[RFI] Question about common mode noise

[Jim Brown]

On 1/20/2025 7:09 PM, Bob WA2SQQ wrote:
> He swears that if the noise increases when the shield is connected, that’s
> a positive indication you have an abundance of common mode noise.
> 
> I’m not so sure this is a valid test. Comments?

It's certainly an indication of failure to do proper grounding and 
bonding. Coax shields should go to a lightning arrestor where they enter 
the building, ideally on the exterior wall of the shack. The arrestor 
MUST be bonded to grounding for the building, which, BY LAW, must be 
bonded to all other grounds in the building, including entry for power, 
telephone, CATV, satellite dish, and any other antennas.

On the operating desk, all parts of the station must have their chassis 
bonded together by short, fat copper, and that must be bonded to the 
grounding for the building at the closest point. And all cable shields 
should be bonded to the chassis AT THE POINT where they enter equipment.

If you've done all this, any common mode current on the shield is going 
to go into that ground bus to all of the driven rods, not into your 
radio or other equipment in your shack.

Study N0AX's excellent ARRL book on the topic, to which I contributed 
extensively, and/or the pdf of slides for talks I've given at Pacifcon, 
Visalia, and to several ham clubs.

73, Jim K9YC