[RFI] wall warts and shielding the DC side - theory?

[Jim Brown]

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