[TowerTalk] Balun on a vertical

[Jim Lux]

At 07:06 AM 10/9/2006, Ian White GM3SEK wrote:
>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.


I can think of cases where you WANT the outside of the coax to be 
part of the antenna.  Consider a portable vertical set up with a few 
radials.  The coax running from the feed point on the ground back 
towards the Tx is just another radial, and would improve the 
performance of the antenna.  In this case, I'd put the choke some 
distance away from the antenna.  (that is, it's just there to keep RF 
from propagating back to the outside of the Tx and you).


>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.


There might be a few design and performance differences, depending on 
construction, regardless of what the beast gets called by its 
manufacturer.  Some isolators/transformers/etc might have more 
parasitic C around the device than others or less series R/L.


>It's a silly situation, though no more so than many other things in the
>English language.
Surely those collapsible fishing poles favored by pedestrial mobile 
people  are used to catch ghotis. (apologies to Shaw)