[RFI] Several USB cables - single toroid

[Jim Brown]

On 2/18/2024 6:45 PM, Pete Smith N4ZR wrote:
> I'm wondering if it is feasible/useful/foolish, with several USB cables 
> going to a single physical location (back of computer, front of 
> computer, hub) to wind the cables that terminate nearby together on a 
> single big 31 core, or do I need one core per cable to be useful ?

Hi Pete,

In general, I recommend one cable per core, primarily because the broad 
(at HF with #31 cores) parallel resonance strongly depends on 
capacitance between turns, conductor diameter, etc. Note also the photos 
and text in my "Killing Noise" application note; guidelines are the same 
for the gear as a source or victim.

IMPORTANT Note: the photos and associated text emphasize the critical 
importance of winding turns in order around the core; turns that 
"crossover" cancel. This is a fact I learned in the long study (2016-18) 
that produced the 2018 HF Cookbook.

I keep a stash of several sizes of #31 clamp-ons for this purpose. For 
these interconnecting cables and to most equipment, I like the 0.75-in 
i.d. part that W6EB turned me on to many years ago with a group purchase 
he organized. While it's larger than needed for smaller cables, it works 
fine, and the more we buy of a single part, the greater the discount 
from industrial vendors (which is the best place to buy them).

For about five years, I've been making most of my component purchases 
from Arrow, one of several good vendors who offer free overnight 
shipping for orders over $50 for anything that's in stock. When I need 
to add stuff to an order to hit that limit, I just add some cores. :)

73, Jim K9YC