NisrUNITED STATES

DEPARTMENT OF COMMERCE

NATIONAL INSTITUTE OF STANDARDS

AND TECHNOLOGY

NATL INST OF ST4N0 » TECH R.I C

A111D3 MT7flflE

NIST

PUBLICATIONS

NIST Technical Note 1347

OCfOO

NIST Calibration Procedure di

for Vertically Polarized

Monopole Antennas,

30 kHz to 300 MHz

D. G. Camel!

E. B. Larsen

J. E. Cruz

D. A. Hill

Electromagnetic Fields Division

Center for Electronics and Electrical Engineering

National Engineering Laboratory

National Institute of Standards and Technology

Boulder, Colorado 80303-3328

\ kV^

U.S. DEPARTMENT OF COMMERCE, Robert A. Mosbacher, Secretary

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY, John W. Lyons, Director

Issued January 1991
National Institute of Standards and Technology Technical Note 1347

Natl. Inst. Stand. Technol., Tech. Note 1347, 28 pages (January 1991)

CODEN:NTNOEF

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1991

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, DC 20402-9325
CONTENTS

Page

Abstract 1

1 . INTRODUCTION 1

2 . THEORETICAL BASIS 2

3. STANDARD FIELD METHOD FOR CALIBRATING MONOPOLES 4

3 . 1 Description of the Equipment 4

3 . 2 Test Procedure 5 3 . 3 Antenna Factor 6

3 . 4 Calibration Uncertainty 7

4

.

CONCLUSIONS 8

5 . REFERENCES 8

APPENDIX A: SAMPLE TEST REPORT FOR A VERTICAL MONOPOLE 10

iii

NIST CALIBRATION PROCEDURE FOR VERTICALLY POLARIZED

MONOPOLE ANTENNAS, 30 KHZ TO 300 MHZ

D.G. Camell, E.B. Larsen, J.E. Cruz, and D.A. Hill

Electromagnetic Fields Division

National Institute of Standards and Technology

Boulder, CO 80303

This report describes the theoretical basis and test procedure

for vertically polarized monopole antenna calibrations at the

National Institute of Standards and Technology (NIST) . The

standard field method applies the theoretical equations of a vertical monopole antenna to calculate the vertical electric

field. This method is used at the NIST open field site in the

frequency range of 30 kHz to 300 MHz. The uncertainty in the

antenna factor of the antenna under test (AUT) is now ± 1 dB.

Key words: antenna factor; calibration; electromagnetic field;

monopole antenna; open field site; standard field.

1 . INTRODUCTION

The purpose of this report is to provide documentation for the method,

equipment, and facility employed by NIST for calibrating vertically

polarized monopole antennas over the frequency range of 30 kHz to 300 MHz

.

The facility is located at the Department of Commerce Boulder Laboratories,

Boulder, Colorado.

The method used at NIST for calibrating vertically polarized monopole

antennas is called the standard field method [1-4]. This method involves

establishing reference cw fields at specified field strength levels for

specified distances from the transmitting monopole on the NIST open-field

site. This measurement site is used over the frequency range of 30 kHz to

300 MHz for vertically polarized antennas.

At the NIST field site a thin cylindrical monopole antenna is used to

transmit a cw electromagnetic field. The voltage supplied to the

transmitting antenna is measured. The field strength is calculated in terms

of this applied voltage, the length and diameter of the transmitting

antenna, the distance between the transmitting antenna and the AUT, and the
height of the AUT above the ground plane. The ground plane is assumed to be

perfectly conducting.

2. THEORETICAL BASIS

The geometry for a thin cylindrical monopole antenna located over a perfect ground is shown in figure 1. The monopole has length i and is

assumed to have a sinusoidal current distribution [5]. The monopole

radiates vertical and horizontal electric field components, E and E , and a

z p horizontal magnetic field component, H , . Only the vertical electric field

<P E (in V/m) is required for calibration of vertically polarized monopole

antennas, and it is given by [5]

-J30I^ -J^^l -^^^2 -J^^O

E = . rJis [- + - 2cos(0Jt) ], (1)

z sinifiJi) r^ r- r- ^

2 2 1/2 where r^ = [d + (z - i) ] ^

rj2 , „v2,l/2 r^ = [d + (z + i) ] /

r^2 2,1/2

r^ = [d + z ] /

^ = 2n/X,

I„ (in A) is the base current of the monopole, and A is the free -space

wavelength. All distances and the wavelength are given in meters. By image

theory the EM fields in the half space above the ground plane are the same

as the fields of a center- fed dipole of length 22 in free space. The

expression for E in eq (1) assumes a sinusoidal current distribution on the

monopole [5], and this assumption is valid for i < A/4.
The input impedance of the monopole Z (in fi) is one half that of a

dipole of length 2i in free space:

Z - Z./2. (2)

mon 1

where Z. (in Q) is the dipole impedance as given by Schelkunoff

' s mode

theory [6, eq (108) on page 433]. At sufficiently low frequencies

(i < A/8), the monopole is electrically short, and the impedance can be

calculated from its capacitive reactance X:

Z == jX « l/(jwC ). (3)

mon -^ ' ^-^ a

The monopole capacitance C is given by [6]

cl

„ 55.63 i , a

'~ in(i/a) - 1' ^^

where a is the monopole radius and C is in pF if i is in m.

The monopole impedance as given by eq (2) has been verified

experimentally by measurements with a commercial vector impedance meter.

Calculated and measured values of the monopole reactance at low frequencies

are shown in table 1. The monopole length is 2.5 m, and the radius is 0.813

mm. At all frequencies the measured reactance was within 5 % of the

calculated reactance.

The base current of the monopole is difficult to measure directly; so the base voltage V- (in V) is measured. The base current is then calculated

from

I^ = V^/Z . (5) mon

The vertical electric field as given by eq (1) has been compared with

measurements made with a small probe which had been calibrated in the known
field of a TEM cell. The agreement between the calculated and measured

electric field was within ± 1 dB.

3. STANDARD FIELD METHOD FOR CALIBRATING MONOPOLES

3.1 Description of the Equipment

The NIST field site facility is designed around a grounded flat screen

mesh on a slab of concrete 30 m wide by 60 m long. A tunnel under this

concrete slab leads to a room approximately 3 m by 3 m in size, where the rf source and other equipment are located. Several small (15 cm) diameter

tunnels leading from this room to various locations under the ground screen

convey rf transmission cables and ac power cords to devices operating on the

ground screen. The cables, being underground, do not interfere with the EM

fields. The NIST standard transmitting antennas consist of telescoping

monopoles, with adjustable length. Monopole antennas should be used as gain

standards only on large, highly conductive ground screens [7]. Figure 2 shows the NIST field site with the locations of the transmitting and

receiving antennas.

The length of the transmitting monopole is adjustable from 0.25 m to a maximum of 2.5 m. The electrical length of the fully extended antenna is

1/4 wavelength at 30 MHz, but only 0.00025 wavelength at 30 kHz. At

frequencies above 30 MHz, the antenna length is reduced to 1/4 wavelength.

At frequencies below 30 MHz, the antenna length is kept at its maximum of

2.5 m. The base diameter of the monopole is 0.5 cm, and the diameter

decreases to a minimun of 0.13 cm with the monopole fully extended. Figure

3 shows a picture of a standard transmitting monopole antenna and mount.

The transmitted signal is produced by a signal generator, amplifier,

low-pass filter, low- loss transmission line, and the transmitting monopole.

Figure 2 shows the equipment used for a calibration at the field site. An

rf voltmeter with a 50 Q tee connector is used to measure the level of the

transmitting voltage. A generic list of equipment used on the ground screen

is given in table 2. The cables used are 50 Q coaxial with type N

connectors
3.2 Test Procedure

NIST tests monopoles and field strength meters or receiving probes at

specified frequencies and heights above the ground. When a device to be

tested arrives, the serial numbers are logged, a test number is assigned,

and a test folder is issued. The unit is inspected to see if it is

operating correctly. Before the actual testing is done, a review of the

anticipated data and procedure details are clarified. The dimensions of the

AUT are measured and logged for use in the calculations. The test data are usually reported in terms of the antenna factor, antenna gain, or antenna

output, such as detected voltage. The most common is antenna factor.

All of the serial numbers on the AUT and equipment are recorded, and a sketch is made of the calibration set up. If the test requires an unusual

arrangement, a photograph of the system is taken. Header titles are written

for each column of data to be taken (along with units) , and any pertinent

information is noted on the data sheet(s) . A sample data sheet is given in

figure 4

.

If the distance between the transmitting and receiving antennas is not

specified, the distance is usually set at 15 to 25 m. If the antenna height

above ground is not specified, the AUT is placed at the height of its normal

operation, which for monopoles is on the ground plane. The equipment is

connected as shown in figure 2. The rf voltmeter and spectrum analyzer are calibrated by the NIST microwave calibration service (test 61190S) [8] and

verified before each test by means of calibrated power meters.

A standard EM field is generated by the NIST transmitting monopole in

terms of the monopole base current and distance to the AUT. The standard

field is calculated at the center of the AUT from eq (1) . The output of the

AUT is measured with a calibrated spectrum analyzer or in terms of the AUT's

meter dial indication. The antenna factor is then determined by the ratio

of the calculated (standard) field strength to the AUT response, as described in Section 3.3. A sample test report is shown in the Appendix.
3 . 3 Antenna Factor

The antenna factor K is defined as

K=|E^A5ol. (6)

where E = calculated field strength, /iV/m, and

V _ = AUT voltage pickup across the 50 n load, ;iV.

The value of the vertical component E of the electric field is computed

from eq (1). It is assumed that the horizontal distance d is large enough

that E is constant over the length of the AUT.

The antenna factor of the AUT is usually given in dB and is calculated

from the expression

^dB - ^dB - ^dB - I-dB <^>

where K, = antenna factor of the AUT, dB, do

E = calculated vertical electric field strength, dB/iV/m, dB

V,_ = AUT voltage across the 50 Q load, dB/xV, and

UD

Lj = cable loss between the AUT and the 50 Q receiver, dB

dB

The theoretically expected antenna factor of a receiving monopole above

a perfect ground, connected to a 50 fi receiver, is given by

Z

1 1 mon , ,, / o \ K= |1 +-^IAeff. (8)

where Z is given by eq (2). The effective height h ^^ of the receiving

mon a J err monopole is given by [5]
1 - cos(/3i^)

where Z is the length of the receiving monopole. At low frequencies where

the receiving monopole is electrically short, pZ « 1 , h ^^ = -? /2 , and

K = Z /(25 i ).

mon r For example, consider aim monopole of 0.5 cm diameter at frequencies

below 10 MHz where the monopole is electrically short. Then h ^^ ~ . 5 m

and the capacitance ^ C = 11 pF. Thus the impedance and the antenna factor

mon ^ 14470 579 are given ° by Z ~ -i (_, ,.,, ) and K ~ „ .,,, , where F.MHz is the frequency in ' mon -^ F.MHz F,MHz ^

megahertz. This example shows that the antenna factor of an electrically

short monopole is inversely proportional to frequency.

3.4 Calibration Uncertainty

For calibrations at the NIST field site, at frequencies from 30 kHz to

300 MHz, the following error estimates apply.

(1) A source of error is the uncertainty of the standard field value at the

open field site. This is due mainly to uncertainty in the value of the

base current in the transmitting monopole. Also, signals from radio

stations and other transmitting antennas may cause additional errors. These additional errors can be avoided by viewing the spectrum analyzer

or receiver and stopping the measurement during any extraneous signals.

The estimated possible error in the standard field is ± 0.4 dB.

(2) Another source of error is the disturbance of the field caused by

interconnecting cables, nearby buildings, etc. This error has been

checked and is estimated to be less than ± 0.2 dB.

(3) Other sources of uncertainty are associated with antenna alignment,

measurement of antenna separation distance, and NIST calibrations of

the spectrum analyzer and rf voltmeter. The error due to these sources

is estimated to be less than ± 0.4 dB.
The overall uncertainty of the calibration is the sum of those listed above,

± 1.0 dB.

A comparison of calculated and measured antenna factor for a vertical

monopole antenna is shown in figure 5 for frequencies from 150 kHz to 15

MHz. The measurements were performed on a cylindrical monopole with a length of 1 m and a radius of 0.4 cm. The transmitting monopole had a length of 2.5 m and a radius of 0.813 mm, and the antenna separation was 20

m. The calculated value of the antenna factor was obtained from eq (8)

The difference between the calculated and measured values is less than 1 dB

at all frequencies from 150 kHz to 15 MHz.

4. CONCLUSIONS

NIST provides calibration of vertically polarized monopole antennas

over the frequency range of 30 kHz to 300 MHz. A standard (known) field is

established using a vertical monopole antenna over the NIST ground screen.

The present uncertainty in the measured antenna factor is the sum of

± 0.4 dB for uncertainty in the standard field, ± 0.2 dB for disturbance in

the field due to nearby objects, and ± 0.4 dB for other sources of

uncertainty for a total uncertainty of ± 1.0 dB.

5 . REFERENCES

[1] IEEE standard methods for measuring electromagnetic field strength for

frequencies below 1000 MHz in radio wave propagation. IEEE No. 302;

1969.

[2] IEEE standard test procedures for antennas. ANSI/IEEE Std. 149-1979;

1979.

[3] Nahman, N.S.; Kanda, M. ; Larsen, E.B.; Crawford, M.L. Methodology for

standard electromagnetic field measurements. IEEE Trans. Instr. and

Meas., IM-34: 490-503; 1985.

[4] Kanda, M. ; Larsen, E.B.; Borsero, M. ; Galliano, P.G.; Yokoshima, I.;

Nahman, N.S. Standards for electromagnetic field measurements. Proc

IEEE, 74: 120-128; 1986.
[5] Jordan, E.G.; Balmain, K.G. Electromagnetic Waves and Radiating

Systems. Englewood Cliffs, N.J.: Prentice-Hall; 1968.

[6] Schelkunoff, S.A.; Friis, H.T. Antennas, Theory and Practice. New York:

Wiley; 1952.

[7] FitzGerrell, R.G. Limitations on vertically polarized ground-based

antennas as gain standards. IEEE Trans. Ant. Propagat . , AP-23: 284-286;

1975.

[8] NBS calibration services users guide 1986-88. Nat. Bur. Stand. (U.S.)

Spec. Pub. 250, 1986 Edition; 1986.
APPENDIX A: SAMPLE TEST REPORT FOR A VERTICAL MONOPOLE

U.S. DEPARTMENT OF COMMERCE

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY

NATIONAL ENGINEERING LABORATORY

Boulder, Colorado 80303

SPECIAL TEST

Broadband Active Monopole Antenna

Manufacturing Company

Model ABC, Serial No. xxx

Submitted by:

Submitting Company

City, State 12345-6789

I Description of Calibration

The Submitting Company electric-field antenna is an active broadband monopole

antenna system designed to operate in the 30 kHz to 300 MHz frequency range.

The antenna factor to be used for measuring the vertical component of an electric (E) field was determined by Immersing the receiving antenna under

test (AUT) in a known field 10 meters from the NIST transmitting monopole. A

block diagram of the instrumentation used for this calibration is given in the

attached figure. The receiving antenna output was connected to a calibrated

cable and 50 q spectrum analyzer. This spectrum analyzer was used as a frequency-selective voltmeter to measure the pickup at each frequency tested.

The calibration of vertical monopole antennas at NIST is done at an outdoor

field site which has a 30 m x 60 m (100 ft x 200 ft) conducting screen mesh

stretched over a concrete slab. An underground room is located beneath the

concrete for the transmitter and monitoring Instrumentation. The strength of

the calibrating field is calculated at each frequency in terms of the base

current in a thin transmitting vertical monopole antenna. The vertical

component of E field, as a function of base current and position, is given by

the equation

30J^

^^" sin(pc)

+ — -2cos(pC) (1)

Page 1 of 4

Test No. ABCDEF

Date of Test:

10
Active Monopole Antenna

Manufacturing Company

Model ABC, Serial No. xxx

where

E^ = vertical component of standard E field, RMS V/m,

lo = monopole antenna base current. A,

Q = height of transmitting monopole, m,

p

= 27r/wave length, m'S

ri = distance from top of transmitting monopole to field point, m,

r2 = distance from image of top of monopole (in ground) to

field point, m,

ro = distance from bottom of monopole to field point, m.

The above equation is number (10-72) on page 323 of Jordan [1].

The base current of the transmitting monopole antenna is calculated by the

equation

where

Vo = measured base voltage of the transmitting monopole antenna, V,

Zin = magnitude of the monopole input impedance, q.

A 2.5 meter transmitting monopole was used to generate a standard field at

each frequency. At frequencies below one-tenth of the self-resonant

frequency, the radiation resistance is negligible and the input impedance is

only capacitive reactance. The capacitance of a thin electrically-short whip

can be calculated from the formula

^_ 55.63{

a

where

C = monopole input capacitance, pF,

Q = monopole height, m,

a = monopole radius, m.

The above equation is twice that of eqn (11) on page 306 of Schelkunoff [21.

Page 2 of 4

Test No. ABCDEF

Date of Test:

11
Active Monopole Antenna

Manufacturing Company

Model ABC, Serial No. xxx

At frequencies above 1/10 the self-resonant frequency, the input impedance of

the monopole (Z in) was calculated from Schelkunoff 's "mode" theory using

equation (108) on page 433 of reference [2]. A check of the calculated E field was made at each frequency with a passive whip antenna with a known

antenna factor. The rf voltage on the transmitting monopole ranged from 0.5

volts at 300 MHz up to 80 volts at 30 kHz.

The antenna factor (K), in dB, was determined by the equation

K=E^-V (4)

where

Eo = field strength calculated from equation (1), expressed

in dBmV/meter,

V = voltage delivered to the calibrated 50 Q spectrum

analyzer, in dBmV.

II. Calibration Data

The measured values of antenna factor (K) with the antenna rod fully extended

are given in the following table

Table 1. Antenna Factor of the Active Monopole Antenna, Model ABC,

Serial No. xxx.

Frequency Antenna Factor

(MHz) (dB)

0.030 571

0.100 8.2

0.500 5.9

1.00 5.7

5.00 5.5

10.0 4.9

30.0 3.9

100.0 2.1

300.0 1.0

Page 3 of 4

Test No. ABCDEF

Date of Test:

12
Active Monopole Antenna

Manufacturing Company

Model ABC, Serial No. xxx

An unknown field strength (E) may be measured with the calibrated monopole

antenna by the expression E = K + V, The data in this report are for use in

making cw measurements only, and not necessarily valid for broadband

interference.

The calibration uncertainty of the antenna factors given is ± 1.5 dB.

III. References

[1] "Electromagnetic Waves and Radiating Systems," E. C.

Jordan, Prentice-Hall, Inc., 1950.

[2] "Antennas, Theory and Practice," S. A. Schelkunoff and H,

T. Friis, John Wiley & Sons, Inc., 1952.

For the Director

National Institute of Standards

and Technology

Test performed by:

Motohisa Kanda, Ph. D.

Group Leader, 723.03

Fields Characterization Group

Electromagnetic Fields Division

Dennis Camell, Engineer

(303) 497-3321

Page 4 of 4

Test No. ABCDEF

Date of Test:

Reference: P.O. No 12345

13
Table 1. Reactance of a Vertical Monopole (^ = 2.5 m, a = 0.813 mm)

Frequency (MHz) Theoretical Reactance (Q) Measured Reactance (Q)

0.5 - 16 093 - 16 100

1.0 - 8 046 - 8 225

2.0 - 4 023 - 4 100

5.0 - 1 609 - 1 600

7.5 - 1 073 - 1 020

14
Table 2. List of equipment for the open-field site

50 n signal generator, 30 kHz to 300 MHz.

Amplifier, 50 dB gain maximum.

Rf voltmeter with coaxial tee connector, 300 V maximum.

Calibrated spectrum analyzer, covering the frequency range.

Assorted low pass filters, attenuators, and low loss cables with

type N connectors.

15
field

point

image

^

ground

plane

Figure 1. Geometry for a transmitting vertical monopole over a ground

plane

.

16
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A y* UNITED STATES DEPARTMENT OF COMMEPCE

National Institute of Standards and Technology

(formerly National Bureau of Standards]

325 Broadway

BoL.iOer. CoioracJo 80303-3328

KIMO or MEASUREMENT

IMSTHUMEKT TESTED Ve-vVicJ

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19
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70

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30

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Figure 5. Comparison of calculated and measured antenna factors for aim

vertical monopole.

20
SL-114A

5-90) U.S. DEPARTMENT OF COMMERCENATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY

BIBLIOGRAPHIC DATA SHEET

PUBUCATION OR REPORT NUMBER

NIST/TN-1347

2. PERFORMING ORGANIZATION REPORT NUMBER

3. PUBUCATION DATE January 1991

TITLE AND SUBTITLE NIST Calibration Procedure for Vertically Polarized Monopole Antennas, 30 kHz to 300 MHz

AUTHOR(S)

D. G. Camell, E. B. Larsen, J. E. Cruz, and D. A. Hill

PERFORMING ORGANIZATION OF JOINT OR OTHER THAN NIST. SEE INSTRUCTIONS)

U.S. DEPARTMENT OF COMMERCE

NATIONAL INSTITUTE OF STANDARDS AND TECHNOLOGY

BOULDER, COLORADO 80303.3328

7. CONTRACT/GRANT NUMBER

8. TYPE OF REPORT AND PERIOD COVERED

SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (STREET. OTY, STATE. ZIP)

). SUPPLEMENTARY NOTES

ABSTRACT (A 200-WORD OR LESS FACTUAL SUMMARY OF MOST SIGNIFICANT INFORMATION. IF DOCUMENT INCLUDES A SIGNIFICANT BIBUOQRAPHY OR

UTERATURE SURVEY. MENTION IT HERE.)

This report describes the theoretical basis and test procedure for vertically polarized

monopole antenna calibrations at the National Institute of Standards and Technology

(NIST). The standard field method applies the theoretical equations of a vertical

monopole antenna to calculate the vertical electric field. This method is used at

the NIST open field site in the frequency range of 30 kHz to 300 MHz. The uncertainty in the antenna factor of the antenna under test (AUT) is now + 1 dB.

KEY WORDS (6 TO 12 ENTRIES; ALPHABETICAL ORDER; CAPITAUZb ONLY PROPER NAMES; AND SEPARATE KEY WORDS BY SEMICOLONS)

antenna factor; calibration; electromagnetic field; monopole antenna; open field

site; standard field

t. AVAILABIUTY a

UNUMITED

FOR OFFICIAL DISTRIBUTION. DO NOT RELEASE TO NATIONAL TECHNICAL INFORMATION SERVICE (NTIS). ORDER FROM SUPERINTENDENT OF DOCUMENTS, U.S. GOVERNMENT PRINTING OFFICE,

WASHINGTON, DC 20402. ORDER FROM NATIONAL TECHNICAL INFORMATION SERVICE (NTIS), SPRINGFIELD. VA 22161.

14. NUMBER OF PRINTED PAGES

28

15. PRICE

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