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Tweakers' Asylum Tweaks for systems, rooms and Do It Yourself (DIY) help. FAQ. |
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Tweakers' Asylum Tweaks for systems, rooms and Do It Yourself (DIY) help. FAQ. |
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In Reply to: RE: The AC safety-earth carries lots of noise. posted by jea48 on January 8, 2008 at 18:27:53:
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.
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Follow Ups
- The wires are usually long compared to the wavelengths. - Al Sekela 19:46:56 01/08/08 (2)
- RE: The wires are usually long compared to the wavelengths. - jea48 21:08:27 01/09/08 (1)
- The cases are not designed to exclude relevant noise. - Al Sekela 21:31:08 01/10/08 (0)
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