Duncan Lindsay -MSC Valencia- wrote:
>Once again, one does not use a balun on a vertical antenna because it
>is an
>un-balanced antenna !! You use a UN-UN or an RF Choke at the feed
>point. Balun's are for balanced-to-unbalanced, antennas & coax !! Or
>did I forget something in Antennas 101 in college ?
You're right to say that a vertical antenna is unbalanced. All verticals
are unbalanced by definition - they cannot ever by symmetrical with
respect to their environment because one end of the antenna is pointing
down at the earth while the other is pointing up into space.
Since a vertical always has this basic lack of symmetry, there is no way
to connect a feedline directly to the antenna without setting up
common-mode currents that will make the feedline radiate. A coax-fed
vertical always needs some kind of device to prevent the outer surface
of the coax behaving as an unwanted part of the antenna.
But the same device can be called by more than one name, depending on
what it's being used for.
The device we're looking for can be called a "feedline choke",
"common-mode choke" or "line isolator" - those names are almost always
correct. When used with a symmetrical antenna such as a horizontal
dipole, exactly same device can be called a "1:1 current balun"... but
not when it's connected to a vertical.
It's a silly situation, though no more so than many other things in the
English language.
--
73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek
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