From: Dave Emery <die@die.com>
Date: Mon, 9 Nov 1998 06:12:51 +0800
To: Jim Choate <ravage@EINSTEIN.ssz.com>
Subject: Re: TEMPEST laptops (fwd)
In-Reply-To: <199811050611.AAA10989@einstein.ssz.com>
Message-ID: <19981108155839.C15373@die.com>
MIME-Version: 1.0
Content-Type: text/plain



On Thu, Nov 05, 1998 at 12:11:20AM -0600, Jim Choate wrote:

> 
> 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.

	Lasers are a technology for generating light, not kinds of
radio waves.  And masers are a fairly obscure technology for amplifiying
weak microwave signals in cryogenicly cooled devices.  Both deal with
electomagnetic radiation and obey Maxwells laws ....

> 
> > Microwave ovens work by having the waves bounce around inside a box. Any
> > significant hole or crack (up to roughly half the wavelength) would let the
> > waves out.
> 
> Depends on the size of the hole and location. In most microwave ovens there
> are definite dead-spots (corners and the exact center of the area are
> notorius).

	Most microwave ovens incorperate a motorized metallic device
called a stirrer that sits directly in the beam of energy from the
magnetron and is designed to reflect microwave energy bouncing around
the cavity and change the standing wave pattern as it rotates, resulting
in much more even distribution of energy.   Without the stirrer hots
spots would be much worse...


> 
> > An open top box will not work.
> 
> If the microwaves (for example) are transmitted parallel to the open side it
> might very well work just fine. It's going to depend on a variety of
> factors that will preclude such a blanket statement from being valid.
> 
	Difraction becomes very significant for openings near in scale to
the wavelength of the energy in question, thus the edges of the top will
act to scatter energy in all directions...

> 
> The absolute magnitude isn't really important.
> 
> Most of the signals that are emitted by a computer are not in the 100dB
> dynamic range (@2x=3db that's a signal range of 1:33) , more likely 40-50db
> if that. For a TTL (5V) signal it barely covers 3dB (LOW is <2.5v and a
> high is >=4.75). There simply is no way in hell a signal with a 3dB range is
> going to emit a rf signal that is 100dB.

	This makes no sense whatsoever.  The EM radiation takes place
when changes in the current flowing happen.   Thus radiation occurs only
when the TTL signal changes state, not during either its high voltage or
low volage state.  The amplitude of the current step is determined by
the impedance of the circuit and how fast the logic switches and how
great the voltage or current swing is , and how effectively it gets
radiated is determined by the geometry of the conductor and its
sourounding ohjects.  EM radiation results only from changes in the
magnetic and electric fields, not from their steady state values.   Thus
it is entirely meaningless to talk of the change in steady state values
as only "3 db" when no radiation results from either the steady high or
steady low value.   And indeed near the conductor, the energy radiated
from the current step will be 100 db greater than the ambiant (decibels
are relative units, thus it is meaningless to talk of "radiating a rf
signal that is 100 db").


-- 
	Dave Emery N1PRE,  die@die.com  DIE Consulting, Weston, Mass. 
PGP fingerprint = 2047/4D7B08D1 DE 6E E1 CC 1F 1D 96 E2  5D 27 BD B0 24 88 C3 18