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From: hughes@math.Berkeley.EDU (eric hughes)
Newsgroups: sci.electronics,rec.ham-radio
Subject: Re: Building a simple home circuit to read data off remote crt screen
Keywords: tempest, emc, rfi, correllator, pll
Message-ID: <19393@agate.BERKELEY.EDU>
Date: 24 Jan 89 00:02:41 GMT
References: <2010@cpoint.UUCP>
Sender: usenet@agate.BERKELEY.EDU
Reply-To: hughes@math.Berkeley.EDU (eric hughes)
Followup-To: sci.electronics
Organization: UCB Mathematics Department
Lines: 46
In-reply-to: die@cpoint.UUCP (David I. Emery)

In article <2010@cpoint.UUCP>, die@cpoint (David I. Emery) writes:
>
>	A year or so ago I saw a demo on TV (on ABC's Nightline I think) of
>what was refered to as a simple device that could be built with $25
>worth of parts that could recover an image of the text on a CRT display
>belonging a nearby PC or terminal from its spurious electromagnetic radiation.
>
>	The demo, done in London to circumvent strong US government opposition
>to demoing this technology on commercial TV, showed a somewhat noisy but
>quite readable image of text on the screen of a PC about 25 feet away from
>the loop antenna used to pick the signal up.
>
>	Does anyone know what the $25 circuit consists of ?
>Can it really be built for $25 ?

This very topic is the cover story in the Winter 1988 issue of _Abacus_ 
(vol 5 no 2).  The title of the article is "The Tempest over Leaking
Computers."  The author is Harold Joseph Highland.  Allow me to
quote a paragraph from the article:

     "Anyone can put together a primitive eavesdropping unit for
somewhat over $100.  All you need is a portable television set with a
voltage-controlled oscillator (VCO) tuner (one with no detents or
stops between the channels and no fine tuning control), a dipole
antenna with a 75-ohm balum (a balance transformer to match the
impedance of the antenna with that of a television set's cable), and a
75-ohm cable to connect the set to the antenna's balum.  The unit will
have a limited range, between 20 and 40 feet, and the reception may be
poor because the picture will be unsteady.  If extensions are used
with the vertical and horizontal hold controls for finer tuning,
picture quality will improve.  With some knowledge of television
design and construction, you could build a more efficient unit at
higher cost.  The circuit diagram of a TV set with a VCO tuner is
available in Sams Photofact #2218, a series used by many television
repair shops.

This equipment was designed by a man named Wim van Eck.  He "noted
that with slightlymore sophisticated and expensive equipment, it was
possible to intercept data from a VDT screen up to 1 kilometer away."

The article is a kind of weird mix between gossip, basic
electromagnetic theory, technical details, and government regulations.
Thoroughly interesting.

Eric Hughes
hughes@math.berkeley.edu