[HCARC] Faraday Cage

[Kerry Sandstrom]

Gary,

I've spent many years working in and on shielded enclosures.  There are 
several reasons why it wouldn't work.  First of all, EMP and geomagnetic 
storms are low frequency events.  The geomagnetic storms have large 
frequency components of perhaps a few Hertz.  EMP from a high altitude 
nuclear detonation has frequency components almost entirely below 1 MHz and 
largely below 100 kHz.  The only way to shield from these low frequencies is 
with a large amount of ferrous metal, steel!  Copper won't do nor will 
aluminum.  Commercial (and expensive) shielded enclosures are only rated 
down to 100 kHz, and even at 100 kHz the attenuation is several 10's of dB 
below what it is at 1 MHz.  Modern shielded enclosures are often constructed 
of particle board sandwiched between sheets of copper plated steel.  Even 
with the steel, they provide almost no attenuation of magnetic fields. 
Shielded enclosures also have power line filters, waveguide below cutoff 
filters for air vents, and very strict methods for bringing coaxial and 
fiber optic cables in.  Telephone lines are seldom brought in but if they 
are they also require filters.  The filters are typically rated for 100 dB 
attenuation above 14 kHz.  Traditional copper wire screen rooms are even 
less effective at low frequencies.  I suspect aluminum screen would be less 
effective than copper screen.  The screens I've seen were about 1/8" or 
less.

I am quite familiar with pulse weapons.  Trust me, they are not a problem 
for us.  Their range is extremely limited unlike a high altitude EMP which 
covers a radius of 1000's of miles.  Unless you are an important target or 
the weapon is inaccurate by a 100 miles I don't think we have anything to 
worry about.  Incidently, the field strength level normally used for EMP 
testing is 50 kV/m.  For comparison, the electric field suceptibility level 
(MIL-STD-461) is about 200 V/m, some 48 dB less.

The main threat from both EMP and magnetic storms is conducted energy on 
transmission lines, power lines and telephone lines that are perhaps 100's 
of feet to miles long.  Unless you have excellent filters on all those lines 
and the filters are properly installed on a shielded enclosure, they will be 
ineffective.  The most effective way to protect your ham station from EMP is 
to diconnect everything: power cords, microphones, keys, antennas, etc. 
Several years ago FEMA or CD or somebody like that did testing on ham gear 
and basically concluded that it is survivable if all wires are disconnected.

By the way, a magnetic compass works just fine in a sheilded enclosure. 
Yes, a General did ask!

This information is all available in the technical literature on EMI/EMC 
(electromagnetic interference/elctromagnetic compatibility).  QST has had a 
few articles on EMP effects and protection during the cold war years.  The 
last real US test of EMP was a device called "Starfish" about 61-62 time 
frame.  It was launched from Johnston Atoll and was above the horizon in 
Hawaii.  A 1976 Los Alamos Scientific Laboratory publication lists Starfish 
as 1.4 Megatons at an altitude of 400 km.   This was the most powerful and 
except for a series of very small experiments called Argus, highest EMP 
test.  The various test ban treaties have prevented any more test of high 
altitude EMP by all parties.

That should keep you busy for a while!

Kerry