I woulk like to know why the shield in the sugnal cables should be grounded only at one end?

 

Ideally it would be grounded throughout the entire length. The exception would be in RF work when forming a 1/4-wave transformer, or 4:1 coaxial balun, or 1/4-wave stub filter and the like. Can you provide an example of where it is grounded at only one end?

 

The shielded cables I've worked with consist of a foil wrap around the insulated conductors in the cable (but under the outer cable sheath), with an uninsulated conductor running the length of the cable. (This is what n9xv is talking about, "grounded throughout the entire length".) It's designed to keep elctromagnetic interference (EMI) out of your cable conductors.

When the shield is grounded at one end, it provides a path for EMI, or essentially "noise", to go. The foil keeps any noise that's trying to induce itself into the circuit out of the conductors, and the uninsulated "ground" wire conducts the noise to a ground. (This in no way counts as any kind of a system grounding.)
If you were to ground both ends of your shield, you could essentially turn you noise shield into a noise antena.

I've experienced many problems with erratic signals, and poor operation of equipment that was traced to shielded wires that were either grounded at both ends, or not grounded at all.

 

Grounding at a single point has nothing to do with signal quality, but rather to make sure your grounded surface is not a conducting surface. Allowing 2 points of connections permits your grounded surface to now carry current between the 2 points of interconnection.

A single point of connection does not permit this and you get the reference voltage only on the grounded surface.

Say the 2 gound points of connection are slightly different voltages. One is 0 volts the other is 0.06 volts. Not much of a difference at all. But to a wire with little resistance, this is more than enough to get a good current flowing. Now you have a signal wire with another wire wrapping around it that is carrying current and making its own magfield at the same time.

 

Originally posted by Brandon@May 9 2005, 07:54 AM
Now you have a signal wire with another wire wrapping around it that is carrying current and making its own magfield at the same time.

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This is exactly what I was talking about when I said that grounding both ends can turn the shielded cable into a "noise antena". I've always had a hard time explaining shielding, so I think in the future I'll borrow from your more technically specific description, Brandon.