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Path: utzoo!mnetor!seismo!rutgers!ames!ptsfa!well!msudoc!umich!itivax!m-net!michael
From: michael@m-net.UUCP (Michael McClary)
Newsgroups: sci.electronics,sci.physics
Subject: Re: Laser eavesdropping
Message-ID: <1218@m-net.UUCP>
Date: Sat, 25-Apr-87 04:12:26 EDT
Article-I.D.: m-net.1218
Posted: Sat Apr 25 04:12:26 1987
Date-Received: Sat, 2-May-87 09:24:14 EDT
References: <503@sw1e.UUCP> <704@brl-sem.ARPA> <16143@sun.uucp> <2632@phri.UUCP> <3275@burdvax.PRC.Unisys.COM>
Reply-To: michael@node.UUCP (Michael McClary)
Organization: McClary Associates, Ann Arbor, MI
Lines: 62
Keywords: modulation
Xref: mnetor sci.electronics:636 sci.physics:1287

In article <3275@burdvax.PRC.Unisys.COM> hal-k@burdvax.UUCP writes:
>> ... the Russians presented the American embassy folks with some sort of
>> carved wood plaque of an american bald eagle, or something like that, as a
>> gift...
>> It never occured to them that the sheet metal plate on the back of it might
>> actually be a microwave reflector...  
>
>I remember reading about the same event.  As I recall it was a desk ornament
>that was a resonant microwave cavity.  When the microwave energy was sent to
>it the response was a sound modulated signal of the sounds within the room.
>These signals were picked up by a ham radio operator in England who reported
>it to the authorities, eventually leading to the discovery of the device.

Yep (except it was an enormous wooden plaque of the great seal - with a few
tiny holes under the eagle's beak to pass the sound to the bug.)

Saw an ilustration of the gadget in a newspaper.  Looked like this in the
side view:

    H---------------H
    H   =========   H
    H       H       H
    H       H H     H
    H       H H=========================================================
    H       H H     H
    H       H       H
    H=======U=======H

The business on the left is a cylinder with the axis up the center of the
t-shaped thing in the middle.  Think of it as a metal top from an aerosol
can (the small type, about as big around as a stack of quarters), with
a metal diaphragm stretched over the opening, and a nail with a big head
that almost touches the diaphragm.  This forms a tuned cavity resonator.
The "nailhead" and the diaphragm form a (variable) capacitor that greatly
lowers the cavity's resonance, and tunes it back-and-forth slightly as
the diaphragm moves.

Think of the long horizontal thing as about the size and shape of a thin
knitting needle stuck through a hole in the side of the cavity (and
insulated where it passes through).  The long section forms an antenna,
and the T-shaped end couples strongly to the cavity.  A good length is a
quarter-wavelength for the resonant frequency.

(Now shrink the whole thing until you're using a cut-down thimble for
the aerosol can lid, and you've got it.  I hear they did one that looked
like an olive on a toothpick, too, and stuck it in a martini.)

If you transmit a carrier a little to one side of the resonance, this
thing will reflect it.  As the diaphragm moves back-and-forth, tuning
the cavity, it will reflect more or less of the carrier, effectively
amplitude-modulating it with audio from the room.  (Much like slope-
detecting an FM signal, but backwards - you modulate the frequency of
the filter, not the carrier.)

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