[RFI] Mystery noise

[Jim Brown]

On 8/13/2021 11:04 AM, David Eckhardt wrote:
> But, I have a rather monstrous common mode choke on the new 
> refrigerator/freezer power cord.  

Remember that effective common mode chokes work by exhibiting a low Q 
(broadband) resonance, so should be tuned (wound) to the range where 
suppression is needed.

> At least that was accessible unlike 
> the clothes washer/dryer and dish washer, and a few additional SMPS's in 
> other appliances.
> 
> And.....if I get serious about a specific radio astronomy associated 
> project, only the computer serving that project is on.  All others are 
> off and the internet is OFF, including the wireless router. Not just 
> OFF, but denied AC power!

A lot of our network infrastructure can be suppressed by choking 
connecting wiring, including, but not limited to, their power, coax from 
CATV internet and CAT5 between modem and router. DSL is a more difficult 
kettle of fish, as the service cable can radiate. Noise from their SMPSs 
can be eliminated by replacing them with suitably rated linear supplies. 
I go a step further by using the linear supplies to float-charge sealed 
lead-acid batteries.  I do the same with outboard video monitors. Mine 
were purchased about five years ago when Samsung and several other 
vendors were using 14VDC, and work fine in the 12-14 range.

I've used nothing but Thinkpads since about 2005. They're pretty good 
for RFI, although I recall one vintage that was nasty on 2M. Running 
with only their built-in screen and a wireless mouse eliminates more 
wiring that could radiate. I always choke PSU wiring and cables to 
external monitors.

73, Jim K9YC