On 9/25/2025 8:27 PM, David Gilbert wrote:
2. A common mode choke at the output of the rig to keep RF that has
been picked up by the coax from getting into the rig. Choking common
mode current at the antenna doesn't necessarily keep the coax run to the
rig from picking up unwanted RF. The coax shield is an antenna and
making it look like an open at one end doesn't keep it from being an
antenna.
Hi Dave,
You're right about pickup on the feedline, BUT: that should NOT matter
if 1) Proper bonding and grounding is done for electrical safety and
lightning protection per NEC, which includes, but is not limited to,
everything bonded to everything and feedlines grounded and lightning
protected at a bonded entry panel; and 2) the Station, especially the
operating desk, is properly bonded and grounded. If it helps, there's
something seriously wrong with Bonding Grounding.
I think I read into your observation that you're talking about a
feedline that goes straight to the amp or the radio without that proper
bonding and grounding. >
3. A common mode choke, like any other inductor, isn't going to have
much effect at a current minimum, so placing common mode chokes at
current maximum(s) along the coax seems like it would be good additional
protection ... and they aren't necessarily located at either end of the
coax. Note that this would be the current maximums for the common mode
current, not necessarily for the differential current. It would
probably require a field probe to find those points if they exist.
Yes. EZNEC will show that if you model enough of the antenna system to
include it. That observation is the basis for my tutorial on the
placement along the feedline of stubs to reduce amplifier harmonics, and
why what W2VJN published on that topic missed the mark, and why I
published a more correct method in NCJ a year later (and confirmed by
measurements in my station).
It has been my experience that if you give NEC ALL of the information,
it will provide very accurate results. The challenge is to SEE all of
the information, which requires both visual powers of observation and a
thorough understanding what things can contribute to the system. N6LF
published a great piece maybe 15 years ago showing that even a tower
that is relatively short as a fraction of a wavelength can become
parasitic to a nearby vertical antenna. Rudy's work is some of the most
important stuff published in the last 30 years on antennas and radial
systems.
73, Jim K9YC
Thoughts?
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