TEMPEST in a teapot

A note discussing the prevention of electromagnetic eavesdropping of personal computers.

Grady Ward (grady@netcom.com)
public key verification by PK server, finger, or by request

Version 1.0 22 March 93

TEMPEST is the code name for technology related to limiting unwanted electromagnetic emissions from data processing and related equipment. Its goal is to limit an opponent's capability to collect information about the internal data flow of computer equipment. Most information concerning TEMPEST specifications is classified by the United States Government and is not available for use by its citizens.

The reason why TEMPEST technology is particularly important for computers and other data processing equipment is the kinds of signals components in a computer use to talk to each other ("square waves") and their clock speeds (measured in megahertz) produce a particularly rich set of unintentional signals in a wide portion of the electromagnetic spectrum. Because the spurious emissions occupy so wide a portion of that spectrum, technologies used to block one portion of the spectrum (as pulling the shades closed on a window to stop the visible light portion) are not necessarily effective in another portion.

Unintentional emissions from a computer system can be captured and processed to reveal information about the target systems from simple levels of activity to even remotely copying keystrokes or capturing monitor information. It is speculated that poorly protected systems can be effectively monitored up to the order of one kilometer from the target equipment.

This note will examine some practical aspects of reducing the susceptibility of your personal computer equipment to remote monitoring using easily-installed, widely available after-market components.


One way of looking at TEMPEST from the lay person's point-of-view is that it is virtually identical to the problem of preventing electromagnetic interference ("EMI") by your computer system to others' radios, televisions, or other consumer electronics. That is, preventing the emission of wide-band radio "hash" from your computers, cabling, and peripherals both prevents interference to you and your neighbors television set and limits the useful signal available to a person surreptitiously monitoring.

Viewing the problem in this light, there are quite a few useful documents available form the government and elsewhere attacking this problem and providing a wealth of practical solutions and resources. Very useful for the lay person are:

Radio Frequency Interference: How to Find It and Fix It. Ed Hare, KA1CV and Robert Schetgen, KU7G, editors
The American Radio Relay League, Newington , CT
ISBN 0-87259-375-4 (c) 1991, second printing 1992
Federal Communications Commission Interference Handbook (1991)
FCC Consumers Assistance Branch
Gettysburg, PA 17326
MIL-STD-188-124B in preparation
(includes information on military shielding of tactical communications systems)
Superintendent of Documents
US Government Printing Office
Washington, DC 20402

Information on shielding a particular piece of consumer electronic equipment may be available from the:

Electronic Industries Association (EIA)
2001 Pennsylvania Ave NW
Washington, DC 20006

Preventing unintended electromagnetic emissions is a relative term. It is not feasible to reduce to zero all unintended emissions. My personal goal, for example, might be to reduce the amount and quality of spurious emission until the monitoring van a kilometer away would have to be in my front yard before it could effectively eavesdrop on my computer. Apartment dwellers with unknown neighbors only inches away (through a wall) might want to even more carefully adopt as many of the following suggestions as possible since signal available for detection decreases as approximately the inverse square of the distance from the monitoring equipment to your computer.


Start with computer equipment that meets modern standards for emission.

In the United States, the "quietest" standard for computers and peripherals is known as the "class B" level. (Class A level is a less stringent standard for computers to be use in a business environment.).

You want to verify that all computers and peripherals you use meet the class B standard which permits only one-tenth the power of spurious emissions than the class A standard. If you already own computer equipment with an FCC ID, you can find out which standard applies. Contact the FCC Consumers Assistance Branch at 1-717-337-1212 for details in accessing their database.

Once you own good equipment, follow the manufacturer's recommendations for preserving the shielding integrity of the system. Don't operated the system with the cover off and keep "slot covers" in the back of the computer in place.


Use only shielded cable for all system interconnections.

A shielded cable surrounds the core of control wires with a metal braid or foil to keep signals confined to that core. In the late seventies it was common to use unshielded cable such as "ribbon" cable to connect the computer with, say, a diskette drive. Unshielded cable acts just like an antenna for signals generated by your computer and peripherals. Most computer manufacturer supply shielded cable for use with their computers in order to meet FCC standards. Cables bought from third-parties are an unknown and should be avoided (unless you are willing to take one apart to see for yourself!)

Try to avoid a "rat's nest" of wire and cabling behind your equipment and by keeping all cables as short as possible. You want to reduced the length of unintended antennas and to more easily predict the likely paths of electric and magnetic coupling from cable to cable so that it can be more effectively filtered.


Block radiation from the power cord(s) into the house wiring.

Most computers have an EMI filter built into their body where the AC line cord enters the power supply. This filter is generally insufficient to prevent substantial re-radiation of EMI voltages back into the power wiring of your house and neighborhood. To reduce the power retransmitted down the AC power cords of your equipment, plug them in to special EMI filters that are in turn plugged into the wall socket. I use a model 475-3 overvoltage and EMI filter manufactured by

Industrial Communication Engineers, Ltd.
P.O. Box 18495
Indianapolis, IN 46218-0495
ask for their package of free information sheets

(AC and other filters mentioned in this note are available from a wide variety of sources including, for example, Radio Shack. I am enthusiastic about ICE because of the "over-designed" quality of their equipment. Standard disclaimers apply.)

This particular filter from ICE is specified to reduce retransmission of EMI by a factor of at least 1000 in its high-frequency design range. Although ideally every computer component using an AC line cord ought to be filtered, it is especially important for the monitor and computer CPU to be filtered in this manner as the most useful information available to opponents is believed to come from these sources.


Block retransmitted information from entering your fax/modem or telephone line.

Telephone line is generally very poorly shielded. EMI from your computer can be retransmitted directly into the phone line through your modem or can be unintentionally picked up by the magnetic portion of the EMI spectrum through magnetic induction from power supplies or the yoke of your cathode ray tube "CRT" monitor.

To prevent direct retransmission, EMI filters are specifically designed for modular telephone jacks to mount at the telephone or modem, and for mounting directly at the service entrance to the house.

Sources of well-designed telephone-line filter products include ICE (address above) and

Box 82
Randolph, OH 44265

Your phone company or telephone manufacturer may be able to supply you with free modular filters, although the design frequencies of these filters may not be high enough to be effective through much of the EMI spectrum of interest. Keep telephone lines away from power supplies of computers or peripherals and the rear of CRTs: the magnetic field often associated with those device can inductively transfer to unshielded lines just as if the telephone line were directly electrically connected to them. Since this kind of coupling decreases rapidly with distance, this kind of magnetic induction can be virtually eliminated by keeping as much distance (several feet or more) as possible between the power supply/monitor yoke and cabling.


Use ferrite toroids and split beads to prevent EMI from escaping on the surface of your cables.

Ferrites are magnetic materials that, for certain ranges of EMI frequencies, attenuate the EMI by causing it to spend itself in heat in the material rather than continuing down the cable. They can be applied without cutting the cable by snapping together a "split bead" form over a thick cable such as a power cord or by threading thinner cable such as telephone several times around the donut-shaped ferrite form. Every cable leaving your monitor, computer, mouse, keyboard, and other computer peripherals should have at least one ferrite core attentuator. Don't forget the telephone lines from your fax, modem, telephone or the unshielded DC power cord to your modem. Ferrites are applied as close to the EMI emitting device as possible so as to afford the least amount of cable that can act as an antenna for the EMI.

Good sources for ferrite split beads and toroids include

Amidon Associates, Inc.
P.O. Box 956
Torrance, CA 90508
(ask for their free information sheet)
Palomar Engineers
P.O. Box 462222
Escondido, CA 92046
(ask for their free RFI information sheet)

and Radio Shack.


Other practical remedies.

Other remedies that are somewhat more difficult to correctly apply include providing a good EMI "ground" shield for your computer equipment and other more intrusive filters such as bypass capacitor filters.

You probably ought not to think about adding bypass capacitors unless you are familiar with electronic circuits and digital design. While quite effective, added improperly to the motherboard or cabling of a computer they can "smooth out" the square wave digital waveform -- perhaps to the extent that signals are interpreted erroneously causing mysterious "crashes" of your system. In other cases, bypass capacitors can cause unwanted parasitic] oscillation on the transistorized output drivers of certain circuits which could damage or destroy those circuits in the computer or peripherals. Also, unlike ferrite toroids, adding capacitors requires actually physically splicing them in or soldering them into circuits. This opens up the possibility of electric shock, damage to other electronic components or voiding the warranty on the computer equipment.

A good EMI ground is difficult to achieve. Unlike an electrical safety ground, such as the third wire in a three-wire AC power system, the EMI ground must operate effectively over a much wider part of the EMI spectrum. This effectiveness is related to a quality known as electrical impedance. You desire to reduce the impedance to as low a value as possible over the entire range of EMI frequencies.

Unlike the AC safety ground, important factors in achieving low impedance include having as short a lead from the equipment to a good EMI earth ground as possible (must be just a few feet); the gauge of the connecting lead (the best EMI ground lead is not wire but woven grounding "strap" or wide copper flashing sheets; and the physical coupling of the EMI into the actual earth ground. An 8 ft. copper-plated ground may be fine for AC safety ground, but may present appreciable impedance resistance to an EMI voltage. Much better would be to connect a network of six to eight copper pipes arranged in a six-foot diameter circle driven in a foot or two into the ground, electrically bonded together with heavy ground strap and connected to the equipment to be grounded via a short (at most, several feet), heavy (at least 3/4-1" wide) ground strap.

If you can achieve a good EMI ground, then further shielding possibilities open up for you such as surrounding your monitor and computer equipment in a wire-screen Faraday cage. You want to use mesh rather than solid sheet because you must preserve the free flow of cooling air to your equipment. Buy aluminum (not nylon) screen netting at your local hardware store. This netting typically comes in rolls 36" wide by several feet long. Completely surround your equipment you want to reduce the EMI being careful to make good electrical bonds between the different panels of netting and your good earth ground. I use stainless steel nuts, bolts, and lock washers along with special non-oxidizing electrical paste (available from Electrical contractors supply houses or from ICE) to secure my ground strapping to my net "cages". A good Faraday cage will add several orders of magnitude of EMI attenuation to your system.


Checking the effectiveness of your work.

It is easy to get a general feeling about the effectiveness of your EMI shielding work with an ordinary portable AM radio. Bring it very close to the body of your computer and its cables in turn. Ideally, you should not hear an increased level of static. If you do hear relatively more at one cable than at another, apply more ferrite split beads or obtain better shielded cable for this component. The practice of determining what kind of operating system code is executing by listening to a nearby AM radio is definitely obsolete for an well-shielded EMI-proof system!

To get an idea of the power and scope of your magnetic field emissions, an ordinary compass is quite sensitive in detecting fields. Bring a compass within a few inches of the back of your monitor and see whether it is deflected. Notice that the amount of deflection decreases rapidly with distance. You want to keep cables away from magnetic sources about as far as required not to see an appreciable deflection on the compass.



If you start with good, shielded equipment that has passed the FCC level B emission standard then you are off to a great start. You may even be able to do even better with stock OEM equipment by specifying "low-emission" monitors that have recently come on the market in response to consumer fears of extremely low frequency ("ELF") and other electromagnetic radiation. Consistently use shielded cables, apply filtering and ferrite toroids to all cabling entering or leaving your computer equipment. Finally, consider a good EMI ground and Faraday cages. Beyond this there are even more effective means of confining the electrical and magnetic components of your system through the use of copper foil adhesive tapes, conductive paint sprays, "mu metal" and other less common components.

Copyright (c) 1993 by Grady Ward. All Rights Reserved.
Permission is granted for free electronic distribution.


Copyright 1994-1996 Quadralay Corporation. All rights reserved.

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