It is impossible to give one answer about common-mode
impedance of a baluns. It would only be a wild guess without
knowing the type of balun and many other things.
I tested a Hygain BN-4000 two days ago to verify the ratings, and
as little as 500 volts of potential from end-to-end of the balun on 80
meters causes it to get so hot the ferrite reaches Curie
temperature. The W2DU balun is even worse! Baluns like this could
easily be damaged when feeding a floating driven element. They
don't even make good dipole baluns on 1.8 MHz, despite the
ratings and claims.
Unless you have a balun wound from coaxial cable with an air core,
it is unlikely the balun will "be happy" with 160 RF flowing common
mode into the driven element. There is no set answer you can
depend on other than to say ferrite core baluns can be a real
problem and should be avoided.
Because of that, I'd get rid of any choke baluns with ferrite cores.
The elements at the center of any yagi can be safely grounded to
the boom through a choke wound from something like #16 or
heavier wire. The choke should have an impedance several times
the elements center impedance on the lowest frequency the yagi is
used on.
Certainly around 300 ohms XL would be very safe. That would be
20 turns of wire on a one-inch diameter two-inch long form for 10
MHz and higher bands. I'd wind the choke with insulated #14 wire
(house electrical wiring would be OK) on a PVC pipe form, and seal
it somehow.
The one catch is you have to be careful with linear or stub loaded
elements to find the electric center point of the element. That point
will be at the middle of the conductor that crosses the boom.
73, Tom W8JI
W8JI@contesting.com
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