Re: [TowerTalk] Balun Recommendation

["K1TTT" <K1TTT@ARRL.NET>]

>I originally challenged this statement from Jim: "But SWR has NOTHING to do
with dissipation in a common mode choke." I still maintain that statement is
>wrong.

I would go with something like this:
In SOME cases SWR may be an indicator (but not direct cause) of changes in
the common mode current that a choke would see.  In other cases it has no
relationship to it.  

I think the misconception that they are directly related comes from stories
where adding a choke near the feedpoint changes the swr because it
essentially stops the current on the shield that was in parallel with the
antenna current which kind of cuts off part of the radiating antenna... in
cases like that it makes it look like the common mode current is affecting
the swr so the user assumes the reverse must be true and therefore the choke
is affected by the swr.  

Take this generalized case that could lead to such an anecdotal
observation... a person puts up a dipole where coax goes right to the
feedpoint and goes off almost parallel to the side of the dipole connected
to the center conductor.  This is likely a very bad case for picking up
large currents on the shield.  It would also normally result in high swr
because of the proximity of the 'hot' side of the antenna and the feedline
shield.  Now the person adds a choke near the feedpoint and sees a change in
swr... but they may also notice the choke getting hot at high power since it
is working hard to block that big current...  they get a suggestion to move
the feedline so it is perpendicular to the antenna and when they do the swr
improves and the choke doesn't get so hot any more.  So obviously the change
in swr prevents the choke from overheating.  Where in fact both effects were
the result of changing the coupling between the feedline and the antenna.

David Robbins K1TTT
e-mail: mailto:k1ttt@arrl.net
web: http://wiki.k1ttt.net
AR-Cluster node: 145.69MHz or telnet://k1ttt.net

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