John,
The other guys are correct here -- what you're asking about is correctly called
a common-mode RF choke. It has no effect on the current flowing inside the
coax,
but it chokes off any current that wants to flow on the outside of the coax.
The
common-mode choke can be "wire-wound" (five or six turns of coax coiled at the
feedpoint) or a ferrite sleeve around the coax at the feedpoint. Both are
effective.
But is it necessary? When feeding a gamma match, the answer is typically no.
Just make sure the coax isn't allowed to run parallel to the elements, and the
likelihood of your getting common-mode currents on your coax is quite low. How
would you know if you really needed a common-mode choke? When you have
common-mode RF currents on the coax feedline, you can have difficulty or
"weirdness" in trying to load the antenna, or you can have "RF in the shack"
where various objects are coupled to the "hot" coax feedline and give you a
burn
when you touch them. (Common-mode currents on the coax can also cause pattern
distortion in a sharp yagi, but are unlikely to have any effect on the pattern
of a 3-element yagi.) In these cases, a choke can be the cure -- but I'd try to
figure out what is coupling the RF to the outside of the coax in the first
place. If that effort fails or the problem cannot be otherwise remediated, then
a common-mode choke will often solve your problems.
Having said that, it can't HURT anything to use a common-mode choke in the
feedline, even if you don't need it.
Bill W5WVO
John Geiger wrote:
> I hope I can get some clarification or advise from list members on
> this. I have a Cushcraft A50-3S 3 element 6m yagi and am getting
> ready to do a little antenna work this week. Currently I have the
> coax running directly to the SO239 on the yagi, as the manual states.
> Would adding a coax balun (a few turns of coax by the feedpoint)
> improve performance in any way? I have seen this recommended for
> other antennas but the Cushcraft manual never says anything about it.
> It does seem that it would keep any stray RF off of the coax shield,
> correct?
>
> 73s John AA5JG
>
>
>
>
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