On Thu, 15 May 2008 09:17:38 -0500, K4SAV wrote:
>A vertical with a good radial system fed with coax doesn't require a
>balun (or choke) to reduce common mode currents because it is an
>unbalanced antenna and so is the feedline.
Actually, a common mode choke IS good practice on the coax feeding a
vertical. It prevents coupling of noise between the power system ground
and the antenna. It also prevents the outside of the coax acting as
another radial, which could put RF in the shack. I have common mode
chokes on all my antennas. As a result, my 160M vertical is about as
quiet as my 110 ft high dipole, and both antennas hear fairly well! Of
course, my Beverages usually hear better, because they're directional.
And they have chokes on them too.
As to MATCHING the vertical. I'm using, and strongly recommend, the
technique of using top loading to tune the antenna long (that is, to
make it enough longer than a quarter wave that it's impedance is 50 +JX
on the frequency where you want to use it, then add a capacitor of -jX
to tune out the inductance. It isn't practical to use this in some
QTHs, but when you can, it works VERY well.
Jim Brown K9YC
TowerTalk mailing list