An UN-UN is un balanced to unbalanced. Is that what you have ? A vertical
is unbalanced.
It isn't that simple. Most Ham antennas are neither perfectly balanced nor
perfectly unbalanced. They are somewhere between.
For example, a 1/4 wave groundplane with four perfect radials is mostly
unbalanced, but still has considerable voltage between the radial center
point and other things.
This is why, if we build a perfect 1/4 wave groundplane and do not use a
choke balun or some other form of common mode decoupling, antenna SWR will
go all over the place with coax or mast length.
By the time we get 15 or 20 (or maybe more) proper radials, it is almost
perfectly unbalanced and common mode is not an issue, but less than that
becomes an increasing problem.
I probably would not use an unun on a vertical unless the vertical had a
pretty good ground, or unless I had a common mode choke of some type to
decouple the feedline.
A current balun functions the same as a common mode choke. We just call it a
"balun", but it really is an isolator for common mode. It doesn't care,
within its design limits, if the load is perfectly balanced, perfectly
unbalanced, or someplace between. It might be labeled a "balun", but it is
directly interchangeable for an unun of the same ratio and has the
additional feature of decoupling the feedline for common mode.
A voltage balun forces unbalanced voltages, and cannot be exchanged with an
unun. It has no common mode isolation, and will actually aggravate common
mode problems on a vertical.
With a vertical with less than very good ground system and coax leaving
below that ground, I would either use a current balun or some other form of
common mode choke with matching. With a perfect system, I'd just use a form
of L network or (reluctantly) an unun by itself.
73 Tom
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