On 9/25/2024 4:32 PM, Greg Troxel wrote:
Thanks, but I was trying to ask about endfed behavior in theory, rather
than how to proceed (which I have read about and realize is empirical as
to exactly where and how much you need).
If there is RF current in a conductor, it will radiate. The greater the
current, the more it will radiate. The better the antenna formed by the
wiring, the better it will radiate. Remember that a counterpoise is part
the antenna -- it carries antenna return current. In a simple end-fed
wire, the current on the both sides of the feedpoint is the same, and
varies along both wires like any other antenna, and is forced to near
zero at the ends.
Perhaps another way to phrase it:
In your experience, given a wall wart that plugs in directly, with a dc
cord, do you find that adding say 2000 ohms of common-mode choking
impedance to the dc cord, right at the device, will significantly reduce
the common-mode current that the device creates on the mains wiring,
least typically? If so, is it reasonable to think of this as "end fed
antenna, once counterpoise is taken away, doesn't work as well"?
The short answer is that there are an infinite number of ways that all
of the wires that can be interconnected on both the power side and the
DC side in real installations. Remember that all of these wires are part
of an antenna that can be very simple (one wallwart, one thing it powers
with no other wires connected) or very complex, with multiple wires on
the equipment side. Think radios, or a home entertainment system, or a
computer hookup. In other words, it's rarely as simple as an endfed
antenna. Real life problems are rarely as simple as those we study in
school. (Which is also true in the rest of life.)
And the effectiveness of any choke will depend on the impedance at the
point on the antenna where we place it. If it's at a low current point,
like the feedpoint of an endfed half-wave, it's much less effective than
if it's at a high current point, like the feedpoint of a quarter-wave
with radials or a counterpoise.
My advice is to start with a choke on the DC side of every PSU. That may
or may not be enough (and what is enough on one or more bands may not be
on others. Use a sniffer like what I talked about in another post
yesterday to see if the noise is present on other wiring, If so, choke
it too. The noise produced by real world sources often varies a lot with
frequency, and antennas work differently with frequency and length.
There's a photo in the ARRL Handbook of a 10-outlet AC power
distribution box that I built with the components to standard electrical
backboxes for ganged switches and outlets, with a lot of PSUs for
computers and other equipment. There's a choke on the AC input to that
box, and on the DC side of each of the PSUs. The photos are also Fig 30b
in k9yc.com/RFI-Ham.pdf
The key to much of this is understanding how antennas work. If you need
a refresher, the ARRL Handbook and Antenna Book are great resources. The
first parts of almost every topic starts out with the fundamental
concepts and works on to more complex stuff.
Hope this helps.
73, Jim K9YC
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