Irv Finkleman wrote:

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There is an air core balun described in Hints and Kinks 12th Edition
12 trifilar turns (three wires of identical length) of #12 or 14 ecw
close wounds on a 1 inch od form. It's a 1:1, but I would think that
you could try experimenting and produce a 4:1 without too much trouble.

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My original balun from 30+ years ago was an airwound balun, commercially
manufactured by B&W. It handled 250 watts AM from 80 through 10 meters.

If I recall correctly it was two bifilar wound coils, close spaced, about 10
inches long with the series parallel jumpers for 4:1 impedance ratio.

/s/ DD W1MCE