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

[Jim Brown]

On 9/25/2024 10:09 AM, David Eckhardt wrote:
> The RFI, as you may realize, is generated internally due to the switching
> power conversion.  The days of RFI quiet real transformers which contain
> iron and copper are unfortunately gone.
> 
> One side connects to the grid and the other, DC or LV side, connects to
> your equipment.  Any appropriate ferrites clamped onto either side will do
> some good.

Not quite. I don't know of any ferrite parts that by simply clamping 
them onto cables will do much to reduce radiation of RF trash below VHF. 
Ferrite parts are parallel resonant devices, and for most "suppression" 
parts, that resonance is in the VHF range, simply because that's where 
meaningful FCC Rules kick in. To be effective at HF or MF, multiple 
turns must be wound through the effective part to lower the resonance to 
the part of the HF spectrum where suppression is needed. Thanks to the 
nature of currently available products, which exhibit relatively wide 
manufacturing tolerances, the only REPEATABLY effective ferrite material 
I know of (and I've been looking for 20 years) is Fair-Rite #31. It is a 
MnZn material, with relatively low Q. Thanks to mfg tolerances, the Q of 
NiZn materials is too high for repeatability of designs/recommendations.

An engineering development and tutorial of all of this is in 
k9yc.com/RFI-Ham.pdf

I strongly agree with everything else in David's excellent post.

   But realize, ferrites are a band aid.  The Chinese engineers
> and suppliers do not spend additional funds on the wall warts to alleviate
> RFI.  Again, them's 'r do facts of ta'day.

Another important point -- the LAST thing we should be doing is going 
"inside the box" of an existing product. Anyone who's been involved with 
product development knows that there are a lot of detailed decisions 
that can interactively cause things to go much further south. :)

73, Jim K9YC