[Boatanchors] 2nd Newbie Q

[Gary Peterson]

“I've noticed that some construction articles involving the use of shielded cables advise that the shield be connected at only one end but they don't say which end, i.e. near or far. Also, when is this practice most useful and why?
Bob  AG5X”

Others have correctly stated that connecting the shield on both ends may create a “ground loop.”  If current is flowing through the shield, noise may be induced in the wires that one intends to protect.  This applies to balanced pairs, where the shield is not necessary in order to propagate the signal from the source to the load.  When building broadcast plants, with audio wiring within a hundred-fifty feet of a tower radiating 25 kW at about 1 MHz, experimentation revealed that the best success was most often achieved with the shield grounded at the source end, rather than the load end.  Sometimes, I had to ground the source end and ground the load end of the shield through a capacitor having low reactance at the operating frequency.  This capacitor effectively grounded the shield, for RF, at both ends, while breaking any DC or 60 Hz AC path.

Unbalanced cables (i.e., usually having RCA connectors) utilize the shield as one of the signal conductors.  These are replete with ground loop problems.  Hence, unbalanced cables are rarely able to be used in high RF fields.  Generally, in broadcasting, I avoided anything with RCA connectors for the inputs and outputs. 

Gary, KØCX