> A coaxial coil choke is one attempt at accomplishing this. There are two
> main problems with it. First, it is imprecise in regard to frequency. Current
> thinking advises that the coil should be wound on an appropriate coil form
> (like a round oatmeal carton or something that will work size-wise). Even at
> that, I don't think the frequency response is anywhere close to a good
> current-type or stacked ferrite bead balun.
This subject was debated on Usenet and the guy who designed the coax coil
baluns described in the ARRL Antenna book responded to the "oatmeal box" flat
coil proposals, explaining that he used a serious piece of test equipment to
develop the specs for those baluns and they were scramble wound. The ones
designed for single bands (listed as "very effective" in the book) in fact have
very high common mode impedance. Winding them flat reduces the common mode
impedance rather than increasing it. The notion that scramble winding "shorts
out" the input to the output of the choke (e.g. stated in the Wireman book)
is mistaken. The guy who designed the Antenna book baluns is W7EL, the author
of EZNEC and a guy who really knows baluns.
I'm not sharing this to argue (I agree with you about the tribander), but just
to suggest that coil baluns can be worthwhile and have gotten a bum rap.
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