From: Brian D Williams (talon57@well.com)
Date: Thu Sep 16 1999 - 11:57:18 MDT
From: ronkean@juno.com
>If a 10 foot by 10 foot aluminum sheet were ungrounded and hit
>facewise with 1 MHz (300 meter) EM radiation, the EM field would
>not be attenuated much at all, even right next to the sheet, in
>the center, in the lee (shadow). This is because the sheet is
>much smaller than the wavelength, and ungrounded.
>But if a 10 foot by 10 foot aluminum sheet were ungrounded and hit
>facewise with 1 GHz (0.3 meter) EM radiation, the EM field would
>be much attenuated in the lee of the sheet, for a receiver close
>to the sheet and near the center of the sheet. This is because
>the sheet is much larger than the wavelength, and highly
>conductive.
ECM 101, and of course Ron is correct in these examples, if you use
a mesh, the openings have to be smaller than the wavelength you are
trying to block.
This of course is why you have a perforated metal screen in the
front of your microwave oven, the wavelength is larger than the
openings in the mesh, so they can't escape and you can safely watch
so you don't burn your mac'n'cheese.....
Brian
Member, Extropy Institute, www.extropy.org
Life Extension Foundation, www.lef.org
National Rifle Association, www.nra.org, 1.800.672.3888
Mars Society, www.marssociety.org
Ameritech Data Center Chicago, IL, Local 134 I.B.E.W
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