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

[Jim Brown]

On 9/25/2024 8:41 AM, Greg Troxel wrote:
> A related question is that given a physically small device that plugs
> in, how are these able to generate common-mode current, given the lack
> of a counterpoise for an endfed (which is of course not quite endfed,
> but very off center)?

The antenna, including counterpoise, is all of the wiring to which the 
noise source is connected, like mains wiring, wiring connected to 
whatever the source is connected. The relative field strength will 
depend on the spectrum of what is created inside the device, and the 
radiation efficiency of that random collection of conductors that 
constitute the antenna. :)

Hence my advice when encountering noise is to 1) carefully sniff the 
source and that wiring with a battery operated radio in the frequency 
range of the noise; and 2) start applying chokes to wiring at the source 
until noise is no longer sniffed on nearby wiring. My favorite sniffer 
is a Kenwood TH-F6A talkie with wideband RX. It's not very sensitive, 
but plenty good enough for up-close sniffing. Below 10 Mhz it 
auto-switches to a built-in loop stick, and there's a software switch 
between the loop and the antenna jack.

Inexpensive alternatives sold under the Tecsun brand will get you from 
the AM BC band to 30 MHz and FM broadcast, and one of their higher 
priced models will get you greater sensitivity and the AM aircraft band 
above that. One of those higher priced models (in the $150 range) was 
reviewed by ARRL Labs about 10 years ago.

73, Jim K9YC