[RFI] Several USB cables - single toroid

[K9MA]

On 2/18/2024 8:45 PM, Pete Smith N4ZR wrote:
> I have about 8 USB cables in my station, two going directly to my 
> computer and 6 to a hub.  I suspect some RFI because periodically- 
> every couple of hours - a couple of USB devices - keyboard, mouse, 
> Winkeyer - stop working.  Stopping and re-starting N1MM does not cure 
> it - I have to restart Windows 10, and then it's OK again for a few 
> hours.  There doesn't seem to be any correlation between the band in 
> use and the problem - in fact, sometimes when I come back to the 
> station after hours away, with N1MM left running, what was working has 
> now stopped, and a computer restart is required.
>
> 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 ?
>
One problem I can see with several cables through one core is that the 
currents in the wires may be out of phase, rather that true common mode. 
Imagine the two halves of a zip cord dipole meeting at the core, then 
passing through it as a transmission line. The currents would just 
cancel each other out, resulting in no flux in the core, so the core 
would have no effect on this differential mode current. That current 
might have a big effect on your USB device, though. Now, if each of 
those wires went through a separate core, the currents would be attenuated.

One exception might be a bundle of cables from a common source, if that 
source is very small compared to a wavelength.

73,

-- 
Scott  K9MA

k9ma at sdellington.us