| Originally Posted by Rod Gervais |
However - I think you also got some bad advice...... copper shielding can be pretty effective for electromagnetic fields.
Originally Posted by Speedskater
There is a whole bunch of fine print about the special conditions under which this is true.
Now lets look at some of the conditions when it's true: copper shielding can be pretty effective for electromagnetic fields.
Cables, Transmission Lines, and Shielding for Audio and Video Systems
by Jim Brown http://audiosystemsgroup.com/TransLines.pdf
Let’s take the simple case where our interfering signal is an AM radio station a mile away
transmitting on 1 MHz. We are in the far field, so it is a plane wave, with the
magnetic and electric field in balance, and the wave impedance is 377 ohms. Steel conduit
would provide a reflection loss of 70 dB; copper or aluminum would provide about 110 dB
When our wiring is in the near field of a current source, the magnetic field is much stronger
than the electric field, so the wave impedance will be quite low and there is little if any reflection
loss. A quick examination of Fig 13 shows that the far field begins at about 400 miles
from a 60 Hz source, and about 1,500 ft from a 100 kHz source. Thus, in the real world, we
are always in the very near field of the power system equipment, so the wave impedance for
these fields is quite low!
On the other hand, when our audio system is in the near field of an interference source that
has high voltage but low current, the wave impedance will be quite high. In this situation, the
electric field predominates, and the dominant coupling mechanism will be capacitive. This
allows a thin copper or aluminum shield to be quite effective.
The above from pages 17/18. This is a very technical paper, not light reading at all. If you miss some of the fine print, you'll get it all wrong.