At 10:34 PM +0300 5/8/01, Sampo Syreeni wrote:
>On Tue, 8 May 2001, Tim May wrote:
>
>>>OTOH, Hams tend to work with considerably longer wavelengths, something you
>>>could not project with easily concealed devices. I think it's pretty evident
>>>that if you pour some 100kW on a 20-meter wave, you can wipe out just about
>>>anything electronic.
>>
>>Only if you can couple the power into the target in the right way.
>
>Started thinking about that about two seconds after hitting ^X. 20m is
>certainly too much to make a good counter-example to microwaves, but some
>octave higher a car should make a fine antenna. This still doesn't mean that
>it couldn't act as a Faraday cage at the same time, but the current induced
>might still mess things up, I think.
>
>>Simplest counterexample: Faraday-shielded enclosure. 100KW at 20m will
>>have no effect on devices inside.
>
>This is one of the things I never understood -- ideal Faraday, fine. But how
>about a physical mime, with finite, non-zero resistance and a physical limit
>on the speed at which charge can move around?
This is why "decibels" are on a log scale. While the "ideal Faraday
cage" is like the "ideal op amp," that is, unrealizable in the
physical world, the real world versions of Faraday cages are still
sufficient to attenuate power by a factor of many, many orders of
magnitude. (_Many_!). This means that that "100 KW" you cite now has
to be 100 GW, or even more, to have the same effect.
You should do some simple experiments with ordinary portable radios
in various kinds of Faraday cages. Crank the volume up full blast,
then put the radio into a metal garbage can and close the lid. (You
can test for the effects of sound attenuation, if you think this is
the reason the radio has fallen silent, by using a portable CD or
tape player in the same way, same location.)
I worked twice in my career inside Faraday cages. Once at UC Santa
Barbara in a SQUID lab, once at Intel on sensitive measurement
equipment. Both experiences taught me at a gut level how well Faraday
cages work. And I attended various conferences on nuclear and space
radiation effects where Faraday cages and EMP were discussed at great
length.
If you don't "believe" the theory, you'll have to consult a text on
electromagnetism. Or go on the Web. I can't convince you here.
>I mean, it should definitely
>leak some of the LF inside as the charge carriers cannot move with infinite
>speed along the perimeter of the cage, right?
>
>>I plan to make this my one and only response here to this latest thread.
>
>That's too bad. Iteration would be a positive thing for us newcomers.
Well, I lied, as I am responding again.
As for your above point, charges do not have to move with infinite
speed to attenuate E-fields by ten orders of magnitude. Work out the
numbers, or read the Web pages.
In practical terms, there are all kinds of graphs showing how much
radiated power ("Poynting vectors" and all that E & M stuff) is
inside a conducting sphere as a function of the type of conductor
("skin depth," mesh spacing, etc.).
(Hint: leakage through seals, joints, and mesh holes is vastly more
important than speed of light limitations, at least for pulses much
longer than it takes light to travel around the enclosure.)
My point was that any claim that 100 KW at 20 m aimed at a car will
incapacitate it is not proven.
This is way off of Cypherpunks topics, and it has been done many
times in the past. (About as often as threads about using thermite to
wipe hard disks...let's not start that now, OK?)
As I have recommended several times, please review the literature on
EMP and Faraday shielding and do some calculations to get some feel
for the issues. The December of every year issues of "IEEE
Transactions on Nuclear Science" has papers from the previous
summer's "Nuclear and Space Radiation Effects Conference." Though by
no means basic primer material, enough reading of the papers will get
the ideas across.
--Tim May
--
Timothy C. May [EMAIL PROTECTED] Corralitos, California
Political: Co-founder Cypherpunks/crypto anarchy/Cyphernomicon
Technical: physics/soft errors/Smalltalk/Squeak/agents/games/Go
Personal: b.1951/UCSB/Intel '74-'86/retired/investor/motorcycles/guns
