On 8/4/2019 10:57 AM, Jack Brindle wrote:
I have to argue with my friend Jim on this one. The coax itself can be an
antenna, and can pick up RF along its path from the antenna to the radio.
Yes, it can, and it is good practice to break up feedlines to high
dipoles to prevent their interaction with adjacent vertical antennas; a
choke acts as an egg insulator. I first published this recommendation
about ten years ago. It is also good practice on long runs from
receiving antennas to prevent noise and TX RF from coupling to the
inside of the coax by a limitation of the shield quantified as its
transfer impedance. This is especially important with the relatively
thin shields of CATV coax we commonly use for RX antennas, which are
especially poor shields below VHF. But, again, these chokes should be
along the line outside the shack.
But NOT on a feedline in the shack! Think about it -- coax feedlines
should be bonded to the premises ground system at the point of entry.
The best example is a vertical antenna which will use any metal it can find for
return currents, which includes the coax after the antenna-mounted choke.
An antenna should NEVER depend on the feedline to carry return current,
and an effective choke prevents this. Further, a path for return current
should be provided by some sort of counterpoise, the most effective of
which is a radial system. Also, you're describing a type of vertical fed
from the base. There are many vertical antennas that approximate center
feed. They are discussed in these slides for another talk I've given at
Pacificon, Visalia, and some ham clubs.
http://k9yc.com/VerticalHeight.pdf and in this pdf, which ran in The Jug
several years ago. http://k9yc.com/AntennaPlanning.pdf
This current will combine with the internal shield current at the first place
they are connected (which very well may be the SO239 on the back of the ATU).
If that's the first point of connection, you've got a VERY unsafe
station with respect to lightning! RUN, do not walk, to buy N0AX's
recent ARRL book on Grounding and Bonding, to which I contributed
extensively. Or study these slides for a talk I've given at Visalia,
Pacificon, and several local clubs. You'll find it referenced in Ward's
book.
http://k9yc.com/GroundingAndAudio.pdf
<http://k9yc.com/GroundingAndAudio.pdf>
A choke at this point will protect against the problem.
The only thing chokes do in the shack is reduce RF current from coupling
to or from poorly designed equipment by a mechanism called "The Pin One
Problem" by the audio professional who first described it, Neil Muncy,
ex-W3WJE (SK) in 1994. I've seen it in virtually every piece of ham
gear where I've looked for it, including all the Elecraft products I've
examined, even the KRC2 band decoder.
73, Jim K9YC
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