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APR 2 1 1969 370

Calibration Principles and Procedures

For Field Strength Meters (30 Hz to 1 GHz)

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U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards
NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards1 was established by an act of Congress March 3, 1901. Today, in addition to serving as the Nation's central measurement laboratory,

the Bureau is a principal focal point in the Federal Government for assuring maximum application of the physical and engineering sciences to the advancement of technology in industry and commerce. To this end the Bureau conducts research and

provides central national services in three broad program areas and provides central national services in a fourth. These are: (1) basic measurements and standards,

(2) materials measurements and standards, (3) technological measurements and

standards, and (4) transfer of technology.

The Bureau comprises the Institute for Basic Standards, the Institute for Materials

Research, the Institute for Applied Technology, and the Center for Radiation Research.

THE INSTITUTE FOR BASIC STANDARDS provides the central basis within the

United States of a complete and consistent system of physical measurement, coor- dinates that system with the measurement systems of other nations, and furnishes

essential services leading to accurate and uniform physical measurements throughout

the Nation's scientific community, industry, and commerce. The Institute consists

of an Office of Standard Reference Data and a group of divisions organized by the

following areas of science and engineering

Applied Mathematics—Electricity—Metrology—Mechanics—Heat—Atomic Physics—Cryogenics2—Radio Physics2—Radio Engineering2—Astrophysics2—Time

and Frequency.2 THE INSTITUTE FOR MATERIALS RESEARCH conducts materials research leading to methods, standards of measurement, and data needed by industry, commerce,

educational institutions, and government. The Institute also provides advisory and

research services to other government agencies. The Institute consists of an Office of

Standard Reference Materials and a group of divisions organized by the following

areas of materials research:

Analytical Chemistry—Polymers—Metallurgy — Inorganic Materials — Physical

Chemistry.

THE INSTITUTE FOR APPLIED TECHNOLOGY provides for the creation of appropriate opportunities for the use and application of technology within the Federal Government and within the civilian sector of American industry. The primary functions

of the Institute may be broadly classified as programs relating to technological meas- urements and standards and techniques for the transfer of technology. The Institute

consists of a Clearinghouse for Scientific and Technical Information,3 a Center for

Computer Sciences and Technology, and a group of technical divisions and offices organized by the following fields of technology:

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THE CENTER FOR RADIATION RESEARCH engages in research, measurement,

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problems of other agencies and institutions. The Center for Radiation Research consists of the following divisions:

Reactor Radiation—Linac Radiation—Applied Radiation—Nuclear Radiation.

1 Headquarters and Laboratories at Gaithersburg, Maryland, unless otherwise noted; mailing address Washington, D. C. 20234.

2 Located at Boulder, Colorado 80302.

3 Located at 5285 Port Royal Road, Springfield, Virginia 22151.
UNITED STATES DEPARTMENT OF COMMERCE

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

NBS TECHNICAL NOTE 370

ISSUED MARCH 1969

Calibration Principles and Procedures

for Field Strength Meters 30 (Hz to 1 GHz)

Harold E. Taggart

and

John L Workman

Radio Standards Engineering Division

Institute for Basic Standards

National Bureau of Standards

Boulder, Colorado 80302

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 $1.25.
FOREWORD

This report was written to help those people engaged in the calibration of field strength meters at frequencies from 30 Hz to 1 GHz.

The methods and procedures outlined in this report were developed and

have been evaluated at the National Bureau of Standards. The step-bystep procedures were written and illustrated so that technical personnel

can easily duplicate the equipment setups and perform the calibrations.

A commercial field strength meter requiring all phases of calibration was selected for writing the step-by-step calibration procedures.

The selection of the meter does not indicate endorsement or disapproval

of the product by the National Bureau of Standards.

The recommended equipment used in the calibration systems are

types whose specification meet the requirements of the calibration systems

Other types whose specifications are equivalent or better than those listed

can be used.

The writing of this report was sponsored by the Department of the

Air Force, Rome Air Development Center, Griffiss Air Force Base,

New York, under Contract No. AF30(602) -4383.
TABLE OF CONTENTS

ABSTRACT vi

1.0 INTRODUCTION _ _ _ _ _ i 1.1 The Basic Field Strength Meter _ _____ 1 1.2 Calibration of the Receiver _______ 4

1.3 Calibration of the Antenna _______ 5

1.4 Components of the Standard Antenna Set _ _ _ 6

2.0 TWO-TERMINAL RF VOLTMETER CALIBRATION _ 10

2.1 General Description and Characteristics of

RF Micropotentiometers ________ 10

2.2 Calibrated Signal Generator ----___ 21

2.3 Two-Terminal RF Voltmeter Calibration

Uncertainties ------______ 21

2.4 Procedure for Calibrating the NF-105 as a

Two-Terminal RF Voltmeter- ______ 22

2.4.1 DC Calibration of the Micropotentiometer - 22

2.4.2 Calibration of the NF-105 Receiver as a Two-Terminal RF Voltmeter Using the RF

Micropotentiometer _«-_-__ 26

3.0 CALIBRATION OF FIM SIGNAL ATTENUATORS _ - 41

3.1 General Discussion __________ 41

3.2 Attenuator Ratio Measurement Uncertainties 42

3.3 Procedure for Calibrating the NF-105

Signal Attenuators __________ 42

4.0 CALIBRATION OF THE FIM OVERALL LINEARITY. 51

4.1 General Discussion __________ 51

4.2 Overall Linearity Measurement Uncertainties - 52

4.3 Procedure for Calibrating the Overall Linearity

of the NF-105- ---.. 52

111
5.0 CALIBRATION OF LOOP ANTENNAS --------59

5.1 General Discussion ------------59

5.2 Standard -Fie Id Method -----------64

5.3 Standard -Antenna Method ----------68

5.4 Injection Method ____________ 70

5.5 Loop Antenna Measurements Uncertainties 75

5.6 Calibration Procedure for Loop Antennas

(30 Hz - 30 MHz) Using the Standard-Field Method - 75

5.6.1 Calibration Site ------------76

5.6.2 Basic Setup -------------76

5.6.3 Calculation of the Calibrating Field ----- 79

5.6.4 Calibration Procedure Using the Standards

Field Method ----- ___82

5.6.5 Uncertainties -------------86

5.7 Calibration Procedure for Loop Antennas

Using the Injection Method ----------86

5.7.1 Basic Setup -------------87

5.7.2 Calculation of the Injection Voltage ----- 90

5.7.3 Calibration Procedure Using the Injection

Method _____92

5.7.4 Uncertainties ------------- 93

6.0 CALIBRATION OF DIPOLE ANTENNAS 95

6.1 General Discussion ------------95

6.2 Calibrating Dipole Antennas Using the StandardAntenna Method _--_-_------_ 98

6.3 Other Methods of Calibrating Dipole Antennas - - -107

6.4 Dipole Antenna Measurements Uncertainties - - - - 108

6.5 Calibration Procedure for Dipole Antennas

(30 - 1000 MHz) Using the Standard -Antenna Method - 110

6.5.1 Calibrating Site - - - - - - - - - - - -111

6.5.2 Basic Setup ------------- 112

6.5.3 Calibration of the Voltage Standard _ _ _ - -115

6.5.4 Calibration of the Antenna Crystal Diode - - - 117

6.5.5 Calibration of the Unknown Receiving Antenna - - 118

6.5.6 Uncertainties- ----------- -119

IV
7.0 THE CALIBRATION REPORT - ----------124

7. 1 General Discussion - ------_____ 124

7.2 Two-Terminal RF Voltmeter Calibration ----- 125

7.3 Signal Input Attenuator Calibration ------- 125

7.4 Overall Linearity Calibration --------- 129

7.5 Loop Antenna Calibration ---------- 129

7.6 Dipole Antenna Calibration ----------133

REFERENCES _-_._. I49

DECIBELS vs VOLTAGE and POWER 150

D„C. CALIBRATION UNIT - 151
ABSTRACT

The National Bureau of Standards has been calibrating many types

of field strength meters and their related antennas at frequencies up to

1000 MHz for several years. Various techniques of measurement have

been investigated regarding accuracy, calibration time, ease of operation,

and reliability. This report discusses some of these calibration techniques in detail, and in some cases lists step-by-step procedures for performing the complete calibration. Typical calibration data are used to

take the reader from the beginning of the calibration to the completed test

report. All measurement setups are clearly illustrated, listing the equipment necessary to perform the calibration. Methods of calibrating two

basic types of antennas are described: (1) the loop antenna at frequencies

from 30 Hz to 30 MHz and (2) half -wavelength dipole antennas from 30 to

1000 MHz. Two methods of calibrating loop antennas are discussed in detail, the standard-field method and the injection method. The standardantenna method of calibrating dipole antennas is fully described. Measurement uncertainties of various methods are discussed. This report was

written to help technical personnel actively participate in antenna calibration work. Hopefully this information will help people to set up new

calibration services and to improve existing calibrating facilities.

Key Words: Calibration Procedures, Field Strength Meters,

Loop Antennas, Dipole Antennas.

VI
CALIBRATION PRINCIPLES AND PROCEDURES FOR

FIELD STRENGTH METERS (30 Hz to 1 GHz)

by

Harold E. Taggart and John L. Workman

1.0 INTRODUCTION

The purpose of this report is to present a discussion of principles

and procedures for calibrating field strength meters at frequencies from

30 Hz to 1000 MHz. The field strength meter has sometimes been called

"field intensity" meter. The word intensity connate s power in optics and

radiation; therefore, the use of field strength is favored over field intensity. However, the abbreviation "FIM" is commonly used for field

strength meter and for simplicity will be used throughout this report. The

FIM consists of a receiver for measuring r-f voltage and an antenna for

detecting an electromagnetic field radiated from a distant source. Methods

of calibrating the receiver as an r-f voltmeter will be discussed. At frequencies from 30 Hz to 1000 MHz, loop and dipole antennas are commonly

used to accurately measure electric and magnetic fields. Methods of calibrating these antennas will be discussed and step-by-step procedures will

be detailed.

The techniques used to calibrate field strength meters must duplicate as nearly as possible the conditions under which the instrument will

be used. The determination of field strength can readily become complicated by factors such as polarization, ground effect, reflections, multipath propagation, and near-fields, just to mention a few. An awareness of

these problems is necessary in order to accurately measure radiated fields

and to calibrate the field strength meter.

1. 1 The Basic Field Strength Meter . The field strength meter is basically a calibrated receiver and an

antenna of known characteristics. The calibrated receiver usually
consists of precision attenuation circuits for changing the signal level

in known increments, an internal calibration standard to provide a means

of setting the gain to known positions, and a readout system of some type

usually calibrated in microvolts or decibels above one microvolt.

Figure 1.1 is a block diagram of this basic field strength meter. The

antenna may be a loop,, a rod, a dipole, a horn or other type of antenna

depending on the carrier frequency and on measurement requirements.

w

RF

y&r

Attn.

RF Amp.

and

Mixer

IF

Attn.

IF

Amp.

Metering

Circuits

D.C.

1 i Attn.

Detecior

Calib. StandardReceiver Gain

Audio

Amp. ^<T6

Figure 1.1. Block Diagram of a Field Strength Meter.

The signal attenuators are usually one of three types or a combination of these. The three types are (1) radio frequency (r-f)

attenuators, (2) intermediate frequency (IF) attenuators, and (3) direct

current (d-c) attenuators. The r-f attenuators must have good frequency

response characteristics; the IF attenuators ordinarily are used at only

one frequency, the intermediate frequency; therefore, their design characteristics are usually less stringent. The d-c attenuators are often

located in the d-c metering circuits, hence the name. These attenuators

produce the same end result which is to attenuate the measured signal in
known amounts, but each accomplishes the task in a different manner.

An awareness of the type of attenuator used is often beneficial during

certain measurements.

The readout system usually consists of a panel meter calibrated

in microvolts or decibels above 1 microvolt. Some panel meters are

single decade ( to 20 decibels); others are double (0 to 40 decibels).

The linearity of the readout system is of concern when making accurate

measurements. It is not uncommon to accumulate errors of 1 to 2 decibels in overall linearity. The chief sources of error in overall

linearity seem to be in the IF amplifier section and the mixer section.

The internal calibrating standard is utilized to provide a known

amount of energy to the receiver so that the receiver gain can be adjusted for proper meter indication. Several types of internal calibrating

standards are used. The choice is dependent somewhat on the frequency

range of the meter and on the design purpose of the meter. There are

three basic types commonly used: (1) impulse generators, (2) noise

diodes, and (3) sine wave reference signals. The impulse generator is

lightweight, can be used at a wide range of frequencies, and has an output calibrated in decibels per megahertz bandwidth (dB MHz) , which

makes it convenient for direct substitution in broadband measurements.

The noise diode generates random noise, is lightweight, and is useful

over a wide frequency range. The sine wave reference signal is used in

several forms, from a wide-range signal generator to a limited-range,

one -level oscillator. The sine wave calibrating standard is normally

larger and more complex in design; however, it is usually more stable

and inherently more accurate.

dBMHz is a unit of measure for expressing voltage in terms of

decibels above 1 microvolt per MHz of receiver bandwidth.
1. 2 Calibration of the Receiver . Three basic measurements are required to calibrate the receiver.

They are (1) calibration of the receiver as a two-terminal r-f voltmeter,

(2) calibration of the signal attenuators, and (3) calibration of the receiver's overall linearity. The two-terminal r-f voltmeter calibration

can be performed with a reliable unbalanced voltage standard such as the

r-f micropotentiometer . This low-impedance device will establish

known voltages at levels from 1|J.V to 100 mV at frequencies from d-c to

1 GHz with uncertainties from 1 to 5 percent depending on level and

frequency.

The signal attenuators and the overall linearity of the receiver

can be calibrated in terms of a good precision r-f attenuator. Dissipative step-type attenuators that work satisfactorily are available in this

frequency range. An attenuator with a 60 decibel range in 0. 1 decibel

increments will calibrate most commercial field strength meters. The

receiver signal attenuators are usually calibrated in increments of 20

decibels; therefore, an 80 decibel range is not required for the standard

attenuator. The readout system of the receiver is calibrated in terms

of the standard attenuator so that it will read correctly over the range

designed. This range is usually 20 or 40 decibels. When calibrating

the readout which is usually a panel meter, the corrections obtained

include the non-linearities in the r-f amplifiers, the mixer, the IF

amplifiers, the metering circuits, and the panel meter; therefore, the

term "overall linearity" applies. Figure 1.2 is a block diagram of a

system for calibrating the signal attenuators and overall linearity that

can be used at frequencies from 30 Hz to 1000 MHz. The r-f substitution method of measuring attenuation is employed; therefore, the

standard attenuator must be capable of performing over a wide frequency range. The termination on the standard attenuator is important

if maximum accuracy is to be achieved. At the higher frequencies it is
advisable (if possible) to calibrate the standard attenuator with the output termination connected to the standard attenuator. This eliminates

errors that may be encountered due to impedance variations. A calibration system such as this can perform calibrations with uncertainties of

not more than 0. 1 dB per 20 decibels if used carefully.

The overall errors of the FIM receiver include the uncertainties

of the two-terminal voltmeter calibration, the signal attenuator calibration, and the overall linearity calibration. A 2 percent uncertainty

in each calibration could amount to a maximum of 6 percent under the

worst conditions.

Standard

Attenuator

Output

Termination

(20-dB Pad)

RF

Signal

Generator

Input

Termination

(10-dBPad)

Signal Attenuators

I J V

RF

Voltmeter

Receiver under test

Figure 1.2. Measurement of Overall Linearity and

Signal Attenuator Ratios.

1. 3 Calibration of the Antenna . Two methods of calibrating loop antennas are discussed in this

report: (1) the standard-field method, and (2) the injection method [1].

The standard-field method is the most accurate and can be used over a wide frequency range. The injection method is not as accurate as the

standard -field method, but it is much faster and can be performed in the
laboratory. The selection of the method will depend upon the accuracy

requirements of the calibration.

The standard -antenna method of calibrating horizontallypolarized dipole antennas from 30 to 1000 MHz is discussed in detail

in this report. Other methods are discussed only briefly. Detailed procedures of the standard -antenna method are listed herein.

Note . To use the methods of calibration described above, a set of

standard antennas is required. Such a set has been developed by the

National Bureau of Standards for the Department of the Air Force, and

it will be assumed that a comparable set of antenna standards are

available

The Standard Antenna Set is designed to calibrate loop and dipole

antennas at frequencies from 30 Hz to 1000 MHz. Loop antennas can be

calibrated at frequencies from 30 Hz to 30 MHz, and horizontallypolarized dipole antennas can be calibrated at frequencies from 30 to

1000 MHz. This system is designed, primarily, to calibrate field

strength and interference meters and their associated antennas, although

other antenna standards can also be calibrated with this system.

The d-c Calibration Unit in this system is an accurate d-c milliammeter that is used to calibrate components of the antenna system, but

it can also be used as an accurate d-c supply from 1 to 1000 milliamperes

for other laboratory needs. It is an excellent device for calibrating

thermoelements that are frequently used in rf-dc devices such as r-f

micropotentiometers or other similar instruments [2],

1.4 Components of the Standard Antenna Set . A photograph of the complete antenna set is shown in Figure 1.3.

The components are placed in front of their respective packing container.

Each component is numbered and named in the accompanying list.
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Component List

Item No,

30 to 300 MHz Dipole Rods

30 to 300 MHz Transmitting Antenna

30 to 300 MHz Standard Receiving Antenna

300 to 1000 MHz Transmitting Antenna

300 to 1000 MHz Standard Receiving Antenna

300 to 1000 MHz Dipole Rods

30 to 300 MHz Balanced Voltage Standard

300 to 1000 MHz Balanced Voltage Standard

Balun Transformer (30 to 300 MHz)

D C Calibration Unit

Low Frequency Balun Transformers (30 Hz to 30 MHz)

D C Millivoltmeter

Loop Matching Unit

Standard Transmitting Loop

Slide Wire Potentiometer

RF Micropotentiometers

Injection Network for Loop

Tripod With Case

Miscellaneous Cables

Megahertz Tape

Figure 1.4 shows the dipole antennas packed in their carrying case.

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20
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2.0 TWO-TERMINAL RF VOLTMETER CALIBRATION.

2. 1 General Description and Characteristics of the RF

Micropotentiometer.

The r-f micropotentiometer, which is a stable, accurate, and

reliable working standard in the microvolt region, is a low impedance

source of a known r-f voltage. This device, as shown in Figure 2. 1,

consists of a radial resistor in series with a thermoelement which is

enclosed in a special housing. The radial resistors range from 1 milliohm to 1 ohm, depending on the range desired. These resistors are

thin film resistors made of platinum, platinum gold, or silver, depending on the desired resistance. The thermoelement is the UHF type

with a straight -through heater and the plane of the thermocouple leads

is positioned 90 with respect to the plane of the heater.

Figure 2.2 is a schematic diagram of the r-f micropotentiometer.

It can be seen from this diagram that this device is a source of voltage,

V, across the resistor, R. The output voltage, V, is determined by the

current and the size of the radial resistor, R, ( V = IR). This device

is calibrated at d-c, and can be used to generate known r-f voltages at

frequencies as high as 1000 MHz. The thermocouple output caused by

the current, I, flowing through the heater of the thermoelement is an

indication of this output voltage [2],

In order to cover the range from 1 microvolt to 100,000 microvolts, various combinations of resistor and thermoelement sizes must

be used. Table I presents a typical set of combinations which will cover

the above voltage range.

The resistance of the radial resistor is chosen below 1 ohm to

minimize the error introduced by loading the output with a 50-ohm

system. The size of the thermoelements do not exceed 100 milliamperes

in order that the input power requirements may be minimized. Also,

the larger thermoelements have an appreciable error due to skin effect

at the higher frequencies. These reasons govern the selection of values

that appear in this table.

10
Figure 2.1. RF Micropotentiometer,

Thermoelement Thermocouple

Heater

Radial Resistor

Input Output

Figure 2.2. Schematic Diagram of RF Micropotentiometer,

11
The determining factor in the selection of micropotentiometer

elements is the required output voltage. The voltage often required to

calibrate a field strength receiver as a two-terminal r-f voltmeter is

100 microvolts. Table I shows that a combination of a 27 milliohm

resistor and a 5 miliamp thermoelement can be used to obtain

a voltage of 100 microvolts. This exact combination may not be available and another combination selected. The value of the radial resistors

listed in Table I are merely nominal values that may be used to give the

desired voltage. Other values can be used. It is the magnitude of the

current through the resistor and the impedance of the resistor that determines the magnitude of the voltage. To assist in this selection,

several guidelines and examples will be given.

TABLE I: COMBINATIONS OF RF MICROPOTENTIOMETER

ELEMENTS TO COVER 1-100,000 MICROVOLTS

NOMINAL VOLTAGE

RANGE

SUGGESTED THERMOELEMENT

RESISTANCE OF SIZE

RADIAL RESISTOR

MICROVOLTS MILLIOHMS

1-5 1 5-15 3

15-45 9

45-135 27

135-400 81

400-1,200 240

1,200-3,600 360

3,600-11,000 440

11,000-33,000 660

33,000-100,000 1,000

MILLIAMPERES

5

5

5

5

5

5

10

25

50

100

12
Normally a low -valued resistor should be selected. This reduces

errors due to the loading effect on the resistor by the 50 -ohm system.

Resistors greater than 1 ohm are not usually recommended when calibrating 50 -ohm receiver systems such as the NF-105.

A thermoelement should be selected which has a current rating

equal to or higher than the computed current. It is best if the current

rating of the thermoelement and the computed current are nearly equal.

This gives a greater output from the thermoelement and better resolution,

The direct current required to produce the desired output voltage may be computed using Ohm's Law as follows:

where I, = the required direct current in amperes,

d-c

V = the desired output voltage in volts, and,

R = the resistance of the radial resistor in ohms.

Example:

Desired output voltage, V = 100 microvolts (100 x 10 volts).

-3

Resistance of radial resistor, R = 10.58 milliohms (10.58 x 10 ohms).

Required direct current, I, = ?.

d-c

-6 -3

V 100 x

10*" volts = 9.452 x 10" amperes

d-c R 10.58 x 10~3 ohms = 9.452 milliamperes

The direct current is 9.452 milliamperes; therefore, a 10-milliampere

thermoelement is required.

13
Thus far a method- of selecting the r-f micropotentiometer has

been described. Unfortunately the impedance of the radial resistor

(called resistance at d-c) is not constant with frequency. This change in

impedance is usually gradual with frequency. At the lower frequencies

(from d-c to approximately 300 MHz) the change may be only 1 to 2

percent; at the higher frequencies (300 to 1000 MHz) it may be as high

as 10 to 20 percent. The r-f micropotentiometer current is measured

using direct current and any change in the impedance of the radial resistors will in turn cause an error in the output voltage. This difference

in the d-c calculated output voltage and the actual r-f output voltage is

called the RF-DC difference, and is normally expressed in percent on

calibration reports . A sample NBS Report of Calibration is shown on

pages 15 and 16.

The RF-DC difference (RF-DC correction factor) should be used

to attain maximum accuracy. It can sometimes be ignored at the lower

frequencies (when it is less than 1 or 2 percent) and used only at the

higher frequencies.

14
foim NtS-443 (»«"'») C A MPT F

U. S. DEPARTMENT OF COMMERCE oaivlj-ljl

NATIONAL BUREAU OF STANDARDS

INSTITUTE FOR BASIC STANDARDS

BOULDER, COLORADO 80302

REPORT OF CALIBRATION

RF MICROPORTENTIOMETER

Submitted by:

The measurements on this instrument were performed under ambient

conditions of approximately 23 C and 40 percent relative humidity.

Determinations were made of the difference between the r-f and d-c

output voltages required for the same thermocouple output. The r-f

and d-c voltage determinations were made with the resistive element

terminated in 50-ohms. The resistance of the resistive element was

measured with d-c and found to be 10. 58 milliohms when terminated

in 50-ohms. The observed differences are given in the table below,

where a positive sign indicates that the r-f output voltage of the RF

Micropotentiometer is greater than the d-c value.

Page 1 of 2

Test No.

Date:

15
SAMPLE

RF Micropotentiometer

The RF-DC difference values are correct within the percentages pre'

sented in the table.

Frequency

MHz

RF.-DC Difference

percent

Accuracypercent

5 -1.4

100 -2.1

300 -1.3

400 - -0.7

500 +0.1

700 +2.0

900 +3.8

Page 2 of 2

Test No.

Date:

Reference:

For the Director,

Institute for Basic Standards

High Frequency Calibration Services

Radio Standards Engineering Division

16
The RF-DC correction factor may be used to correct the calculated currents, I. , as follows: d-c

100 - k T , 4 Xc

= luu" Vc (2)

where

I = corrected current in milliamperes,

k = RF-DC difference in percent, and

I, = calculated direct current in milliamperes

Example

Desired output voltage, E = 100 microvolts

Resistance of radial R = 10.58 milliohms

resistor,

Frequency =700 MHz

Determine the correct current for 100 microvolts at 700 MHz,

From the preceding example on page 13 the direct current required,

I , for 100 microvolts was calculated to be 9.452 milliamperes.

From the Report of Calibration on page 16 the RF-DC difference at

700 MHz is +2.0 percent. The corrected current,I , can now be calculated as follows:

= 100 -k

c 100 d-c

100 - (2.0) 9.452 = 9.263

100

= 9.263 milliamperes at 700 MHz.

17
The measurement of the direct current through the thermoelement and the radial resistor can be illustrated in Figure 2.3.

-Thermocouple Output

DC Milliammeter

|—Thermoelemen

I I RF Micropotentiometer

Radial

Resistor

Figure 2»3. DC Calibration of RF Micropotentiometer.

The d-c milliammeter should be a high quality laboratory type

instrument capable of measuring direct current to within 0.25 to

0.5%. This current measurement can be accomplished by other methods

as illustrated in Figure 2.4 which is a circuit diagram of a highly accurate

direct-current milliammeter that utilizes a precision resistor and an

accurate millivoltmeter.

18
Thermocouple Output

• vyw\A/—vwwv—

Input

•Thermoelement

RF Micropotentiometer > RF Output

Precision Slide Wire

Potentiometer

^-Radial

^ Resistor

Figure 2.4. Circuit Diagram of a Precision DC Milliammeter.

R_ and R are multi-turn potentiometers, R is a precision 1-ohm resistor, and the millivoltmete r is a precision slide wire potentiometer.

The current flowing through the r-f micropotentiometer is the same as

the current flowing through the precision resistor, R . By accuratelymeasuring the voltage drop across R the current flow can be determined

by simple application of Ohm's Law. One milliampere of current flowing

through a 1-ohm resistor will have a 1 -millivolt voltage difference across

it.

19
A block diagram of a typical two-terminal r-f voltmeter calibration

is illustrated in Figure 2.5.

RF 1Watching RF

er

Receiver

Under Test

Generator Network Micropotentiomel

Frequency

Meter

dc

Millivoltmeter

Figure 2.5. Typical Two-Terminal RF Voltmeter Calibration.

The frequency meter may not be required if the dial indicators

are within 1 percent on the generator. The matching network may not be

required if sufficient power is available from the generator; however,

matching stubs are usually required at the higher frequencies in order

to achieve sufficient power to the r-f micropotentiometer. The d-c

millivoltmeter can be any high quality meter with a 10 -millivolt full

scale deflection. A low -impedance (10-100 ohms) meter is preferred.

The accuracy of this meter is not important, but the repeatability and

resolution are very important.

20
2. 2 Calibrated Signal Generators . There may be times when r-f micropotentiometers are not available as voltage standards to calibrate the receivers. A calibrated

signal generator can be used to calibrate the receivers but with greater

uncertainty which depends on the signal generator used. Most field

strength meters have a nominal 50-ohm input impedance; therefore,

a calibrated 50-ohm signal generator can be used to inject known voltages

into the receiver.

2. 3 Two-Terminal RF Voltmeter Calibration Uncertainties . The uncertainties in establishing known voltages at these low

levels (10 (0.V to 100 mV) is a function of frequency, i.e. , the higher the

frequency the greater the uncertainty. One of the more accurate means

of establishing known voltages is to use the r-f micropotentiometer . By

using a calibrated micropotentiometer and all corrections, voltages can

be generated with uncertainties from 3 to 10 percent, depending on the

frequency. The following table lists the uncertainties as a function of

frequency that can be obtained by using calibrated r-f micropotentiometers:

Frequency range

MHz

Uncertainty

%

d-c to 100 3-4

100 to 500 4 - 6

500 to 1000 6-10

If a signal generator is used in place of the r-f micropotentiometer, the uncertainty is usually increased by at least twice the uncertainty listed above. It is obvious that at the higher frequencies the

21
calibration uncertainty will exceed 1 decibel. At the lower frequencies

the uncertainty should be within 1 decibel.

2.4 Procedure for Calibrating the NF-105 as a Two-Terminal RF

Voltmeter . 2.4.1 DC Calibration of the Micropotentiometer

The following equipment is required to provide this calibration;

Slide Wire Potentiometer - range, to 160 mVuncertainty ±0.1% of reading.

DC Calibration Unit from the Standard Antenna Set

(see page 151).

DC Millivoltmeter - range to 10 mV.

Calibration Procedure

DC Calibration Unit ( !-IOOmA d-c Supply)

Thermocouple

Output

Slide Wire

Potentiometer

RF MicroPotentiometer

/

Radial Resistor

DC Millivoltmeter

Figure 2.6. RF Micropotentiometer Calibration

System Cabling Diagram.

22
1 . Turn the Function Switch of the DC CALIBRATION UNIT to

the "OFF" position. Turn the "TEST-CALIB" switch of the

"1-10-MA DC SUPPLY" to "TEST".

2. Using the appropriate cable connect the PRECISION SLIDE

WIRE POTENTIOMETER to the "TO PRECISION S.W.

POTENTIOMETER" jacks of the DC CALIBRATION UNIT as

shown in Figure 2.6.

3. Connect the appropriate cable from the "1-10-MA DC

SUPPLY" jacks of the DC CALIBRATION UNIT to the r-f

input Type N connector of the RF MICROPOTENTIOMETER.

The radial resistor N connector of the RF MICROPOTENTIOMETER should have nothing connected to it.

4. Connect the RG-55 split tail cable from the BNC terminal on

the RF MICROPOTENTIOMETER to the terminals on the DC

MILLIVOLTMETER

CAUTION: Watch polarity.

5. Zero the SLIDE WIRE POTENTIOMETER.

6. From Example 1, page 24, obtain the value for I of the first

frequency and multiply it by 1 ohm to obtain the voltage to be

set into the SLIDE WIRE POTENTIOMETER

7. Set the SLIDE WIRE POTENTIOMETER to read the voltage

obtained in step 6.

8. Turn all of the "DC ADJUST" knobs on the DC CALIBRATION

UNIT fully counterclockwise.

9. Turn the Function Switch to the "1-10 MA" position.

23
10. Turn the "COURSE DC ADJUST" knob clockwise until the

DC MILLIAMMETER reads just under the value for the

corrected current I . c

11. Switch the "TEST-CALIB" switch of the "1-10 MA DC

SUPPLY" to the "CALIB" position.

12. Depress the contact button on the SLIDE WIRE POTENTIOMETER and adjust the "DC ADJUST" knobs of the DC CALIBRATION UNIT until the SLIDE WIRE POTENTIOMETER is

balanced.

13. Record the reading of the DC MILLIVOLTMETER. See

Example 1, page 24.

14. Repeat steps 5 through 13.

EXAMPLE 1 From Section 2. 1 it was found that I, 3 9.452 milliamperes. The

dc

RF-DC correction was taken from the curve on page 25, Figure 2.7.

This curve was drawn from the data in the Report of Calibration for the

r-f micropotentiometer. Using equation, (2), the corrected current is

obtained for each frequency above 600 MHz. The table below shows the

value of I obtained from the calculations. Also shown are sample readc ings of the d-c millivoltmete r (step 13, above).

Frequency

MHz

RF--DC difference

%

Ic

mA

Sample

Millivoltmete r Readings

9.452 7.71

700 +2.0 9.263 5.73

800 +2.8 9.187 5.45

900 +3.8 9.093 4.99

1000 +5.0 8.979 4.48

24
X

o

c

a>cr

Figure 2.7. Frequency vs RF-DC Difference for the

RF Micropotentiometer.

25
2.4.2 Calibration of the NF-105 Receiver as a Two-Terminal RF

Voltmeter Using the RF Micropotentiometer.

2.4.2. 1 Frequencies to be Calibrated for the TX Tuning Unit.

14 - 25 kHz Band

14

17

21

23

25

25-62 kHz Band

25

35

45

55

62

62 - 150 kHz Band

62

85

105

125

150

The following equipment is required to provide this calibration:

Signal Generator - frequency range, 30 Hz to 30 MHz

output, 3 volts into 50 ohms

DC Millivoltmeter - range to 10 mV.

Calibrated RF Micropotentiometer.

26
Calibration Procedure

Signal Generator

DC Millivoltmeter

RF Micropotentiometer

OnJ

s*

SU- 105 Switching Unit

RF Resistor

Thermocouple Output

(7mV or less)

Figure 2.8. Two -Terminal RF Voltmeter Calibration

System Cabling Diagram.

1. Connect an 18 -inch length of RG-9 cable from the output of the

SIGNAL GENERATOR to the input N connector on the RF

MICROPOTENTIOMETER.

CAUTION: SIGNAL GENERATOR output should be in low output

position to protect the RF MICROPOTENTIOMETER.

2. Connect the 18-inch RG-55 split tail cable from the DC MILLIVOLTMETER to the BNC connector on the RF MICROPOTENTIOMETER.

27
CAUTION: Watch polarity of the DC MILLIVOLTMETER.

3. Connect the green 30-foot cable from the radial resistive N

connector of the RF MICROPOTENTIOMETER to the BNC

connector marked "FROM ANTENNA" on the SU-105

SWITCHING UNIT.

4. Turn off RECEIVER and plug TX TUNING UNIT into the MAIN

UNIT. Turn on RECEIVER. Allow RECEIVER to warm up

an hour before starting calibration.

5. Connect the 6-inch RG-55 COAXIAL CABLE from the terminal

on the TX TUNING UNIT marked "TO ATTENUATOR" to the

terminal on the MAIN UNIT marked "ATTN OUTPUT".

6. Connect an 18 -inch length of RG-55 COAXIAL CABLE from

the terminal on the TUNING UNIT marked "IMPULSE

GEN OUTPUT" to the terminal on the SU-105 SWITCHING

UNIT marked "TO IMPULSE GENERATOR".

7. Connect an 18-inch length of RG-55 COAXIAL CABLE from

the terminal on the MAIN UNIT marked "SIGNAL INPUT" to

the terminal on the SU-105 SWITCHING UNIT marked

"TO SIGNAL INPUT".

28
8. Set the controls in the following positions:

SU-105

SWITCHING UNIT

MAIN UNIT

CONTROL

Calibrate -Read Switch

POSITION

"READ'

Function Switch "CARRIER"

"SLIDEBACK" Full CCW

"IF GAIN" Midway

"SIGNAL INPUT ATT." "20 dB"

"METER" "INT"

TX HEAD

"SIG. ATT" "NORM"

"BFO" "OFF"

"IMPULSE GEN" "OFF"

Band Switch n in

Positions of all other controls are of no importance for start of

calibration.

9. Set the SIGNAL GENERATOR frequency to 14 kHz.

10. Apply r-f to MICROPOTENTIOMETER until an indication on

the DC MILLTVOLTMETER is given that is just below the

Millivoltmeter indication recorded during d-c calibration

of the MICROPOTENTIOMETER. See Example 1, page 24.

CAUTION: Never exceed the millivolt indication on the DC

MILLIVOLTMETER for any given frequency, as it is possible

to burn out the thermoelement in the MICROPOTENTIOMETER

11. Tune the RECEIVER until a maximum indication is shown on

the Read-Out Meter. If the meter indication is off scale, reduce or increase the "IF GAIN" until an on-scale indication is

obtained. Again tune for a maximum indication.

29
12. Switch the Function Switch of the RECEIVER to "METER BAL"

position. Adjust "METER BAL" knob until the Read-Out Meter

reads 0.

13. Switch Function Switch to the "ZERO ADJ" knob until the

Read-Out Meter reads 0.

14. Repeat steps 12 and 13 until both steps indicate without

further adjustment.

15. Switch the Function Switch to the "CARRIER" position.

16. Increase the SIGNAL GENERATOR output until the DC MILLIVOLTMETER indication is the same as the indication recorded

during the d-c calibration of the r-f micropotentiometer.

CAUTION: Increase the SIGNAL GENERATOR output slowly. There

is a delay in meter indication of the DC MILLIVOLT - METER caused by the time required for the heater of

the thermoelement in the MICROPOTENTIOMETER to

change temperature.

17. Retune the receiver for maximum indication.

18. Adjust "IF GAIN" control until a full-scale indication "20 dB"

on the Read-Out Meter is observed. This gives a total reading

of "40 dB", 20 dB on the Read-Out Meter, plus 20 dB in

"SIGNAL INPUT ATTENUATOR". This is equivalent to 100 uV.

19. Change switch on SU-105 SWITCHING UNIT from "READ" to

"CALIBRATE".

20. Change Function Switch from "CARRIER" to "PEAK".

21. Turn on the impulse generator in TX TUNING UNIT by changing

the "IMPULSE GENERATOR" switch from "OFF" to "ON".

30
22. Adjust the "IMPULSE GENERATOR LEVEL" by rotating the

outer knob clockwise to increase the level in 10 dB increments, and the inner knob clockwise to increase the level

in 1 dB increments. Adjust the "IMPULSE GENERATOR

LEVEL" until the Read-Out Meter indicates between 19 and

20 dB.

23. The difference between the Read -Out Meter indication and the

20 dB point in tenths of a dB, plus the readings from the

"IMPULSE GENERATOR LEVEL" knobs gives the

"CAL SETTING".

EXAMPLE

Reading

Outer Knob

Inner Knob

Tenths of dB between needle

and 20 dB mark

Cal Setting

70 dB

4 dB

0.3 dB

74.3 dB

24. Record the "CAL SETTING". See Figure 2.10, page 38,

25. Repeat steps 10 through 24 three times. Average the three

"CAL SETTINGS" for a final "CAL SETTING".

26. Change the SIGNAL GENERATOR to next frequency listed on

page 26 and repeat steps 10 through 26.

31
2.4.2.2 Frequencies to be Calibrated for the TA Tuning Unit

0.15 - 0.36 MHz Band

0.15

0.22

0.29

0.36

0.36 -0.86 MHz Band

0.36

0.50

0.65

0.86

0.86 -2.1 MHz Band

0.86

1.2

1.6

2.1

2.1 - 5.2 MHz Band

2.1

3.0

4.0

5.2

5.2 - 12.7 MHz Band

5.2

7.5

10.0

12.7

12.7 - 30 MHz Band

12.7

18.0

24.0

30.0

32
The following equipment is required to provide this calibration:

Signal Generator - frequency range 0. 15 to 30 MHz - output 3 volts into 50 ohms.

Calibrated RF Micropotentiometer.

DC Millivoltmeter - range to 10 millivolts.

Calibration Procedure

O

j

]

Signal Generator

RF Micropotentiometer

DC Millivoltmeter

SU- 105 Switching Unit

RF Resistor

Thermocouple Output

( 7 mV or less )

Figure 2.9. Two-Terminal RF Voltmeter Calibration

System Cabling Diagram.

1. Connect an 18-inch length of RG-9 cable from the output of the

SIGNAL GENERATOR to the input N connector on the RF

MICROPOTENTIOMETER.

CAUTION: SIGNAL GENERATOR output should be in a low output position to protect the MICROPOTENTIOMETER.

2. Connect the 18-inch RG-55 split tail cable from the DC Millivoltmeter to the BNC connector on the MICROPOTENTIOMETER.

CAUTION: Watch polarity of the DC Millivoltmeter.

33
3. Connect the green 30-foot cable from the radial resistive N

connector of the MICROPOTENTIOMETER to the BNC connector

marked "FROM ANTENNA" on the "SU-105 SWITCHING UNIT".

4. Turn off RECEIVER and plug the TA TUNING UNIT into the

MAIN UNIT. Turn on the RECEIVER. Allow RECEIVER to

warm up an hour before start of calibration.

5. Connect the 6-inch RG-55 COAXIAL CABLE from the terminal on the TA TUNING UNIT marked "TO ATTENUATOR"

to the terminal on the MAIN UNIT marked "ATTN OUTPUT".

6. Connect an 18-inch RG-55 COAXIAL CABLE from the terminal on the MAIN UNIT marked "IMPULSE GEN" to the

terminal on the SU-105 SWITCHING UNIT marked

"TO IMPULSE GENERATOR".

7. Connect an 18-inch RG-55 COAXIAL CABLE from the terminal on the MAIN UNIT marked "SIGNAL INPUT" to the

terminal on the SU-105 (SWITCHING UNIT) marked

"TO SIGNAL INPUT".

CONTROL

SU-105

SWITCHING UNIT

MAIN UNIT

TA HEAD

Calibrate-Read Switch

Function Switch

"SLIDEBACK"

"IF GAIN"

"SIGNAL INPUT ATT"

"METER"

"SINE WAVE OSC - IMPULSE GEN SW"

Band Selector

POSITION

"READ"

"CARRIER"

Full CCW

Midway

"20 dB"

"INT,"

"OFF"

.15 - .36'

Position of all other controls are of no importance for start of

calibration.

34
8. Apply the r-f signal to the RF MICROPOTENTIOMETER until

an indication on the DC MILLIVOLTMETER is given that is

just below the indication recorded during the d-c calibration

of the RF MICROPOTENTIOMETER.

CAUTION: Never exceed the d-c calibration indication on the DC

MILLIVOLTMETER as it is possible to burn out the

thermoelement in the MICROPOTENTIOMETER.

9. Tune the RECEIVER until a maximum indication is shown on

the Read-Out Meter in the MAIN UNIT. If the meter is off

scale, reduce or increase the "IF GAIN" until an on-scale

indication is obtained. Again tune for a maximum indication.

10. Switch the Function Switch to the "METER BAL" position.

Adjust the "METER BAL" knob until the meter reads 0.

11. Switch Function Switch to the "ZERO ADJ". Adjust "ZERO ADJ"

knob until meter reads 0.

12. Repeat steps 10 and 11 until both steps indicate without

further adjustment.

13. Switch Function Switch to "CARRIER" position.

14. Increase the SIGNAL GENERATOR output until the DC MILLIVOLTMETER indication is the same as the indication given in

the MICROPOTENTIOMETER calibration.

CAUTION: Increase the f IGNAL GENERATOR output slowly. There

is a delay in tne DC MILLIVOLTMETER indication

caused by the time required for the heater of the thermoelement in the MICROPOTENTIOMETER to change

temperature.

35
15. Check tuning for maximum indication.

16. Adjust the "IF GAIN" control until a full scale indication,

20 dB, on the Read-Out Meter of the receiver is observed.

This gives a total reading of 40 dB, 20 dB on the Read-Out

Meter, plus 20 dB in the "SIGNAL INPUT ATTENUATOR".

This is equivalent to 100 uV.

17. Change switch on SU-105 SWITCHING UNIT from "READ" to

"CALIBRATE".

18. Change Function Switch from "CARRIER" to "PEAK".

19. Turn on the Impulse Generator in the MAIN UNIT by changing

the "SINE WAVE OSC." - "IMPULSE GEN." switch from

"OFF" to "IMPULSE GEN.".

20. Adjust the "IMPULSE GEN. OUTPUT dB ABOVE 1 |aV/Mc"

attenuators until the Read-Out Meter reads as close to full

scale as possible using the 10 -dB steps of the attenuator.

21. Adjust the "ADD TO IMPULSE GEN. OUTPUT" control until a

full-scale meter reading (20 dB) is obtained on the Read-Out

Meter of the receiver.

22. Add the dB reading from the "ADD TO IMPULSE GEN. OUTPUT" meter to the "IMPULSE GEN. OUTPUT dB ABOVE

1}aV/Mc" attenuator setting to obtain the CAL SETTING as

shown on the next page.

36
"ADD TO IMPULSE GEN. OUTPUT"

Meter Reading + 3.2 dB

"IMPULSE GEN. OUTPUT dB ABOVE

lfiV/Mc"

Attenuator Setting 7 0.0 dB

Cal Setting 73.2 dB

'ADD TO IMPULSE GEN. OUTPUT"

Meter Reading - 1 . 2 dB

"IMPULSE GEN. OUTPUT dB ABOVE

luV/Mc"

Attenuator Setting 8 0.0 dB

Cal Setting 78.8 dB

24. Record the CAL SETTING. See Figure 2.10, page 38.

25. Repeat steps 8 through 24 three times. Average the three CAL

SETTINGS. See Figure 2.10, page 38.

26. Change the frequency of the SIGNAL GENERATOR to the next

frequency listed on page 32 and repeat steps 8 through 25.

37
oBSERvat.on S„eet U. S. DEPARTMENT OF COMMERCE

NBS255

NATIONAL BUREAU OF STANDARDS

kind op measurement RF Voltmeter Calibration

„AWl2-l2-66

Sheet No. *

Observer JLW

i N<=TRUMFNTT«Tr„ NF-105 Ser. No. 1965 TA tuning unit t«tnn 920362

Frequency POO" Tb-mpfrathbp 'CI Humidity %. MM.

ain set in i

Barometer

Imp. gen. controls: 10 cycles , P.R.R. @ xlOO, g jeak pos., attn. in

20 dB position, voltage readings taken in "carrier" pos., input voltage - 100 \iY

Band Freq. Input Impulse Generator Settings

MHz voltage Ave.

nv dB dB dB dB

/ n.ifT loo 9JT.2 9£2. XJT./ ?<r.2

O.TOL <*?.L fl.L ?3. L 93. L

D.?1

0.3L

2 O.IL 79.2 79.3 7*2 79. ZL

O.X-3

D.90

OAj 1 r

Figure 2.10. Sample Data Sheet for RFVoltmeter Calibration.

38
2.4.2,3 Frequencies to be calibrated for the T-l, T-2, and T-3

Tuning Units.

T-l TUNING UNIT

20-65 MHz Band

20

30

35

50

65

65 - 200 MHz Band

65

100

150

200

T-2 TUNING UNIT

200 - 400 MHz Band

200

250

300

350

400

T-3 TUNING UNIT

400 - 700 MHz Band

400

500

600

700

39
T-3 TUNING UNIT

700 - 1000 MHz Band

700

800

900

1000

The following equipment is required to provide this calibration:

Signal Generator - frequency range 20 to 1000 MHzoutput 500 millivolts into 50 ohms

Calibrated RF Micropotentiometer

DC Millivoltmeter - range to 10 millivolts

The calibration procedure for the T-l, T-2, and T-3 Tuning

Units is the same as for the TA Tuning Unit.

40
3.0 CALIBRATION OF FIM SIGNAL ATTENUATORS

3. 1 General Discussion

The signal attenuators of the FIM are calibrated using a straightforward r-f substitution technique. This is illustrated in Figure 3.1.

The signal attenuators are calibrated in terms of a standard attenuator.

As the signal attenuation of the FIM is reduced, attenuation is added

with the standard attenuator to maintain a pre -selected level on the

readout of the FIM. V The change in attenuation of the standard attenuator

indicates the correct attenuation of the signal attenuators. From this

a correction factor can be determined.

Standard

Attenuator

Output

Termination

(20-dBPad)

RF

Signal

Generator

Input

Termination

(10-dBPad)

Signal

Attenuators

i „

'

RF

Voltmeter

Receiver under test

Figure 3. 1. Measurement Of Signal Attenuator Ratios.

The standard attenuator must be a calibrated r-f attenuator with a 60-dB range in 0. 1 dB increments. The 20-dB output pad is important

to the standard attenuator because it provides the standard attenuator

with a constant terminating impedance and serves as an isolator. The

10 -dB pad at the input of the standard attenuator provides some isolation

between the signal generator and the standard attenuator, and helps provide stable signal levels. The 10-dB pad may be removed if necessary

for the calibration of higher steps of the signal attenuators, but a close

watch must be maintained on the input signal level.

41
The signal attenuators should be calibrated at a high and a low

frequency, and also at a medium frequency, if the frequency range is

large or the signal attenuators are frequency sensitive. If an FIM

has IF attenuators and there are several intermediate frequencies, it

is usually wise to be certain all are calibrated.

3. 2 Attenuator Ratio Measurement Uncertainties . The chief requirements of an accurate attenuation measurement

system include stable calibrated standard attenuators, excellent shielding of all components from the r-f source to the detector, and stable

r-f sources. Shielding cannot be overemphasized, because the lack of

adequate shielding can introduce an appreciable error. The range of

attenuator measurements associated with FIM calibrations can be as

high as 100 decibels, and extreme care must be taken with all components to insure accurate measurements. One source of r-f leakage

that is often overlooked is the r-f connector. It is often necessary to

wrap them in a shielding material such as aluminum foil when making

high-level attenuation measurements. By using a calibrated standard

attenuator and a properly-designed system, measurements can be made

to within 0,1 dB + 0.3% of the attenuation in decibels (which is approximately 2 percent) from d-c to 400 MHz, and 0. 1 dB + 0.5% of the attenuation in decibels (which is approximately 3 percent) from 400 to 1000 MHz,

3.3 Procedure for Calibrating the NF«»105 Signal Attenuators:

Calibration frequencies:

150 kHz

300 MHz

1000 MHz

42
The following equipment is required to provide this calibration:

Signal Generator - frequency range 150 kHz to 1000 MHzoutput 500 millivolts into 50 ohms

Standard Attenuator - 60 dB maximum attenuation with

0. 1 dB steps

RF Voltmeter - frequency range 150 kHz to 1000 MHz

Calibration Procedure

Signal Genera

RF Voltmeter

SU-105 Switching Unit

Figure 3.2. Signal Attenuator Calibration

System Cabling Diagram.

43
1. Connect the T -Connector to the output of the SIGNAL GENERATOR,

2. Connect the probe of the RF VOLTMETER to the T -Connector.

3. Connect a short length of RG-9 cable from the female N connector of the T -Connector to the female N connector of the 10 -dB

pad attached to the input of the STANDARD ATTENUATOR.

4. Connect one end of the green 30-foot RG-55 cable with an N-toBNC adapter to the female N connector of the 20«dB pad on the

output of the STANDARD ATTENUATOR. Connect the other end

of the green 30-foot RG-55 cable to the BNC-female connector

marked "FROM ANTENNA" on the SU-105 SWITCHING UNIT.

5. Turn off RECEIVER and plug the TX TUNING UNIT into the

MAIN UNIT.

6. Turn on RECEIVER. Allow a one-hour warm-up period before

starting the calibration.

7. Connect the 6-inch RG-55 cable from the terminal on the TX

TUNING UNIT marked "TO ATTENUATOR" to the terminal

on the MAIN UNIT marked "ATTEN. OUTPUT".

8. Connect an 18 -inch length of RG-55 cable from the terminal

on the TX TUNING UNIT marked "IMPULSE GEN OUTPUT"

to the terminal on the SU-105 SWITCHING UNIT marked "TO

IMPULSE GENERATOR".

9. Connect an 18-inch length of RG-55 cable from the terminal on

the MAIN UNIT marked "SIGNAL INPUT" to the terminal on

the SU-105 SWITCHING UNIT marked "TO SIGNAL INPUT".

44
10. Set the controls in the following positions:

CONTROL

SIGNAL GENERATOR

STANDARD ATTENUATOR

SU-105

SWITCHING UNIT

MAIN UNIT

TX TUNING UNIT

POSITION

"MODULATION "CW"

SELECTOR"

"VERNIER "1 MICROATTENUATOR" VOLT

10 dB STEP DRUM "0 dB"

1 dB STEP DRUM "0 dB"

2 to 2. 9 dB STEP DRUM "2.5 dB"

Calibrate-Read Switch "READ"

Function Switch "CARRIER"

"SLIDEBACK" Full CCW

"IF GAIN" Midway

"SIGNAL INPUT "20 dB"

ATT."

"METER" "INT."

"SIG ATT" "NORM"

"BFO" "OFF"

"IMPULSE GEN." "OFF"

Band Switch "3"

Position of all other controls are of no importance for start of

calibration.

11. Set the frequency of the SIGNAL GENERATOR to 150 kHz.

12. Increase the output from the SIGNAL GENERATOR to approximately

3 millivolts.

13. Tune the RECEIVER for maximum indication on the meter. If an

off-scale reading is observed, reduce or increase the SIGNAL

GENERATOR output until an on- scale reading is acquired. Again,

tune the RECEIVER for maximum deflection.

45
14. Calibrate the RECEIVER in accordance with the instruction

manual using the "CAL SETTING" found in Section 2.4 for

150 kHz.

15. Switch the READ, CALIBRATE SWITCH on the SU-105

SWITCHING UNIT to "READ".

16. Tune RECEIVER again for maximum indication.

17. Place the "SIGNAL INPUT ATTENUATOR" in the "80 dB"

position.

18. Increase the output of the SIGNAL GENERATOR until an onscale reading of the meter is observed.

NOTE: The 10-dB pad on the input of the STANDARD ATTENUATOR

may be removed for this one step to obtain an on-scale

reading.

CAUTION: Do not exceed the maximum voltage rating of the

STANDARD ATTENUATOR.

19. Adjust the SIGNAL GENERATOR output until a convenient

reference point on the meter is obtained. If possible a fullscale or 20-dB reading is best, but if this is not possible, then

any point or marking on the upper half of the meter that can be

accurately read.

20. Add 20 dB with the STANDARD ATTENUATOR.

21. Change the "SIGNAL INPUT ATTENUATOR" from the "80 dB"

position to the "60 dB" position.

22. Adjust the STANDARD ATTENUATOR until the meter on the MAIN

UNIT again reads the reference point of step 19.

23. Record the reading from the STANDARD ATTENUATOR. See

Figure 3.3, page 50

46
24. Change the "SIGNAL INPUT ATTENUATOR" back to the "80 dB"

position.

25. Remove the added attenuation from the STANDARD ATTENUATOR.

26. The meter on the MAIN UNIT should again read the reference

point of step 19. If the meter needle does not come back to

the reference point, the measurement will be in error and have to

be repeated. The RF VOLTMETER reading should be monitored

throughout the measurement to maintain the output voltage from

the SIGNAL GENERATOR.

27. Steps 16 through 26 should be repeated three times and the results

of step 23 averaged. See Figure 3.3, page 50

28. Adjust the SIGNAL GENERATOR output until a convenient

reference point on the meter is obtained. If possible a full-scale or 20-dB reading is best, but if this is not possible, select any

point on the upper half of the meter that can be accurately read.

29. Add 40 dB of attenuation with the STANDARD ATTENUATOR.

30. Change the "SIGNAL INPUT ATTENUATOR" from the "80 dB"

position to the "40 dB" position.

31. Adjust the STANDARD ATTENUATOR until the meter on the

MAIN UNIT again reads the reference point of step 28.

32. Record the reading from the STANDARD ATTENUATOR. See

Figure 3.3, page 50.

33. Change the "SIGNAL INPUT ATTENUATOR" back to the "80 dB"

position.

34. Remove the added attenuation from the STANDARD ATTENUATOR.

47
35. The meter on the MAIN UNIT should again read the reference

point of step 28. If the needle does not come back to the

reference point, the measurement will be in error and have

to be repeated. The RF VOLTMETER reading should be

monitored throughout the measurement to maintain the output voltage of the SIGNAL GENERATOR constant.

36. Repeat steps 28 through 3 5 three times and average the result

of step 32. This will be used later as a cross check. See

Figure 3.3, page 50.

37. Replace the 10-dB pad to the input of STANDARD ATTENUATOR

if it has been removed.

38. Reduce the output of the SIGNAL GENERATOR by 20 dB.

39. Set the frequency of the SIGNAL GENERATOR to 150 kHz.

40. Place the "SIGNAL INPUT ATTENUATOR" in the "60 dB"

position.

41. Tune the receiver for maximum indication on the meter. If

an off-scale reading is observed reduce or increase the

SIGNAL GENERATOR output until an on-scale reading is

acquired. Again, tune the receiver for maximum deflection.

42. Adjust the output of the SIGNAL GENERATOR until a fullscale reading (20 dB) is obtained on the meter.

43. Add 20 dB to the STANDARD ATTENUATOR.

44. Change the "SIGNAL INPUT ATTENUATOR" from the "60 dB"

position to the "40 dB" position.

45. Adjust the STANDARD ATTENUATOR until the meter on the

MAIN UNIT again reads full scale (20 dB).

48
46. Record the reading from the STANDARD ATTENUATOR. See

Figure 3.3, page 50,

47. Change the "SIGNAL INPUT ATTENUATOR" back to the

"60 dB" position.

48. Remove the added attenuation from the STANDARD

ATTENUATOR.

49. The meter on the MAIN UNIT should return to full scale

(20 dB). If the needle does not come back to full scale (20 dB)

the measurement is in error and must be repeated.

50. Repeat steps 41 through 49 three times and average the results. See Figure 3.3, page 50.

51. The average results of steps 23 and 50 when added together

should be the same as the average results of step 32. If there

is a difference greater than 0.1 dB, the measurements will

have to be repeated until agreement is reached. See

Figure 3.3, page 50.

52. The above procedure is repeated for the rest of

"SIGNAL INPUT ATTENUATOR" positions.

53. The Calibration Procedure is repeated for the remaining two

frequencies. The TX TUNING UNIT is replaced by the

appropriate tuning unit for each frequency.

49
observat.cn sheet U. S. DEPARTMENT OF COMMERCE

NBS255

NATIONAL BUREAU OF STANDARDS

K,H„nr M»g,p.u«r Attenuator Calibration

n»TF 12-12-66

Sheet No. u , observer jec

i NeTRIIHFNTT«Tcn NF-105 Ser. No. 1965 TA Tunine Unit Tfst No 920362

FREQUENCY 15U fcHZ BnnM Tfmpfbstiihf •<": HllMiniTY % Barometer MM.

Gain standardized, function switch in "carrier"

GPO : 1966 — 217-724

Signal Standard Attenuato: ' Reading

Attn. Pos 9 Ave.

dB dB dB dB dB

fo 0-0 -?£ O-GOS O-O-^.iT ?..T

Lo 20-/-3.I ? 0-1-2.0 -20-I-?.} 23. 7

Lo 0-O-l.h 0-0-lb O-O?^ iX

40 2Q-0-2.Z QO-V-XZ 2 0-0 O.S

h 22X

4o

So

QO

<ZQ

4-o

&0

Figure 3.3. Sample Data Sheet For Attenuator Calibration,

50
4.0 CALIBRATION OF THE FIM OVERALL LINEARITY

4.1 General Discussion

The readout system of the receiver, which is usually a panel

meter, indicates the field strength either in microvolts or decibels above

1 microvolt. Either of these units can be used for field strength measure

ments . The ideal FIM will be perfectly linear, and the panel meter will

always indicate the correct reading. However, nonlinearities do exist

and can cause errors of several decibels in some cases. These nonlinearities can exist in the r-f section (including the r-f amplifiers and

mixer), the IF amplifiers, the metering circuits, and the panel meter.

Because the linearity of the FIM is measured from the signal input

connector to the panel output meter, it is called overall linearity and

includes corrections for all nonlinearities within the FIM. Experience

has shown that most of the nonlinearities will occur in the IF section of

the receiver, although this is not always the case. It is usually advisable

to calibrate the FIM at sufficient frequencies to include each IF amplifier.

The measurement setup for measuring overall linearity is the

same as the setup for calibrating the signal attenuators. Overall

linearity is essentially an attenuator ratio measurement relating the

change in input voltage to the FIM to the indicated voltage on the panel

meter. If the input signal is changed by 10 decibels, the panel meter

should indicate the exact same change. If the panel meter indicates a change of 10.5 decibels, a linearity error exists.

A point on the panel meter is selected (usually full scale) as being the reference point for all measurements. This point is used to

reference corrections to all other indicator points on the panel meter.

The calibration is performed by starting out with a full scale indication

51
A point on the panel meter is selected (usually full scale) as

being the reference point for all measurements. This point is used to

reference corrections to all other indicator points on the panel meter.

The calibration is performed by starting out with a full scale indication

on the panel meter and increasing attenuation in the standard attenuator

while recording the amount of attenuation required to make the panel

meter indicate various readings. A sufficient number of points on the

panel meter should be calibrated in order that the indicated panel meter

readings can be plotted against the corrected panel meter readings.

4.2 Overall Linearity Measurement Uncertainties . The chief requirements of an accurate overall linearity measurement system are e s s entially the same as those for the attenuator calibration system. These include stable r-f sources, good shielding of the

entire system, and high-quality components. By using a calibrated

standard attenuator and a properly designed system, measurements can

be made to within 0.1 dB + 0.3% of the attenuation in decibels (which is

approximately 2 percent) from d-c to 400 MHz, and 0. 1 dB + 0.5% of the

attenuation in decibels (which is approximately 3 percent) from 400 to

1000 MHz.

4.3 Procedure for Calibrating the Overall Linearity of the NF-105 . Calibration frequencies

TX Tuning Unit 90 kHz

TA Tuning Unit 1 . 5 MHz

30 MHz

Tl Tuning Unit 200 MHz

T2 Tuning Unit 300 MHz

T4 Tuning Unit 1000 MHz

52
The following equipment is required to provide this calibration:

Signal Generator - frequency range, 90 kHz to 1000 MHzoutput, 500 millivolts into 50 ohms.

Standard Attenuator - 60-dB maximum attenuation within 0. 1 dB

steps.

RF Voltmeter - frequency range 90 kHz to 1000 MHz.

Calibration Procedure

Signal Generator

RF Voltmeter

SU- 105 Switching Unit

Figure 4.1. Overall Linearity Calibration

System Cabling Diagram.

53
1. Connect the T-Connector to the output of the SIGNAL

GENERATOR, as shown in Figure 4.1.

2. Reduce the output from the SIGNAL GENERATOR to its lowest

output.

3. Connect the probe of the RF VOLTMETER to the T-Connector.

4. Connect a short length of RG-9 cable from the T-Connector to

the 10-dB pad attached to the input of the STANDARD ATTENUATOR.

5. Using an adapter, connect one end of the green 30-foot RG-55

cable to the 20-dB pad on the output of the STANDARD ATTENUATOR,

Connect the other end to the connector marked "FROM ANTENNA"

on the SU-105 SWITCHING UNIT.

6. Turn off the RECEIVER and plug the TX TUNING UNIT into the

MAIN UNIT

7. Turn on the RECEIVER.

8. Allow the RECEIVER to warm up for an hour before starting

calibration.

9. Connect the 6-inch RG-55 cable from the terminal on the

TX TUNING UNIT marked "TO ATTENUATOR" to the terminal

on the MAIN UNIT marked "ATTEN. OUTPUT".

10. Connect an 18-inch RG-55 cable from the terminal on the

TX TUNING UNIT marked "IMPULSE GEN OUTPUT" to the

terminal on the SU-105 SWITCHING UNIT marked "TO IMPULSE

GENERATOR".

11. Connect an 18-inch RG-55 cable from the terminal on the MAIN

UNIT marked "SIGNAL INPUT" to the terminal on the SU-105

(SWITCHING UNIT) marked "TO SIGNAL INPUT".

54
12. Set the controls in the following positions:

CONTROL

SIGNAL GENERATOR „w~TM A "MODULATION

SELECTOR"

POSITION

i 'r* ixT-i i CW*

"VERNIER "1 MICROATTENUATOR" VOLT

STANDARD ATTENUATOR

10-dB STEP DRUM "0 dB"

1-dB STEP DRUM "0 dB"

2 to 2.9 dB "2.5 dB"

STEP DRUM

SU-105

SWITCHING UNIT

Calibrate -Read Switch "READ"

MAIN UNIT Function Switch "CARRIER"

SLIDEBACK" Full CCW

"IF GAIN" Midway

"SIGNAL INPUT "20 dB"

ATTN."

"METER" "INT."

TX TUNING UNIT

"SIG. ATT." "NORM"

"BFO" "OFF"

"IMPULSE GEN." "OFF"

Band Switch "3"

Positions of all other controls are of no importance for start

of calibration.

13. Tune the SIGNAL GENERATOR to 90 kHz.

14. Increase the output from the SIGNAL GENERATOR to approximately 3 millivolts.

55
15. Tune the RECEIVER for maximum indication on the meter. If

an off-scale reading is observed, reduce or increase the output

of the SIGNAL GENERATOR as required to obtain an on- scale

reading and tune for maximum indication on the meter.

16. Switch the READ CALIBRATE SWITCH on the SU- 105 SWITCHING

UNIT to the "CALIBRATE" position.

17. Reduce the output of the SIGNAL GENERATOR to its lowest

output

18. Calibrate the RECEIVER in accordance with the instruction manual

using the CAL SETTING for this frequency found in the TWOTERMINAL RF VOLTMETER CALIBRATION Section 2.4.

19. Switch the READ CALIBRATE SWITCH on the SU-105 SWITCHING

UNIT to the "READ" position.

20. Re-tune the RECEIVER for maximum indication on the meter.

21. Increase the output of the SIGNAL GENERATOR until a full scale

(10 |jlV) indication is obtained on the meter.

22. Note voltage level on the monitor RF VOLTMETER.

23. Add attenuation with the STANDARD ATTENUATOR until a meter

reading of 9 (J-V is observed on the RECEIVER.

24. Record the amount of attenuation required from the STANDARD

ATTENUATOR to obtain step 23. See Figure 4.2, page 58.

25. Take out the added attenuation from the STANDARD ATTENUATOR

and see if the meter reads full scale (10 uV) . If a full-scale

indication is not obtained, the measurement is in error and steps

20 through 24 must be repeated.

56
26. Repeat steps 20 through 24 three times and average the results.

The results are recorded as shown in Figure 4.2, page 58.

27. Repeat steps 20 through 24 for the other meter indications. This

will change the meter indication of step 23 from 9 to 8 to 7 to 6,

et cetera.

28.' The calibration procedure is repeated for each tuning unit with

its required frequency. The TA Tuning Unit has two required

frequencies because of the two IF strips used in the tuning unit.

57
Observation sheet NBS 255 U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

Date 12-12-66

Sheet no. . Observer JEC

Kind of measurement <Dverall Linearity

Instrument Tested NF-105 fi,N. lQfc* TX Tuninp wBa ^ non«o

Frequency. "U kflz room Temperature -'C. HUMIDIT

3—-- — — — tar

Y % Barometer

NO. /t.v_fut,

("lain standardised, aftn. pos . = 40 dR

GPO : ,966 0-2,7-724 TefereilCe 10 (J.V Meter Standard1 Attn . .q^tlting

Reading Ave.

.. (HV) dR dR dR dB

-

10 O-o-l.S G-O-l.Z 0-0-A.C 2..r

? o-/-a.i 0-/-3.3 0-1-21 :?. ^? v &-2-a.? 0-2-7.4 0-2-3.1 4.3

7 5:^

L ?.?

*

9./

4 /A?

3 /3.?

1 /JTc?

1 ^?^: u

1

Figure 4.2. Sample Calibration Data Sheet for Overall Linearity.

58
1.0 CALIBRATION OF LOOP ANTENNAS

5.1 General Discussion.

The loop antenna is used to measure radiated electromagnetic

ifields at frequencies from a few hertz to approximately 30 MHz. The

loop antenna consists of one or more turns of wire that are normally

placed in an electrostatic shield surrounding the turns. The loop antenna

responds to the magnetic component of the radiated field. If the relationship between the magnetic field and the electric field is known, either

field can be determined in terms of the other. The two fields are related in free space by the equation

E = ZH, (3)

where

and

E = the electric field strength in volts per meter,

Z = intrinsic impedance of free space, 120 n ohms,

H = magnetic field strength in amperes per meter.

Figure 5.1. Far -field Radiation Pattern

Loop Antennas

59
The far -field radiation pattern of a loop antenna is illustrated in

Figure 5.1. It is apparent that this pattern possesses certain directional properties, which are useful in direction -finding measurements.

When the plane of the loop is placed in a vertical position (relative to the

earth's surface), the loop is oriented for vertical polarization, and when

the plane of the loop is placed in a horizontal position, the loop is oriented

for horizontal polarization. It is necessary to orient the loop for maximum signal strength when measuring radiated fields. This condition

exists when the axis of the loop is perpendicular to both the electric

field vector and the line joining the receiver and the transmitter.

The directional properties of loop antennas are valid only if the

current distribution in the loop is uniform. Loops that are large

electrically, i.e., the physical dimensions are large compared to the

wavelength, usually do not have a uniform current distribution. Loop

symmetry is important in order to maintain a balanced output from the

antenna. Stray capacities can cause unbalance in loop antennas if they

are not uniformly distributed. In order to maintain symmetry and

balance, and reduce other undesirable effects, a shield is usually placed

around the turns of the loop. The shield must be made of a nonmagnetic material such as copper or aluminum and have a small air gap

at the top center of the antenna. A sketch of such a loop is illustrated

in Figure 5.2.

60

/.
AIR GAP

ELECTROSTATIC

SHIELD

Figure 5.2. Sketch of a Loop Antenna Showing The Air Gap

and Electrostatic Shield.

The effective length of an antenna can be related to field strength

and induced antenna voltage by the following equation:

_V

I

(4)

eff

where

E = the electric field strength in volts per meter,

V = the induced antenna voltage in volts,

and

& 9 tt = the effective length of the antenna in meters.

By rearranging (4) to E (£eff ) = V., it is apparent that the greater

the effective length of the antenna, the greater will be the induced antenna

61
voltage. The effective length of a loop antenna may be calculated by the

following equation:

V - %* (5)

where

t9tt = the effective length in meters,

A = the area of the loop in square meters,

N = the number of turns,

and

X = the free-space wavelength in meters

According to this equation, the effective length is directly proportional to the area and the number of turns. However, there are

limitations that prevent excessive effective lengths. The diameter of the

antenna must be electrically small ( d « \) for equation (5) to be valid.

Also, excessive number of turns can produce resonances that will seriously limit the useable frequency range of the loop. These are two of

several factors that must be considered when designing and using loop

antennas.

Most loop antennas used with commercial field intensity meters

are designed to operate over a large frequency range such as the LP-105

loop antenna for the NF-105 FIM. The LP-105 loop is designed to operate

from 0.15 to 30 MHz. Because it must work as high as 30 MHz, the

natural resonance of the loop must be above 30 MHz. This loop consists of a single turn of approximately 12 inches in diameter. The

effective length of such a loop is quite small.

The output of a loop is usually balanced (symmetrical with respect

to ground,) and the input of most FIM' s is an unbalanced 50-ohm coaxial

input. The impedance of a loop antenna is not constant, but it is a function

62
of frequency. In order to connect a balanced loop to an unbalanced receiver, a network of some kind is required. The network design will depend on the type of loop and receiver input circuit. Several designs may

be used; Figure 5.3 shows the design of three commercial FIM's.

(a)

Pi

0- *A/W -t-VsAr-^ —

S

k

rf Amp.

Input Circuit Of The LP-105 Loop Of The NF-105 FIM

(b)

E—

M

Input Circuit Of The Multi-Turn Loop Of The PRM-1 FIM

(c)

-nnjjRfwv

¥

Input Circuit of the Nems -Clark 120 FIM

Figure 5.3. Different Types of Loop Antenna Input Circuits,

63
Figure 5.3(a) shows the input circuit of the LP -105 loop of the NF-105

FIM. The LP-105 is a single-turn shielded loop connected to the receiver by means of a 30-foot coaxial cable. Figure 5.3(b) shows the input circuit of the shielded multi-turn loop of the PRM-1. The loop is

connected to the receiver by means of a very short twinax transmission

line. Figure 5.3(c) is the input circuit of the Nems-Clark 120 FIM. This

loop is a shielded multi-turn antenna that is made as an integral part of

the FIM; therefore, it is not detachable. In general, a loop antenna and

receiver that are designed to function over a large frequency range will

not be as sensitive as one that is designed for a smaller frequency range.

Loop resonances, excessive stray capacities, lack of symmetry, and small

effective lengths are but a few of the factors that affect the design characteristics of a broadband loop antenna.

The calibration of loop antennas used with field intensity meters

is necessary when measuring radiated electromagnetic fields. There

are a number of techniques that can be used to calibrate loop antennas.

A discussion of all calibration techniques is beyond the scope of this

paper. Only selected calibration techniques will be discussed. These

include the standard-field method, the standard-antenna method, and the

injection method. Each technique has its advantages and disadvantages.

Some methods are accurate and slow; others are less accurate but faster.

Each method is discussed further in the following sections.

5.2 Standard-FieId Method. The standard-field method is used to calibrate loop antennas from

30 Hz to 30 MHz. Basically, this technique involves placing the antenna in

a known standard field and determining a calibration factor or antenna coefficient from the magnitude of the known field and the FIM reading. The

standard -field method is favored over the standard-antenna method for

calibrating loop antennnas, because it is more convenient, and calibrations

64
can be performed rapidly.

The standard -fie Id method calibrates loops in terms of a freespace quasi-static magnetic field produced by a single -turn, unshielded,

balanced transmitting loop of known radius carrying a known current.

The magnitude of the field produced by a single -turn circular loop is

given by the following equation [3,4, 10]:

where

and

- ^fflj,./-

^^

E = equivalent free-space field strength in rms volts per meter,

r x = radius of the transmitting loop in meters,

rg = radius of the receiving loop in meters (if the receiving loop

is rectangular, use the radius of a circle having same area).

d = axial spacing in meters between the coaxial loops,

I = transmitting loop current in rms amperes,

X = free -space wavelength in meters,

The actual value of the qua si- static field, H, produced by the loop

is expressed in terms of the equivalent electric field, E, that would exist

in a free-space radiation field by the relationship E = ZH, where Z is

the impedance of free space, 120tt ohms. Equation (6) is only valid for

determining the equivalent free, space electric field strength, where

r x ,rg , and d are electrically small compared to X. The loop spacing

should be a minimum of four times the radii of the larger of r x or r§ for

(6) to be valid within one percent.

65
The magnitude of the field is a function of frequency (wavelength) as indicated by (6); however, the induction field component under

the radical which is the frequency correction term has a negligible

effect at frequencies below approximately 5 MHz. Below 5 MHz this

term can be omitted without appreciable error if d is less that 2 meters.

Typical loop antenna configurations are illustrated by Figure 5.4. The

transmitting and receiving loops are positioned coaxially to each other

at a spacing of 1 to 2 meters. The spacing is determined by the desired

magnitude of the calibrating field and the frequency.

TRANSMITTING

LOOP

RECEIVING

LOOP

Figure 5.4. Loop Antenna Configuration For Calibration.

To meet the free -space requirements of (6), the calibration

site should be in an area that is relatively free of any sizeable metallic

objects that might influence or distort the calibration field. Normally,

if the calibrating area is cleared of metallic objects for about two or three times the loop spacing, d, there should be no appreciable effect.

The calibrating site should, therefore, be in an area relatively free of

any objectionable metallic objects such as overhead power lines, shielded rooms, or steel reinforced walls. A non -metallic building with no

66
overhead wiring makes a satisfactory calibration site. Figure 5.5 show!

a typical loop calibration setup at the NBS loop calibration site.

Figure 5.5. Typical Loop Calibration Setup at NBS.

The diameter of the NBS transmitting loop is approximately

20 centimeters, and the loop current is normally 100 milliamperes

The current is measured by means of a vacuum thermoelement mounted

in the top center of the loop. ' 'his d-c calibrated thermoelement is believed to measure r-f current to well within one percent at frequencies

up to 30 MHz. The output of the thermocouple is measured with a highquality millivoltmeter or digital voltmeter. The loop input is balanced

by means of a high-quality balun transformer. The term balun comes

from the combination of balanced and unbalanced. The balance error of

this transformer with respect to ground does not exceed 2 percent. The

67
uniformity of the loop current is also a function of the loop circumference.

If the loop circumference is no greater than X/16, the loop current will

be essentially uniform. Therefore, it is apparent that the current distribution presents problems only at the higher frequencies. It may be

possible to drive the loop from an unbalanced source at the lower frequencies without serious accuracy degradation.

Another point of consideration is the position of the loop antenna

relative to the receiver case when the loop antenna is mounted directly

to the case. The receiver case will distort the measured field, but the

distortion may not be the same for the quasi-static field (calibrating field)

as the radiation field. To minimize this source of error, it is advisable

to specify the position of the loop relative to the receiver case if the

proximity of loop is within one loop diameter of the receiver case.

5.3 Standard-Antenna Method . The standard -antenna method of calibrating loop antennas has been

compared with the standard-field method and good agreement between the

two obtained. A standard receiving loop antenna was constructed at NBS

for use at 10 MHz. It consisted of a single turn, and was approximately

20 cm in diameter, with a point contact silicon crystal diode built into the

center of the loop and an R-C filter network connected at the terminals.

This type of antenna [5] is illustrated in Figure 5.6.

To calibrate a loop antenna using the standard-antenna method, the

calibrating field is measured first using a standard antenna. The antenna

under test is then substituted in place of the standard antenna. From

these data the antenna coefficients can be determined.

68
CRYSTAL RECTIFIER

H^T * D.C.

MILLIVOLT

<* METER

Figure 5.6. Diagram of Standard Receiving Loop Antenna.

The d-c output of the crystal is measured with a precision,

high-impedance millivoltmeter and the induced antenna voltage, V* » can be determined from the RF-DC voltage characteristics of the crystal.

The magnitude of the electric field strength can be calculated from

equations (4) and (5) pages 61 and 62.

69
A photograph of the receiving loop used at 10 MHz is shown in

Figure 5.7.

Figure 5.7. Standard Receiving Loop Used at 10 MHz.

5.4 Injection Method . Another technique that can be employed to calibrate loop antennas

is the injection method. A low-impedance voltage source (less than 0. 1 ohm)

is used to inject a known voltage in series with the loop. By calculating

the effective length of the loop and knowing the injection voltage, the

antenna coefficient can be determined. The chief advantage of this method

is that it can be performed in the laboratory, thus the need for a special

calibrating site is eliminated. There are, however, definite limitations

and disadvantages to this method. It is more frequency sensitive than

70
some others, and the calibration results are not as accurate as others,

such as the standard-field method.

Electric field strength can be related to voltage and effective

length by equations (3) and (4), pages 59 and 61. The injected voltage

from the low-impedance source can be substituted for the induced antenna

voltage, V i ; thus, the equivalent electric field strength can be determined in terms of the injected voltage and loop effective length.

A circuit diagram showing how the voltage can be injected in

series with the loop antenna and the FIM receiver is illustrated in

Figure 5.8.

Injection Voltage £> b <~

Loop

FIM Receiver

l_. .J

Figure 5.8. Injection Voltage Circuit Diagram.

The voltage is injected between points a and b, and should be injected

without unbalancing or disturbing the loop and receiver circuits . The resistor between points a and b represents the low-impedance source used

to inject the voltage. This impedance should be as low as possible.

71
The r-f micropotentiometer is a good voltage standard, and it

has a very-low internal impedance (usually several milliohms). These

properties are highly desirable in order to inject an accurate voltage

into the loop without significantly disturbing the loop and receiver circuit.

The resistive element of the r-f micropotentiometer serves as the resistor illustrated in Figure 5.8.

When the loop is connected to the FIM receiver by means of a

balanced twin cable, an injection network is necessary to adapt the unbalanced output of the r-f micropotentiometer to the balanced terminals

of the loop. When the loop is connected to the FIM receiver by means of

a piece of coaxial cable such as with the NF-105 and the LP-105 loop,

(balun built into the loop) it is necessary to modify the loop circuit slightly

so that the voltage may be injected. The ground side of the loop is lifted

and brought out of the case with an N Type coaxial connector. For normal

use a shorting cap is used to complete the circuit to ground. When calibrating the loop, the shorting cap is removed and the r-f micropotentiometer

is connected to the Type N connector as shown in Figure 5.9. This places

the radial resistor of the r-f micropotentiometer in series with the loop

as shown in Figure 5.8. Figure 5.9 shows a photograph of an NF-105

loop being calibrated using the injection method. Notice the r-f micropotentiometer being used to inject the voltage into the loop.

72
Figure 5.9. NF-105 Loop Being Calibrated Using the

Injection Method

73
A sketch of the injection network for balanced loops is shown in Figure 5. 10.

The r-f micropotentiometer is used to inject the voltage using the injection network.

TO LOOP

t

MICROPOT

ill II . TWIN CONNECTOR

(FEMALE)

TWIN CONNECTOR

(MALE)

TO RECEIVER

Figure 5. 10. Sketch Of The Injection Network For

Balanced Loops.

Injection networks designed for use with standard signal generators are

commercially available. Most signal generators are not the best voltage

standards; therefore, the uncertainty of the loop calibration is increased

by the additional uncertainty of the signal generator which may be as much

as 1 decibel or more.

The uncertainty with which loop antennas can be calibrated using

the injection network varies with the design characteristics of the loop

being calibrated and with frequency (the higher the frequency the greater

the uncertainty). A multi-turn loop cannot be calibrated as accurately as

a single -turn loop. This is due to several factors such as calculation of

the effective length, stray capacities, and circuit unbalance. Comparisons

74
of the injection method with the standard -fieId method show that the injection method can be used with approximately a 10 percent measurement

uncertainty at frequencies as high as 25 MHz with antennas such as the

LP-105. A 10 percent measurement uncertainty is often sufficient and

the time saved can make this technique quite useful.

5. 5 Loop Antenna Measurement Uncertainties . The standard-field method of calibrating loop antennas is the

most versatile and most accurate technique for calibrating loop antennas.

It is believed that known fields can be generated to accuracies of 3 to

5 percent from 30 Hz to 30 MHz. The National Bureau of Standards

offers a calibration service for loop antennas at frequencies from 30 Hz

to 30 MHz with uncertainties of 3 percent at frequencies to 5 MHz and

5 percent from 5 to 30 MHz. The standard-antenna method provides a technique that is perhaps comparable in accuracy to the standard -fieId

method except that it is limited in frequency and is very difficult to use.

The injection method is a convenient technique, because it can be performed in the laboratory using readily-available laboratory equipment

but, it has certain frequency limitations as well as uncertainties of

approximately 10 percent. The standard-field method offers the most

accurate technique; the injection method offers the most convenient

technique.

5.6 Calibration Procedure for Loop Antennas (30 Hz to 30 MHz)

Using the Standard-Field Method . The standard-field method of calibrating loop antennas can be

used at any frequency from 30 Hz to 30 MHz. This section of the report

describes in detail the measurement setups and step-by-step procedures

for calibrating loop antennas. The procedures outlined here are applicable

to most types of field strength meters

75
5.6.1 Calibration Site.

In order to meet the necessary environmental conditions for

establishing standard fields* the calibration site should be in an area

relatively free of any sizeable metallic objects that might influence or

distort the calibration field. Normally, if the calibrating area is cleared

of metallic objects within a distance of about two or three times the loop

spacing, there should be no appreciable effect. The calibrating site

should, therefore, be in an area relatively free of any objectionable

metallic objects such as overhead power lines, shielded rooms, or steel

reinforced walls. A non-metallic building with no overhead wiring makes

a satisfactory calibration site.

5.6.2 Basic Setup.

The standard transmitting loop and the receiving loop are

positioned coaxially with the planes of the two loops parallel to one another.

The height of the standard transmitting loop is adjusted until the axes of

both loops are at the same height. Figure 5.11 shows the loops positioned

for calibration. The spacing between the loops is determined from the

equation for the standard field. This distance is measured from the

electrical center of the transmitting loop to the electrical center of the

receiving loop. The electrical center is normally the same as the physical

center. This spacing is critical and must be measured accurately.

76
Figure 5.11. Loops Positioned for Calibration.

A wooden table is a convenient surface to make the setup. No

equipment other than the two loops should be on the table during the

measurements. If the receiving loop is attached directly to the receiver,

the loop and receiver must be on the table and the position of the receiver case observed. The position of the case of the receiver should be

recorded in the calibration data so that the user in the field may duplicate

the conditions under which the receiver and loop were calibrated.

77
SIGNAL

GENERATOR

Receiving

Loop

FIELD STRENGTH

METER

Figure 5.12. Diagram of the Measurement System

for Calibrating Loop Antennas.

Figure 5.12 shows the diagram for the measurement system. The

cable from the thermocouple output of the thermoelement at the top of the

standard loop to the millivoltmeter is a shielded twin lead cable. The

length of the RG-22 twin cable from the input of the standard loop to the

balun transformer is critical. A 15-inch twin cable is generally used.

If longer lengths of cable are used, a resonant condition can exist that will

introduce errors into the measurements. Select the balun transformer in

accordance with the calibration frequencies.

The following equipment is required to provide this calibration:

Standard Antenna Set

Signal Generator - frequency range, 30 Hz to 30 MHzoutput, 3 volts into 50 ohms.

78
5.6.3 Calculation of the Calibration Field.

Equation (6) page 65 is used to calculate the calibrating fields

2 - The term [l+(2ird/X) ] . of equation (6) is the frequency correction

factor; below 5 MHz it is of no significance (less than 1%) and can be

omitted. The following examples show the calculation of the calibrating

field below 5 MHz:

(a) If the following conditions exist,

1. Standard transmitting loop diameter = 20 cm ±0.1 cm

2. Receiving loop diameter - 28.8 ± 0.6 cm,

3. Standard transmitting loop current = 100 mA,

4. Spacing between the standard transmitting loop and the

receiving loop =1.0 m,

then

60rrr|l E = [d'+rl+rS] 3/ 8

_ (60)(3.1416)(0.01)(0.1)

" (1. 0+0.0 1+0. 02074)3/ 8

E = 180.1 mV/m.

(b) If the following conditions exist,

1. Standard transmitting loop diameter = 20 cm ±0.1 cm

2. Receiving loop diameter = 90 cm ± 4 cm

3. Standard transmitting loop current = 100 mA

4. Spacing between standard transmitting loop and the receiving loop = 2m,

then the magnitude of the calibrating field is 21.80 mV/m.

79
Above 5 MHz the frequency correction of the calibrating field becomes significant (greater than 1%). The following example illustrates

the significance of the frequency correction term for a calibrating field

at 1 . 5 meters with a magnitude of 54.53 mV'/m at frequencies below 5 MHz

The frequency correction term is 1.048 at 10 MHz which is approximately

5 percent.

Examplefor a frequency of 10 MHz,

60nr|l

" [ d

3 + r? + r|] 3/

3

E = 54.53 mV/m

= (54.53) (1.048)

i + ,^f

1 + /(2)(3.1416)(1.5) \

3

\ 30 ; j

i*

E = 57.15 mV/m.

Although the calibrating field changes for each frequency, the transmitting loop current and the spacing between loops remain the same throughout the calibration.

80
The table below lists the frequency correction factors and the

corrected calibration fields for frequencies above 5 MHz using

54.53 mV/m as the calibrating field, and d = 1.5 m.

Frequency

MHz

Frequency Correction

Ht)]

Corrected

Calibrating Field

E

mV/m

5.2 1.013 55.25

7.5 1.027 56.02

10.0 1.048 57.16

12.7 1.077 58.71

18.0 1.149 62.65

24.0 1.252 68.29

30.0 1.374 74.93

81
5.6.4 Calibration Procedure Using the Standard-Field Method.

5.6.4.1 DC Calibration of the Standard Loop Thermoelement

The following equipment is required to provide this calibration:

DC Millivoltmeter - range, to 10 millivolts

Slide Wire Potentiometer - range, 100 mVuncertainty ± 0.1% of reading.

DC Calibration Unit from the Standard Antenna Set.

Calibration Procedure

DC Millivoltmeter

Thermoelement^

Standard Transmitting Loop—/

/^s r"v o o ®°«cr 1 1 6 VL

Slide Wire 99 OOO Input

Potentiometer D C Calibration Unit

Figure 5.13. Cabling Diagram for the DC Calibration

of the Standard Loop Thermoelement.

1) Connect the DC CALIBRATION UNIT and the DC MILLIVOLTMETER to the STANDARD LOOP as shown in Figure 5. 13.

2) Place the DC CALIBRATION UNIT Loop Switch in the "TEST"

position and then place the Function Switch in the "LOOP"

position.

3) Adjust the d-c control knobs until the required calibrating

current is read on the d-c milliammeter, (usually 100 mA).

82
4) Place the Loop Switch in the "CAL" position.

5) Using the SLIDE WIRE POTENTIOMETER as a milliammeter,

readjust the d-c control knobs until the required calibrating

current is read on the SLIDE WIRE POTENTIOMETER.

(1 mV on the SWP is equal to 1 mA).

6) Record the thermoelement output which is indicated on the

DC MILLIVOLTMETER.

5.6.4.2 Calibration of the Receiving Loop Antenna.

The following equipment is required to provide this calibration:

Standard Antenna Set

DC Millivoltmeter - range, 0-10 millivoltmeter

Signal Generator - frequency range, 30 Hz to 30 MHzoutput, 3 volts into 50-ohms.

Calibration Procedure

1) Connect the STANDARD TRANSMITTING LOOP and the

RECEIVING LOOP and RECEIVER as shown in Figure 5.14,

Thermoelement

Standard

Transmitting Loop

Balanced Input

Signal

Generator

Balun Transformer

Matching Network

NF 105 Receiver

* Matching Network Is Not Needed

At The Lower Frequencies

Figure 5. 14. Cabling Diagram for the Calibration

of the Receiving Loop Antenna.

83
2) Apply r-f to the STANDARD TRANSMITTING LOOP until

the DC MILLIVOLTMETER reads just below the meter indication

recorded in the d-c calibration of the thermoelement

(step 6, page 83).

3) Tune the RECEIVER for maximum indication.

4) Turn off the r-f to the STANDARD TRANSMITTING LOOP.

Calibrate the gain of the RECEIVER in accordance with the

instruction manual.

5) Reapply the r-f to the STANDARD LOOP until the indication

of the DC MILLIVOLTMETER is the same as the recorded

indication of the d-c thermoelement calibration

(step 6, page 83).

6) Measure the field with the RECEIVER and record the data.

See Figure 5.15, page 85.

7) Repeat this procedure for each frequency.

8) The determination of the antenna factor is found in Section 7.

84
observat.onshcet U. S. DEPARTMENT OF COMMERCE NBS 255 NATIONAL BUREAU OF STANDARDS

KiNnnrM^nprurNT Loop Antenna Calibration

n*TF 12-12-66

4

Sheet No. obsfrvfb je c

i NSTM,MTMT-r«Tm NF-105 Ser. No. 1965; LP-105: TA TUNING UNITr-,. « 920362

Frequency Room Tfmper»thbf 'f. Unumi-rv % barometer MM.

Gain standardized with the signal attenuator in 40 dR nosition

«po: 19660-217-724 Std . loop current = 100 ma

Band Freq. Loop Attn. Meter Reading

Spacing Position Ave.

MHz metfirs HR llV u.V u.V M-V f"-¥

/ 0.tf Lo l*o I.LX I.Lo /.u /£/

O.ZZ /.?9 1.1* 1,9V i> n

(0,29 -?•!!, 7. If 2.3&. 7.3L

O.IL

2 D.3L 2.*9 7tS0 7.49 2.49

0.5

O

-

r ?.4 7.3 7 -3.3J 7,19

D.Lf

gxl *< r

Figure 5.15. Sample Data Sheet for Loop

Antenna Calibration.
5.6.5 Uncertainties.

With proper care it is believed that known fields can be generated

with uncertainties within 0.5 dB or less (approximately 6%) using the proper

Set of Standard Antennas.

Several areas that are most critical to the overall performance of the

measurement system are listed below:

1) The spacing between the loops is a critical measurement.

A 1 percent error in measurement will give ^approximately a

3 percent error in the standard field.

2) The diameter of the standard transmitting loop must be known

accurately. A 1 percent error in this measurement will give

a 2 percent error in the standard field.

3) The measurement of the diameter of the receiving loop can

give an appreciable error. The error is dependent upon the

spacing of the loops. The greater the spacing the smaller

the error. The examples given in the calculation of the calibrating field section show two calibrating fields calculated

using two different spacings. The 1 meter spacing example

shows a receiving loop measurement with tolerance of

±0.6 cm while the example using 2 meter spacing shows the

receiving loop measurement with a tolerance of ±4.0 cm.

Both sets of tolerances were determined using a 1 percent

change in the calculated calibrating field as the limit of error.

4) Care also must be taken that no operator error is introduced

into the operation of the FIM.

5.7 Calibration Procedures for Loop Antennas Using the Injection Method . Loop antennas are usually connected to the FIM receiver by means

of a twin cable (balanced input) or by means of a coaxial cable (unbalanced

input). The design of the instrument determines the type of input used. The

basic calibration setup for these two types of inputs is different when using

the injection method. As the name implies,the injection method injects a voltage into the loop using an r-f micropotentiometer as the source of voltage,

An injection network is used to inject the voltage into the balanced-input

loop. The injection network is not required with an unbalanced -input loop.

86
5.7.1 Basic Setup.

The injection network is illustrated in Figure 5. 10. Notice the

two balanced twin connectors for connection to the loop and receiver and

the unbalanced type N connector for the r-f micropotentiometer

Figure 5.16 illustrates the injection network connected to the r-f

micropotentiometer, the loop, and the receiver. The resistor, R, represents the low impedance radial resistor of the r-f micropotentiometer

injecting the voltage between points a and b.

RF Micropotentiometer

Input >

Loop

r^\

Injection

Network

To Receiver —

Figure 5.16. The RF Micropotentiometer Connected to a Loop

"With a Balanced Input Using the Injection Network.

The injection network is not used when calibrating loops with unbalanced inputs. The voltage is injected into the loop by performing a minor modification to the loop. The modification consists of disconnecting

the ground side of the loop and bringing it out of the case through the center

conductor of a type N receptacle. For normal use of the FIM, a type N

shorting cap is placed on the receptacle to complete the ground circuit of the

87
loop. "When calibrating the loop, the shorting cap is removed and the r-f

micropotentiometer is connected to the type N receptacle. This places the

r-f micropotentiometer radial resistor in series with the loop as shown in

Figure 5. 17.

Loop

RF Micropotentiometer

Input > r*^ n

r

To Receiver

Figure 5. 17. The Micropotentiometer Connected to a Loop

with an Unbalanced Input.

Figure 5.18 is a photograph of an unbalanced loop being calibrated

using the injection method. The r-f micropotentiometer is connected to

the type N receptacle that was added in the minor modification.

88
Figure 5.18 A Loop With an Unbalanced Input Being Calibrated

Using the Injection Method.

89
5.7.2 Calculation of the Injection Voltage.

The injection voltage is the same as the induced antenna voltage,

Vt, which is related to E and £eff as follows:

Vi = E (£,„) (7)

where

Vj, = the induced antenna voltage in volts,

E = the electric field strength in rms volts per meter, and

ltt{ = the effective length of the antenna in meters as found in

equation on (5) page (62).

Example:Find the required injection voltage which is the same as the induced antenna voltage, Vi , equivalent to a field strength of 57. 16 mV/m

at 10 MHz for a one -turn loop with a radius of 14.4 cm:

2ttAN 2n 8 r

8 N _ (2) (3 . 1416) a (0.02074)(1)

*•" " X ' \ 30

= 0.01365 meters,

therefore,

Vi = E t9tt = (0.05716)(0. 01365)

Vi = 0.0007802 volts = 780. 2uV.

90
The following table shows the effective length and induced antenna voltage,

Vj , for a one turn loop with a radius of 14.4 cm:

Frequency ^tt Vt

MHz millimeters

0.2047

microvolts

0.15 36.9

0.22 0.3002 54.1

0.29 0.3957 71.3

0.36 0.4913 88.5

0.50 0.6823 123.0

0.65 0.8870 160

0.86 1.174 211

1.2 1.638 295

1.6 2.183 3 93

2.1 2.866 156

3.0 4.094 223

4.0 5.458 298

5.2 7.096 3 92

7.5 10.23 573

10.0 13.65 780

12.7 17.33 1018

18.0 24.56 1539

24.0 32.75 2236

E

millivolts per meter

180.1

V

54.53

55.24

56.00

57.16

58.73

62.65

68.27

91
5.7.3 Calibration Procedure Using the Injection Method.

The following equipment is required to provide this calibration:

Signal Generator - frequency range 30 Hz to 25 MHzoutput, 3 volts into 50 ohms,

DC Millivoltmeter - range, 0-10 mV

Standard Antenna Set

Calibration Procedure

1) Select the RF MICROPOTENTIOMETER that will give the desired injection voltage. Calibrate the RF MICROPOTENTIOMETER with d-c for the desired voltages.

2) Connect the RF MICROPOTENTIOMETER to the loop and FIM

receiver - use the injection network, if required.

3) Apply r-f to the RF MICROPOTENTIOMETER until the desired voltage is obtained.

4) Tune the RECEIVER for maximum indication.

5) Reduce the applied r-f to its lowest level. Calibrate the gain

of the RECEIVER in accordance with its instruction manual.

6) Reapply the r-f to the MICROPOTENTIOMETER .

7) Measure this voltage with the FIM and record the results on

the data sheet as illustrated in Figure 5, 19.

Determination of the Loop Antenna Factor.

The antenna factor using the injection method is determined in

essentially the same manner as with the standard-fie Id method. The

magnitude of the calibrating field, E s , is calculated using the injection

voltage as V t and equation (7).

The magnitude of the measured field, Effl , is determined from

the meter readings (see Figure 5.19), and the antenna factor, K, is the

ratio of E 9 to Ea or

K = -I

1- (8)

92
The data sheet in Section 7.5, Figure 7.4, illustrates how the

data from Figure 5.19 can be used in calculating the antenna factor.

The antenna factor can be expressed as a ratio or in decibels.

5.7.4 Uncertainties.

The injection method can be used with approximately a 10 percent

calibration uncertainty level at frequencies as high as 25 MHz. By

calibrating the injection method in terms of the standard-field method,

it may be possible to use the injection method up to 30 MHz.

93
obsErv*t,onshEET U. S. DEPARTMENT OF COMMERCE bat* 12-12-66

NATIONAL BUREAU OF STANDARDS Cum. Mrt 1

Observer JLW

Kind of Measurement Loop Antenna Callhra.ti on

|NSTRUMFNT -r^-r^ NF-105 Ser. No. 1965: LP-105: TA TUNING UNITt^no q?.036?.

Frequency Bnnu Tfupfpitiibf •<": Hi JMIDITY MM.

in 40

% Baromfter

Gain standardized with the signal attenuator dB position

GPO : 1966 —217-724 Band Freq. Injection E, Atten,

s- Meter \

HVt > eading

Voltage Position

Ave.

MHz M-V raV/m dB VN HV nv

/ a/r 3k 9 /<?*7 £<? j.Wk i.LO LUI

£22 &./ / 99 /.9% l>9tr

A2? 7/3 2.3 L ?.?b~ 73 L

A?9 ?t.r

2 0.3 £ JPfr.5" ?+* 3.4-9 2.49

0.5"© 123. o _\ f 2.3<7 3.? ? 1.29

a^r 1L0.O

D.9L ?//. \ r\»

Figure 5.19» Sample Data Sheet for

Loop Calibration.

94
6.0 CALIBRATION OF DIPOLE ANTENNAS.

6.1 General Discussion.

Dipole antennas are commonly used to measure the electric

component of radiated electromagnetic fields at frequencies from 20

to 1000 MHz. The most popular dipole is the half-wave dipole. It is

simple in construction, rugged, and possesses several desirable

electrical properties that enhance its value for accurately measuring

electric fields.

The far-field antenna pattern of a half-wave dipole is similar

to the loop antenna pattern except that the half-wave antenna has slightly

more directivity. Figure 6.1 shows the far-field antenna patterns of a

half-wave dipole and a small loop antenna. The beam width between

the half-power points is

small loop antenna [6] .

o o the half-power points is 78 on the half-wave dipole and 90 on the

Figure 6.1. Far-Field Antenna Patterns of a Half-Wave

Dipole and a Loop Antenna.

95
Placing a half-wave dipole antenna in a horizontal position

relative to the earth' s surface orients the antenna for horizontallypolarized waves, and placing the antenna in a vertical position orients

it for vertical polarization.

The output terminals of a half-wave dipole are balanced with

respect to ground, and when connected to an unbalanced 50-ohm FIM

receiver, as is often the case, a balun transformer is necessary. The

balun also can be used to match the antenna to the receiver. The

impedance of a self-resonant half-wave dipole antenna is approximately

73 ohms. This value, however, is seldom applicable in most practical

situations. Since it is only valid for an infinitely thin antenna in freespace, and these conditions are seldom achieved. The height of the

antenna above the ground, as well as the ground constants (conductivity

and dielectric constant), can significantly influence the calibration of

the antenna. When calibrating dipole antennas, the height of the antenna

above ground should always be specified so that the user may duplicate

the height. In general, it is best to operate 0.5 wavelength or more

above the ground to minimize the ground effect errors. These errors

can be 10 percent or more for heights below 0.6 wavelengths. Figure 6.2

shows the computed error for a half-wavelength horizontally-polarized

dipole at different wavelengths above average ground [7] .

96
1.0 2.0

ANTENNA HEIGHT, WAVELENGTHS

Figure 6.2. Computed Ground Effect Errors for a Half-Wavelength

Horizontally-Polarized Dipole at Different Wavelengths

Above Average Ground.

97
6.2 Calibrating Dipole Antennas Using the Standard-Antenna Method.

The standard-antenna method is used by the National Bureau of

Standards to calibrate horizontally-polarized dipole antennas at frequencies from 30 to 1000 MHz.

The calibration is performed by radiating a horizontally-polarized

field of convenient magnitude from a dipole antenna. A standard

receiving antenna (capable of accurately measuring the magnitude of

this field) is placed in the far-field at a fixed height above the ground,

and the magnitude of the electric field is measured. The transmitting

antenna power is held at a constant level, and the antenna being calibrated is substituted in place of the standard-antenna. The antenna

factor is then determined by equation (8), page 92.

The NBS standard receiving antenna consists of a self-resonant,

half -wave dipole antenna with a high impedance balanced voltmeter built

into the center of the antenna. The balanced voltmeter is composed of

a selected point-contact silicon crystal diode as the detector and an

R-C filter network. The d-c output of the crystal can be measured

with any high quality high impedance millivoltmeter. Figure 6.3 is a

photograph of an experimental standard receiving dipole antenna showing

the silicon crystal diode and the R-C filter network. This particular

antenna is used at frequencies from 400 to 1000 MHz. The antenna mount

is made of PTFE and the antenna rods are 1/16-inch diameter silver

plated brass. The ends of the antenna rod are threaded internally so

that extensions may be added to change the operating frequency of the

self-re sonant dipole.

Polytetrafluorethylene

98
Figure 6.3. Experimental Crystal-Diode Receiving Dipole.

The crystal diodes are carefully selected to optimize characteristics such as frequency insensitivity, temperature stability, voltage

sensitivity, r-f impedance, and shunt capacity. Several types have been

evaluated. For the lower frequencies (30 to 400 MHz) a ceramic cartridge type diode is used; for the higher frequencies (400 to 1000 MHz)

a glass encapsulated, subminiature diode is often used.

The frequency response of each crystal diode is evaluated using

the basic setup illustrated in Figure 6.4. The bolometer and power

meter are carefully calibrated so that a constant power level may be

applied to the diode mount. The bolometer is also the 50-ohm termination for the system. The 20 -dB pad serves as an isolator. The low

pass filter is used to minimize errors that might be attributed to

99
generator harmonics, and the d-c output of the crystal diode is

measured with an accurate potentiometer or other high impedance

millivoltmeter

(O Low Pass

Filter 20 dbPad

Diode Bolometer

Mount

Power

\2y Mount Meter

Vlillivoltmeter

Figure 6.4. Measurement System for Evaluating the

Frequency Response of Crystal Diodes.

The diode mount is made from a Type N T-connector as

illustrated in Figure 6.5. A cutaway on the left shows how a ceramic

cartridge diode is positioned in the mount. The dielectric material

supporting the center conductor is PTFE. The d-c output of the crystal

is filtered by approximately 100 picofarads formed by the T-connector

body and the cylindrical mass on the diode output. Two-mil PTFE

tape was used to form the dielectric of this capacitor. The VSWR of

this mount is less than 1.05 up to 1000 MHz. The diode is removed

from the mount by removal of the PTFE-covered cylindrical mass on

the diode output. When the subminiature glass diodes are placed in the

mount, small adapters are placed on each end.

100
Figure 6.5. Cutaway of Diode Mount Used for

Evaluating Frequency Response.

The frequency response of two types of diodes is plotted in

Figure 6.6. The power level applied to the diodes was held constant

and the d-c output of the diode plotted as a function of frequency. These

two curves represent only the frequency response of selected diodes and

are not typical of all diodes of these types. The response of the ceramic

diode is flat to approximately 500 MHz and physically adapts very well to

the lower frequencies (30 to 400 MHz). The glass diode is flat to approximately 800 MHz and has approximately 3 to 4 percent frequency correction

at 1000 MHz. This type is more suitable for the higher frequencies

(400 to 1000 MHz). Both the ceramic and glass diodes have been used up

to 1000 MHz, but the frequency correction of the ceramic diode is considerably higher at 1000 MHz, and it does not possess the physical qualities

of the smaller glass diode. The frequency response of both mixer and

video detectors have been evaluated, and there does not appear to be any

significant difference between the two in this particular application.

101
o

Ia

cr

a:o

o

>-o

3O

LU

q:

8

6

4

2

Mixer

Miniature Glass

Video Detector

Ceramic Case

200 400 600 800 1000

FREQUENCY MHz

Figure 6.6. Frequency Response Curves of

Selected Crystal Diodes.

The temperature coefficients of these diodes are not uniform. The

majority of those tested have a decreasing d-c output with increasing

temperature, but some have been tested that are just the opposite. In

order to reduce errors due to temperature changes the diodes are calibrated as a balanced r-f voltmeter at a temperature as close as possible

to that at which they are used. Temperature corrections can be applied if

necessary.

The crystal-diodes are calibrated as a balanced r-f voltmeter by

applying a known balanced voltage to the diode and measuring its d-c

output. This is illustrated in Figure 6.7. The balanced voltage standard

is attached to the diode mounted in the antenna by means of gold plated

clips. The d-c output of the diode is measured with a potentiometer. The

voltage standard is used at 50 MHz and has an output of 150 millivolts.

The output of the voltage standard is controlled by changing the input voltage

102
in known amounts by means of a precision step-attenuator. The 50-MHz,

150 mV output of the balanced voltage standard has been referenced to an

NBS unbalanced voltage standard and is believed to have an uncertainty

of 1 or 2 percent.

Figure 6.7. Balanced Voltage Standard Attached

to Crystal Diode Mounted in Antenna.

The electric component of a field can be expressed in terms of the

induced antenna voltage and the effective length of the antenna, l>%it . The

effective length of a half-wave dipole (assuming sinusoidal current distribution) can be calculated from the relationship

Ioff

X , ttI = — tan —r—

IT X

(9)

103
where I is the antenna half-length in meters. The induced antenna

voltage is measured with a reasonably high impedance balanced voltmeter; therefore, the loading effect on the antenna is negligible and its

terminal voltage is essentially the same as the open-circuit voltage

(referred to the center terminals). The magnitude of the field can be

determined by the relationship

£-?>-", (io)

where

E = electric field strength in volts per meter,

\^c = open-circuit antenna voltage in volts, and

^9 tt = effective antenna length in meters.

In order to achieve a self -re sonant, half-wave, dipole antenna of

finite thickness, it is necessary to shorten the overall length of the

antenna. The shortening is a function of the antenna rod diameter and

frequency. The percent shortening may be determined by the following

equation [8]:

M , . 2708 .,,, % shortening = ——— ,, (11J Kg

where

Ke = 120 [log.(^) - 1], and

d = antenna rod diameter in centimeters.

The percent shortening of the NBS antennas is usually between 4 and 6

percent, depending on the length to diameter ratio. The transmitting

antenna is also a self-resonant, half-wave antenna identical in physical

dimensions to the receiving antenna. The transmitting antenna is fed

with a balanced input by means of a balun transformer.

104
The antenna measurements are performed at a nearby calibration

site. A picture of this site is shown in Figure 6.8. The power lines are

buried underground, and all sizeable metallic objects are minimized.

Figure 6.8. Antenna Calibration Site.

The transmitting and receiving antennas are spaced a minimum of

3 wavelengths apart. The distance is usually 100 feet at frequencies

from 30 to 400 MHz and 40 feet at frequencies from 400 to 1000 MHz.

The calibrating equipment is housed in a console in the larger building.

Buried coaxial cables connect the r-f power sources to the transmitting

antennas. Figure 6.9 is a picture of the NBS calibration setup for a

dipole antenna. The standard receiving antenna is mounted in the foreground, and the transmitting antenna is mounted in the background. The

field strength receiver and its antenna (laying on the ground) are near the

105
operator. The field strength meter antenna is substituted in place of the

standard antenna after the strength of the calibrating field is determined.

Antennas submitted for calibration are normally calibrated at a height

of 10 feet or more above the ground, because the presence of the ground

will have an effect on the antenna impedance and thus an effect on the

antenna factor. If used at lower heights it is advisable to use additional

corrections [7],

Figure 6.9. Typical NBS Dipole Antenna Calibration Setup.

106
6.3 Other Methods of Calibrating Dipole Antennas . Another method of calibrating dipole antennas is the standard -

field method where the field is determined in terms of the transmitting

antenna parameters, incident and reflected signals, and the field site

characteristics. This technique requires more knowledge of the surrounding environment and is more difficult and time-consuming than the

standard-antenna method. This method is used at NBS to compare

antenna calibration technique s [ 9] . A modification of the standard-antenna method has been used for

experimentation purposes and compared with the standard-antenna

method. The current in a receiving antenna is measured using a thermoelement, and the electric field strength is determined in terms of

antenna current, dipole impedance, and dipole effective length. Double

expanded styrofoam was used to support the self-resonant dipole constructed of 28-gauge wire. The small diameter wire more nearly approaches the ideal antenna. The thermoelement heater serves as a low impedance load for the antenna.

The desired open-circuit antenna voltage may be determined by

the following relationship:

V,oc Vl(Z, +zl ) z L (12)

where

VL = load voltage,

Vac = open-circuit voltage,

Z L = load impedance, and

Z A = antenna input impedance

107
This technique has certain inherent disadvantages and is difficult

to accomplish. It was compared with the standard -antenna method at

frequencies to 500 MHz with agreement to within 5 percent [5],

A calibration can be performed by calibrating the antennas in

terms of receiver accuracy as a two-terminal r-f voltmeter, antenna

balun transformer characteristics, and the assumption that the antenna

is ideal. This method is easy to perform because the measurements

can be performed in the laboratory, but the results are not very encouraging. Agreement with the standard -antenna method was no better

than 15 to 25 percent at some of the higher frequencies.

6.4 Dipole Antenna Measurement Uncertainties . The standard-antenna method of calibrating dipole antennas has

been compared with the standard -field method at selected frequencies up

to 1000 MHz in order to determine the uncertainty. The agreement between the two methods varied from 5 percent at the lower frequencies

[4] to 10 percent at the higher frequencies. The establishment of

accurately-known standard-fields at the frequencies within this range is

difficult and requires a large flat area (several acres) without interfering

objects such as trees or power lines. This type of location is often not

readily available nearby. The standard -antenna method of calibrating

dipole antennas is easier to perform, and the calibrating site requirements are less stringent.

Other techniques that have been investigated and compared with the

above-mentioned techniques include the measurement of the receiving

antenna current, and the determination of the electric field strength in

terms of the antenna current, dipole impedance, load impedance, and

effective length. Thermoelements are used to measure the antenna current.

108
The measurements using this technique are in reasonably good agreement with the standard -antenna method, but the technique has certain

inherent disadvantages, and is somewhat difficult to accomplish.

It is believed that dipole antennas can be calibrated to within a measurement uncertainty of 12 percent or 1 decibel (using the standardantenna method) . Horizontal polarization is recommended for this

calibration, although it is believed to be essentially the same for

vertical polarization.

109
6.5 Calibration Procedure for Dipole Antennas (30" 1000 MHz) Using

the Standard-Antenna Method . This section of the report describes the measurement setups and

step-by-step procedure for calibrating half -wave horizontally-polarized

dipole antennas at frequencies from 30 to 1000 MHz. The following frequencies are those generally used for the calibration of dipole antennas:

Frequency

MHz

30

35

50

65

100

150

200

250

300

400

500

600

700

800

900

1000

The system can be used at other frequencies between 30 and

1000 MHz, but it is necessary to make the antenna rods for each

frequency selected.

110
6.5.1 Calibration Site.

The selection of a calibration site is important, because a poor

calibration site can introduce considerable errors in the measurements.

The ideal calibration site is a flat open area containing underground

power lines and cleared of all obstructions for at least 300 feet in all

directions. The ideal site is not often available and it is necessary to

select a site that as nearly approaches the ideal site as possible. Avoid

large metallic structures that might distort the calibrating field or change

the antenna characteristics. Non-metallic buildings can be tolerated in

the calibrating vicinity without undue deterioration of the calibrating field.

Figure 6.10 shows a calibration site for dipole antennas that has nonmetallic structures in the calibrating area. This site has proved to be

satisfactory. When calibrating dipole antennas, REMEMBER, you are

trying to calibrate the antenna under as nearly the same conditions as the

Figure 6.10. Calibration Site for Dipole Antennas,

111
antenna will be used in the field. A calibration that is valid for onlyone set of measurement conditions is not desirable for general field use.

6.5.2 Basic Setup.

The Set of Standard Antennas contains two sets of transmitting

and receiving antennas. One set of dipoles is used to calibrate from

30 to 300 MHz; these are called the low-frequency dipoles. The other

set, called the high-frequency dipoles, is used to calibrate from 300 to

1000 MHz.

Figure 6.11 shows a diagram of a basic calibration setup.

Both the transmitting and receiving antennas are placed at the height (h)

of 10 feet above the ground with the rods of the dipoles parallel to each

other. The spacing (d) between the two antennas is not critical but should

be a minimum of 2 to 3 wavelengths. A convenient spacing is 100 feet

for frequencies from 30 to 300 MHz and 30 to 40 feet at frequencies from

300 to 1000 MHz.

112
STANDARD

RECEIVING

ANTENNA

SLIDE WIRE

POTENTIOMETER

BALUN

TRANSFORMER

RF

VOLTMETER

TRANSMITTING

ANTENNA

^_LOW PASS

FILTER

Figure 6.11. Diagram of Basic Calibration Setup

for Dipole Antennas.

The transmitting antenna is connected to a signal generator through

a low-pass filter and a balun transformer. The low-pass filter reduces

the harmonics from the signal generator. A low-pass filter may not be

required if the generator harmonics are 40 decibels below the fundamental. The balun transformer is used to connect the unbalanced signal

generator to the balanced transmitting antenna. The r-f voltmeter is

used to monitor the output of the signal generator.

The output from the standard receiving antenna is connected to

the slide wire potentiometer. The twin-lead cable is brought straight

113
down the antenna mast to the ground and across the ground to the slide

wire potentiometer.

The unknown receiving antenna is mounted to a mast so that it will

be at the same height above the ground, when placed in position for

measurement, as the standard receiving antenna. The unknown antenna

must be mounted so that when substituted in place of the standard antenna

the distance to the transmitting antenna is the same. A large error can

be introduced if the unknown receiving antenna is not positioned in the

same position as the standard receiving antenna.

Figure 6.12. Typical Field Calibration Setup

for Dipole Antennas.

Figure 6. 12 shows a photograph of a calibration setup under normal

field conditions. The transmitting antenna and the standard receiving antenna

are shown. Some liberty has been taken in the spacing between the two

antennas so that more detail can be shown in this photograph.

114
6.5.3 Calibration of the Voltage Standard.

The following equipment is required to provide this calibration:

DC Millivoltmeter - range to 10 mV

Slide Wire Potentiometer - range, to 150 mV

uncertainty ±0.1% of reading,

Standard Antenna Set

Calibration Procedure.

1) Select the proper voltage standard; the low -frequency dipole

uses one voltage standard; the high-frequency dipole uses

another.

2) Turn the Function Switch of the DC CALIBRATION UNIT to

the "OFF" position.

3) Connect the equipment as shown in Figure 6. 13 (a). The

high-frequency dipole voltage standard (300 to 1000 MHz) is

connected directly to the DC CALIBRATION UNIT as shown

in Figure 6.13 (b). The LOW- FREQUENCY DIPOLE VOLTAGE

STANDARD (30 to 300 MHz) is connected to the DC CALIBRATION UNIT by means of a 6 -foot twin cable and a female

to-female adapter (in lid of the DC Calibration Unit).

4) Turn the Function Switch of the DC CALIBRATION UNIT to

the "DIPOLE VOLTAGE STD" position and the "DIPOLE

VOLTAGE STD." switch to the "CAL" position.

5) Increase the d-c output of the DC CALIBRATION UNIT until

the SLIDE WIRE POTENTIOMETER reads 150 millivolts.

6) Record the thermocouple output on the DC MILLIVOLTMETER.

This setting represents the 150 mV output of the voltage

standard.

115
Dipole Voltage Standard

Slide Wire Potentiometer DC Calibration

Unit

Mill ivoltmeter

Figure 6.13(a). Diagram Showing the DC Calibration

of the Dipole Voltage Standard.

Figure 6. 13(b). Photograph Showing the DC Calibration

of the Dipole Voltage Standard.

116
6.5.4. Calibration of the Antenna Crystal Diode.

The following equipment is required to provide this calibration:

Signal Generator - frequency range 50 MHzoutput, 2 volts into 50-ohms,

Slide Wire Potentiometer - range, to 150 mV

uncertainty ±0.1% of reading,

Step Attenuator - 60 dB maximum attenuation with

0. 1 dB steps,

DC Millivoltmeter - range 0-10 mV

Calibration Procedure

Signal Generator

Dipole Antenna with Crystal Diode

Dipole Voltage Standard

Balun Transformers

Step Attenuator

RF Voltmeter DC Millivoltmeter Slide Wire

Potentiometer

Figure 6. 14. Diagram of Antenna Crystal Diode

Calibration Setup.

117
1) Connect the setup as shown in Figure 6. 14, page 117.

2) Set the frequency of the SIGNAL GENERATOR to 50 MHz.

3) With the STEP ATTENUATOR in the zero position, apply

r-f until the thermocouple output reads the same as noted

in step 6 on page 115.

4) Note the reading of the RF VOLTMETER. This reading will

be used throughout the calibration to maintain a constant

r-f level.

5) Measure the d-c output from the crystal diode with the

SLIDE WIRE POTENTIOMETER. Record the measurement

as shown in Figure 6. 15.

6) Add 3 dB of attenuation to the STEP ATTENUATOR. Readjust

the SIGNAL GENERATOR output so that the RF VOLTMETER reads the same as noted in step 4.

7) Again, measure the d-c output from the crystal diode with

the SLIDE WIRE POTENTIOMETER. Record the measurement as shown in Figure 6. 15.

8) Steps 6 and 7 are repeated until the d-c output from the

crystal diode is less than 5 millivolts.

9) Figure 6. 16 shows an example of how the data from

Figure 6.15 can be plotted. It shows the r-f voltage

applied to the crystal diode plotted against the d-c output

of the crystal.

6.5.5 Calibration of the Unknown Receiving Antenna.

1) The rods of the TRANSMITTING ANTENNA, the STANDARD

RECEIVING ANTENNA, and the unknown receiving antenna

are adjusted in length for the calibration frequency.

2) Set up the equipment in accordance with the Basic Setup,

page 112 and Figure 6.11.

3) Apply r-f to the TRANSMITTING ANTENNA.

4) The STANDARD RECEIVING ANTENNA is placed in position,

and the field measured. The output of the SLIDE WIRE

POTENTIOMETER is recorded. See sample data sheet

Figure 6.17.

118
5) The unknown receiving antenna is now substituted in place

of the STANDARD ANTENNA, and the field strength is

measured with the Field Strength Meter. Record the

measurement as shown in Figure 6. 18.

6) Steps 4 and 5 are usually repeated three times. The RF

VOLTMETER is used to monitor the level of the generator

during the measurement.

7) The same procedure is repeated for each calibration

frequency.

8) The determination of antenna factors is found in Section 7.

6.5.6 Uncertainties

Dipole antenna calibrations uncertainties can be performed

within 12 percent or 1 decibel using the standard -antenna method.

Precautions to observe during measurements .

1) The standard receiving antenna and the unknown receiving

antenna should be mounted so that when positioned they are at the same

height and spacings.

2) If possible, avoid calibrating during large temperature

variations. The crystal diodes are temperature sensitive and if calibrated at one temperature and used at a much different temperature,

errors will be introduced. It is advisable to calibrate the crystal diode

at nearly the same temperature as it will be used.

3) A constant monitor of the power to the transmitting antenna

is recommended during calibration to avoid errors because of undesirable

level changes.

119
oBsERvat,oN shEEt U. S. DEPARTMENT OF COMMERCE date 12-12-66

NATIONAL BUREAU OF STANDARDS €tutnrr M„ 7

ftBSFBUre J EC

kino of measurement CrystalDiode Calibration

Instrument Tc-«T«rr« Xtal 1N832A tfstnn 920367

Frequency _>U JVLHZ Rnnu Tpmpfritmrf 'C. HUMIDITY % Barometer MM.

GPO : 1966 — 217-724 Crysta] Output Atten. RF

Ave Input

mV-dc mV-dc mV-dc mV-dc dB mV-rf

It?J 39.

131.1 131.1 O J TO. O

97S <m S7-7 97 L 3 /0L.7L

iT3.3 X21 £3.3 5?-1 L 2T.X

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17.1 n.i /7.3 173 /5L 17.7

%0 */ ?/ */ /C 7^.7

4.L 4,/. 4.J, 4-M /7 l*.9

Figure 6.15. Sample Data Sheet for Crystal

Diode Calibration.

120
20 30 50 100 APPLIED RF- MILLIVOLTS

300

Figure 6. 16. Sample Curve of the Applied RF Voltage

to the Crystal Diode Plotted Against the

DC Output of the Crystal.

121
Observation sheet NBS 255

KIND OP MEASUREMENT

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

Dipole Antenna Factor

nm 12-12-66

Sheet No. 6

Observer JEC

instrument T«Tm Std . Rx Antenna C rvstal Output tpstn 920362

Frequency . Room Te"peratuan 'CI Humidity MM.

^832A

% Barometer

Crystal 11

GPO : 1966 — 217-724 Rand Freq, Cryst: l1 Output Frequency ^orrftct^rl Induced

Correctioi 1 Crystal Antenna

Output voltage

MT-T7. mV-df rr\Y-c\r. mV' -r\r. mV-Hn Vi

mV-rf

1 ^00 JT% tfjr.o tto 1.0 tto 9d.

500 17. ZL 47.2 47. X — r 47.2 7/>,

Loo

100

2 loo ?4.p f4. f4.Q &99 fZ.TL / 03.0

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loo

/OG

Figure 6 e l7. Sample Data Sheet for Dipole

Antenna Calibration

122
obsErv*t.onshEEt U. S. DEPARTMENT OF COMMERCE date 12-12-66

NBlS 255 NATIONAL BUREAU OF STANDARDS «. HI7irr Mrt 5 Orsprufp J XL C

KiNnnrM^mrMrvr Dipole Antenna Calibration

iNSTpiiucNT Trerm NF - 1 5 S.N. 1965 TfstNn 920362

Frequency Ronu Tfupfbatiirf 'CZ Humidity % MM.

position

Barometer

Gain standardized in 20 dB attenuator

GPO : 1966 —217-724 Band Freq. Attn. Mete r Reading (Average)

Position Ave.

MHz dB jiV nv nv nv

i * oo 90.0 574 s:z 5T3 r?

K00 0.1 7.3 P. 3 0.1

UOQ

100

2 J 00 3.2 7.1 2 1 3.3

Zoo /f /4 A3 /.*

^oo

/ 1?0 _^ r

Figure 6,18. Sample Data Sheet for Dipole

Antenna Calibration.

123
7.0 THE CALIBRATION REPORT.

7.1 General Discussion . Upon completion of all measurements required for the calibration

of an FIM, a very necessary and important task is the presentation of

the calibration data to the user. This is usually accomplished by means

of a calibration report. The report must be concise and simple but complete so that all calibration data are fully understood. A fully calibrated

FIM is of no value if the calibration data are not understood or utilized

by the user.

The typical FIM such as the NF-105 is a rather complex piece of

measuring equipment. Measurements can be made by direct reading or

by slideback, and there is peak detection and average detection. It is

important that the calibration report explain fully the conditions under

which the calibration was performed.

In the previous sections of this report examples of calibration data

have been given for attenuation measurements, overall linearity, etc.

In this section we are going to prepare a calibration report using the data

sheets from the earlier sections. The report will be divided in the

following manner:

1 . Two-terminal RF Voltmeter Calibration

2. Attenuator Calibration

3. Overall Linearity Calibration

4. Loop Antenna Calibration

5. Dipole Antenna Calibration

A typical NBS Calibration Report of an NF-105 FIM is included in

Section 7.6. This example is shown to help guide the user in preparation

of the calibration data.

124
7.2 Two- Terminal RF Voltmeter Calibration.

The two-terminal voltmeter calibration was very straightforward:

100 microvolts were applied to the input of the receiver, and the receiver

gain adjusted so the receiver read correctly; a setting of the impulse

generator, equivalent to the 100-microvolt input, was then determined

by substitution. A typical data sheet for a two-terminal r-f voltmeter

calibration is shown in Figure 7.1. Note that all pertinent comments

regarding the instrument are written on the data sheet. This information

should be included in the final calibration report. The extreme righthand column marked "AVE" in Figure 7. 1 is the data listed in the calibration report.

7.3 Signal Input Attenuator Calibration . Each attenuator step of the FIM has been measured, and now a

corrected value or correction factor must be determined for each step.

The two-terminal r-f voltmeter calibration was performed using the

"20" dB attenuator step and "20" dB point on the panel meter. This

establishes these two as the reference points for the attenuator and overall linearity correction data respectively. The "20" dB attenuator will

be exactly 20.0 dB, and the panel meter reading will be exactly 20.0.

Other attenuator steps and panel meter readings will be referenced to

these two points.

The data sheet illustrated in Figure 7 . 2 shows the same data as

recorded in Section 3.0, except that it has now been completed showing the

the corrected values that are to be used on the calibration report. The

"difference" column shows the measured value of each attenuator step.

If the frequency is high and corrections must be applied to the standard

attenuator readings, these corrections must be applied to the "difference"

column. From the "difference" column the corrected attenuator settings

can be determined by mere addition or subtraction. Notice the "20" dB

125
oBSErvat,onshEEt U. S. DEPARTMENT OF COMMERCE date 12-12-66

NATIONAL BUREAU OF STANDARDS 9utm. Mo 1

OBSERVER JLW

KiNnnrMF^mrMPNT RF Voltmeter Calibration

Instrument TKrm NT?1 - 1 5 Spi- 1Mr> IQA^ T A tuninrr n•-.;+ tfstnn 920362

FREQUENCY Rortu TruPFDiTiPRF °C1 Humidity «/.. Barometer MM.

Imp. gen. controls: 10 cycles, P.R.R. (cD xl 00, pain set in1 Deak r>os. . atrn. in

20 dB position, voltage readings taken in "carrier" pos., input voltage - 100 ixV.

GPO : 1966 —217-724 Band Freq. Input Impulse Generato r Sftttinas

MHz voltage Ave.

fiV dB dB dB dB

/ Q./JT / QQ 9JT.-2 9£2 n: / 9j,:z

0.27 Q £, Lt %!(> tt.L ?1.L

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Figure 7.1. Sample Data Sheet For RF Voltmeter Calibration,

126
obsErvat,on sheet U. S. DEPARTMENT OF COMMERCE date 12-12-6.6

NBS 255 NATIONAL BUREAU OF STANDARDS «. uirirT No 2

Observer <j^-^>

KiMn or Mr^.mrurMT Attenuator Calibration

iM^M^-rT^TTM NF-105 Ser. No. 1965 T-X Tuning Unit o 920362

FREQUENCY MM. - *'e;n kHz JxilzJ RnnuTrMPPBjTiiPF T. Humidity V. Rabomftfr

Gain standardized, function switch in "carrier"

GPO : 1966 —217-724

Signal Standa: -d Attemia- or Rp^rlin y Differ. Corrected Voltage

Attn. Pos

D Ave. Attn. Set. Correction

dB dB dB dB dR dB dB Factor

%o O-0-7S n-o-xf D-O-ls 7,4"

1,0 3/5-/OJ 70-1-2.0 70-1-7.1 7.2. ot 2 a $7 %(?.4- J.0+J

L o O-0-2S GO lb O-O-IS 2.<r 55.? 0.977

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40 -39.X (0.977

70 J9.X 720.0 /, D

7 o

VD.? -OS 0.944-

Figure 7.2. Sample Data Sheet For Attenuator Calibration.

127
attenuator step is the reference attenuator and is 20.0 dB. The "40"

dB step is determined by adding the 19.8 difference between the "20" and

"40" steps to the 20.0 step giving 39.8. The "60" dB step is determined

by adding the 20.0 difference between the "40" and "60" steps to the

corrected "40" step (39.8 + 20.0 = 59.8). Notice the corrected value

of the "0" dB step; it is -0.5, because the difference between the "0"

and the "20" steps is greater than 20.0 dB. The "0" corrected step is

determined by subtracting the "difference" column from the corrected

"20" dB step, 20.0 - 20.5 = -0.5.

The units of the calibration reports can be either in decibels or in

microvolts. The choice is usually determined by the user or the calibrator. If microvolts are selected, the corrected attenuator settings

in decibels must be converted to an equivalent correction factor applicable

to microvolts. This can be done easily by means of a decibel to voltage

ratio table. The following relationship may be used to convert the corrected attenuator setting in decibels to an equivalent voltage correction

factor:

, voltage ratio of the corrected attn. setting voltage correction factor = :—° : — TT. voltage ratio of the attn. setting

Example: Find the voltage correction factor for the "80" dB step which

has a corrected attn. setting of 80.4 dB.

voltage correction factor = ' n ^^ = 1.047

The voltage correction factor should always be a number very close

to 1.0. If it is not close to 1.0, a check should be made to determine if

an error was made or if the attenuator is defective. The voltage correction factor for each step is shown in the extreme right-hand column of

Figure 7.2. The sample observation sheet, shown on page 127, illustrates

128
how the calibration results are reported in both decibels and microvolts.

Normally, the calibration results are reported in either decibels or

microvolts. Note that the calibration report explains how to use the

correction factors -- see paragraph at top of page 143.

7.4 Overall Linearity Calibration . The overall linearity calibration data are presented in a format

similar to the attenuator calibration data. The meter reading can be

corrected in terms of decibels or microvolts. A typical data sheet is

illustrated in Figure 7.3. The panel meter was calibrated in microvolts.

The column marked "Ave" is the standard attenuation readings between

the different points on the panel meter. The "difference" column is the

actual attenuation between the reference point (10 microvolts) and

successive points on the panel meter. By converting this attenuation in

the "difference" column to a voltage ratio and dividing the reference

point (10 microvolts) by this voltage ratio, the corrected meter readings,

in microvolts, are obtained. Any point on the panel meter can be made

the reference point; the 10-microvolt point was selected because of

the two-terminal r-f voltmeter calibration.

If decibels are selected for the panel meter calibration, the

"difference" column decibel figures can be used directly to add or subtract in order to obtain the corrected meter readings in decibels. The

overall linearity calibration report is shown on page 144 . Note that

the calibration report gives instructions regarding use of the calibration

data.

7.5 Loop Antenna Calibration . A calibrated loop antenna is one that has an antenna coefficient or

antenna factor assigned at selected frequencies so that the loop antenna

can be used to accurately measure field strengths when connected to a

129
Observation sheet NBS 255

Kind of Measurement.

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

Overall Linearity

date 12-12-66

Sheet no. . Observer . JEC

instrument t«tTM NF - 1 5 S.N. 1965 TX Tuning Unit

Frequency " u kHz room temperature *c. humidity__

Gain standardized, Attn, pos. - 40 dB

Test no. 920362

_%. Barometer m

GPO : 1966 —217-724 reference 10 M-V Meter Standarc Attn. Setting Difference Voltage Corrected

Reading Ave, Ratio Meter Read.

(M-V) dB dB dB dB dB teY)

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Figure 7.3. Sample Data Sheet For Overall Linearity Calibration.

130
calibrated receiver. A plot of the antenna factors permits accurate

measurements at all frequencies. The antenna factor expresses the

ratio of the magnitude of the correct field strength to the measured

field strength. This number can be expressed as a direct ratio or in

decibels. If the measurements are expressed in volts per meter, the

antenna factor is a multiplier. If the measurements are expressed in

decibels, the antenna factor is additive. The example used herein is

for the LP- 105 loop antenna and is computed in terras of volts

per meter.

In Section 5.0 a sample data sheet was shown illustrating the

measurement data of the loop antenna. A new data sheet, shown in

Figure 7.4, is required to complete the necessary calculations of the

antenna data. The meter reading of the receiver must be corrected

using the overall linearity calibration data, and the attenuator setting

of the receiver must be corrected using the attenuator calibration data

so that the magnitude of the field measured by the FIM can be accurately

determined. Column A of Figure 7.4 contains the uncorrected meter

readings transcribed from the initial data sheet. Column B shows the

meter readings after the overall linearity corrections have been made.

Column C is the corrected attenuator setting. Column B and C are used

to determine the measured field, E m

. The standard calibrating field,

Eg

, is listed in Column E. The antenna factor, K, is determined by

dividing Es by Em as shown in Column F.

The calibration report lists the antenna calibration results

on pages 145 and 146. The applicability of all calibration corrections

is shown on the first page of the calibration report.

131
Observation sheei NBS 255

Kind of Mea

Instrument

Frf;q"ency

U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

suREMENT Loop Antenna Calibration

DAT! Shee

Obse

12-12-66

5

T Nn -"

BVFP JEC

TrsTcn NF -105 S.N. 1965; IP-105: TA Tuning Unit r«T K,rt 920362

Rfinu TFMPFR4TIIBF ' CI HiiMiniTV % IFTFB MM.

2 for 1.5 meter soac

Baron

E for 1 meter spac ing = 180.1 mV/ra; I ing = 54-3 mV/rn

GPO : 1966 — 21" -724 A B C D E F

Band Freq. Uncorr. Correct Correct Measured Stri Field Ant. Coef.

Meter Meter Attn. Set Field E„ E, E„/EM =K 3

MHz uY H.V mV/m mV/m

/ O.lf I, LI l.+L / fOOO i>*L yffA/ /2J.4.

0.22. 1.1% '1.1/ /.9/ ?f 3

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Figure 7.4. Sample Data Sheet For Loop Antenna Calibration.

132
7.6 Dipole Antenna Calibration . The dipole antenna calibration data are presented in essentially

the same format as the loop antenna data. The dipole antenna factor

can also be expressed as a direct ratio or in decibels. The compilation

of the calibration data is somewhat more complex than with the loop

antenna, because the standard -antenna calibration data must also be

handled.

Four data sheets are recommended:

1. For the FIM, see Figure 7.5.

2. For the standard receiving antenna, see Figure 7.6.

3. For the crystal diode calibration, see Figure 7.7.

4. For the calculation of the standard field E 8

, see Figure 7.8.

5. For calculation of the antenna factor, see Figure 7.9.

Each sheet is self-explanatory. The final analysis is made on the

antenna factor data sheet, Figure 7.9. The effective length is calculated

by . The induced antenna voltage, Vj , is obtained from Figure 7.6,

The standard calibrating field, E s , is determined in terms of Vj and

the effective length. The measured field, Em , is determined by the

corrected meter reading and the corrected attenuation columns. The

antenna factor is then calculated by the relationship, K = E s /E m . The

presentation of the data in the calibration report is shown on pages 147

and 148.

133
obSErvat,on shEET U. S. DEPARTMENT OF COMMERCE date 12-12-66

NATIONAL BUREAU OF STANDARDS „„,,„ hln 5

kind of MrAsiiRFMENT Dipole Antenna Calibration

Instrument T«Tim NF-105. S.N. 1965 tfctn n 920362

Frequency RnnM TrupFRi-niBF T. HiiMiniTV %, BAROMETER MM.

Gain standardized in 20 dB attenuator position

GPO : 1966 — 217-724 Band Freq. Attn. Mete]' Reading Average)

Position Ave.

MHz dB H-V H-V HV H-V

/ ^OO 90.0 S.4 T.3 A". 3 ^.3

SOO 2.3 ?>3 2 1 2.3

loOD

1O0

2. loo 3.3 23 3.1 7.3

^oo IA IA l>? 1,4-

9oo

/ & e o 1

'

Figure 7.5. Sample Data Sheet for Dipole

Antenna Calibration.

134
obsErvat,onshEEt U. S. DEPARTMENT OF COMMERCE NBS 255 NATIONAL BUREAU OF STANDARDS

kind op measurement Dipole Antenna Calibration

„„J2-l2-66

Rue-c-t Mn ° ObsfbvTM JEC

. MCT mt t«TM Std. Rx Antenna Crystal Output _M Q 920362

Frequency »«/-»« TrupFBlTllPP *C HnuiniTV «. Barometer MM.

Crystal 1N832A

GPO : 1966 O —217-724 Band Freq. Crystal Output Fr^qii^"f

rCnrrPftP'! Induced

Ave Correctioi i Crystal Antenna

Output Voltage

MHz mV-dc mV-dc mV-dc mV-dc Vi

mV-rf

/ $00 %o.o fO.f? 90. o f.o 90. o 90.o

SOO S7.f $*.l $Z.o t.o r*. n 70. O

boo

loo

7 loo ft.f 91. f 94.9 D.Sj 9+.o / 02. O

9oo 19.9! 19. f? J9.p 0.9? J9.4- i*-r

<7 / 66

Figure 7.6. Sample Data Sheet for Dipole

Antenna Calibration.

135
obsErvat,on sheet U. S. DEPARTMENT OF COMMERCE date 12-12-66

NATIONAL BUREAU OF STANDARDS „HCTrr Nrt 7

OBSFBVFR JEC

K,«n TM m^^.^m^ C rystal Diode Calibration

Instrument Tc-QTE-n Xtal 1N832A tfbt k o 920367

Frequency bU JVlriZ Rru-tM Tfmpfratiipf 'C Humidity %. Rabometer MM.

GPO : 1966 —217-724 Crystal Output Atten. RF

Ave. Input

mV-dc mV-dc mV-dc mV-dc dB mV-rf

1*9./ l?9.2 iyr.1 IJ*? / J-S&

n? £7 L 97 J 57. L 3 J0LTL

51.3 52.3 53.1 £7.3 U 7S.Z.

V<i ?/, o vt ?l>l ? 53. Z.

/7.3 17,1 17,3 173 )Z 37.7

9.0 *./ ?,/ 9,1 JT 7C1

47. +M f L 4.L !? /*.f

Figure 7.7. Sample Data Sheet for Crystal

Diode Calibration.

136
OBSERVAT.ONSHHET U. S. DEPARTMENT OF COMMERCE date 12-12-66

NBS 255 NATIONAL BUREAU OF STANDARDS Sum hln 7

Obsfbvfb J XL C

„ Dipole Antenna Calibration Kind of Measurement c

. NSTBMuTMTT«Tm Std. Rx Antenna Measured Field .,„, K „ 920362

Frequency Room Temperature *C Humidity % Barometer mm.

GPO : 1966 —217-724 Band Freq. Induced Effective

Ant. Volt. length

iL

Vi eff Vi

I

eff

MHz mV-rf meters mV/m

1 4oo 90.0 0.239 1^4.7

5G o lo.o 0J9f 1LL.S*

1*00

"Jon

7, loo /ol.'o (9JZ7 757. 7

<*0o ?0.9 (2.1(9 z.rs-.r ^00

/ noo

Figure 7.8. Sample Data Sheet for

Computation of E ,

137
obsErVat,onshEEt U. S. DEPARTMENT OF COMMERCE

NBS255

NATIONAL BUREAU OF STANDARDS

K, Nn«P M««, «««« Dipole Antenna Calibration

niTM 12-12-66

Sheet No. ° , Obbfbwb J lL C

lNSTRUMENTT«Trn NF - 1 5 S . N . 1965 TPSTNO.920362

FREO'IPJC-Y Hi"" TFMPFBAT1IBC ' Ct HiiHiniTv % Barometer MM.

GPO : 1966 — 217-724 Band Freq

Corrected Corrected Measured Std. Ant. Facte ir Meter Attn. Set Field, E Field, E E /E = K

Reading

M 3 3 M MHz M-V [iV mV/m mV/m

/ too S-.o / D&DQ SO. 134.7 f*.7

sot? 7J / oooo 2/, <? ?lla- J7.lT

U oo

7 00

2 7 oo 9?0D -?/.n 7r/

Pf.3

ftoo A* yioo 17. L ?.*-*.? 3 0.JT

*7 0n

J ODD

Figure 7.9. Sample Data Sheet for Dipole

Antenna Factor.

138
FOfiM NBS-443

(MV. 7-65 ) U. S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

INSTITUTE FOR BASIC STANDARDS

BOULDER, COLORADO 80302

REPORT OF CALIBRATION

NOISE AND FIELD INTENSITY METER

Singer CompanyModel NF- 10 5, Serial No. 1965

Submitted by:

The data in this report are for use in making rms cw measurements

only and are not valid for broadband interference measurements.

Field strength may be determined with the loop and dipole antennas by

using the following equation:

E = KMA

where E = unknown field strength in microvolts per meter,

K = antenna factor,

M = corrected meter reading, and

A = corrected attenuator setting.

A minimum operating time of one hour was allowed before any measure

ments were made. The 60 Hertz ac supply voltage was approximately

115 v.

Page 1 of 10

Test No. 920362

Date: December 12, 1968

139
Noise and Field Intensity Meter

Singer CompanyModel NF-105, Serial No. 1965

TWO-TERMINAL RF VOLTMETER CALIBRATION

This calibration was performed by applying 100 microvolts to the end of

the green 30-foot coaxial cable connected to the "ANTENNA" connector

on Switching Unit SU-105 with the SIGNAL INPUT ATTENUATOR in the

"20" dB position, the function switch in the "CARRIER" position, and the

IF GAIN adjusted to an output meter reading of 10 microvolts. The impulse generator settings were determined with the SIGNAL INPUT

ATTENUATOR in the "20" dB position and the function switch in the

"PEAK" position, referenced to an output meter reading of 10 microvolts. The pulse repetition rate of the impulse generator was set at

1000 Hz.

The uncertainty of the calibrating voltage was believed to be within 5 percent

to 400 MHz and within 10 percent from 400 to 1000 MHz.

Tuning

Unit

TX/NF-105

Band

Impulse

Generator

Frequency Setting

kHz decibels

14.0 92.6

17.0 91.8

21.0 90.7

23.0 90.1

25.0 90.3

25.0 92.4

35.0 90.2

45.0 90.1

55.0 90.8

62.0 91.4

62.0 91.0

85.0 91.8

105.0 98.3

125.0 96.8

150.0 93.6

Page 2 of 10

Test No. 920362

Date: December 12, 1968

140
Noise and Field Intensity Meter

Singer CompanyModel NF-105, Serial No. 1965

Tuning

Unit

TA/NF-105

Band

Impulse

Generator

Frequency Setting

MHz decibels

0.15 85.2

0.22 83.6

0.29 80.5

0.36 80.9

0.36 79.2

0.50 76.5

0.65 73.6

0.86 73.7

0.86 79.8

1.2 76.7

1.6 77.0

2.1 76.8

2.1 73.5

3.0 72.3

4.0 72.1

5.2 73.0

5.2 71.9

7.5 73.5

10.0 73.6

12.7 72.0

12.7 74.0

18.0 72.4

24.0 72.9

30.0 72.9

Page 3 of 10

Test No. 920362

Date: December 12, 1968

141
Noise and Field Intensity Meter

Singer CompanyModel NF-105, Serial No. 1965

T\ining

Unit

Tl/NF-105

Band

T2/NF-105

T3/NF-105

impulse

Generator

Frequency Setting

MHz decibels

20 56.1

30 55.9

35 54.5

50 55.8

65 54.2

65 56.2

100 56.8

150 56.4

200 56.2

200 46.4

250 46.3

300 45.8

350 48.0

400 46.3

400 41.4

500 43.6

600 43.6

700 43.0

700 43.7

800 42.2

900 42.9

1000 43.5

Page 4 of 10

Test No.

Date:

920362

December 12, 1968

142
Noise and Field Intensity Meter

Singer CompanyModel NF-105, Serial No. 1965

SIGNAL INPUT ATTENUATOR CALIBRATION

This calibration was performed with the function switch placed in the

"CARRIER" position, and the IF GAIN adjusted in accordance with the

two-terminal r-f voltmeter calibration data. The corrected attenuator

settings are referenced to the "20" dB attenuator setting. The corrected

attenuator settings may be determined by multiplying the attenuation

multiplier by the correction factor. The uncertainty of this calibration

is believed to be within 2 percent at 0.015 and 300 MHz and within 3

percent at 1000 MHz.

Attenuator Se

Is

tting Attenuation

Multiplier

x 1

CCorrection Fadtor

decibe 0.015 MHz

0.94

300 MHz

0.54

1000 MHz

0.83

20 x 10 1.00 1.00 1.00

40 x 10

2

0.98 1.00 1.05

60 x 10

3

0.98 1.02 1.01

80 x 10

4

1.05 1.05 0.97

Page 5 of 10

Test No. 920362

Date: December 12, 1968

143
Noise and Field Intensity Meter

Singer Company

' Model NF-105, Serial No. 1965

OVERALL LINEARITY CALIBRATION

The overall linearity was measured from the "ANTENNA" connector

on Switching Unit SU-105 to the panel output meter, referenced to a meter reading of 10 microvolts. The measurements were made with

the function switch placed in the "CARRIER" position, the SIGNAL

INPUT ATTENUATOR in the "20" dB position, and the IF GAIN adjusted in accordance with the two-terminal r-f voltmeter calibration

data. It is recommended that the linearity calibration data for the

TA/NF-105 Tuning Unit be used as follows: The Band 3 corrections

are for use with all bands having a 455 kHz IF (Bands 1 and 3); the

Band 6 corrections are for use- with all bands having a 1600 kHz IF

(Bands 2, 4, 5, and 6). The uncertainty of this calibration is believed

to be within 2 percent at 0.090, 1.5, 30, 200, and 300 MHz and within

3 percent at 1000 MHz.

Corrected Meter Reading

Indicated TX/NF-105

Band 3

TA/NF-105 Tl/NF-105

Band 2

T2/NF-105

Band 1 T3/NF-105

Meter Band 3 Band 6 Band 2

Reading 0.090 MHz 1 . 5 MHz 30 MHz 200 MHz 300 MHz 1000 MHz

microvolts

1 1.03 0.72 1.15 0.98 0.46 0.70

2 1.95 1.93 2.13 1.95 1.59 1.67

3 2.82 2.95 3.07 2.92 2.69 2.68

4 3.85 4.07 4.17 3.94 3.80 3.65

5 4.73 5.07 5.19 5.01 4.79 4.67

6 5.89 6.09 6.16 6.09 5.90 5.79

7 6.84 7.16 7.25 7.16 7.05 6.86

8 8.13 8.32 8.41 8.13 8.31 8.15

9 9.12 9.33 9.33 9.12 9.28 9.13

10 10.00 10.00 10.00 10.00 10.00 10.00

Page 6 of 10

Test No. 920362

Date: December 12, 1968

144
Noise and Field Intensity Meter

Singer CompanyModel NF-105, Serial No. 1965

ANTENNA CALIBRATION

The loop antenna was calibrated in terms of a quasi- static magnetic

field. The dipole antennas were calibrated in terms of a horizontallypolarized field for an antenna height of 10 feet. Approximate corrections

for other heights may be obtained from NBS Research Paper RP 2062.

LOOP ANTENNAS

The IF GAIN was adjusted with the function switch placed in the "PEAK"

position and the SIGNAL INPUT ATTENUATOR in the "40" dB position

using the impulse generator settings listed on pages 2 and 3 of this report. The antenna factors were determined with the function switch

placed in the "CARRIER" position. The uncertainty of the calibrating

field was believed to be within 3 percent to 5 MHz and within 5 percent

from 5 to 30 MHz.

LG-105 Loop Antenna

Tuning Band

Unit

TX/NF-105 1

Frequency Antenna

kHz Factor

14.0 81.4

17.0 66.9

21.0 51.1

23.0 43.6

25.0

25.0 52.1

35.0 34.9

45.0 30.2

55.0 30.0

62.0 30.9

62.0 23.2

85.0 22.4

105.0 41.5

125.0 30.2

150.0 20.3

Page 7 of 10

Test No. 920362

Date: December 12, 1968

145
Noise and Field Intensity Meter

Singer CompanyModel NF-105, Serial No. 1965

LP- 105 Loop Antenna

Tuning Band Frequency Antenna

Unit MHz Factor

TA/NF-105 1 0.15 123.4

0.22 94.3

0.29 77.3

0.36 70.1

2 0.36 70.4

0.50 51.7

0.65 41.1

0.86 37.6

3 0.86 44.8

1.2 33.8

1.6 31.3

2.1 29.5

4 2.1 26.5

3.0 19.9

4.0 17.5

5.2 18.2

5 5.2 14.6

7.5 13.0

10.0 11.9

12.7 9.5

6 12.7 11.6

18.0 8.3

24.0 9.0

30.0 8.4

Page 8 of 10

Test No. 920362

Date: December 12, 1968

146
Noise and Field Intensity Meter

Singer CompanyModel NF-105, Serial No. 1965

DIPOLE ANTENNA

The IF GAIN was adjusted with function switch placed in the "PEAK"

position and the SIGNAL INPUT ATTENUATOR in the "20" dB position

using the impulse generator settings listed on page 4 of this report.

The antenna factors were determined with the function switch placed in

the "CARRIER" position. The uncertainty of the calibrating field was

believed to be within 10 percent to 400 MHz and within 12 percent from

400 to 1000 MHz.

DM-105-T1 Antenna

Tuning Band F requency

Unit MHz

Tl/NF-105 1 30

35

50

65

2 65

100

150

200

DM- 105-• T2 Antenna

Tuning Band F requency

Unit MHz

T2/NF-105 1 200

250

300

350

400

Antenna

Factor

1.03

1.06

1.69

1.83

1.98

3.07

4.80

5.09

Antenna

Factor

4.48

7.17

7.17

8.07

9.75

Page 9 of 10

Test No.

Date:

920362

December 12, 1968

147
Noise and Field Intensity Meter

Singer CompanyModel NF-105, Serial No. 1965

DM-105-T3 Antenna

Tuning Band Frequency Antenna

Unit MHz Factor

T3/NF-105 1 400 6.7

500 17.5

600 16.6

700 23.0

2 700 24.3

800 20.5

900 27.4

"' 1000 36.1

For the Director

Page 10 of 10

Test No. 920362

Date: December 12, 1968

148
REFERENCES

1» Diamond, H. , K.A. Norton, and E.G. Lapham, On the accuracy

of radio field -intensity measurements at broadcast frequencies,

NBS J. Res. 21_, December 1938.

2. Ries, F.X., and G. Rebuldela, High frequency microvolt measurements, ISA Conference 1963, Preprint No. 37.2.63.

3. Greene, F.M., Calibration of commercial radio field-strength

meters at the National Bureau of Standards, NBS Circular 517, 1951

4. Jean, A.G., H.E. Taggart, and J.R. Wait, Calibration of loop

antennas at VLF, NBS J. Res. 6j3, No. 3, 1961.

5. Taggart, H.E., Field strength calibration techniques at the

National Bureau of Standards, IEEE Trans. EMC, EMC -7 , No. 2,

June 1965.

6. Klaus, J.D., Antennas (McGraw-Hill Book Co. , Inc., New York,

N.Y.), 1950.

7. Greene, F.M., Influence of the ground on the calibration and use

of VHF field -intensity meters, NBS J. Res. 44, February 1950.

8. Schelkunoff, S.A., Theory of antennas of arbitrary size and shape,

Proc, IRE 2_9, No. 9, 1941.

9. Greene, F.M., and M. Solow, Development of very-high-frequency

field intensity standards, NBS J. Res. 44, May 1950.

10. Greene, F.M., The near-zone magnetic field of a small circularloop antenna, NBS J. Res., 71C , No. 4, December 1967.

149
DECIBELS vs. VOLTAGE AND POWER

The Decibel Chart below indicates DB for an/ ratio of voltage or power up to 100 DB. For voltage ratios greater than 10 (or power ratios greater than 100) the ratio con be broken down into two products, the DB found for each

separately, the two results then added. For example: To convert a voltage ratio of 200:1 to DB: 200:1 VR equals the product of 100:1 and 2:1. 100:1 equals 40 DB; 2:1 equals 6 DB. Therefore, 200: 1 VR equals 40 DB + 6 OB or 46 DB.

Voltage Power Voltage Power Voltage Power Voltage Power Voltage Power Voltage Power

Ratio Ratio -db + Ratio Ratio Ratio Ratio -db + Ratio Ratio Ratio Ratio -db + Ratio Ratio

1 .0000 1.0000 1.000 1.000 .4467 .1995 7.0 2.239 5.012 .1995 .03981 14.0 5.012 25.12

.9886 .9772 .1 1.012 1.023 .4416 .1950 7.1 2.265 5.12S» .1972 .03890 14.1 5.070 25.70

.9772 .9550 .2 1.023 1.047 .4365 .1905 7.2 2.291 5.248 .1950 .03802 14.2 5.129 26.30

.9661 .9333 .3 1.035 1.072 .4315 .1862 7.3 2.317 5.370 .1928 .03715 14.3 5.188 26.92

.9550 .9120 .4 1.047 1.096 .4266 .1820 7.4 2.344 5.495 .1905 .03631 14.4 5.248 27.54

.9441 .8913 .5 1.059 1.122 .4217 .1778 7.5 2.371 5.623 .1884 .03548 14.5 5.309 28.18

.9333 .8710 .6 1.072 1.148 .4169 .1738 7.6 2.399 5.754 .1862 .03467 14.6 5.370 28.84

.9226 .8511 .7 1.084 1.175 .4121 .1698 7.7 2.427 5.888 .1841 .03388 14.7 5.433 29.51 .9120 .8318 .8 1.096 1.202 .4074 .1660 7.8 2.455 6.026 .1820 .03311 14.8 5.495 30 20

.9016 .8128 .9 1.109 1.230 .4027 .1622 7.9 2.483 6.166 .1799 .03236 14.9 5.55? 30.90 .8913 .7943 1.0 1.122 1.259 .3981 .1585 8.0 2.512 6.310 .1778 .03162 15.0 5.623 31.62

.8810 .7762 1.1 1.135 1.288 .3936 .1549 8.1 2.541 6.457 .1758 .03090 15.1 5.689 32.36

.8710 .7586 1.2 1.148 1.318 .3890 .1514 8.2 2.570 6.607 .1738 .03020 15.2 5.754 33.11 .8610 .7413 1.3 1.161 1.349 .3846 .1479 8.3 2.600 6.761 .1718 .02951 15.3 5.821 33.88

.8511 .7244 1.4 1.175 1.380 .3802 .1445 8.4 2.630 6.918 .1698 .02884 15.4 5.888 34.67

.8414 .7079 1.5 1.189 1.413 .3758 .1413 8.5 2.661 7.079 .1679 .02818 15.5 5.957 35.48

.8318 .6918 1.6 1.202 1.445 .3715 .1380 8.6 2.692 7.244 .1660 .02754 15.6 6.026 36.31 .8222 .6761 1.7 1.216 1.479 .3673 .1349 8.7 2.723 7.413 .1641 .02692 15.7 6.095 37.15

.8128 .6607 1.8 1.230 1.514 .3631 .1318 8.8 2.754 7.586 .1622 .02630 15.8 6.166 38.02

.8035 .6457 1.9 1.245 1.549 .3589 .1288 19 2.786 7.762 .1603 .02570 15.9 6.237 38.90 .7943 .6310 2.0 1.259 1.585 .3548 .1259 9.0 2.818 7.943 .1585 .02512 16.0 6.310 39.81

.7852 .6166 2.1 1.274 1.622 .3508 .1230 9.1 2.851 8.128 .1567 .02455 16.1 6.383 40.74

.7762 .6026 2.2 1.288 1.660 .3467 .1202 9.2 2.884 8.318 .1549 .02399 16.2 6.457 41.69

.7674 .5888 2.3 1.303 1.698 .3428 .1175 9.3 2.917 8.511 .1531 .02344 16.3 6.531 42.66

.7586 .5754 2.4 1.318 1.738 .3388 .1148 9.4 2.951 8.710 .1514 .02291 16.4 6.607 43.65

.7499 .5623 2.5 1.334 1.778 .3350 .1122 9.5 2.985 8,913 .1496 .02239 16.5 6.683 44.67

.7413 .5495 2.6 1.349 1.820 .3311 .1096 9.6 3.020 9.120 .1479 .02188 16.6 6.761 45.71 .7328 .5370 2.7 1.365 1.862 .3273 .1072 9.7 3.055 9.333 .1462 .02138 16.7 6.839 46.77

.7244 .5248 2.8 1.380 1.905 .3236 .1047 9.8 3.090 9.550 .1445 .02089 16.8 6.918 47.86

.7161 .5129 2.9 1.396 1.950 .3199 .1023 9.9 3.126 9.772 .1429 .02042 16.9 6.998 48.98 .7079 .5012 3.0 1.413 1.995 .3162 .1000 10.0 3.162 10.000 .1413 .01995 17.0 7.079 50.12

.6998 .4898 3.1 1.429 2.042 .3126 .09772 10.1 3.199 10.23 .1396 .01950 17.1 7.161 51.29

.6918 .4786 3.2 1.445 2.089 .3090 .09550 10.2 3.236 10.47 .1380 .01905 17.2 7.244 52.48

.6839 .4677 3.3 1.462 2.138 .3055 .09333 10.3 3.273 10.72 .1365 .01862 17.3 7.328 53.70

.6761 .4571 3.4 1.479 2.188 .3020 .09120 10.4 3.311 10.96 .1349 .01820 17.4 7.413 54.95

.6683 .4467 3.5 1.496 2.239 .2985 .08913 10.5 3.350 11.22 .1334 .01778 17.5 7.499 56.23

.6607 .4365 3.6 1.514 2.291 .2951 .08710 10.6 3.388 11.48 .1318 .01738 17.6 7.586 57.54

.6531 .4266 3.7 1.531 2.344 .2917 .08511 10.7 3.428 11.75 .1303 .01698 17.7 7.674 58.88

.6457 .4169 3.8 1.549 2.399 .2884 .08318 10.8 3.467 12.02 .1288 .01660 17.8 7.762 60.26

.6383 .4074 3.9 1.567 2.455 .2851 .08128 10.9 3.508 12.30 .1274 .01622 17.9 7.852 61.66 .6310 .3981 4.0 1.585 2.512 .2818 .07943 11.0 3.548 12.59 .1259 .01585 18.0 7.943 63.10

.6237 .3890 4.1 1.603 2.570 .2786 .07762 11.1 3.589 12.88 .1245 .01549 18.1 8.035 64.57

.6166 .3802 4.2 1.622 2.630 .2754 .07586 11.2 3.631 13.18 .1230 .01514 18.2 8.128 66.07

.6095 .3715 4.3 1.641 2.692 .2723 .07413 11.3 3.673 13.49 .1216 .01479 18.3 8.222 67.61 .6026 .3631 4.4 1.660 2.754 .2692 .07244 11.4 3.715 13.80 .1202 .01445 18.4 8.318 69.18

.5957 .3548 4.5 1.679 2.818 .2661 .07079 11.5 3.758 14.13 .1189 .01413 18.5 8.414 70.79

.5888 .3467 4.6 1.698 2.884 .2630 .06918 11.6 3.802 14.45 .1175 .01380 18.6 8.511 72.44

.5821 .3388 4.7 1.718 2.951 .2600 .06761 11.7 3.846 14.79 .1161 .01349 18.7 8.610 74.13

.5754 .3311 4.8 1.738 3.020 .2570 .06607 11.8 3.890 15.14 .1148 .01318 18.8 8.710 75.86

.5689 .3236 4.9 1.758 3.090 .2541 .06457 11.9 3.936 15.49 .1135 .01288 18.9 8.811 77.62 .5623 .3162 5.0 1.776 3.162 .2512 06310 12.0 3.981 15.85 .1122 .01259 19.0 8.913 79.43

.5559 .3090 5.1 1.799 3.236 .2483 .06166 12.1 4.027 16.22 .1109 .01230 19.1 9.016 81.28

.5495 .3020 5.2 1.820 3.311 .2455 .06026 12.2 4.074 16.60 .1096 .01202 19.2 9.120 83.18

.5433 .2951 5.3 1.841 3.388 .2427 .05888 12.3 4.121 16.98 .1084 .01175 19.3 9.226 85.11 .5370 .2884 5.4 1.862 3.467 .2399 .05754 12.4 4.169 17.38 .1072 .01148 19.4 9.333 87.10

.5309 .2818 5.5 1.884 3.548 .2371 .05623 12.5 4.217 17.78 .1059 .01122 19.5 9.441 89.13

.5248 .2754 5.6 1.905 3.631 .2344 .05495 12.6 4.266 18.20 .1047 .01096 19.6 9.550 91.20

.5188 .2692 5.7 1.928 3.715 .2317 .05370 12.7 4.315 18.62 .1035 .01072 19.7 9.661 93.33

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151

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