From: Jim Choate <ravage@EINSTEIN.ssz.com>
Date: Fri, 6 Nov 1998 05:52:46 +0800
To: cypherpunks@EINSTEIN.ssz.com (Cypherpunks Distributed Remailer)
Subject: Re: TEMPEST laptops (fwd)
Message-ID: <199811052117.PAA14255@einstein.ssz.com>
MIME-Version: 1.0
Content-Type: text



Forwarded message:

> Date: Thu, 5 Nov 1998 11:56:20 -0500
> From: Petro <petro@playboy.com>
> Subject: Re: TEMPEST laptops (fwd)

> >It depends on the frequency. Last time I checked a laser or a maser (both
> >are radio waves strictly speaking) travel LOS. The scattering comes from
> >beam divergence and incidental refractions and reflections from the
> >molecules in the air and supported detritus.
> 
> 	And both Lasers and Masers scatter under certain conditions. If you
> can see the laser, it is scattering a bit of [energy light photons]

All EM radiation scatters *IF* the wavelength of the signal is comparable
to the wavelength of the particulate. If the particulate is less than about
1/3 wavelength it won't significantly scatter.

The reason that dust in the air scatters lasers (and masers don't have this
problem because the wavelength is too long) is that the particles are
significantly larger than the wavelenght of the signal. It is directly
analogous to moon-shine. In astronomy this is related to something called
the Poynting Effect.

This is also the reason that IR radiation goes through smoke and fog while
visible light won't. Raleigh Scattering is the reason that the fog appears
white.

In photography and radar mapping this is what limits the resolution of the
bounced signal as well.

You can build an experiment to demonstrate this with peg-board, pegs, and
some rope. Lay the peg-board on a table. Place a peg on one edge and connect
the rope. then along the line of the rope place other pegs at equal but
different distances and try to get a transverse plane wave to travel down
the rope. Note the relative wavelenght of the wave in the rope to the
spacing of the pegs. It takes practice to get it to work correctly so don't
give up to easy.

> 	I was under the (apparently false) impression that things like
> animal bodies, and ordinary building materials (like wood, Lathe &
> plaster/drywall) wouldn't stop or significantly bounce the beams back the
> way you didn't want them to go.

Depends on the material, angle of incidence, water content, frequency of the
signal, transmissivity of the material at that frequency, etc.

In the 10cm radar (used by tanks and ground troops to find each other) even
leaves will bounce a signal.

> 	I was thinking more of controling the direction of the RF, rather
> than trying to completely supress it.

Exaclty. The issue then is what do we do with the signal once we've got it
directed where we want it. For EM signals we want to get them to a ground
plane so they go away and don't exist anymore.

If it doesn't exist anymore you can't very well tap it.

> 	We're not cooking Hotdogs here, and we aren't using (for the most
> part) microwaves. Yes, you can heat a hotdog on a PII, but that is more
> from heat radiation than RF.
> 
> 	Microwave ovens also leak RF.

True enough, but the physics are applicable. Don't confuse cause with
effect.

As to microwave ovens leaking, yes but they leak orders of magnitude less
which makes them orders of magnitude harder to detect. That is the point to
TEMPEST after all, take advantage of the 1/r^2 law so the mallet has to be
sitting right in your lap to get a good field strength.



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