NBSIR 75-813

ELECTRONIC EAVESDROPPING TECHNIQUES AND EQUIPMENT

R. N. Jones

Electromagnetics Division

Institute for Basic Standards

National Bureau of Standards

Boulder, Colorado 80302

June 1975

Prepared for:

National Institute for Law Enforcement and Criminal Justice

Law Enforcement Assistance Administration

Department of Justice

Washington D. C. 20537

NBSIR 75-813

ELECTRONIC EAVESDROPPING TECHNIQUES AND EQUIPMENT

R. N. Jones

Electromagnetics Division

Institute for Basic Standards

National Bureau of Standards

Boulder, Colorado 80302

June 1975

Prepared for:

National Institute for Law Enforcement and Criminal Justice

Law Enforcement Assistance Administration

Department of Justice

Washington D. C. 20537

U.S. DEPARTMENT OF COMMERCE, Rogers C. B. Morton, Secretary

NATIONAL BUREAU OF STANDARDS Richard W Roberts Director

ELECTRONIC EAVESDROPPING TECHNIQUES AND EQUIPMENT

CONTENTS

Page

1. INTRODUCTION 1

2. RADIATING DEVICES AND RECEIVERS 2

2.1 General 2

2.2 Operating Frequencies 3

2.3 Power Output, Operating Time and

Operating Distance 10

2.4 Crystal Control vs. Free Running Oscillators 16

2.5 Automatic Volume Control 16

2.6 Modulation and Audiofrequency Response 17

2.7 Frequency Deviation 17

2.8 Packaging 18

2.9 Ease of Detection 18

2.10 Antennas 19

2.11 Propagation Conditions 20

2.12 Body-Mounted Transmitters •••• 21

2.13 Telephone Tapping Transmitters 24

2.14 Receivers 25

2.15 Advantages and Disadvantages of Body-Worn

Transmitters and Receivers 28

3. TESTS OF RADIATING DEVICES 29

3.1 Laboratory Tests of Body-Worn Transmitters 29

3.2 Field Tests of Body-Worn Transmitters 43

4. NON-RADIATING DEVICES 55

4.1 General 55

4.2 Telephone Taps 56

4.3 Concealed Microphones 58

4.4 Advantages and Disadvantages of Wired Surveillance

Systems 62

5. TAPE RECORDERS 62

5.1 Open-Reel Tape Recorders 62

5.2 Cassette Tape Recorders 63

5.3 Telephone Recording Actuator 63

6. CONCLUSIONS AND RECOMMENDATIONS 63

APPENDIX A—ANNOTATED BIBLIOGRAPHY 66

iii
FIGURES AND TABLES

ELECTRONIC EAVESDROPPING TECHNIQUES AND EQUIPMENT

Page

Figure 1. Model communication system 4

Figure 2. Length (in air) of quarter-wavelength antennas

for the frequency range 25 to 600 MHz 6

Figure 3. Normalized range as a function of signal loss

in decibels 13

Figure 4. Battery test circuit.. 15

Figure 5. Series-connected telephone transmitter 26

Figure 6. Parallel-connected telephone transmitter 27

Figure 7. Decay of output power and battery voltage for

transmitter no. 1 51

Figure 8. Output spectrum of transmitter no. 1 32

Figure 9. Decay of output power and battery voltage for

transmitter no. 2 33

Figure 10. Output spectrum of transmitter no. 2 34

Figure 11. Decay of output power and battery voltage for

transmitter no. 3 35

Figure 12. Output spectrum of transmitter no. 3 36

Figure 13. Decay of output power and battery voltage for

transmitter no. 4 37

Figure 14. Output spectrum of transmitter no. 4 38

Figure 15. Decay of output power and battery voltage for

transmitter no. 5 39

Figure 16. Output spectrum of transmitter no. 5 40

Figure 17. Decay of output power of transmitter no. 6

using alkaline and carbon-zinc batteries 44

Figure 18. Test setup for field performance studies of

body-worn transmitters 46

Figure 19. Transmitter position for measurement no. 2 48

Figure 20. Transmitter position for measurement no. 3 49

Table 1 Laboratory Test Results 30

iv
FOREWORD

The National Bureau of Standards (NBS) has established

a Law Enforcement Standards Laboratory (LESL) for the National

Institute of Law Enforcement and Criminal Justice (NILECJ) of

the Law Enforcement Assistance Administration, Department of

Justice. LESL's function is to conduct research that will

assist law enforcement and criminal justice agencies in the

selection and procurement of quality equipment.

In response to priorities established by NILECJ, LESL

is (1) subjecting existing equipment to laboratory testing

and evaluation and (2) conducting research leading to the

development of several series of documents, including national

voluntary equipment standards, user guidelines, state-of-theart surveys and other reports.

The Electromagnetics Division of NBS prepared this report

under the direction of Marshall J. Treado, Program Manager

for Communication Systems and Jacob J. Diamond, Chief of LESL.

This document, Electronic Eavesdropping Techniques and

Equi pment , describes various techniques and types of equipment

used for electronic voice surveillance. It is intended to

serve as a background reference and information guide for

users and prospective users of equipment.

v

ELECTRONIC EAVESDROPPING TECHNIQUES AND EQUIPMENT

1. INTRODUCTION

Voice surveillance using electronic techniques has become

an essential activity of law enforcement organizations at

almost all levels of government. Often the lives and safety of

law enforcement officers, and others as well, depend upon the

reliability of miniaturized and concealed communications devices.

Furthermore, the apprehension of criminals and the successful

prosecution of criminal cases can depend upon evidence obtained

by these techniques. These considerations clearly indicate the

importance of this type of equipment.

The advertising, manufacture, and distribution of equipment

intended for use in surreptitious information-gathering is

controlled by title III of the "Omnibus Crime Control and Safe

Streets Act of 1968" (Public Law 90-351) as amended. One of

the objectives of this law is the prohibition of surreptitious

information-gathering by all except law enforcement agencies.

In addition to protecting the public against the undesirable

use of such devices, the law has made it more difficult for law

enforcement agencies to procure and use them legitimately.

Nevertheless, this type of equipment is being manufactured,

purchased, and used. One discouraging aspect of the situation

is that some of the devices being manufactured and sold are of

very poor quality and are immensely overpriced.

IMPORTANT NOTICE

All transmitting equipment must comply with the licensing

and operating requirements of Part 89 of the Federal Communications

Commission (FCC) Rules and Regulations, including Section 89.117(b)

which requires type acceptance. Such equipment may not be

marketed prior to a grant of type acceptance by the FCC. Proscriptions concerning the marketing of radi of requency devices

are contained in Volume II, Part 2, Subpart I of the FCC Rules

and Regulations. (See Title 47, Code of Federal Regulations,

Part 89, Subpart A and Part 2, Subpart I.) Court-ordered

eavesdropping requiring radi of requency devices may be accomplished

either by the use of type accepted transmitters operating on

specific frequencies authorized to the licensee, or by use of

low power transmitters which conform to the requirements of

Part 15, Subpart E of the FCC Rules and Regulations.
This report has a two-fold purpose: first, to help inform

law enforcement personnel concerning the application and

functioning of undercover communications equipment and, second,

to be the forerunner of a voluntary standard to be used in the

selection, evaluation, and procurement of such equipment.

The information contained in this report has been derived

from a number of sources including law enforcement personnel,

manufacturers, publications, and laboratory measurements. This

report is concerned with the types of devices available and

their application, capabilities, advantages, and disadvantages.

An attempt has been made to bring together a body of

information which will assist the police officer in performing

his duties safely and effectively. Although the variety of

circumstances under which such equipment may be used is

seemingly unlimited, there are some basic technical rules,

facts, and techniques which are generally applicable in most

situations. Understanding and utilizing scientific techniques

can go a long way toward the realization of success in an undercover surveillance situation.

While the topic of immediate interest is surveillance or

offensive activity, coun tersurvei 1 1 ance or defensive activity

cannot be ignored completely because of the strong influence

these activities have on each other. Being involved with

either of these activities requires a sound knowledge of the

other. While a detailed treatment of coun tersurvei 1 1 ance is

not within the scope of this report, it will be mentioned where

it is of importance in the understanding of surveillance

equipment and its use.

The equipment and techniques discussed herein are concerned

only with the interception of voice or audio communication,

and, therefore, no information is included on video or optical

techniques. An annotated bibliography is provided as a guide

to further reading.

2. RADIATING DEVICES AND RECEIVERS

2.1 General

In this category is the miniaturized transmitter which may

be worn on the body, concealed in some stationary location, or

concealed in a vehicle or portable container of almost any

type. Because of this versatility, the miniature transmitter

is used in a wide variety of situations. These transmitters

2
are particularly useful in circumstances where unrestricted

mobility is required. In contrast to hard-wire devices (see

section 4.0), the miniature transmitter is most often used when

operation is required for only a relatively short period of

time, such as a few minutes or hours rather than days or weeks.

This time limitation is imposed primarily by the lifetime of

the power supply.

The use of miniature transmitters brings into play a great

many factors which can cause performance to vary widely in

seemingly similar circumstances. In this context, it is useful

to look at a complete communications system involving the

transmitter, the propagation medium and the receiver. Figure 1 is highly oversimplified; relaying information successfully

from one location to another is an involved procedure. It has

been estimated by users of typical equipment that the causes of

failure are approximately as follows:

Because surveillance needs often arise on short notice and

adverse conditions are frequently encountered, it is imperative

that persons using surveillance equipment become familiar with

all aspects of its use. The importance of well-trained

personnel can hardly be overemphasized.

2.2 Operating Frequencies

With few exceptions, miniature transmitters used in police

work operate in the following four frequency ranges:

Power supply failure (batteries)

Operator error

Equipment failure (circuitry)

50%

45%

5%

25

88

150

400

50 MHz

120 MHz

174 MHz

512 MHz

3
Transmi tter Propagation Medium Receiver

1 . Power supply

(usually batteries)

2. Electronic circuitry

such as the oscillator,

modulator, amplifiers,

and filters

3. Antenna

1 . Atmosphere 1 ncl udi ng all

physical obstructions and other

radio signals

1 . Power supply

2. Electronic circuitry such as the local oscilla

tor, speaker,

amplifiers and

f 11 ters

3. Antenna

Fi gure 1 Model communication system.

4
Some experimental work has been conducted over the past few

decades to determine which frequencies are best from a propagation standpoint, but the investigators have not produced

conclusive evidence indicating that one frequency is better

than another in a majority of circumstances. One such

investigation was conducted by Rice [17], who comDared

propagation into buildings at 35 '1Hz with that at 150 MHz. In

this study, 150 MHz was found to be slightly better on a statistical basis, but the difference was so small that the

issue must still remain in doubt as far as any specific

situation is concerned. Other work was reported by Huber [7]

regarding the propagation behavior of frequencies from 0.1 to

1.0 GHz as related to communication from the inside of

automobiles. This study clearly showed some freauencies to be

superior to others, but factors other than freauency influenced

the measurements so that definite conclusions concerning

frequency selection could not be drawn.

For a number of reasons, the trend has been toward the

higher frequencies. The use of higher freauencies is

advantageous because shorter antennas are easier to conceal.

Figure 2 shows the length of quarter-wavelenqth antennas in air

for frequencies in the range from 25 to 600 MHz, with the

specific frequency bands mentioned above shown as the solid

portions of the curve. Note that at 25 MHz a quarterwavelength antenna would be 300 centimeters ( a Dproxi ma te 1

y

10 feet) long, while at 500 MHz it is only 15 centimeters

(approximately 1/2 foot) long. The use of antennas shorter

than a quarter-wavelength reduces radiation efficiency, thus

reducing the effective communication range.

Interference must also be taken into consideration in

frequency selection. Both man-made and natural interference

decrease significantly with increasing frequency, whicn further

strengthens the tendency toward the use of higher freauencies.

As more use has been made of radios for communication purposes,

the lower frequency bands have become more crowded, constantly

increasing the chances for interference or interception. *\ s a precaution, an FM receiver can be used to monitor the frequency

spectrum in the location where the undercover operation is to

take Dlace, and an unused frequency can be selected.

The use of higher freauencies improves the signal-to-noise

ratio. As solid state technology improves and better devices

and tecnniques are developed, the use of frequencies above the

UHF region will almost certainly become common practice for

surveillance work.

3
1000

800

600

400

n 200

i >-o

_ o

- o

UJ

OS

100

80

60

40

20

10

\

50 100 150 200 250 300

t/4 - CENTIMETERS

Figure 2. Length (in air) of quarter-wavelength antennas

for the frequency range from 25 to 600 MHz.

6
2.2.1 25-50 MHz Band

The use of frequencies in this band for surveillance

purposes is decreasing. There are several reasons for this.

Principally, this band offers poor security compared to higher

frequency bands. The 25 to 50 MHz band is very heavily used,

and this has greatly increased the possibility of either

undesired detection or interference from other signal sources.

In addition, these frequencies are reflected by the night-time

ionosphere, further increasing the problems of interference and

security. These disadvantages, coupled with the requirement

for fairly long antennas which are difficult to conceal, have

caused surveillance personnel to abandon these frequencies for

higher ones.

Harmonic radiation is an additional disadvantage of

operation in the 25-50 MHz band. Transistor oscillators are

characteristically rich in odd-number harmonics, and unless the

output is filtered, their use can lead to difficulties for

other users. The third harmonics of frequencies between 29.3

and 40 MHz fall within the 88 to 120 MHz band. Unfiltered

transmissions can thus result in detection by receivers

operating in the 88 to 108 MHz commercial FM band or

interference with aircraft communication.

2.2.2 38-120 MHz Band

This frequency range includes the commercial broadcast FM

band extending from 38 to 108 MHz. Receivers of good quality

are mass produced and may be purchased at prices far below

those of receivers of comparable quality on other frequency

bands. Also, some of these receivers can be tuned to operate

above the commercial FM band, at frequencies as high as 120

MHz. For these reasons, the 88 to 120 MHz frequency range is

attractive, and has been widely used for surveillance purposes.

There are, several reasons however, why such use is

undesirable. With millions of receivers in this band in

constant use, it is evident that this frequency range affords

less security than almost any frequency band that could be

chosen. Also of importance is the fact that the commonly

available commercial receiver is not crystal controlled and,

therefore, more subject to drift with the attendant loss of

signal strength. A second reason for avoiding this frequency

band is that the portion from 103 to 120 MHz is used for

7
aircraft landing systems, and any interference, especially in

the vicinity of airports, can be very dangerous. Some

electronic eavesdropping equipment manufacturers produce and

sell equipment which operates at these frequencies, and

articles have been written which openly advocate use of the

aircraft band [3]. While there may be no intention to create a hazard, it is evident that once a device is sold, the

manufacturer loses all control over its use. The best policy

is not to use equipment operating in this range. It is not

only a potential hazard, but is also in violation of FCC and

FAA regulations.

2.2.3 150-174 MHz Band

Surveillance equipment utilizing these frequencies is

usually more sophisticated in design than that designed to

operate at lower frequencies. Most of the higher quality bodymounted transmitters and receivers manufactured during the past

few years operate within this band. When comparing this

frequency range with the lower ones already discussed, several

advantages appear. First, better security exists because there

are fewer receivers in existence. Next, interference is lower

because of lighter communication traffic, lower incidence of

ionospheric reflection, and lower levels of natural and manmade noise. Finally, higher radiation efficiencies can be

achieved using shorter antennas.

2.2.4 400-512 MHz Band

Even lower interference is likely in this band and

detectors which might lead to discovery are relatively rare.

However, some reception difficulties can be expected in areas

where physical obstructions are numerous. The shorter antennas

required may be an overriding consideration because, at these

frequencies, a quarter-wavelength antenna can be easily

concealed.

2.2.5 Summary

Following is a summary of the significant points regarding

the four frequency ranges:

8
25 to 50 MHz

Advantages

For sane power, possibly a. better coverage in b.

crowded areas.

c

.

d .

Disadvantages

Recuires longer antennas.

A much-used freauency band,

with consequently poor security.

Sky wave propagation at night

may result in undesired signal

transmission or reception.

Third harmonic radiation.

88 to 120 "Hz

Advantages

a. Low cost receivers a. available. b.

c

.

Di sadvantaqes

Violates FCC regulation.

Minimum se c u r i ty

;

easily monitored because of

availability of receivers.

108 to 120 MHz may interfere

with aircraft navigation and

communication.

1 50

A dvantages

a. Quarter- wavelength

antennas short enough for

concealment on body.

b. Good equipment available.

c. 3 o o d security compared to

lower frequencies.

to 174 MH zDisadvantages

a. EauiDment is expensive.

b . Band heavily used in many

areas of the country

.

400 to 512 MHz

Advantages Disadvantages

a. Rarely monitored; best a. Equipment is expensive,

from security standpoint. b. Few sources of equipment.

b. Requires very short

antennas.

c. Low level of interference.

9
2.3 Power Output, Operating Time and Operating Distance

The maximum practical distance between transmitter and

receiver for satisfactory communication can be very Important

in surveillance applications. This distance is dependent upon

a number of things including transmitter power output, gains of

transmitting and receiving antennas, and receiver sensitivity.

Still other factors are the local conditions such as the

terrain, the conductivity and dielectric constant of the earth,

the building density, and the heights of the antennas. The

fact that any or all of these factors may vary with time and

location makes accurate prediction of operating distance nearly

impossible. General statements are often made about the

operating distance or coverage area of a transmitter, but it is

unwise to rely heavily upon information of this type. Such

statements are likely to be based on a few specific experiences

in a given locality, and performance may be found to differ

widely 1n different situations. Claims about operating

distance are not dependable criteria for determining the

effectiveness of a transmitter under a given set of conditions

and should not be used to determine the superiority of one

transmitter over another.

There are two approaches to objective measurement of

transmitter output. One is to make comparative field strength

measurements under carefully standardized conditions, and the

other is to measure the transmitter power into a 50 ohm

termination. Either approach may present difficulties. The

field strength approach requires a measurement site free of

obstruction and a receiver or field strength meter. The

intensity of the field radiated by each transmitter must be

measured and plotted over a period of time. This method

presents several problems, including the determination of the

proper type antenna to use to achieve accurate data, the

necessity for receiver calibration 1n the case where the

transmitters to be compared operate on different frequencies,

and the possibility of other radio signals on the same

frequency leading to erroneous field strength measurements.

Clearly, this is not an ideal approach unless field test

facilities and highly skilled personnel are available.

I 0
Using the power output measurement approach 1s much

simpler, but not without possible complications. The procedure

is to connect a good quality power meter to the antenna output

terminal of the transmitter and record the power over a specified time interval. However, power meters do not yield

accurate results unless the output impedance of the transmitter

matches the input impedance of the power meter. The input

impedance of most power meters is 50 ohms. Providing the

output impedance of the transmitter is also 50 ohms, an adapter

from the transmitter output connector to the power meter input

connector is usually all that is needed to make the

measurement. If the Output impedance of the transmitter is not

50 ohms, some sort of a matching or tuning network is

necessary. The tuning network is inserted between the

transmitter and power meter and is varied until the power meter

reading is maximized, after which the measurement can proceed.

It is common to encounter miniature surveillance

transmitters which are neither 50 ohms at the antenna output

connector nor equipped with a coaxial connector. Some do not

have a ground connection but instead have only a screw terminal

for the attachment of an antenna wire. With such a combination

of output connectors and terminal conditions, the task of

making meaningful measurements becomes extremely difficult.

There is a need for standardization if there is to be

meaningful and objective comparison of equipment performance.

Ideally, there should be a convenient and straightforward means

of measuring transmitter power output without ambiguity, and

specifications should spell out not only maximum transmitter

power output but also some indication of how this power

decreases with transmission time.

One final, important point regarding the measurement of

transmitter power output using a power meter is the manner in

which the power is distributed over the frequency spectrum.

Because most power meters utilize a broadband resistive

element, such as a bolometer, as the sensing device, the power

measurement alone will not provide sufficient information. If

a particular transmitter has an output spectrum rich in

harmonics, it will not be distinguishable from a second

transmitter which shows the same power output but has had the

harmonic frequencies drastically suppressed. It is possible to

have a situation where as much as one-half of the power

radiated is at the harmonic and other spurious frequencies.

Such spurious radiation is detrimental in that it substantially

increases the risk of detection.
Figure 3 [9] is a plot of operating range versus signal

loss in decibels (dB), with the point 1.0 on the x-axis of the

graph representing a maximum range, whatever this may be in

actual distance. It can be seen that a decrease of 6 dB in the

received signal strength corresponds to an effective reduction

of this range by one-half. A loss of 20 dB reduces the range

by a factor of 10. This illustrates how drastically the

operating range can vary from one situation to another,

especially when it is realized that many things such as building density, antenna types and orientations, and antenna

position with respect to the body of the wearer can all

contribute losses of several dB. Thus, claims about operating

range do not offer a reliable basis on which to compare

equipment. The best procedure is to make comparative

measurements under identical conditions. Only then can

superior performance be recognized.

2.3.1 Batteries

Most body-worn transmitters and receivers can be equipped

with a number of different types of batteries. Even though

there are wide differences in their chemical composition, these

batteries are near enough to the same physical shape and size

that they may be interchanged or adapted to fit. This permits

the user to select the best type for a particular application.

Manufacturers of the equipment usually recommend the most

advisable type to use. The importance of battery quality is

paramount, and the user should insure that the use of poor

batteries is avoided. Detailed information on batteries is

available [8]. Even recently purchased batteries may have

already suffered sufficient shelf-life deterioration to cause

significant degradation in the performance of a transmitter or

receiver.

With undercover surveillance equipment, such as body-worn

transmitters and receivers, the two most frequently used types

of batteries are the alkaline and the mercury primary

batteries. Ni ckel -cadmi urn batteries, which are rechargeable,

have not as yet found wide acceptance in undercover work. The

carbon-zinc or common flashlight battery is generally not

suitable because of its low capability for meeting high-current

drain requirements. Alkaline and mercury batteries are

desirable mainly because they are able to supply relatively

large amounts of energy for their size. Alkaline batteries are

less expensive and more readily available than mercury

batteries. However, body transmitters using mercury batteries

usually have a less rapid decay in power output (see figures 11

and 15) .

I 2
RANGE

Figure 3. Normalized range as a function of signal loss in decibels.

I 3
A good measure of the condition of a battery is its ability

to sustain a moderate current drain. One test that may be used

on a 9-volt alkaline battery is to determine whether it will

supply 75-30 mil 1 i amperes of current for a period of 45

seconds. This can easily be done using the circuit shown in

figure 4. If the battery is at an acceptable state of charge,

the voltmeter indication should not drop below 9 volts for a period of 45 seconds.

It is not appropriate to test mercury batteries in the same

manner because mercury batteries sustain a nearly constant

voltage under load until they are almost completely discharged.

Therefore, even though a mercury battery has undergone

considerable use and is near the end of its lifetime, the

current drain test might not indicate it. At present there

appears to be no simple and reliable method for determining the

s tate-of-charge of a mercury battery. Mercury batteries,

fortunately, have a shelf-life of approximately two and onehalf years. The best way to be confident of the freshness of a mercury battery is to make sure that its open terminal voltage

is what it should be and to use batteries with fairly recent

dates of manufacture. To avoid the possibility of battery

failure during an operation requires vigilance. Points to

remember about batteries include the following:

a. Know the milliampere hours the battery can supply and

the transmitter current drain.

b. Make sure the batteries are fresh. If possible, use a fresh battery for each mission.

c. Keep a log of battery life expended.

d. For improved shelf-life, batteries should be stored at

temperatures well below normal room temperature.

Neither the alkaline nor the mercury batteries are

rechargeable and, once discharged, must be discarded. Care

must be taken with mercury batteries to assure that they are

not burned because toxic vapors are emitted which can endanger

life. One further point regarding batteries is that their

contents are corrosive and they should not be stored in an instrument for longer periods of time than necessary. Corrosion

can cause serious damage to an expensive item of eouipment.

I 4
©

1 20ft

—vw9 VOLT

Figure 4. Battery test circuit.
Another important consideration is the effect of ambient

temperature on battery operation and life. Some types of

batteries suffer severe reductions in performance when

subjected to either extreme heat or extreme cold. One example

is the carbon-zinc (flashlight) battery, which suffers in its

ability to perform when temperatures are below 0°C (32°F) or

above 50°C (122°F). Also, the mercury battery is a poor

performer at temperatures below 5°C (41°F), but is an excellent

performer at high temperatures. See reference [8].

2.4 Crystal Control vs. Free-Running Oscillators

Undercover transmitters and receivers should be crystalcontrolled because free running oscillators drift in frequency

and can be changed by modulation, making it difficult to

sustain optimum performance over long periods of time without

retuning. Crystal -control 1 ed equipment should operate with a frequency tolerance of a few parts per million. This tolerance

varies with frequency; however, tolerances of ten parts per

million are typical in the 150 MHz range.

In some instances, the requirement for miniaturization may

be so critical that the extra size incurred by the addition of

crystal -control circuitry cannot be tolerated. In these

situations, extremely small and very low-powered transmitters

of a size comparable to a one-inch section of a common lead

pencil can be used. In equipment this small, hearing aid

batteries are used, and transmitter output power is typically

in the vicinity of 10 mW. Frequency drift in devices not

having crystal control will typically be of the order of 10 kHz

per hour under normal operating conditions.

2.5 Automatic Volume Control

It is evident that voice modulations reaching the

transmitter will be at different intensities, depending on the

proximity of each speaker to the microphone. The wearer of the

transmitter will of course be much closer than the other

speakers. Consequently, it is probable that the wearer's voice

will be too loud while others may barely be heard. This brings

about the critical need for automatic volume control circuitry

in the transmitter so that all those talking may be heard

distinctly.

I 6
2.6 Modulation and Audiofrequency Response

Because of circuit simplicity, good fidelity and the

relatively low power drain, frequency modulation is used almost

exclusively in undercover transmitters, although there appears

to be no reason why other types of modulation could not be used

in order to provide better security.

Most of the frequency components of the human voice are in

the range from 300 Hz to 3000 Hz. This is the audiofrequency

response for which the equipment should be designed. Broader

responses are not only unnecessary but may even be detrimental

,

because sounds could be transmitted which may interfere with

the intelligibility of the conversation being monitored.

2.7 Frequency Deviation

Federal Communications Commission (FCC) regulations

pertaining to public safety communications equipment authorize

a maximum frequency deviation of +5 kHz for FM systems. It is

important that surveillance transmitters be designed to satisfy

this requirement, not solely because of the FCC regulations,

but because the transmitters must be compatible with the

receiving equipment commonly used in law enforcement

communications. Clarity of reproduction is another reason for

using equipment with the proper frequency deviation and

modulation bandwidth. Using bandwidths narrower than those

specified may create difficulty in identifying the speakers,

while the use of broader bandwidths does not Improve voice

reproduction and may introduce unwanted sounds. It is

particularly important that surveillance recordings be clear

and intelligible because of their possible use as court

evidence. In general, +5 kHz frequency deviation and 3 kHz

audiofrequency response provide adequate voice reproduction.

In surveillance work problems can arise related to the loudness

of the modulating voices and the action of the automatic volume

control (AVC) circuits, which may, in some cases, produce

recordings unsuitable as court evidence.
2.8 Packaging

Miniature transmitters have been packaged or concealed in

almost every way imaginable, as dictated by specialized needs.

Lamps, plugs, chair legs and flower pots have all been used to

conceal a transmitter. Some have achieved fame and attracted

international attention, such as the one concealed in the shoe

of a diplomat and the one in the seal above an embassy desk.

These are all custom-made devices, which for the most part

would be unavailable to a police department unless it had

direct contact with a facility which could produce them. Some

of these devices are available, but information regarding

specific suppliers is not easily obtained.

Good quality transmitters and receivers can be packaged in

small ruggedized containers with dimensions on the order of 3"

x 2" x 3/4", and can be either body worn or concealed in some

unlikely location. The particular situation dictates the

necessary concealment precautions and in this the agent must

use his own judgment. If he cannot wear heavy clothing or

outer garments, concealment may be a severe problem.

Electronic packaging has to be considered, especially in

the sense that the devices used should be repairable and the

circuitry should be accessible for trouble-shooting purposes.

Devices which are potted in an epoxy envelope, although

weatherproof, are not repairable in the event of circuit

failure. It is characteristically the very low-priced and

lower quality devices that are manufactured in this way.

Epoxy-potted transmitters usually operate in the 100 to 120 MHz

range

.

2.9 Ease of Detection

Detection may occur by visual observation, physical search

(friski^o), or by the use of electronic defensive

(countermeasures ) equipment. Little can be said of the

likelihood of detection by visual or physical means except that

the smaller the device is, the less apt it is to be discovered.

Obviously, discovery is dependent upon how cleverly it is

concealed.

Detection by electronic methods deserves more thorough

attention. Consider the following three factors for the

bearing they have on detection:

I 8
Operating Frequency: As has already been mentioned,

transmitters operating in the 83 to 103 MHz band are more

apt to be detected due to the fact that receivers covering

this band are extremely common. It does not require a very

vivid imagination to picture an undercover agent in a situation where someone is tuning through this frequency

band and picks up the broadcast of voices and sounds from

within the room, or hears the oscillatory squeal of energy

fed back to the receiver by a concealed transmitter. These

frequencies should be avoided.

Power Output: High power output increases operating range.

But the greater the power radiated, the wider the area of

coverage, with the consequence that detection is more

probable. Therefore, it is advisable to avoid using a onewatt transmitter, for example, where a one-quarter watt

unit would do. Where there is a reasonable choice, it is

much wiser to improve receiver sensitivity or antenna

efficiency than to increase transmitter power.

Harmonics: Good equipment radiates energy primarily at the

fundamental frequency, and filters out and suppresses the

harmonics. This makes it less likely that the energy being

radiated will affect other equipment, such as television

receivers, which may draw suspicion and lead to discovery.

It is not unreasonable to expect the level of radiated

harmonic and other spurious energy to be at least 35 dB

below that of the fundamental output of the transmitter.

Devices may be discovered by detection of the energy

radiated at their harmonic frequencies because antenna

efficiency may be better at the harmonic frequencies than

it is at the fundamental frequency. Searching by

electronic methods is often done by starting at the highfrequency region and working downward.

2.10 Antennas

Overall system performance can be greatly affected by the

type of antenna used, the type of installation, and the

conditions of operation. Monopole antennas are used

exclusively with body-worn transmitters, because they are

omnidirectional and of very simple construction. For many

reasons, other antenna configurations are impractical.

I 9
One important fact to consider is that best communication

usually occurs between two monopole antennas when they are

oriented parallel to one another; i.e., polarization should be

the same in both antennas. Either they both should be vertical

or both horizontal. Although this is generally true, local

distortions in field configuration caused by conducting objects

such as steel in a building can cause significant variations.

Normally, the optimum length for an omnidirectional

monopole antenna is approximately one-quarter wavelength at the

operating frequency. However, this is modified somewhat in the

case of antennas placed next to the body. Because of the added

capacitance between the antenna and the body of the wearer,

resonance occurs when the length of the antenna is 4 or 5

percent shorter than one-quarter wavelength.

The requirements that the antenna be both small and located

next to the body constitute very severe handicaps to efficient

communication. Antennas which are short compared to a quarter

wavelength are not efficient. In addition, the human body is a good absorber of radio energy. These constraints combine to

place very severe limitations on the distances over which

satisfactory communication can be achieved. Given ideal

antenna conditions, it is entirely possible to maintain good

communication with transmitters of one watt or less at

distances of several miles. However, when antennas are

shortened and placed next to the body, the range may drop to a few hundred feet or less, depending upon local propagation

conditions.

The subject of body-worn antennas is a complex one. One

study of this topic has been prepared for the Law Enforcement

Assistance Administration by the Aerospace Corporation of El

Segundo, California [9].

2.11 Propagation Conditions

The propagation medium is an important component of the

communications system. This is the part of the system over

which the operator has almost no control. An accurate

prediction of how well a system will work in a given situation

is not possible to any high degree of accuracy without actually

making field strength measurements. This is usually

impractical, so the next best thing is to examine some of the

variables and do those things which will enhance the

probability of success.

20
Two factors which affect the performance of an antenna are

the conductivity and the dielectric constant of the ground.

These vary widely around the country, and can make a difference

in field strength of as much as several dB. The moisture

content of the soil is also a factor, as higher amounts of

moisture enhance propagation. While these factors are not

controllable, they are mentioned for general interest.

Some factors are controllable by the operators. One way to

improve the received signal from a vertical monopole is to

elevate the receiving antenna. Improvements of several

decibels in received signal strength can be realized by

choosing listening post locations so that the angle between

transmitter and receiver is 30 to 45 degrees from the

horizontal

.

A transmitter or receiver does not perform well when placed

inside an automobile. The metal enclosure acts as a shield

which attenuates high-frequency electromagnetic energy. If

there were no windows in the ca^, or an outside antenna, there

would be essentially no transmission or reception at all. The

metal used as building reinforcement also reduces the strength

of a transmitted signal. Even the average home may seriously

hamper i nsi de-to-outsi de communication because of the use of

foil-backed insulation in the walls. A transmitter enclosed in

a metal container is completely ineffective. This fact was

once used in an extortion case involving a money drop. The

criminal, suspecting that a transmitter was concealed in the

package, placed it in a metal suitcase and thereby escaped

because the transmitter was useless. The wearer of a transmitter may position himself near openings such as nonmetallic doors or windows, thereby avoiding the shielding

effects of unseen metal barriers. The use of such tactics

comes with experiencp and a general understanding of the

behavior of radio energy.

2.12 Body-Moun ted Transmitters

There are a number of other features which may be important

in the field use of bod v-mounted transmitters and these are

discussed below.

2.12.1 Polarity Protection

When installing batteries in a body-mounted transmitter, it

is very important that the batteries be oriented for correct

polarity. Most units employ alkaline batteries equipped with

snap-on connectors. These batteries can not be installed with

the incorrect polarity because the connectors at the positive

and negative terminals are different from each other and the

battery will not fit the receptacle unless it is correctly

oriented.

21
Occasionally a body-worn transmitter may be encountered

which utilizes mercury batteries. A group of four of five

individual cells are usually held in place by a cardboard or

plastic sleeve, and two such groups are required to power the

transmitter. Because the mercury cells are usually not equipped wi

snap-on connectors like the alkaline batteries, it is possible

to install them with reversed polarity.

There are at least two possible consequences of Imposing

the incorrect voltage polarity. First, the transmitter will

not operate and the operator may not know why. Next, the

transmitter may be permanently damaged. This can happen with

either mercury or alkaline batteries because even a very brief,

momentary exposure to reversed polarity can result in

irreversible damage. Whether or not such damage will result

depends upon the circuit, and better quality units have

built-in polarity protection.

2.12.2 Antenna Connector

A common trouble spot in body-worn transmitters is the

connector between the antenna lead and the transmitter. A wide

variety of connectors are used by various manufacturers and

most have both advantages and disadvantages. There are the

simple plug-in microphone jack types, the screw-on types, and

the twist-lock types. Two considerations are important. First

is the possibility that the lead will be pulled out of the

socket during use, will work loose, or will otherwise break the

connection. Second is the ease of making repairs if the

antenna lead is broken. Some units combine the microphone wire

and the antenna wire into one lead.

2.12.3 Microphone

Although microphones will be discussed in detail in a later

section, it is worthy to note here that two different mounting

schemes are used. Some miniature transmitters have the

microphone built into the case of the transmitter, while others

use a microphone connected by a cable. If the microphone is

mounted in the case, there should be a dirt seal to protect it

against clogging and subsequent damage or loss of sensitivity.

A piece of foam-like material is a satisfactory protector for

this purpose.

22
Most of the better transmitters use the el ec trodynami c type

of microphone, which has excellent sensitivity and adequate

frequency response for voice communication. Electret

microphones of adequate quality are a relatively recent

development. They provide better frequency response and may be

expected to find increased use in undercover equipment. Both

types are rugged, but electret microphones have a limited

lifetime of about three years of operation at maximum

sensitivity. Should this be overcome and costs made

competitive with the el ectrodynami c types, electret microphones

could come into common use.

2.12.4 Location of On-Off Switch

Because miniature transmitters are often located where they

are subject to being jolted, or where other materials may rub

against them, it is important that the on-off switch be of a positive type requiring a fairly strong force to operate and

that it be in a location where it is not apt to be moved

accidentally. In some situations it may be desirable to make

it either difficult or impossible for the wearer to turn the

switch off while the surveillance operation is under way. Some

manufacturers make special provisions for this but may not

indicate it in their specifications.

2.12.5 Ruggedness

Body-worn equipment should be constructed to withstand

severe physical shocks. Construction should be such that

dropping from chest height to a concrete floor will not

permanently affect operation. This requires either an allmetal case of heavy gauge or very high performance plastic. If

plastic materials are used, they should not soften when warm or

shatter in severe cold.

2.12.6 Size

The primary factor limiting the smallness of a transmitter

is the size of the batteries required to supply the power. In

general, a crystal -con trol 1 ed transmitter with batteries,

having an output in the 0.2 to 1 watt range, can be contained

in a volume slightly smaller than a cigarette package. In such

a transmitter, the batteries occupy approximately 75 percent of

the total volume, and the crystal control feature adds

additional bulk. Transmitters no larger than the eraser

portion of a common lead pencil are available, but these

devices are not crystal controlled, have a power output of only

a milliwatt or so, and their greater frequency instability

restricts their application and dependability. Such low-power

transmitters are not suitable for applications where they must

be placed next to a large power absorber such as the human

body. They may be used as room bugs where long distances to a listening post are not involved.

23
2.13 Telephone Tapping Transmitters [15]

There are several types of telephone tapping transmitters

usable for monitoring both sides of a telephone conversation.

In general, they are not crystal controlled and have a power

output ranging from a few milliwatts to perhaps one quarter

watt; many of them utilize the telephone line as an antenna.

All types may be located either by their radiated signal or by

visual inspection. The poorer ones may be found by the current

they draw from the telephone lines. If the current drain 1s

sufficient, it can give an indication of trouble on the line

and draw the attention of the telephone company repair

facilities. Some difficult problems are encountered in

evaluating the output power of telephone tapping transmitters.

Because of the antenna arrangements they utilize, 1t is not

feasible to make output power measurements into a 50 ohm load,

and it becomes necessary to rely on field strength measurements

to determine the radiated power.

2.13.1 The Drop-In Telephone Transmitter

This device looks very much like the mouthpiece unit of a telephone. The carbon microphone mouthpiece drops out easily

if the retaining ring over the mouthpiece is unscrewed. The

drop-in transmitter may be substituted 1n its place very

quickly and the telephone returned to apparently normal

operation. The telephone transmitter draws its power from the

telephone line and operates only when the receiver 1s lifted

from the cradle. Both sides of the telephone conversation

modulate the rf carrier frequency of the telephone transmitter

which may be received and recorded at a remote location. One

of the main advantages of the device 1s that it takes only a few seconds to install, and the better ones are so cleverly

constructed that they may be difficult to recognize by anyone

who is not extremely familiar with the telephone equipment.

The drop-in transmitter does not require a separate antenna but

instead utilizes the telephone line for this purpose. However,

it is possible to detect the use of this type of transmitter by

means of impedance measurements or by measuring the current

drain while the telephone is in use.

24
2.13.2 The Series -Connected Telephone Transmitter

Any accessible location on the telephone line to be tapped,

such as a terminal block, may be used as an installation point

for a series-connected transmitter. Therefore, direct access

to the telephone instrument is not renuired. The circuit in

figure 5 illustrates the method of connection. The polarity is

not important, so the transmitter may be installed in eitner

side of the line. Because series-connected transmitters draw

current from the telephone line, they can be detected in the

same manner as drop-in transmitters.

2.13.3 The Parallel-Connected Telephone Transmitter

Figure 6 illustrates the method used to connect a parallel

transmitter to a telephone line. Again, the polarity is

usually not important, but there are several differences

between this installation and the series transmitter. Because

the parallel transmitter is powered by a separate battery, it

draws no current from the phone line and is essentially

undetectable by impedance or current measurement techniques.

As with any battery-powered device, it has a limited operating

time. The transmitter is activated when the receiver is

1 if ted.

2.14 Receivers

Receivers used in undercover work are not always packaged

in miniaturized form because in many instances concealment is

not required. The sensitivity of a receiver has almost nothing

to do with its physical size. Receiver components and

measurable characteristics which distinguish good quality

equipment from poor quality equipment are discussed below.

2.14.1 Sensitivity

Sensitivity is probably the most important characteristic

of receiver Quality. It is a measure of how weak a signal can

be detected, and therefore is of first-order importance in

determining the operating range of an undercover receiver.

There are two common methods of specifying this information.

The first is in terms of microvolts for 20 dB nuietina [13].

Tne second is in terms of microvolts for 12 dB SI NAD [8].

Either is an acceptable method, although the 12 dB S I NAD

Drovides a better measure of true receiver performance because

it is made under modulated conditions. As a general rule of

thumb, one may add 0.1 microvolt to the 12 dB SI NAD sensitivity

figure to obtain a comparable sensitivity for a receiver rated

on the basis of 20 dB quieting. Although this is not exact, it

does provide a quick means of comparing two receivers which are

specified in tnese different ways. For examnle, a good

receiver should be capable of at least 0.4 microvolt

sensitivity at 12 dB SI NAD or 0.5 microvolt for 20 dB quieting.

25
SERIES

TRANSMITTER

TELEPHONE LINE

Figure 5. Series-connected telephone transmitter.

26
TELEPHONE LINE

PARALLEL

TRANSMITTER

ANTENNA

Figure 6. Paral 1 el -connected telephone transmitter.

27
2.14.2 Frequency Control

The receiver should be equipped with a crys tal -control 1 ed

local oscillator for automatic frequency control. Otherwise,

it will tend to drift or detune, causing fading of the received

signal. This is especially important if the receiver is to be

unattended for extended periods of time.

2.14.3 Adjacent Channel Rejection

Adjacent channel rejection, also termed selectivity, is

achieved through the use of good filter networks which reject

signals at nearby frequencies, preventing unwanted

i n terference

.

2.14.4 Antenna

Much the same considerations apply to receiving antennas as

to transmitting antennas, particularly if they are worn on the

body (section 2.10). Where the receiver is not body-worn, one

would normally use antennas which have higher gains and/or

di rec ti vi ties.

2.15 Advantages and Disadvantages of Body-Worn Transmitters

and Receivers

Advantages

1. Completely portable (small

and lightweight).

2. Quickly installed.

3. Highly versatile.

Disadvantages

1. Limited period of operation.

2. Limited and uncertain range

of operation.

3. Can be located by electromagnetic sensors.

4. Will not relay information

when surrounded by a conducting shield such as a car

body, airplane, or metal

sui tease

.

5. Quality (intelligibility) of

transmitted information may

vary.

6. Subject to locally generated

interference and atmospheric

noise.

7. Personal contact with subject

often necessary.

28
3. TESTS OF RADIATING DEVICES

3.1 Laboratory Tests of Body-Worn Transmitters

A limited number of measurements were performed to evaluate

some of tne equipment which is now commercially available to

law enforcement agencies for electronic surveillance work.

Five transmitters were tested to determine some of their

operating characteristics. All units were of the FM crystalcontrolled type, operating on either mercury or alkaline

batteries and capable of output powers of one watt or less.

The units were tested for

as a function of time, spuriou

and dc current drain and effic

frequency stability, output power

s emissions, AM hum and noise,

i e n c y .

The testing was performed under laboratory conditions of

approximately 23°C (73°F), 40 percent relative humidity, and

atmospheric pressure of 630 mm Hg, with each transmitter

radiating into a 50 ohm load. Table 1 shows some of the test

results. Others are described below. The following units were

tested

.

=1 --Nominal rf output 1 watt, carrier frequency 165.188 MHz,

frequency modulated, powered by three 4.2 volt mercury

batter i es

.

=2-- Nominal rf output 250 milliwatts, carrier frequency 165.188

MHz, frequency modulated, powered by three 4.2 volt mercury

batter i es

.

#3--Nominal rf output 250 milliwatts, carrier frequency 151.625

MHz, frequency modulated, powered by two 7 volt mercury

batter i es

.

-4- - 1Nominal rf outDut 10 milliwatts, carrier frequency 159.300

MHz, frequency modulated, powered by one 9 volt alkaline

battery

.

#5--Nominal rf output 50 milliwatts, carrier freauency 154.890

MHz, phase modulated, powered by one 9 volt alkaline

battery

.

^6--:iominal rf output 100 milliwatts, carrier frequency 154.890

MHz, frequency modulated, powered by one 9 volt alkaline

battery

.

29
Table 1 Laboratory Test Results

Parameter #1

TRANSMITTER

#2 #3 #4 #5

Frequency

Stabi 1 i ty

parts per

mi 1 1 i on

Harmoni c Suppression

AM Hum & Noise

D.C.

Current

Ef f i ci ency

into 50 n Load

3 ppm over

period of

4 hr.

45 m i n

.

33 dB

57.7 dB

230 ma at

10.0 volts

65.9%

5 ppm over

period of

2 hr.

45 m i n

.

33 dB

52.8 dB

97.8 ma 0

9.3 volts

32.4%

5 ppm over

period of

3 hr.

45 m i n

.

31 dB

50.1 dB

56 ma @

11.0 vol ts

34.1%

6 ppm over

period of

3 hr.

45 mi n

.

6 dB

24.9 dB

34 ma 0

9.3 volts

2.6%

1 ppm over

period of

45 m i n

.

30 dB

25.1 dB

74 ma @

9.3 volts

6.2%

30
TIME - HOURS

Figure 7. Decay of output power and battery voltage for

transmitter # I .
TRANSMITTER #1

POWER APPROXIMATELY 1 WATT

FREQUENCY (f Q

) = 165.188 MHz

Figure 8. Output spectrum of transmitter #1

32
TIME - HOURS

Figure 9. Decay of output power and battery voltage

transmitter #2.

33
TRANSMITTER #2:

POWER APPROSIMATELY 250 MILLIWATTS

FREQUENCY (f ) = 165.188 MHz

1 i i J •

4f

o

Figure 10. Output spectrum of transmitter #2.

34
Figure 11. Decay of output power and battery voltage

for transmitter #3.

35
TRANSMITTER #3:

POWER APPROXIMATELY 250 MILLIWATTS

FREQUENCY (f

Q

) = 151 .625 MHz

11

1 dB

_

1 ll ||

f 2f 3f 4f 5f 6f

0 ooooo

Figure 12. Output spectrum of transmitter #3.
3

0 12 3 4

TIME - HOURS

Figure 13. Decay of output power and battery voltage for

transmitter #4.

37
TRANSMITTER #4:

POWER APPROXIMATELY 7 MILLIWATTS

FREQUENCY (f ) = 159.300 MHz

Figure 14. Output spectrum of transmitter #4.

38
CASE 2

0 1 0 20 30 40 50 60 70 SO

TIME-MINUTES

Figure 15. Decay of output power and uattery voltage Tor

transmitter #5.

39
TRANSMITTER #5:

POWER APPROXIMATELY 25 MILLIWATTS

FREQUENCY (f ) = 154.890 MHz

Figure 16. Output spectrum of transmitter #5.

40
Units #1, #2 and #3 were designed for body-worn use, while

units #4 and #5 were designed for stationary application in a building to provide security against holdups. Therefore, a direct comparison of the first three units with units #4

and #5 is not appropriate. All units were comparable in size

to a cigarette package and therefore could be concealed on a person. With regard to table 1, the harmonic suppression is

the difference in signal strength between the fundamental

output frequency of the transmitter and the next most powerful

component of the radiated spectrum. It is desirable that this

difference be as large as possible; thus, the larger numbers

indicate the better performance. Spectrum-pictures of the

output of the five transmitters are shown in figures 3, 10, 12,

14, and 16. AM hum and noise is a measure of the undesired

amplitude m oJulation produced within the transmitter, and again

the larger numbers indicate the better performance. The

efficiency values given in the last row were calculated from

the relationship,

E = _ll x 100,

p dc

where P r f is the rf power of the transmitter and P^c is the

product of the dc current and dc voltage supplied by the

transmitter batteries. The dc voltage and rf power data are

shown graphically in figures 7, 9, 11, 13 and 15.

Transmitter JH

Figure 7 shows the decay in rf output power due to the

expenditure of battery energy over the operating period. Also

shown on the same graph is the battery voltage variation over

the same time period. The output power was nearly constant for

the first hour and did not fall below the half-power point

until approximately two and one-half hours of operation. Three

4.2-volt mercury batteries were used to power the transmitter,

and it appears that a cell failure may have occurred in one of

the batteries near the end of the first hour. This would

explain the rather significant drop in output power over the

next 30 minutes of operation. This performance is apparently

not typical, as is indicated by the data from t;ie other

transmitters. However, it does illustrate what can happen and

shows how critical battery performance is to reliable

operation. The output spectrum of this transmitter, shown in

figure 3, remained essentially the same over a tnree-nour

period.

4 I-
Transmitter #2

As shown in figure 9, the decay in transmitter output was

approximately linear after the first few minutes of operation,

and the half-power output level was not reached until after 8

hours of operation. A cell failure appears to have occurred at

approximately 7 hours and 45 minutes into the test. The

batteries were three 4.2-volt mercury batteries of the same

type and manufacturer as those used in the test of transmitter

#1 . This would appear to be typical of the expected

performance. Figure 10 shows the output spectrum with the

strongest spurious radiation being the fourth harmonic (660

MHz), which is approximately 33 dB lower than the fundamental

carrier frequency output, f . Transmitter #3

Referring to figure 11, it is apparent that the behavior of

transmitter #3 was much the same as that of transmitter #2.

Operation time to half-power output level was nearly the same.

In this instance, the dc power supply was two 7-volt mercury

batteries, and a cell failure occurred after approximately 6

hours and 15 minutes. The output spectrum (figure 12) shows

the spurious radiation to be down approximately 31 dB below the

fundamental. In contrast to transmitters #1 and #2, the most

intense spurious radiation was evidently not a harmonic of the

fundamental frequency.

Transmitter #4

One 9-volt carbon-zinc battery served as the dc supply for

this transmitter, and its performance was noticeably different

from those of transmitters #1, 2, and 3, which used mercury

batteries. Transmitter #4 was received in a poorly adjusted

condition, as can be seen from figures 13 and 14. For this

reason, the spurious radiation level was only 6 dB below the

fundamental carrier frequency output (figure 14). In this

condition, this transmitter would be unacceptable for

undercover work because of the ease with which it could be

detected by its excessive spurious radiation. Elimination of

unwanted spurious radiation wastes power; when a bandpass

filter was inserted between the transmitter and power meter to

eliminate the spurious energy, the output power showed a very

significant drop. This is shown by the curves labeled case 1 and case 2 in figure 13. Compared to other transmitters using

mercury batteries, it is apparent that a rather large initial

drop in output power occurs soon after turn-on when using

carbon-zinc batteries, although the decay rate after the first

hour or so appears to be comparable. Also, the mercury

batteries do not appear to suffer the radical voltage drops

42
under load evidenced by the carbon-zinc batteries. It should

be noted that the efficiencies of this transmitter and

transmitter number 5 are quite low, which means that the

radiated output energies are very low for the battery powers

being expended.

Transmitter #5

As with transmitter #4, a 9-volt carbon-zinc battery was

used as a dc power source for transmitter #5, and similar

output characteristics were displayed, as shown in figure 15.

However, the initial decay in output power was so severe that a half power level was reached after only a few minutes of

operation. Such a transmitter might serve satisfactorily for

short and intermittent use but would be unsatisfactory for most

undercover missions. As with transmitter #4, a bandpass filter

was inserted and the change in output power noted. Because the

spurious radiation was an acceptable 30 dB below the carrier

output (see fig. 16), there was a much less severe change in

the output upon insertion of the filter, as is illustrated by

the similarity between the curves of case 1 and case 2.

Carbon-zinc batteries are not recommended to power

transmitters in undercover surveillance use because of their

inability to provide adequate power. Both mercury batteries

and alkaline batteries are much more satisfactory.

Transmitter i6

The sixth transmitter was tested to assess the difference

in transmitter performance as a function of the type of battery

used. It was operated until it reached its half-power output

levels, first with a carbon-zinc battery and then with an alkaline battery. Figure 17 is a plot of the resulting data

and shows a half-power lifetime of slightly more than one and

one-naif hours using the alkaline battery compared to only 15

minutes with the carbon-zinc battery. The performance of

mercury batteries is similar to that of the alkaline.

3.2 Field Tests of Body-Worn Transmitters

When the antenna of a transmitter is mounted on a human

body, the effective radiated power of the transmitter is

considerably reduced. To study this phenomenon, an outdoor

test range was established and measurements were made under

simulated field conditions. The data thus gathered are very

interesting, and illustrate the amount of variation and the

complexity of the problem of maintaining dependable

communication using body-worn equipment.

43
Figure 17. Decay of output power of transmitter #6 using

alkaline and carbon-zinc batteries.

44
The measurement setup, using a field strength meter at the

receiver location, is illustrated in figure 18. Transmitter

#1 , described in section 3.1, was used. Measurements were made

in the daytime under warm, dry conditions, and there were no buildings or other obstructions in the area. Initial

measurements were made with the transmitter and its antenna

mounted on a wooden post, with the transmitter and receiver

antennas oriented vertically. A series of measurements were

then made with the transmitter and its antenna mounted in

various locations on a person with the person oriented at

different angles to the receiving antenna. In all cases, the

first position was with the subject facing the receiver with

arms extended downward at the sides. Succeeding observations

were made with the subject rotated clockwise to the successive

90 degree positions. To observe the changes in field strength

due to arm movement, the arms were moved through a variety of

positions including extending them directly overhead, to the

front, and to the sides. The arm movements followed a consistent pattern of continuous motion during which the field

strength fluctuations were observed on the meter. These

fluctuations are shown for measurements 2 through 6; the

approximate variation of the field strength reading is

indicated for each of the four facing positions. In some

instances, arm movement appeared to increase signal strength

perceptibly. The measurement conditions and data observed were

as follows (observed field strengths at the receiver are given

in dB above one microvolt per meter):

Measurement #1 : Transmitter mounted on a vertical

with transmitting and receiving antennas vertical,

observed field strength was 91.4 dB yV/m.

wooden post

The

45
~ 3.05 METERS ( 10 FEET)

I .37 METERS (4.5 FEET)

RECEIVER TRANSMITTER

30.48 meters ( 100 feet)

Figure 18. Test setup for field performance studies of

body-worn transmitters.

46
Measurem ent # 2: Trie transmitter was mounted on the left side

of the person, at approximately waist level (see figure 19).

The antenna extended up the left side of the chest to the

shoulder. The subject was wearing a cotton "T-shirt." The

subject was then oriented at the four quadrant angles to the

transmitter, and the variations due to arm movements were noted

at each position. The transmi tti no and receiving antennas were

oriented vertically.

88.9 dB yV/m

4 ^ 74.9 dB y V/m 87.9 dB yV/m

2

1 71.4 dB yV/m

FACING RECEIVER

ARMS DOWN

IN ALL POSITIONS

Changes in Field Strength, E,

with Arm Movement

Pps it i on

1 -12 to +3

2 -4 to +1

3 _4 to +i

4 -4 to +1

47
48

Measureme nt #3 : The transmitter was tucked into the belt wi

the antenna positioned vertically up the center of the chest

over a "T-shirt" (see figure 20).

69.9 dB yV/m

90.0 dB uV/m 89.9 dB yV/m

1

92.9 dB yV/m

FACING RECEIVER

ARMS DOWN

IN ALL POSITIONS

Changes in Field Strength, E,

with Arm Movement

Pos i ti on

1

2

3

4

Variation (dB)

-6 to 0

-7 to +]

0 to +10

-7 to +1

50
easjjrement #4: The transmitter was tucked into the belt with he antenna arranged vertically un the center of the chest next o the skin. 1 Nt;u

86.4 dB pV/m

67 .9 dB yV/m

-86.9 dB uV/m

1

90.0 dB yV/m

FACING RECEIVER

ARMS DOWN

IN ALL POSITIONS

Changes in Field Strength, E

,

with Arm Movement

Position Variation (dB)

1 -3 to + 1

2 -10 to +2

3 -fi to +£

4 -10 to +2

5 I
Mea surement #5: Both the transmitter and receiver antennas

were oriented hor i zon ta 1 1 y . The transmitter location was at

the waist, with the antenna wrapped around trie waist just above

t h e b e 1 1

.

76.4 dB yV/m

A

84.9 dB yV/m 81 .9 dB yV/m

i 1 89.9 dB yV/m

FACING RECEIVER

ARMS DOWN

IN ALL POSITIONS

Changes in Field Strength, E,

with Arm Movement

Pos itio n Variation (3 B

)

1 -o.5 to n

2 0 to +5

3 -3 to +2

4 -7 to +1

52
^as,urement_£6. The transmitter antenna was oriented Horizontally and the receiver antenna was oriented vertically lae transmitter and its antenna were mounted at th- waist in the same manner as for measurement #5

51 .9 dB yV/m

71.9 dB y Y/m

I

1 71 .4 dB yV/m

FACING RECEIVER

ARMS DOWN

IN ALL POSITIONS

56.9 dB yV/m

Changes in Field Strength, E,

with Arm Movement

Pos ition

1

2

5

4

-9 to +1

-9 to +1

0 to +13

0 to +13

53
Measurement

were

left

towa rd

_J[7_: Both the transmitter and receiver antennas

oriented vertically, with the transmitter mounted on the

hip of the subject and the antenna extended down the thigh

the knee

64.0 dB yV/m

-> A

65.5 dB yV/m 59 . 0 dB y V/m

1 65.5 dB uV/m

FACING RECEIVER

Measure ment #8 : except that the

subject and the

ankle.

The conditions were identical to those in #7

transmitter was mounted at the knee of the

antenna extended down the lower lea toward the

71 .0 dB yV/m

n

69.0 dB yV/m 64.0 dB yV/m

69.0 dB yV/m

Note that measurements #1 through #6 were made on one day and

measurements -!7 and #3 were made on a second day. This allow

the possibility that the radiated power from the transmitter

might not have been equal on the two days. Therefore, it is

not certain that upper-body mounting of the transmitter

provides a larger signal, as the data would seem to indicate.

54
Although these data a tentative conclusions can

re by no means

Le drawn.

concl usi ve , several

(1) When the body is between the transmitting and

receiving antennas, there is a very significant reduction in

the strength of the received signal. Therefore, the wearer

should make every effort to keep himself oriented so that his

transmitting antenna is on the same side of his body as the

receiver.

(2) It is desirable to .have the receivinn antenna oriented

narallel to the transmitting antenna. The data of measurement

#6 show severe signal loss (approximately 25 dB) when the two

antennas are perpendicular to each other.

(3) Arm movement is significant and can have a very

important effect when communication is marginal. It is best to

keen the arms as far away from the antenna as possible.

Folding the arms across a chest-mounted antenna could attenuate

a signal enough to make it unrecoverable.

4. NON- RADIATING DEVICES

4.1 General

Tiie distinguishing feature of non - ra d i a t i n n surveillance

devices is the medium used to convey information from one

location to another. The information is transmitted over

wires, usually telephone wires, instead of being broadcast.

Wired surveillance systems have several distinct advantages

over radiating systems. 'lost notable is the security from

detection by an electronic sweep made by a radio receiver,

tiecause virtually no electromagnetic energy is radiated from

the wired network, this means of discovery is eliminated. A

second very innortant consideration is the virtually unlimited

range that is possible. Although the ranoe of radiating

devices is usually measured in feet, the ranoe of wired systems

can be measured in miles. In addition, wired communication

systems include a source of dc voltage. Many wired

surveillance devices utilize this built-in voltaon source to

eliminate the troublesome and limitinq battery requirement. A final advantage which cannot be overlooked is the reliability

of the communication medium. Wires are not affected by

atmospheric conditions, and signals of consistent and

dependable quality are almost certain, with the additional

benefit tnat natural and man-made radiation interference are

eliminated.

55 •
While wired systems have many advantages, they also have

limitations. Restricted mobility is most often the reason a wired surveillance system is not used. In cases where the

person under surveillance is on the move or located away from a fixed listening post, the use of wired equipment is obviously

not feasible. A second drawback is that prior access to the

premises is required to install listening devices or telephone

taps. Also, in most instances, more advance preparation time

is needed to set up wired equipment, so it is difficult to

respond quickly to emergencies. Where these restrictions pose

insurmountable problems, there is no choice but to use

radiating devices.

In some states there are laws which prohibit telephone

tapping. Therefore, portable and body-worn transmitters and

receivers are essentially the only undercover communications

devices available to law enforcement officers for electronic

survei 1 1 ance

.

4.2 Telephone Taps

Surveillance must often be carried out by using remotely

located tape recording equipment to record telephone

conversations. The fact that telephone companies install extra

wiring in anticipation of future demands for service provides a unique and convenient situation for wiretapping. Almost

invariably, there will be one or more pairs of conductors in a cable which are not in use and which can be appropriated for

surveillance purposes. These pairs are easily located at

various access points in a given building or neighborhood by

someone who is familiar with the telephone system and its

equipment. Thus, not only the telephone itself but the whole

wire communication network offers a limitless range of

possibilities to the skillful technician. Even if a thorough

examination were to determine that the telephone itself was not

being tapped, there would still be no guarantee that a hidden

microphone was not present which was carrying the conversation

within a room to a remote listening post via telephone wiring.

In the following paragraphs, some of the more frequently used

telephone tapping methods and equipment are discussed along

with the methods used to detect them.

56
4.2.1 "Infinity Transmitter" or "Harmonica Bug" [1]

Although the infinity transmitter is installed in the

telephone, it does not monitor telephone conversations.

Instead, it enables someone to listen in on a room conversation

by means of another telephone. Once the device is installed in

a telephone, a listener dials the number of the altered

telephone in the customary manner. However, just before the

dialed telephone rings, an audible note or tone is transmitted

over the phone line. This activates the bug, which opens the

phone line without ringing, thereby enabling the caller to

listen to any conversation taking place in the vicinity of the

bugged telephone. The bug itself is an electronic switch which

closes in response to the audio tone. It is wired in parallel

with the hook switch on the telephone so that, upon being

activated, it prevents the telephone from rinqing and opens the

phone line at the same time. When this bug is in operation,

the telephone will be busy to all other callers even though the

receiver is in the cradle in the usual position. Thus, one

symptom of the presence of such a bug is the receipt of a busy

signal when the phone is known not to be in use. This device

must be installed in the telephone and can be located by

physical search. Defensive equipment is manufactured which

will place a tone sweep on the phone line and detect the

voltage drop from 48 volts to approximately S volts when such a bug is activated. A tone sweep is necessary because these bugs

can be made to respond to a wide range of audio frequencies.

The name "harmonica bug" is derived from the use of a common

harmonica to actuate the device.

4.2.2 Inductive Coupling or "Pick-Up Coils"

A current in a conductor gives rise to a magnetic field

around the conductor; when the current varies either in

magnitude or direction, the magnetic field also changes.

Conversely, if a conductor is immersed in a changing magnetic

field, a current will be produced. This is the principle of

the transformer and is used as a means of tapping a telephone.

Various types of coils and pick-up loops can be employed to

couple to an active telephone line and hence to monitor the

conversation in progress. Many such devices are used in

conjunction with an audio amplifier to actuate a tape recorder

or to modulate a radio transmitter. Such devices do not

produce any measurable loading or draw any detectable extra

current from the line and therefore can not be located by

voltage or current measurements. Physical search is

practically the only means of detecting such a device.

57
4.2.3 Three-Wire Systems

A telephone may be used for both tapping and bugging

simultaneously by the use of a third conductor for bypassing

the hookswitch and thereby activating the mouthpiece for use as

a room bug. Telephone hookswitch bypass is the term sometimes

used to refer to this method of eavesdropping and some highly

sophisticated electronic devices are employed to accomplish it

and to make detection difficult.

4.2.4 Dialed Number Recorder

Although not a device for intercepting oral communication,

the dialed number recorder is used to monitor telephone calls

by providing a record of all numbers dialed from a particular

telephone. These devices usually provide a paper tape printout

of numbers dialed, and in addition may provide the date and

time a call was made. Such devices were once called "pen

registers" because a pen riding on a moving chart was used.

The pen records the groups of pulses emitted by the telephone

dial mechanism as it returns to the rest position. The dialed

number is ascertained by counting the number of pulses in each

group. In telephone systems using touch-tone dialing, a touchtone decoder with a printed readout is used. These instruments

are manufactured mainly for use by the telephone companies.

Application is very simple, requiring only the connection of a lead to each side of the telephone line. No legal implications

are involved, and connection can be made anywhere along the

line between the telephone and the exchange. Detection of the

presence of a dialed number recorder is extremely difficult by

any means other than visual inspection because these devices

have a high input impedance and the line loading effect is

n e g 1 i g i b 1 e .

4.3 Concealed Microphones

Microphones cannot be found by electromagnetic sensing

devices. The only means of discovery is through physical

search. Installation requires access to the premises, but the

degree of concealment is limited only by the imagination of the

installer, the cleverness of his methods, and the time he has

to work. The basic equipment required consists of a microphone

as small as possible, the necessary length of shielded wire,

and a tape recorder. Shielded wire is used where possible to

avoid unwanted pickup such as power line hum or other noise

58
coupled into the circuit. For short lead lengths, very fine

wires can be run along baseboards, window sills, or under floor

coverings. Conducting paint can be used to form a conducting

path on a wall surface. This is done by simply painting thin

conducting stripes on the surface as required, and making

electrical connections at the ends of the painted stripes.

These stripes can then be painted over with regular paint,

making discovery difficult.

Excellent microphones as small as 9.5 mm x 6.35 mm x 4.76

mm (3/8" x 1/4" x 3/16") are available, with sensitivities such

that they can pick up a whisper at 7.5 meter (25 feet). These

can be combined with an amplifier of comparable size so that a listening post can be established several miles away, using a pair of unused telephone wires.

The small dynamic-type microphones commonly used for these

purposes require a column of air (a leak) directly in front of

the pick-up surface so that, in mounting them, a very small

hole must be drilled in the object used for concealment.

Sometimes, plastic tubing threaded through a crack serves this

purpose and aids in concealment. Microphones equipped with a small section of metal tube directly over the microphone leak

opening are available for attaching the tubing.

In the use of a microphone for eavesdropping, it is

important that precautions be taken to avoid rubbing against

the microphone because this introduces noise. This becomes a problem especially when a microphone is used with a body-worn

transmitter which is hidden in clothing. Friction from cloth

rubbing against the microphone is called "clothes noise" and

poses a problem in obtaining good quality sound reproduction.

Miniature surveillance microphones, with their amplifiers,

have characteristics similar to those of the human ear. The

fact that the ear and the microphone can detect the same sound

at the same distance provides a guide to the limits of a physical search for a hidden microphone. There are devices

such as the highly directional shotgun or parabolic microphones

which have greater ranges, but they are much more difficult to

conceal. These instruments can detect normal conversation at

ranges of fifty to one hundred feet. Their increased range is

derived from their directionality and, for some, their acoustic

amplification. This directionality prevents interference from

any direction other than that in which the device is pointed.

One defense against such devices is to add background sounds,

such as music. This, along with speaking in low voices, can

render the recorded conversation unintelligible.

59
Microphone technology is an extensive field, and no attempt

will be made to cover the subject in depth. However, several

types of microphones which are of special interest in

electronic surveillance, and which find fairly wide

application, will be described briefly [2, 14, 16].

4.3.1 Carbon Microphones

The carbon microphone is the type used in the mouthpiece of

the telephone transmitter. It operates on the principle that

the resistance of a package of carbon granules varies as the

external pressure on the package changes. When mounted behind

a vibrating diaphragm and energized with a dc voltage, the

carbon microphone will cause the current in the circuit to vary

in accordance with the sound striking the diaphragm. This

modulating effect enables the voice to be transmitted over the

telephone circuit. Carbon microphones are very sensitive but

have a rather high background noise of the hiss type. They do

not lend themselves to miniaturization as well as other types

of microphones.

4.3.2 Condenser Microphone

Also referred to as an electrostatic microphone, the

condenser microphone utilizes a vibrating diaphram as one plate

of a paral 1 el -pi ate , air-dielectric capacitor. Sound striking

the diaphram produces capacitance variations which, in turn,

produce electrical impulses. Condenser microphones can be made

very sensitive and reproduce sound with excellent fidelity, but

they have the disadvantages of being fragile and sensitive to

vibrations transmitted through solids. For these reasons, they

are generally not suitable for eavesdropping applications.

4.3.3 El ectrodynami c Microphone

Often called simply a dynamic microphone, this is the type

most commonly used in electronic surveillance. These

microphones have many desirable features. They can be very

small, adequately sensitive, rugged, and require no external

power source. The modulated current is generated by the motion

of a coil of wire in the field of a permanent magnet.

60
4.3.4 Electret Microphone

Recent technology [5] has resulted in the development of

miniature electret microphones which have many features useful

in electronic surveillance work. An electret is the electrical

analog to a permanent magnet. A permanently polarized

dielectric material which is sensitive to pressure changes

produced by acoustic energy is used as the sensing element.

Operation is similar to that of the condenser microphone, but

the electret microphone is much more resistant to shock and

solid-borne vibration. Advantages of the electret are that no external bias is required, and 1t has a frequency response

which is superior to those of other types. Superior frequency

response, however, 1s not of first-order importance in

electronic surveillance, where only voice frequencies are of

interest. Electrets perform well under temperature extremes

and may be slightly superior from the standpoint of ruggedness,

but one precautionary note is worthy of mention. While 1t

appears that better materials are being developed which will

diminish the problem, periods of humidity 1n the range of 90

percent or more cause drastic loss of sensitivity. Should this

problem be overcome and costs made competitive with those of

the dynamic types, the electret may become the preferred type

of microphone. At present, however, the electret appears to be

little used, and it may be several years before this situation

changes

.

4.3.5 Induction Microphone

The induction microphone is very similar in principle of

operation to the el ectrodynami c microphone in that both

incorporate a moving conductor in the field of a permanent

magnet. The induction microphone utilizes a fixed coil and a moving piece of magnetic material, whereas the el ectrodynami

c

type employs a moving coil and is often much smaller. A common

example of the induction microphone is the ear piece of the

telephone. These microphones are sometimes used as room bugs

where concealment problems do not require a device as small as the el ectrodynami c type. Because they are used in telephones,

they are inexpensive and readily available; this explains their

common use.

6 I
4.4 Advantages and Disadvantages of Wired Surveillance Systems

Advantages Disadvantages

2. Operating time not limited

by battery life.

1. Unlimited range. 1. Prior access to premises

requ i red

.

2. Lack of mobility.

3. Not subject to radiated

electromagnetic interference .

3. Outlawed in some states,

court order required in

others to use telephone

system

.

4. Not detectable by electromagnetic sensors. 4. Installation requires

5. Personal contact with highly skilled people.

subject not required.

5. TAPE RECORDERS

In gathering legal evidence through electronic

surveillance, the tape recorder is an indispensable item.

Whether the surveillance method is a radiating system involving

transmitters and receivers or a wired system using a telephone

tap or a concealed microphone, a tape recorder is invariably

involved. Many very good tape recorders are available, but

there are apparently none designed specifically for

surveillance purposes. In general, most good tape recorders

are adequate to perforin the functions required by electronic

surveillance, especially in regard to sensitivity and frequency

response. Both open-reel and cassette- type recorders are used.

5.1 Open-Reel Tape Recorders

The main advantage of the open-reel recorder is its large

tape capacity, which allows for very long periods of recording

without the necessity of changing reels. It generally yields

broader frequency response, but this is not required for good

fidelity in voice reproduction since the voice frequency band

is relatively narrow. Important options available on open-reel

recorders include voice-actuation, and automatic shut-off at

the end of a reel so that tape flap noise will not reveal the

presence of a hidden recorder. Temperature extremes are to be

avoided; the temperature inside the trunk of an automobile on a not day, for example, can cause serious operational

difficulties.

62
5.2 Cassette Tape Recorders

The cassette is being used increasingly in spite of its

smaller recording capacity. This is due to the advantages of

small size and ease of operation; it is much quicker and

easier to change and handle the tape. Miniature versions of

the cassette tape recorder are now making their appearance, and

at least one model is available which is small enough for

concealment within clothing, its dimensions being 5 mm x 7.5 mm

x 2.5 mm (2" x 3" x 1"). This will have a significant bearing

on the application of recorders and could conceivably displace

radiating equipment in certain situations.

5.3 Telephone Recording Actuator

In order that a tape recorder not be required to run

continuously, or be monitored continuously, a recorder actuator

may be used. With the actuator, the tape recorder is turned on when the telephone receiver is lifted from the cradle and

turned off when it is replaced. These actuator devices may be

attached to a telephone line at any point between the telephone

and the control switchboard. They are battery powered, so they

create no detectable electrical effect on the telephone line.

These devices utilize the fact that the voltage on the

telephone line drops from 48 volts to approximately 8 volts

when the receiver is lifted from the cradle.

6. CONCLUSIONS AND RECOMMENDATIONS

There are several standards available to gauge the quality

and performance of personal /portabl e electronic equipment [3,

11, 12]. However, there is a degree of uniqueness about

electronic surveillance equipment which would seem to require

standards especially tailored for these devices. As an example, since battery life is so critically important in the

case of body-worn transmitters, it follows that maximum use

should be made of available power. This leads to the need for

some standard for transmitter efficiency. This does not mean

that a simple specification of efficiency based upon power

delivered into a 50-ohm load would be appropriate, because few

of these devices are designed to work into 50 ohms. It would

be desirable to develop some criteria whereby a high level of

efficiency could be assured, thereby providing the maximum

signal strength and transmitting time consistent with the

battery power available.

63
A second component of a standard should be a specification

of operating frequencies. This would include a safeguard

against the use of transmitters operating in the aircraft

landing system bands, which could constitute a very serious

hazard to air travel. A serious accident caused by an

electronic surveillance system operating on instrument landing

system frequencies is too high a price to pay for any evidence

which might be thus obtained. The exclusion of these

frequencies would eliminate the use of the detuned commercial

FH receiver as a listening post, and require the purchase of

receivers manufactured specifically for surveillance work.

Although this is an economic penalty, it will likely pay off

iiandsomely in terms of system performance because many

commercial Ffi undercover receivers are of relatively poor

qua! i ty

.

Finally, the modulation bandwidths characteristic of highfidelity equipment are not required for good quality voice

communication. Therefore, the standard can afford to relax

some of the requirements normally made of equipment used, for

example, in the entertainment industry.

The need for high-quality training for those engaged in

electronic surveillance work is most evident. This work

encompasses a large body of knowledge, much of which is not

found in textbooks and which can therefore be conveyed only by

those with experience.

Much additional work is needed in the area of antenna

design and evaluation, especially of body-mounted antennas

where large variations in received signal strength are

observable.

In the process of gathering information for the preparation

of this report, one fact was outstandingly evident. This was

that information regarding the manufacturers of electronic

surveillance equipment and the places where such equipment

could be purchased, was difficult to obtain. On the positive

side, this would indicate that the present laws governing the

advertising of such equipment are proving effective. From the

side of law enforcement, however, this presents a serious

problem. Where there is a legitimate need for electronic

surveillance, law enforcement personnel need ready access to

current information on the sources for the purchase of

equipment and instruction in its use. Many law enforcement

officials have expressed a desire for a users' guide that could

be consulted for information on a particular type of equipment,

64
who manufactures it, and how the products of the various

manufacturers compare, for the money invested. The development

of objective performance standards is a step in this direction.

65
APPENDIX A- -ANNOTATED BIBLIOGRAPHY

1. Brown, R. H. , "The Electronic Invasion," John F. Rider

Publishing, Inc., New York, 1967.

This book deals with the many types and applications of

devices usable for eavesdropping, as well as those used to

detect the presence of such devices. For almost every

device discussed, there is given the name of its

manufacturer and often the prices as well. Because many

manufacturers of such equipment are small firms, it may be

found that many are no longer in business. The book is

written in a popularized style and contains a great deal of

general information along with some technical information,

including circuit diagrams.

2. Dash, S., Schwartz, R. F., and Knowlton, R. E., "The

Eavesdroppers," Rutgers University Press, New Brunswick,

N.J., 1959.

This is a lengthy book written in three parts, namely the

practice, the tools and the law as they pertain to

eavesdropping. Probably because the book was written prior

to the advent of miniaturized electronics, it does not deal

with radiatinq equipment. However, it is quite

comprehensive and informative on such subjects as video

devices in use up to the date of publication. Also

included is a good bibliography.

3. EIA Standard RS-316, "Minimum Standards for

Portable/Personal Land Mobile Communications FM or PM

Equipment 25-470 MHz," Electronics Industries Association,

11 West 42nd Street, New York, N.Y., July 1965.

4. Farmer, R. A., "SINAD System Design," IRE Transactions on Vehicular Communications, Volume VC-10, Number 1, Anril

1961 , pp. 1 03-1 08.

This paper describes the SliJAD system for evaluating

performance of a mobile communications system as well as explaining its use and advantages in evaluating system

components and parameters such as receiver sensitivity and

selectivity. It also discusses the problems of site noise

and propagation and terrain variations. Specific data arc

given for frequencies in the vicinity of 50, 150, and 450

MHz, which are the frequencies of major interest in

undercover communications work.

66
Fraim, F., and Murphy, P., "Miniature Electret

Microphones," Journal of the Audio Engineering Society,

Vol. 18, No. 5, October 1970, pp. 511-517.

This technical paper discusses the development of

miniaturized electret microphones, tells how they are

constructed and the requirements of the preamplifiers used

with them. The significant performance characteristics of

the electret are discussed along with their strengths and

weaknesses, and comparisons are made to other types of

miniature microphones.

Greene, R. M., Jr., editor, "Business Intelligence and

Espionage," Dow Jones-Irwin, Inc., Homewood, Illinois, Oct.

1 966.

Chapter eleven of this book is entitled "Electronic

Eavesdropping (Bugging): Its Use and Coun termeasures . " It

is an essentially non- technical discussion of both

radiating and wired devices used in undercover Informationgathering activities. No attempt is made to delve into the

engineering and technical characteristics of the devices

described; instead, it gives an overview of the types of

devices in use, a brief description of what they do and how

they are applied.

Huber, J. L., "Signal Attenuation for Passenger Vehicles

Between 0.1 and 1.0 GHz," Proceedings of the 1970 Carnahan

Conference on Electronic Crime Coun termeasures , Bulletin

92, April 1970.

Signal attenuation experienced when transmitted energy was

radiated from the inside of various vehicles was observed

to vary from 3 to 15 dB for a vertically polarized antenna

and from 10 to 30 dB with a horizontally polarized antenna

over the frequency range 0.1 to 1.0 GHz. Wide variations

occurred in a cyclic pattern with the average attenuation

over all frequencies being around 15 dB.

Jesch, R. L., and Berry, I. S., "Batteries Used with Law

Enforcement Communications Equipment, Comparison and

Performance Characteristics," LESP-RPT-0201 . 00 , National

Institute of Law Enforcement and Criminal Justice, U.S.

Department of Justice, GP0 Stock Number 2700-0156, May

1 972 .

67
This report is the result of an extensive literature search

conducted in the field of primary and secondary batteries.

It lists terms and definitions pertaining to batteries and

their characteristics, reviews basic battery principles and

types and assembles performance characteristics of battery

systems into chart form for comparison purposes. Includes

basic precautions and references to pertinent literature.

9. King, H. E., and Yowell, C. 0., "Assessment of Technology

Applicable to Body-Mounted Antennas," prepared for the Lav;

Enforcement Assistance Administration, U.S. Department of

Justice, Contract No. F04701 072-C-0073 , Controlled Access,

March 1973.

This report was written in connection with a development

effort on body-mounted antennas for police transceivers.

Although these antennas are not necessarily designed for

concealment such as those used with surveillance equipment,

there are many common problems and interests in the two

applications. The report contains a great deal of

analytical and measurement information pertinent to the

manufacture and use of surveillance equipment. Subject

headings include electrically small antennas, antenna

types, antenna and the body, and propagation information.

10. Mason, J. F., "Designers Compete for that Snug Automatic

Bug in a Rug," Electronic Design, Vol. 21, No. 20,

September 27, 1973, pp. 22-30.

This popularized article discusses many of the modern

techniques of electronic audio surveillance and

concentrates on devices utilizing the telephone. Both hard

wire and transmitting devices are pictured and briefly

explained. One page of this article shows the circuit

diagram of a common telephone and tells twelve ways of

tapping it. Some discussion of coun termeasure devices is

included near the end of the article, along with some

predictions as to what the future will bring in the

advancement and sophistication of electronic audio

survei 1 1 ance

.

11. NILECJ-STD-0203. 00 , "NILECJ Standard for Personal /Portabl

e

FM Transmitters," National Institute of Law Enforcement and

Criminal Justice, U.S. Department of Justice, (in press).

68
This document establishes performance requirements and

methods of test for frequency modulated personal /portabl

e

transmitters used by law enforcement agencies. Equipment

which meets these requirements is of superior quality and

is suited to the needs of" most law enforcement agencies.

This standard may be referenced 1n procurement documents

and used to determine whether or not purchased equipment

meets stated requirements.

12. NILECJ-STD-0208.00, "NILECJ Standard for Personal /Portabl

e

FM Receivers," National Institute of Law Enforcement and

Criminal Justice, U.S. Department of Justice, (in press).

This document establishes performance requirements and

methods of test for frequency modulated personal /portabl

e

receivers used by law enforcement agencies. Equipment

which meets these requirements is of superior quality and

is suited to the needs of most law enforcement agencies.

This standard may be referenced in procurement documents

and used to determine whether or not purchased equipment

meets stated requirements.

13. Oliver, B. M., and Cage, J. M., editors, "Electronic

Measurements and Instrumentation," Inter-University

Electronics Series, Vol. 12, McGraw-Hill Book ComDany, New

York, N.Y., 1971.

This is a comprehensive book on the theory and techniques

of a very wide variety of electrical and electronic

measurements. Of particular Importance to the subject of

this report 1s chapter fourteen, which deals with

measurements on transmitters and receivers. This chapter

also includes a comprehensive listing of specifications

concerned with radio receiving or transmitting equipment

used for communication or navigation purposes.

Specifications listed include those of the. Electronic

Industries Association (EIA), Aeronautical Radio

Incorporated (ARINC), Federal Communications Commission

(FCC), Military Specifications (MIL) and the Defense

Communications Agency (DCA).

14. Olson, H. F . , "Acoustical Engineering," D. Van Nostrand

Company, Inc., Princeton, New Jersey, 1957, Chapter 8.

69
This book contains a very detailed chapter covering

microphones. Theoretical relationships, equivalent,

mechanical and electrical circuits, and frequency response

curves for the various types are given, along with crosssection drawings to illustrate operating principles. The

book also has an excellent chapter on recording.

15. Pollock, D. A., "Methods of Electronic Audio Surveillance,"

Charles C. Thomas Publishing Co., 301-327 East Lawrence

Avenue, Springfield, Illinois, 1973.

This book is presented as an electronics manual for

detectives. Following an introductory chapter on basic

electricity and electronics, the principles of audio,

telephone and radio frequency communication systems are

explained. A practical applications section on operational

techniques is included, followed by some material on

countermeasures . In the concluding section of the book are

chapters on legal aspects, administrative control and

ethics. An appendix comprising several sections includes a glossary of terms and a partial listing of distributors of

audio surveillance devices and essential electronic

components. The section on operational techniques is quite

explicit in the explanations it gives for installing room

bugs and telephone taps of various kinds.

16. Randall, R. H., "An Introduction to Acoustics," AddisonWesley Press, Inc., Cambridge, Mass., 1951, pp. 240-246.

This is a college text which provides a discussion of

various types of microphones, including carbon, capacitor,

el ec trodynami c , and crystal types, and gives a comparison

of their relative sensitivities.

17. Rice, L. P., "Radio Transmission and Buildings at 35 to 150

mc," The Bell System Technical Journal, January 1959, pp.

197-210.

Work done in an attempt to determine the relative merits of

two frequencies for propagation into buildings is reported

and analyzed statistically. Although not greatly

different, the 150 mc coverage was found to be better than

that at 35 mc in most instances. At both frequencies,

reception was better on upper floors of the building than

on lower floors, although this latter phenomenom may have

been due to the elevation of the transmitting antenna used

in the experiments.

70
Shrader, R. L., "Electronic Communication," Second Edition,

McGraw-Hill Book Company, Inc., New York.

This book Incorporates an electronics and radio

communications theory course Intended to prepare the reader

for examinations required to obtain amateur and commercial

Federal Communications Commission radio licenses. A great

deal of the material is relevant to electronic

surveillance. The book contains an excellent chapter on

antennas, including a good discussion of the propagation of

radio energy.

7 I
MBS-114A (REV. 7-73)

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBSIR 75-813

2. Gov't Accession

No.

3. Recipient's Accession No.

4. TITLE AND SUBTITLE

ELECTRONIC EAVESDROPPING TECHNIQUES AND EQUIPMENT

5. Publication Date June 1975

6. Performing Organization Code

7. AUTHOR(S)

Raymond N. Jones

8. Performing Organ. Report No. Nat'l Bur. of Stds.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No. 2724472

11. Contract/Grant No.

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP) U. S. Department of Justice

Law Enforcement Assistance Administration

Washington, D. C. 20234

13. Type of Report & Period Covered

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here.) This report describes some of the methods and equipment used for electronic

audio surveillance. The subject of countersurveillance (defensive) equipment and

techniques, while not completely excluded, is confined to the minimum necessary

to the understanding of offensive surveillance activity. Both radiating and hard

wire methods are discussed along with the advantages, disadvantages and limitations

of each. Particular attention is given to the subject of body-worn transmitters and

the operational problems associated with their use including batteries and bodymounted antennas. Experimental data is included to illustrate some of the principles

and problems surrounding their use. The conclusion includes recommendations of

various items for inclusion in a standard for body-worn transmitters. A fairly

extensive annotated bibliography is also provided.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons) Body-worn antenna; bugging; eaves dropping ; electronic

surveillance; microphones; receiver; tape recorder; telephone; transmitter; wire18. AVAILABILITY \~2 Unlimited

For Official Distribution. Do Not Release to NTIS

Q 1 Order From Sup. of Doc, U.S. Government Printing Office Washington, D.C. 20402. SD Cat. No. CH

H Order From National Technical Information Service (NTIS)

Springfield, Virginia 22151

19. SECURITY CLASS

(THIS REPORT)

UNCLASSIFIED

21. NO. OF PAGES

20. SECURITY CLASS

(THIS PAGE)

UNCLASSIFIED

22. Price

USCOMM-DC 29042-P74

USUIMM - tKl.