Topband: Unun and Balun on vertical

[Tom W8JI]

> 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