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In Reply to: RE: The AC safety-earth carries lots of noise. posted by Al Sekela on January 06, 2008 at 20:37:07
The connection to earth is a long way away for RF noise.Hi Al. A while back Steve Eddy challenged me to produce any reliable evidence that RF noise can be bled, drained, off to earth ground. I spent a few hours, off and on, on Google trying to find any credible evidence. Do you have any? I even talked to a Ham Radio operator about it.
Just for the heck of it I tried an experiment with 2 well powered walkie-talkies. I took one of the radios and grounded the antenna to a good earth ground. The grounding of the antenna had absolutely no effect on the performance of the radios.
As you know a Faraday cage does not need to be grounded to reject RFI. Don't most CD & DVD players have doubled insulated power wiring, and do not use an equipment ground connection?
Your thoughts appreciated.......
Edits: 01/08/08Follow Ups:
We both know the purpose of the safety-earth wiring, so I won't go into that again.
At 60 Hz, the wire lengths in typical houses are short compaed to the wavelength and there is no transmission line behavior to worry about.
However, at 100 MHz, the wires are long. They behave as poor transmission lines and the RF noise propagates along them. Some of the noise may be absorbed by the earth connection, but most of it is simply reflected at impedance changes. These are the points where the wiring goes through connections or past surrounding metal objects, etc.
A radio antenna may require an earth connection to function properly, but that is because the antenna is physically located at the connection, and is using the earth as a mirror. The long safety-earth wiring in a house would not be suitable for that kind of RF grounding, as you found with your experiment.
Most audio systems have ground loops, as more than one component uses the AC safety-earth. Even if there is no 60 Hz hum from the ground loop, the RF noise present on the AC safety-earth wiring gets into the ground loop and degrades the audio signal. This is why many components are double-insulated: they do not make the safety-earth connection and avoid the ground loop altogether. They are not built as Faraday cages, however, and are just as susceptible to broadcast RF noise as three-wire equipment.
> > They are not built as Faraday cages, however, and are just as susceptible to broadcast RF noise as three-wire equipment. < <
> > > > > > > > > > > > > >I was under the impression that is exactly what the metal enclosure around a CD and DVD player is. I would agree RFI can enter through the power cord. It also can go back out from the equipment back on the power cord and end up on the AC power line..
===============> > The long safety-earth wiring in a house would not be suitable for that kind of RF grounding, as you found with your experiment. < <
I preformed the experiment on a job site. The ground was the central grounding grid point in a large mechanical/electrical room. The earth ground was more than antiquate. I thought for sure when I grounded the antenna the radio would not transmit or receive. But as I said earlier it did not effect the radio one bit.
One thing I remember the Ham operator said about the earth absorbing RFI, he said if that was the case the transmission of radio waves wouldn't work. Especially AM.
Jim
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There are various holes for the CD drawer, the display, ventilation, and the mating edges of the case pieces are not designed to maintain conduction. In addition, the power cord inlet and the audio output jacks may allow noise to get in and out.
See the link.
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