On 9/28/13 8:23 PM, Avery Davis wrote:
Hans,
There are professional techniques for using Aluminum foil to shield a
room, and the trick is to treat the seams and penetrations.
Penetrations should be installed on a conducting plate (AL to match the
foil shield, but galvanized steel should be OK), and should use filtered
or shielded wiring. This website shows AL foil specifically for room
shielding, and shows depictions of seam treatment.
http://www.ramayes.com/aluminum_foil_emi_rfi_shielding.htm
Note that the foil there (2.5 mil) is substantially thicker than
household aluminum foil (0.5-0.6 mil)
.
that does have effect.. aside from the lower resistivity of the thicker
foil, you have to consider skin effect. A shield works by the incident
field inducing a current creating an opposite field in the conducting
plane. If the foil is thinner than, say, 5 skin depths, some of that
current is on the opposite surface, where it can radiate.
At 1 MHz (lightning frequencies) skin depth in copper is 2.6 mils and
aluminum is 3.2 mils
Note that for iron, assuming resistivity of 9.6 micro-ohm-cm (vs 1.67
for copper) and permeability of 100, the skin depth is 0.6 mils.
This is why galvanized steel is a popular shielding material: cheap and
very effective in thin layers.
Here are the seaming methods for AL foil (in order of effectiveness:
1. 3" overlap
2. No overlap, 6" AL seaming tape
3. 3" overlap plus 3" seaming tape
4. 4" wide aluminum tape that has a conductive adhesive.
I have also seen claims of good results using a folded seam.
But for lightning, the best thing you can do is to implement a single
point ground. This is a conductive panel to which all wiring and
cabling goes to before going anywhere else and has surge/lightning
protection and shield bonding, and which has an excellent grounding
bond. LIghtning is much more likely to follow a conductor into the
shack than penetrate a non-conducting wall or roof.
73,
Avery, WB4RTP
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