the ferrite, leaving a few inches to make the connection to the
feedpoint (thus keeping the balun itself from coupling into the
radiating element too badly).

Another possibility: in his book on baluns and ununs, Jerry Sevick
discusses this basic sort of application, and suggests that a 1:1
balun of this sort can be used as an isolation transformer by putting
it 1/2 wavelength back down the coax from the balanced-antenna
feedpoint (a technique which he credits to one of Roy W7EL's
articles). It seems to me that this would work equally well for one
of these 1/4-wave-impedance-transformers-wrapped-into-a-balun. Mount
it back behind the reflector, run 1/2 electrical wavelength of any
convenient coax (or maybe open-wire line?) up to the antenna
feedpoint, and it'd still perform both the balanced-to-unbalanced
conversion, and the proper impedance transformation in that location.

So - does this make sense, or am I being stupid and missing something?

--
Dave Platt AE6EO
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