BW said:
A friend of mine keeps his USB drives in biscuit tins, to protect
against a monster solar flare.
He said there were flares in 1859 and 1921 that would have crippled
lots of electronic devices, and there is a good chance of another
within our lifetime. And when it happens, banking and utilties would
crash, so you
might not be able to rush out a buy a replacement anyway.
So it got me thinking: how many devices are inside a metal box that
can shield them? I had at look at some PC cases. Obviously the wanky
gamer cases with perpex sides are out. And in many cases, the front
panel
is mostly plastic - covers for DVD or floppy bays, and a plastic mesh
air filter lower down.
So what cases have a front that is nearly all metal? And what of the
ventilation holes you see all over cases?
How small do the holes have to be offer protection?
The secret to any form of protection, is knowing what the threat is.
I wouldn't be "reaching for my biscuit tin", without knowing
whether it would truly be effective or not.
*******
Solar flares affect satellites, because the satellite is outside the
protective layers around the Earth (the atmosphere, the magnetosphere).
Solar flares also affect long-distance power transmission lines,
by inducing a current into them. Your computer does not have
long enough dimensions to be affected, but the connection
to the power grid, might be an exposure. (I don't know what
the rise time of such an event is. It could be slow enough,
the HVDC transmission system is destroyed, but other levels
of the power system, open breakers in time.)
If an actual radiation pulse made it through, it would likely
have as much affect on you, as on your computer. There are
other mechanisms, unlikely ones, that can do something like
that. I'd be more concerned about one of these events. Move
to the basement, and cover the ceiling with lead plate.
http://news.discovery.com/space/astronomy/8th-century-earth-was-blasted-by-gamma-rays-130207.htm
If the threat you were protecting against, was E1 from an EMP,
perhaps a Faraday cage constructed around the computer would suffice.
With a rise time of 5 nanoseconds, the fundamental is going
to be somewhere around 200MHz.
http://en.wikipedia.org/wiki/Electromagnetic_pulse
http://en.wikipedia.org/wiki/Solar_flare
Maybe you could find a biscuit tin as big as the computer,
but then, you'd have a *lot* of biscuits to eat.
Your microwave oven door, demonstrates the size of hole
to block 2.45GHz radio signals. Just to give some idea
how it's done. The door is a high pass filter, which cuts
off 2.45GHz or lower. Being a high pass filter, it passes
light (which is a very high frequency by comparison).
Changing the size of the hole, changes the filter corner
frequency. Apparently, the circular shape of the hole,
is so there is no filter performance dependence, on
microwave polarization.
(Figure 5-7 on page 63.)
http://ro.ecu.edu.au/cgi/viewcontent.cgi?article=1480&context=theses
While the fundamental of the E1 of the EMP, might be 200MHz,
you might still want to raise the high pass frequency of
the filter, just to ensure the large magnitude of voltage,
is filtered out. For all I know, a small mesh might still be
called for.
*******
Physicists like Faraday cages. There is an example of
one here. A physicist typically makes the cage large enough,
so the human operator can sit inside (so you can monitor
equipment or make adjustments).
http://upload.wikimedia.org/wikipedia/commons/f/fe/Home-made_Faraday_cage.jpg
We had those kinds of cages in the basement of our university
physics building. The Faraday cage used pure copper window screen,
which admitted fresh air. The seams are done with copper foil.
Stuff was likely soldered together, for good electrical performance.
Really very nice looking construction job. And at the time,
graduate students used to do their homework inside them. The
cage was big enough for a card table and chair, and grad
students would fight over who got to use them (it was a bit
of a joke amongst them - they have a strange sense of humor
anyway - you should try working there sometime...).
Paul
