On 1/13/2019 3:42 PM, Al Lorona wrote:
By the way, whenever Jim says, "...a very good common mode choke at the feedpoint of
an antenna...," he means an antenna fed with coax.
What I actually mean is an antenna matched to its feedline at the
operating frequency(ies).
For the rest of us, of course, that choke would (should) go at the output of
the antenna tuner, whether in the shack or close by.
Actually, for the rest of you, a choke at the shack ONLY reduces
coupling common mode current into the shack, but does NOTHING to reduce
noise pickup on the line, because the feedline is not isolated from the
antenna. Remember -- unless it's choked AT THE FEEDPOINT, that feedline
is part of the antenna. If it's ONLY choked at the tuner, the feedline
is a wire dangling from the antenna that isn't connected to anything on
the other end.
AND if the antenna is not matched, a choke that doesn't fry with TX
power probably isn't doing anything useful. For a discussion of this,
see "Don't Burn Up Your Balun" by retired Antenna Book editor N6BV, from
QST something like 3-4 years ago.
Bottom line -- if you live miles from anywhere with no noise sources
pumping trash, all-band non-resonant antennas with open wire feeders can
be a decent solution -- they transmit just fine if you've done
everything right. But they're a sitting duck for noise because there's
no practical way to choke them.
73, Jim K9YC
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