There are several possible failure modes for baluns. Besides those involving
the magnetic core material, high SWR can lead to excessive voltage between the
windings. Tom, W8JI has written about that in the past. Several suppliers will
sell you teflon tubing that you can use to sleeve the formvar wire in the balun
to significantly increase the breakdown voltage.
(One advises to select teflon tubing one size larger than your wire because
it's otherwise hard to pull in over the wire). I don't have the links
immediately available, but will try to find them and post.
Additionally, balun loss is affected by mismatch. Jerry Sevick has written a
lot about matching the winding impedance to the operating impedance, and the
significant impact that it can have on the loss of the balun. Sleeving the
wires with teflon will change the impedance because the two wires are slightly
farther apart and thus the impedance increases.
-- Tom, N5EG
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