NBS

345

A New Near-Zone Electric Field-Strength Meter

FRANK M. GREENE

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National Bureau of Standards

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THE NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards1 provides measurement and technical information services

essential to the efficiency and effectiveness of the work of the Nation's scientists and engineers. The

Bureau serves also as a focal point in the Federal Government for assuring maximum application of

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accomplish this mission, the Bureau is organized into three institutes covering broad program areas of research and services: THE INSTITUTE FOR BASIC STANDARDS . . . provides the central basis within the United

States for a complete and consistent system of physical measurements, coordinates that system with the measurement systems of other nations, and furnishes essential services leading to accurate and uniform

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associated materials services including mainly reference materials and data on the properties of materials. Beyond its direct interest to the Nation's scientists and engineers, this Institute yields services which are essential to the advancement of technology in industry and commerce. This Institute is or- ganized primarily by technical fields: —Analytical Chemistry—Metallurgy—Reactor Radiations—Polymers—Inorganic Materials—Cryogenics2—Materials Evaluation Laboratory—Office of Standard Reference Materials.

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20234.

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UNITED STATES DEPARTMENT OF COMMERCE • John T. Connor, Secretary

NATIONAL BUREAU OF STANDARDS • A. V. Astin, Director

NBS TECHNICAL NOTE 345

ISSUED NOVEMBER 15, 1966

A NEW NEAR-ZONE ELECTRIC FIELD-STRENGTH METER

FRANK M. GREENE

Radio Standards Laboratory

National Bureau of Standards

Boulder, Colorado

NBS Technical Notes are designed to supplement the Bureau's regular publications program. They provide a means for making available scientific data that are of transient or limited interest. Technical Notes may be

listed or referred to in the open literature.

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C., 20402

Price: 35 cents.

TABLE OF CONTENTS Page

ABSTRACT 1

1. INTRODUCTION 1

2. THE "SEMI-CONDUCTING" PLASTIC TRANSMISSION LINE . 2

2. 1 Errors Caused by Metallic Lines 2

2. 2 The Use of a Non-Metallic Line 3

2. 3 Description of the "Semi-Conducting" Line 3

2. 4 Electrical Characteristics of the Line 4

3. DESCRIPTION OF THE NBS FIELD-STRENGTH METERS . . 5

3. 1 Principles of Operation 5

3. 2 Electrical Design 9

3. 2. 1 General 9

3. 2. 2 The Dipole Antenna 10

3. 2. 3 RF Capacitive Step Attenuator 17

3. 2. 4 Band-Pass Amplifier and Detector . • 19

3. 2. 5 Calibrating Oscillator and Detector 25

3. 2. 6 Remote Indicator Unit 31

3.2.7 The Assembled Field-Strength Meter ..... 35

4. DASA FINAL ACCEPTANCE TESTS 39

4. 1 General 39

4. 2 Low -Frequency Tests 39

4. 3 High-Frequency Tests 43

4. 4 Summary of Tests 45

5. ACKNOWLEDGEMENT 46

6. BIBLIOGRAPHY 47

in

LIST OF FIGURES Page

Fig. 1 Block Diagram of the NBS Field -Strength Meter .... 6

Fig. 2 Cutaway Drawing of the Measuring Dipole 11

Fig. 3 View of the Complete Measuring Dipole 13

Fig. 4 Detector -DC -Output Transfer -Wiring Schematic .... 14

Fig. 5 Circuitry of Capacitive Step Attenuator 16

Fig. 6 Basic Layout of RF Band-Pass Amplifier 18

Fig. 7 Circuit of Band -Pass RF Amplifier (150 to 250 kHz). . . 20

Fig. 8 Circuit of Band-Pass RF Amplifier (18 to 30 MHz) ... 21

Fig. 9 Seven-Pole Butterworth Band-Pass Filter (f = 200 kHz) . 22

Fig. 10 Six-Pole Tschebyscheff Band-Pass Filter (f°= 24 MHz) . 23

Fig. 11 Nominal Overall Selectivity Characteristics of the Two

Band-Pass Amplifiers 24

Fig. 12 Basic Layout of Calibrating Oscillator 26

Fig. 13 Circuit of Low-Frequency Calibrating Oscillator

(150 to 250 kHz) 28

Fig. 14 Circuit of High-Frequency Calibrating Oscillator

(18 to 30 MHz) 29

Fig. 15 Basic Layout of Remote Indicator Unit ....... 30

Fig. 16 Circuit of DC Vacuum-Tube Voltmeter 32

Fig. 17 Circuit of Audio -Amplifier Beat- Indicator ._ 33

Fig. 18 View of the Measuring Dipole and Mounting Gimbal ... 34

Fig. 19 The Complete Near -Zone Field -Strength Meter .... 36

Fig. 20 View of the Remote Indicator Unit 37

Fig. 21 Low -Frequency DASA Acceptance Tests 40

Fig. 22 Vertical Electric Field of a 35-Foot Monopole 42

Fig. 23 Vertical Electric Field of a \/4 Monopole 44

TABLES

Table 1 10

A NEW NEAR-ZONE ELECTRIC FIELD-STRENGTH METER

Frank M. Greene

Abstract-The National Bureau of Standards has recently completed the

development of prototype instrumentation for measuring the electric

field components of complex, high-level, near -zone electromagnetic

fields from 0. 1 to 1000 volts per meter, at frequencies from 150 kHz

to 30 MHz with a present uncertainty of less than ±2 dB. The design of

the NBS meters is based on the use of a novel form of telemetry, employing a completely non-metallic electrical transmission line, which

apparently has not been fully exploited heretofore. This avoids the perturbing effects on the field being measured, usually caused by fieldstrength meters employing metallic RF transmission lines. The design

and performance of the meter are discussed in some detail.

1. Introduction

The National Bureau of Standards, under the sponsorship of the

Defense Atomic Support Agency, has recently completed the development of prototype instrumentation for measuring the electric -field components of complex, high-level, near -zone electromagnetic fields. The

instrumentation is capable of measuring both the magnitude and direction

of elliptically polarized CW electric fields having strengths in the range

from 0. 1 to 1000 volts per meter at frequencies from 150 kHz to 30 MHz.

This represents one step in the effort to develop accurate and meaningful measuring and calibrating instrumentation, for use in evaluating

hazards of electromagnetic radiation to electro-explosive ordnance

devices. It is well known that high-level fields of powerful nearby radio

transmitters may cause premature detonation of missile and rocket type

weapons during storage or loading operations on shipboard, or at other

military installations. It was from the standpoint of improving both the

weapons reliability, as well as the safety of operating personnel and

equipment, that the program was undertaken.
The design of the NBS meters is based on the use of a novel form

of telemetry, employing a completely non-metallic electrical transmission line to avoid perturbing the field being measured. This approach

has apparently not been fully exploited, heretofore. The extremely -high

RF loss of the line used attenuates any RF currents induced on the line

by the surrounding field and essentially eliminates re-radiation from the

line, or unwanted coupling between the line and the measuring antenna.

The high line -loss, however, necessitates miniaturizing the RF portions

of the receiving and calibrating instrumentation and placing them and the

associated battery supplies inside the measuring antenna. The field

information contained in the detected DC-AF output of the receiver is

transmitted over the line to a remote readout unit, where the strength of

the electric -field component parallel to the axis of the antenna is read

directly.

Interim electric -field standards and calibration techniques were

also developed at NBS to evaluate the performance of the field -strength

meters during their development, as well as to provide a tentative calibration of the completed instruments. The present uncertainty of the

standards is believed to be less than ±2 dB, but further development

effort is expected to reduce this to less than ±1 dB.

2. The New "Semi- Conducting" Plastic Transmission Line

2. 1 Errors Caused by Metallic Lines.

In the past, electric field -strength meters have usually made use

of a long metallic RF transmission line to connect the measuring antenna

with its receiver, usually located at a point remote from the antenna.

Such metallic lines often cause large measurement errors, especially

when measuring near -zone fields having complex spatial distributions.

In these cases, a neutral-field path, along which the line can be placed,
either seldom exists, or the orientation of such a path, if it does exist

is usually not known in advance of making the measurements. Under

these conditions, it is difficult or impossible to orient the line so that it

is everywhere normal to existing electric -field components. Thus, the

line not only may perturb the field being measured, but unwanted RF

currents induced on the line can be coupled into the antenna and contribute to the total response of the field -strength meter.

2. 2 The Use of a Non-Metallic Line.

The NBS near -zone meters avoid this difficulty by making use of a special non-metallic balanced transmission line in which the conductor

RF loss has been purposely made extremely high compared to that of the

usual copper line. If the conductors are made of sufficiently high resistance material, the line can be made essentially "transparent" to the

surrounding field. This is to say that the RF currents induced on the

transmission line by the field will be negligibly small, resulting in insignificant reradiation, or that any RF energy propagating along the line

will be heavily attenuated because of the extremely high loss of the line.

It has been found that this can be achieved if the volume resistivity of the

conductor material used is of the order of a million times or more higher

than that of copper. This is roughly midway (on a logarithmic scale)

between the volume resistivity of metals, on the one hand, and that of

insulators such as glass or mica on the other. Such materials can therefore be said to be "semi-conducting. "

2. 3 Description of the "Semi-Conducting" Line.

The material used in the NBS lines is basically polytetrafluoroethylene (PTFE), rendered "semi-conducting" by uniformly dispersing

finely -divided carbon black (approximately 30 percent by weight) throughout the plastic while it is in the semi-fluid state during its manufacture.
Tests at NBS have indicated the PTFE material to be more stable electrically than other types tried, such as silicone rubber, polyethylene,

carbon-coated multi-fiber glass, etc. Other types of conductor materials (including high-resistance metallic alloy wire and thin deposited

metallic films) were examined, but were not found to be practical for

this use, either because of unsuitable electrical characteristics or excessive cost.

Several parallel-conductor balanced transmission lines were made

at NBS using the semi -conducting PTFE material. The line conductors

are in the form of 0. 03 inch diameter monofilaments in place of the usual

copper conductors. They are coated with a thin nylon film (approximately 0. 005 inch thick) to further improve their strength and electrical

stability. Each of the filaments is in turn enclosed in a vinyl-coated

woven fiber-glass sleeve approximately 0. 1 inch O. D. Two such insulated conductors are encased in a polyvinyl chloride outer jacket, for added strength and protection, to form the completed transmission line.

Military Type JJ-048 or PJ-291 twin-conductor plugs are used at the two

ends for connecting to the dipole antenna, and to the remote indicator

unit, respectively. The overall length of the lines is approximately 30

feet.

2. 4 Electrical Characteristics of the Line.

The "semi-conducting" PTFE material used in the line has a volume resistivity of approximately 3. ohm-cm (compared to 1. 7 x 10

ohm-cm for copper), giving a resistance of approximately 20,000 ohms

per lineal foot, or a loop resistance of approximately 1. 2 megohms for a line 30 feet in length. The measured mutual capacitance between the

two line conductors is approximately 10 pF per foot, or roughly 300 pF

for the entire line. The transmission loss, to the desired field-strength

information at DC and low AF, is negligible if a high -impedance DC-AF
vacuum -tube voltmeter is used as the readout device at the receiving

end of the line. The line attenuation increases as the square -root of

frequency, reaching a theoretical as well as a measured value of approximately 53 dB per foot at a frequency of 30 MHz, for the differential

mode of transmission. The common-mode attenuation with the line lying

flat on a metal ground plane turns out to have essentially the same value.

The use of a high-loss, parallel-conductor, balanced transmission line

of this type has been found to reduce the perturbation of the surrounding

field by more than two orders of magnitude below that existing in the

case of a copper line. This renders any effect of the line on the field

wholly negligible in most instances.

A type of "flexural" noise is generated on the "semi-conducting"

transmission line when it is suddenly flexed or moved physically. The

common-mode component of this "noise" may be relatively high, having

a peak value of several volts on open circuit, for a 30 foot line. However,

the differential -mode component is appreciably smaller, having a peak

value of the order of 0. 2 volt with the line open circuited at both ends.

This can be decreased to approximately 0. 02 volt by terminating either

the sending or receiving end of the line in a resistance of 10 megohms.

This noise does not present a problem in the NBS meters where the level

of transmission over the balanced line is in the range 0. 75 to 7. 5 volts, as shown in Table 1.

3. Description of the NBS Field -Strength Meters

3. 1 Principles of Operation.

Two prototype, near-zone, CW electric field-strength meters were

developed for the Defense Atomic Support Agency. These meters cover

the frequency ranges 150 to 250 kHz, and 18 to 30 MHz, respectively,

and are capable of measuring fields over the range 0. 1 to 1000 volts per
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meter (RMS). Both meters make use of a selective TRF receiver to

amplify the voltage induced in the receiving antenna by the component of

the electric field being measured. The detected output of the receiver

is transmitted over the special non-metallic transmission line to a remote indicator unit where the strength of the field is read directly. A

block diagram of the basic circuitry is shown in Fig. 1.

These meters employ the same basic method of receiver-gain

calibration that has been used in CW-type field-strength meters in this

country for many years. Meters of this basic type are somewhat simpler in construction, but, at the same time, have proven themselves to

have much better long-term amplitude stability than present-day socalled "interference" meters.

A CW oscillator is included to permit calibrating the overall receiver gain periodically during a sequence of measurements in the field.

This oscillator is tunable over the same frequency range as the receiver.

This method permits setting the receiver gain each time to the same

value that existed when the instrument was previously calibrated in a standard field.

The receiver portion of the field-strength meter consists basically

of an RF input capacitive step attenuator, a fixed-tuned band-pass RF

amplifier (with manual gain control), and a diode detector. The internal

calibrating oscillator is tunable over the pass -band of the receiver to

permit adjusting the receiver gain to a "standard" value for any signal

frequency within its pass band. The RF output level of the oscillator is

monitored by its own detector. During calibration this output is applied

to the receiver input through a fixed resistive attenuator and transfer

switch, S-l. With the receiver input attenuator set to its reference or "calibrate" position, the receiver gain is manually adjusted until the DC

outputs of detectors 1 and 2 are identical. The exact output level of the
calibrating oscillator is not important and need not be known, nor does

it have to be exactly the same each time a calibration is performed.

However, it is important that the oscillator detector remain stable. In

this way, slow changes in the gain of the receiver (including its detector)

due to variations in ambient temperature, battery supply voltage, component aging, tuning drift, etc. , are compensated for each time the

receiver gain is calibrated.

While not a necessary condition, it can be seen, for simplicity,

that if detectors 1 and 2 have identical characteristics, the loop gain of

the circuitry between them is adjusted to unity each time the fieldstrength meter is calibrated. That is, the overall amplifier gain is

adjusted so that it is equal to the total loss in the two tandem attenuators.

The overall long-term calibration stability of this field-strength meter

is thus seen to depend principally upon the stability of the two attenuators

and the oscillator detector. These components are relatively easy to

stabilize. The RF amplifier need have only relatively good short-term

stability over the period between interim calibrations in the field of

roughly 30 minutes or so. The calibrating oscillator, on the other hand,

only needs to have good short-term amplitude stability over a period of

roughly 60 seconds or so, during calibration procedures in the field, or whenever the instrument is being calibrated in a standard field.

The original design goals, followed in developing these fieldstrength meters, specified the measurement of the field strength produced by a single CW source. If two signals, having different frequencies, were present within the fixed-tuned pass-band of the instrument,

the resulting response would tend toward the peak of the combined, or composite input, rather than the RMS, or average.
3. 2 Electrical Design.

3. 2. 1 General.

The two near -zone TRF electric field-strength meters developed

at NBS-BL consist of the following principal components:

(a) a short hollow cylindrical dipole antenna in which is induced an RF

voltage proportional to the component of the electric field parallel

to the dipole axis;

(b) an RF capacitive step attenuator, contained within the dipole, to

reduce the antenna -output RF voltage to the working level of the

receiver;

(c) a miniaturized solid-state selective amplifier (or receiver) and

CW calibrating oscillator, both contained within the hollow dipole

antenna, along with their respective battery supplies;

(d) a special semi-conducting plastic transmission line, approximately

30 feet in length, over which the detected DC-AF output of the

receiver is transmitted;

(e) a remote -indicator unit where the strength of the desired component of the electric field is read directly on a highly-stable batteryoperated vacuum-tube voltmeter. A miniaturized solid-state audio

amplifier and speaker are included for use as a zero-beat tuning

indicator, when adjusting the frequency of the calibrating oscillator

to that of the field being measured.

Before discussing these components in detail, it is of interest to

examine the range of nominal RF voltage levels over which they will

operate. The components are all directly interrelated in their electrical

design. As an example, the required RF amplifier gain is dictated by

several factors, some of which are: the dipole length as well as its

internal and residual capacitances; the required linearity of response

and dynamic range; the "flexural" noise level of the semi-conducting

plastic transmission line.

9
A list of the various ranges of RF voltage over which the principal

components operate is shown in Table 1 for the range of electric field

strength from 0. 1 to 1000 volts per meter. An explanation of how the

ranges were calculated is given in the following:

Effective Length of Measuring Dipole >Leff = 0.15 m

Induced Voltage in Dipole , V. = E • L6ff Dipole Voltage-Transfer Ratio, V /Vt = Ca /(Ca + C8 ) - 0. 20

Ca = Internal Capacitance of Dipole , C& = 3 pF

Cs = Residual Gap and Circuit Capacitance.., Cs = 12 pF

RF Amplifier Nominal Voltage Gain , Vg /Vx = 100

TABLE 1 DISTRIBUTION OF NOMINAL OPERATING-RF-VOLTAGE LEVELS

(in the TRF field-strength meter)

E(V/m) a(dB) Vt (volts) V (volts) Vj. (volts) V2 (volts) V3 (volts)

0. 1-1. .015-0. 15 .003-0.03 0.003-0.03 0. 3-3.0 0.75-7.5

1.0-10. 20 0. 15-1. 50 0.03-0. 30 0.003-0.03 0. 3-3.0 0.75-7.5

10. -io2 40 1.50-15.0 0. 30-3. 00 0.003-0.03 0. 3-3.0 0.75-7.5

102 -103 60 15.0-150. 3.00-30.0 0.003-0.03 0. 3-3.0 0.75-7.5

E(V/m) = Electric Field Strength, volts per meter

a(dB) = Measuring Dipole Attenuator Setting

V± = Induced Voltage in Measuring Dipole

V = Output Voltage of Measuring Dipole

Yi = Input Voltage to RF Amplifier

V2 = Output Voltage of RF Amplifier

V3 = Output Voltage of Amplifier Detector (DC)

3. 2. 2 The Dipole Antenna.

A cutaway drawing of the measuring dipole is shown in Fig. 2, and

a view of the complete assembly is shown in the upper portion of Fig. 3. The two dipole cylinders are made of gold-plated copper tubing 1-3/8

inches outside diameter, with a 1/32 inch insulating gap between them

at the center. They are aligned and held together by an epoxy-glass

sleeve as shown. The overall length of the antenna is approximately

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13 inches, not including the nylon control knob on each end. A view of

the interior instrumentation with the copper dipole sleeves removed is

shown in the central portion of Fig. 3. The miniaturized, solid-state,

band-pass amplifier (receiver) and its battery supply are in the left half

of the dipole. The tunable CW calibrating oscillator and its battery

supply are in the right half. The RF capacitive step attenuator is located

in the central portion of the dipole, and is comprised of the two rotary

wafer switches on the extreme left and associated fixed capacitors. The

two center wafers comprise a DPDT switch used for transferring the

semi-conducting line to the DC output of either the amplifier detector or

the oscillator detector. The single wafer switch on the extreme right of

center energizes the calibrating oscillator when the attenuator control

knob (located on the extreme left end of the dipole) is in either of the

CALIBRATE positions. At the bottom of Fig. 3 are shown (left to right):

(a) a short piece of the semi-conducting plastic transmission line and

its twin -conductor plug;

(b) an internal dipole -shorting sleeve, used to attenuate strong local

signals while calibrating the amplifier gain;

(c) some of the miniature silver -oxide cells used in the internal battery supplies.

The transmission-line plug connects to the two stainless -steel pins

at the center of the dipole. The detector DC -transfer and RF -filtering

circuitry is shown in Fig. 4. The RF filter is composed of resistors Rl

to R4 and bypass capacitors C3 and C4. It serves to preserve the electrical symmetry or balance of the dipole with respect to the transmission

line, and to attenuate any transverse, as well as common-mode RF pickup that might exist on the line. Special care was exercised in the design

and construction of the measuring dipole, so as to maintain accurate

electrical symmetry. This is essential if the antenna is to respond only

15
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16
to the component of the electric field parallel to its axis. The electrical

symmetry, or balance, of the NBS dipole is such that its response to the

cross -polarized component of the electric field is 40 dB or more below

its principal response.

It can be shown that if the length of the measuring dipole does not

exceed 0. 03 wavelength, its effective length when measuring a field

having a highly -complex spatial distribution will not differ by more than

2 or 3 percent (in the worst case) from the effective length when immersed in a uniform plane wave. Likewise, the interaction error, resulting from coupling between such a short antenna and its image, will

be small provided the loading is light and that measurements are not

made closer than two or three feet to the ground or large metallic objects.

3. 2. 3 RF Capacitive Step Attenuator.

The circuitry and capacitance values used in this attenuator are

shown in Fig. 5. This unit is constructed using special 60-degree

detent, subminiature, rotary wafer switches (glass -silicone wafers and

coin-silver contacts). The fixed capacitors used are low -temperature

coefficient ceramic miniatures. Special capacitance values, which have

been selected to 1 percent tolerance, are used as shown. The input and

output trimmer capacitors (not shown) are 1-10 pF ceramic subminiatures. The step attenuator proper has four positions, 0, 20, 40, and

60 dB, with two additional dB positions, Calibrate A (OSC) and Calibrate B (AMP). In the latter two positions, the calibrating oscillator is

turned ON, and the semi-conducting plastic transmission line can then

be switched between the DC outputs of the oscillator and amplifier detectors while calibrating the amplifier gain.

The attenuator design is based on a dipole internal capacitance of

3 pF and a residual gap and circuit capacitance of 12 pF, as given in

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Table 1. The latter value is the total input-circuit capacitance when the

attenuator is in the 0-dB position. In the 40 and 6C dB positions, the

residual capacitance divides into two parts, 7 pF for the antenna or input

side of the attenuator, and 5 pF for the amplifier or output side. The

overall size of the complete miniaturized attenuator module is approximately 1 inch diameter by 1 inch long.

3. 2. 4 Band-Pass Amplifier and Detector.

The basic layout of this unit is shown in the block diagram of Fig.

6. In order to provide a high input impedance, so as not to appreciably

load the dipole antenna or the capacitive step attenuator, a Metal-Oxide -

Silicon Field-Effect Transistor (MOS-FET) is used in the first stage as an untuned input -impedance converter ( source -follower configuration).

This is an insulated -gate N -channel FET having an extremely high input

resistance of several megohms and a low input capacitance of approximately 2 pF. In order to protect this MOS-FET from burn-out due to

excessive RF or static potentials, two back-biased shunt diodes are used

connected back-to -back across the RF input. This is shown in the schematic diagrams of Figs. 7 and 8, for the low -frequency and high-frequency

amplifiers, respectively.

In the low -frequency amplifier (150 to 250 kHz) the untuned input

stage is followed by a 7 -pole Butterworth band-pass filter, the circuit of

which is given in Fig. 9. This is in turn followed by a 3 -stage, broadband, resistance -coupled amplifier using overall negative RF feedback

to improve gain stability with respect to temperature and supply voltage

variations. In the high-frequency amplifier (18 to 30 MHz) a 6-pole

Tschebyscheff band-pass filter is used to determine the overall selectivity. The circuit diagram of this filter is shown in Fig. 10. The main

amplifier has three stages using wide -band synchronously-tuned interstage transformers, with a low-Q LC tank in the collector circuit of the

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final stage. These low-Q tuned circuits do not contribute appreciably to

the overall selectivity of the high-frequency amplifier, but do contribute

to its stability and serve to reduce the output noise to a satisfactory level

by limiting the bandwidth of this portion of the receiver. No overall negative RF feedback is employed in the design of this amplifier because of

the difficulty in maintaining proper RF phase relations over the passband. However, the gain stability was improved by Zener -regulating the

DC bias supplied to the bases of the three transistors.

The overall gain of both amplifiers can be manually adjusted to the

required operating value, which is approximately 40 dB. The output

detectors of both amplifiers employ two stable silicon diodes in a voltage

doubling circuit as shown. This eases somewhat the voltage -swing

requirements on the final amplifier stage, and also reduces the battery

drain accordingly. Subminiature rigid coaxial lines 1/16 inch O. D. are

used for the RF input and detector DC output leads, as indicated in Fig.

2. The nominal overall selectivity characteristics of the low -frequency

and high-frequency band-pass amplifiers are shown in Fig. 11. The

battery supply for the amplifiers is 12 volts, being composed of silveroxide cells in two paralleled stacks of eight cells each. A silicon diode

is used, as shown, to provide reversed-polarity protection for the amplifier. This supply has a useful life of from 10 to 20 hours. The two bandpass amplifier modules were developed to NBS specifications by a private

manufacturer and are approximately 1-1/4 inches O. D. by 5 inches long,

overall, including the battery supply.

3. 2. 5 Calibrating Oscillator and Detector.

The basic circuit arrangement of the calibrating oscillators is

shown in the block diagram of Fig. 12. The tuning range of each of the

two oscillators is the same as the 3 dB bandwidth of the band -pass amplifier with which it is used, i. e. , 150 to 250 kHz for the low -frequency

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unit, and 18 to 30 MHz for the high-frequency unit. The detailed circuitry of these two oscillators is shown in Figs. 13 and 14, respectively.

Tuning is accomplished by means of a ferrite slug core, driven by a steel

lead screw, as can be seen in Fig. 2. The inductor of the low -frequency

unit is a four -layer progressive-universal winding having 500 turns of

7/44 Litz wire, while that of the high-frequency oscillator is a singlelayer solenoid having 22 turns of No. 20 A. W. G. solid-copper wire.

The windings are molded on the inside of an epoxy form to minimize the

air gap, and to increase the tuning range and Q. Tuning linearity is

achieved by varying the winding pitch along the winding. An amplifier

and AGC system is incorporated to maintain the oscillator RF output

level essentially constant over the tuning range.

The RF output of the oscillator is adjustable to 3. 16 volts and fed

into the input terminals of a fixed 30-dB resistive attenuator pad. When

calibrating the gain of the band-pass amplifier, the dutput of 0. 1 volt

from the pad is applied through a small variable capacitor (adjusted to

approximately 3 pF) which simulates the dipole probe capacitance. The

detector circuit is identical to that used with the band-pass amplifiers

and employs two stable silicon diodes in a voltage -doubling circuit. The

detector monitors the 3. 16 volt RF oscillator level at the input terminals

of the 30 dB pad. Subminiature rigid coaxial lines 1/16 inch O. D. are

used for the RF output and detector DC output leads. The battery supply

is essentially the same as that used with the band-pass amplifier unit,

and uses two paralleled stacks of silver -oxide -cells (nine cells per stack).

This gives a nominal supply voltage of 13. 5 volts for the collector supply

of the three transistors, as shown. A portion of this supply is Zenerregulated and applied to the transistor base circuits for improved amplitude stability over the useful life of the batteries, which is about 20 hours.

The two calibrating -oscillator modules were developed to NBS specifications

27
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by a private manufacturer and are approximately 1-1/4 inches O. D. by

5 inches long, overall, including the battery supply.

3. 2. 6 Remote Indicator Unit.

The basic makeup of this unit is shown in the block diagram of

Fig. 15. It consists, in the main, of a balanced DC vacuum-tube voltmeter with its DC microammeter , on which the magnitude of the electric

field strength is indicated. The VTVM is a battery-operated unit and has

extremely good stability and linearity of response because of the large

amount of negative DC feedback used in its design. It is used over a

single 20 dB input range of approximately 0. 7 5 to 7. 5 volts DC. A schematic circuit diagram of the VTVM is shown in Fig. 16.

The input circuitry of the VTVM is balanced to ground and provides

from 40 to 60 dB rejection of common-mode input in the 60 to 400 Hz

power -frequency range. In addition, a two-section balanced RF filter is

used to suppress common as well as transverse -mode input at RF frequencies. This filter provides at least 40 dB suppression over the range

from 2 to 30 MHz. The input resistance of the VTVM has been adjusted

to approximately 200 megohms, making the DC voltage drop on the semiconducting transmission line less than 1. percent.

An output circuit of the VTVM is used to drive a transistorized

audio amplifier and loud speaker contained in the same case with the

VTVM, as indicated in the block diagram. This unit is of conventional

design and is used as a zero-beat indicator when adjusting the frequency

of the calibrating oscillator to that of the field being measured. It is

always necessary to calibrate the gain of the RF amplifier in the fieldstrength meter at the particular frequency at which a measurement is to

be made. The schematic circuit diagram of the AF amplifier is shown

in Fig. 17.

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FIG. 18 VIEW OF THE MEASURING DIPOLE AND MOUNTING GIMBAL

34
The DC-VTVM and AF amplifier each has its own self-contained

battery supply. The complete remote indicator unit is housed in a drawnaluminum case, using a tight-fitting panel to minimize RF leakage into

the unit. A small drawn -aluminum case is used to enclose the rear side

of the DC microammeter to reduce RF entry through the meter face.

3. 2. 7 The Assembled Field -Strength Meter.

The measuring dipole of the near -zone TRF field -strength meter

is mounted on a hollow fiberglass shaft which is perpendicular to the

dipole axis. This shaft is, in turn, supported in plastic bearings in a circular reinforced polyfoam gimbal, approximately 21 inches in diameter, as shown in Fig. 18. The gimbal has been coated with a thin film

of epoxy to increase its strength. This assembly is mounted on a sturdy

wooden surveyor' s tripod in such a manner that manual adjustments in

azimuth and elevation can be readily made in the orientation of the dipole.

The complete field -strength meter consists of three units:

(a) the tripod -mounted dipole measuring antenna;

(b) the 30-foot "semi-conducting" plastic transmission line;

(c) the battery-operated remote -indicator unit.

A view of the complete operating instrument is shown in Fig. 19,

in which these components can be seen. The balanced transmission line

connects to the dipole through the hollow shaft on its right. A simple

split cylindrical tool is used to facilitate inserting the twin-conductor

transmission-line plug through this shaft. The other end of the line connects to the balanced input of the remote -indicator unit on the right. It

is necessary to observe the correct polarity, as marked, when making

this connection. A close-up view of this unit is shown in Fig. 20.

35
FIG. 19 THE COMPLETE NEAR-ZONE FIELD-STRENGTH METER

36
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FIG. 20 VIEW OF THE REMOTE INDICATOR UNIT

37
- NOTES -

38
4. DASA Final Acceptance Tests

4. 1 General.

The completed NBS near -zone TRF field -strength meters underwent extensive acceptance tests at the U. S. Naval Weapons Laboratory

at Dahlgren, Virginia, during the week from October 18 to October 22,

1965. The tests were conducted by FCDASA and NBS-BL personnel and

were witnessed by several representatives of each of the three branches

of the Department of Defense. Of great assistance in the tests were the

NWL metal ground plane No. 2 (100 by 240 feet in size), made of welded

sections of 3/8 inch steel plate, and the Navy transmitting equipment

with sufficient power to produce the required test field strengths at the

specified frequencies.

4. 2 Low -Frequency Tests.

The low -frequency TRF field -strength meter wa's tested at a frequency of 185 kHz, using vertically-polarized electric fields with

strengths up to 1020 volts per meter. The test fields were established

between a large horizontal "fan" antenna and the metal ground plane.

The "fan" was in the approximate shape of an equilateral triangle, about

100 feet long on a side and supported by three wooden poles. The "fan"

was composed of 14 diverging wires, connected at their common end to

a single -wire feed line, and supported by one of the poles. At their

opposite end, the fanned wires connected to a messenger cable stretched

between the other two poles. The height of the horizontal fan was adjusted to approximately 14 feet above the ground plane at the lowest point

(Station 3), increasing to 18 feet at the three points of support.

A Navy Type AN/FRT-39 transmitter having a CW RF power output

of approximately 2 kilowatts was used to excite the antenna system. The

transmitter was housed in one of the vans located beside the ground plane.

39
SINGLE WIRE FEEDER

VEHICLES

NWL FAN ANTENNA

14 FT. ABOVE GROUND PLANE

AT STATION (z) TEST FREQUENCY -185 kHz

(7) THRU (JO) - TEST STATIONS

A, B a C - 35 FT. WOODEN POLES

SCALE IN FEET

10 5 10

FIELD- STRENGTH MEASUREMENTS

STATION NUMBER © © © © © © © © © © © © © © ©

ANTENNA CURRENT (I

fl ) 13.0 12.9 12.4 13.0 13.0 12.8 13.0 12.6 13.0 13.0 12.7 12.9 13.0 13.0 13.0 FIELD STRENGTH (V/m)

1020 240 440 460 1020 880 210 420 440 1020 920 220 460 980 1020 *(AS MEASURED 4.3 FT. ABOVE GROUND)

UNBALANCE TEST

AT STATION (5)

DIPOLE ORIENTATION 0° 180° 0°

INDICATED V/m 1020 1020 1020

COMMON MODE TEST AT STA. (?) REMOTE INDICATOR AND LINE MOVED AND HANDLED IN STRONG FIELD

NO CHANGE IN INDICATED V/m

RF- SHIELDING TEST AT STA. @

TEST CONDITIONS AMPL. "ON"

(60 db STEP)

AMPL. ON, DIPOLE SHORTED

(Odb STEP)

AMPL. OFF, DIPOLE SHORTED

(Odb STEP)

REMOTE

INDICATOR

UNIT ONLY

INDICATED V/m 1020 1

FIG. 21 LOW-FREQUENCY DASA ACCEPTANCE TESTS

40
The total antenna RF current was measured in the single -wire feed line

at the transmitter and included any stray current that existed to ground.

The antenna layout, the locations of the test stations, as well as the test

results are shown in Fig. 21. All measurements were made with the

center of the dipole antenna of the field -strength meter at a vertical

height of 4. 3 feet above the ground plane. Values of electric -field

strength were measured over the range from 210 to 1020 volts per meter

with good reproducibility. Both the dipole unbalance and the commonmode pickup were found to be negligible (less than 1 percent), as indicated. RF shielding of the dipole was effective to approximately 60 dB, as measured in a 1020 volt-per -meter field. Susceptibility tests were also

performed which are not indicated in Fig. 21. No measurable susceptibility was found to an interfering 22. 91 MHz signal (on alone, or simultaneously), having a strength of 200 volts per meter. The remote

indicator unit was placed in the direct beam of a Type AN/SPS-17 P-Band

(222 MHz) radar, about 70 feet from its antenna which was radiating a peak power of about 7 50 kilowatts. A copper wire several feet long was

attached to the input connector of the remote -indicator unit which was

held by the operator. The meter showed no indication whatsoever, and

the unit was able to withstand the intense RF input without damage.

Another scanning radar was in operation near the test site, but the lowfrequency TRF meter showed no measurable susceptibility when in its

beam.

It was not considered feasible to attempt a theoretical evaluation of

the electric -field strength existing between the "fan" antenna and the

ground plane. This might have been achieved had it been possible to

accurately evaluate the antenna capacitance. However, the uncertain

boundary conditions existing prevented this. Unfortunately, as a result,

there are no values of theoretical field strength with which to compare

these test measurements.

41
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ANTENNA CURRENT: 7.2 AMR

FREQUENCY: 22.91 MHz

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DISTANCE FROM ANTENNA, FEET

FIG. 22 VERTICAL ELECTRIC FIELD OF A 35 FT. MONOPOLE

42
4. 3 High-Frequency Tests.

The high-frequency TRF field -strength meter was tested at a frequency of 22. 91 MHz. This was one of the normally used transmitter

frequencies nearest the center of the passband of the instrument. In the

first group of tests, the electric field was produced by a 35-foot selfsupporting vertical monopole with a base current of 7. 2 amperes. The

antenna was located at the edge of the ground plane near the transmitter

vans. It was excited by a Navy Type AN/FRT-5 transmitter having a nominal power output of approximately 15 kilowatts. No attempt was

made to determine the theoretical field distribution of this antenna because of the uncertain boundary conditions involved.

Field -strength measurements were made at a height of 4. 3 feet

(\ /10) above the ground plane at eleven points extending horizontally in a

line from the antenna across the ground plane to a point 20 feet beyond

the opposite edge (over bare ground). The results of these measurements

are shown plotted in Fig. 22, over the range from 8 to 17 2 volts per

meter.

Similar tests to the above were performed using a self-resonant

quarter -wave vertical monopole located at the center of the metal ground

plane. The same test frequency (22. 91 MHz) was used, with antenna

base currents of 4. 3 and 9. amperes. The antenna was a No. 14 A. W. G.

copper wire, supported on plastic stand-off insulators mounted on a thin

wooden mast (2 inches by 2 inches). A small loop was formed in the wire

at the top to minimize corona discharge, and an RF thermocouple type

ammeter was inserted at the base. The transmission feed line used was

50 ohm Type RG-221/U low -loss coaxial cable, lying on the ground plane

with the sheath grounded near the base of the antenna and the inner conductor connected directly to one of the ammeter terminals. The calculated

43
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ANTENNA HEIGHTU/4): 10.4 FT. ( 3.18 Ml

ANTENNA CURRENT: 4.3 AND 9.0 AMR

FREQUENCY: 22.91 MHz

TupoRFTirAI ( FOR A SINU^riinAI • • •

CURRENT DISTRIBUTION)

MEASURED (i« DAY)

O O O MEASURED (2'Jy DAY)

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FIG. 23 VERTICAL ELECTRIC FIELD OF A X/4 MONOPOLE

140

44
self -resonant antenna length was 10. 4 feet, which included an estimated

3 percent shortening due to end effect.

Field- strength measurements were made on two successive days

at a height of 4. 3 feet above the ground plane, at horizontal distances of

0. 5, 1. 0, 2. 0, and 3. wavelengths from the antenna. The results are

shown in Fig. 23 over the range from roughly 7 to 83 volts per meter.

Also shown for comparison is the theoretical field for this antenna based

on an assumed sinusoidal current distribution. The measurements agree

in general with the theoretical values to within about one dB. The use of

an assumed sinusoidal current distribution introduced some error in the

computed values of field strength, but this is probably not large because

of the small diameter of the antenna (0. 064 inch). The effect of edge

reflections from the ground plane is another factor that may have introduced additional uncertainty, although this is also believed to be small

in this case. The dipole unbalance and common-mode pickup were found

to be negligible (less than 1 percent). No penetration of the field into

either the dipole or the remote -indicator unit could be observed in a 200

volt-per -meter field. No measurable susceptibility was found to an

interfering 185 kHz signal (on alone, or simultaneously), having a strength

of 300 volts per meter. The unit showed no susceptibility to the scanning

radar mentioned in Section 4. 2.

4. 4 Summary of the Tests.

The NBS TRF meters showed the capability of measuring electric

field strengths without difficulty over the entire range from 0. 1 to 1000

volts per meter (RMS). The tests demonstrated the simplicity of operation of the NBS TRF meters and the reproducibility of measurements

free of the perturbing effects of metallic transmission lines usually experienced. There was no observable interaction between the semiconducting transmission line used and the field being measured. In

45
general, the agreement was about ±1 dB between the instrument readings

as calibrated in the NBS (vertical monopole) standard field at Boulder

and those obtained using a similar technique at NWL at Dahlgren. While

this agreement may be somewhat fortuitous, it was also felt to be quite

remarkable when considering all the various factors involved at the two

sites.

5. Acknowledgement

The work described in this paper was performed for the Field

Command, Defense Atomic Support Agency, under DASA Order HD-1102-

3171-0001 (June 20, 1963).

The author wishes to acknowledge the valuable assistance of

Mr. William E. Jessen of NBS-BL in the assembly and performance

evaluation of the instrumentation described herein. The assistance of

Mr. Ezra B. Larsen in the standard -field development work is also

gratefully acknowledged.

46
6. Bibliography

[ 1] Stoker, W. C. , et al, An Investigation of Electromagnetic Coupling

Devices for the Measurement of Noise Fields, Final Report,

Rensselaer Polytechnic Institute, Troy, N. Y. , Signal Corps

Contract No. DA 36-039 sc-207, September 1952.

[2] Abromavage, M. M. , et al, Electromagnetic Coupling to Ordnance

Systems, Report No. N178-7604, August 1961, Jansky & Bailey

Div. , Atlantic Research Corp. , Alexandria, Virginia.

[3] Bartfeld, R. A. , et al, Study of Methods of Improved Measurement

for Electromagnetic Field Components, Final Report, 26 May I960

to 30 June 1962, University of Pennsylvania, Philadelphia, Pennsylvania, USNCEL Contract NBy-3200.

[4] Cowles, W. W. , et al. , Study of Methods of Implementing PoyntingVector Measurements, Final Report 1 July 1962 to 31 December

1964, University of Pennsylvania, Philadelphia, Pennsylvania,

USNCEL Contract NBy-32219.

[5] Harrison, R. P. , and Lewis, E. A. , A Method for Accurately

Measuring the Vertical Electric Field Strength of a Propagating

VLF Wave, IEEE Trans, on I & M, VIM-14, Pl/2, pp. 89-97,

March-June 1965.

[6] Clark, D. B. , et al, Electromagnetic Field-Strength Measurements

in a Quasi -Absolute Monopole Field, Final Report, August 1963 to

October 1964, TR-R-361, U. S. Naval Civil Engineering Lab. , Port

Hueneme, California, March 1965.

[7] Cowdell, R. B. , HERO Ground-Plane Measurements and Development of the Near -Field Field -Intensity Meter, Final Report No.

2667-7191, 30 April 1965, and Addendum No. 1, July 19 65,

Genistron, Incorporated, College Park, Maryland.

47

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