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NBSIR 77 - 868

DO NOT REMO"^"

THE CHARACTERISTICS OF BROADBAND, ISOTROPIC

ELECTRIC FIELD AND MAGNETIC FIELD PROBES

Electromagnetics Division

Institute for Basic Standards

National Bureau of Standards

Boulder, Colorado 80302

November 1977

Motohisa Kanda

OTP

NBSIR 77 - 868

THE CHARACTERISTICS OF BROADBAND, ISOTROPIC

ELECTRIC FIELD AND MAGNETIC FIELD PROBES

Motohisa Kanda

Electromagnetics Division

Institute for Basic Standards

National Bureau of Standards

Boulder, Colorado 80302

November 1977

U.S. DEPARTMENT OF COMMERCE, Juanita M. Kreps, Secretary

Sidney Harman, Under Secretary

Jordan J. Baruch, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Acting Director

CONTENTS

Page

1 . INTRODUCTION 1

2. ELECTRIC FIELD PROBE PERFORMANCE 1 2.1 Low Frequency Electric Field Probe (LEP) — 2 2.2 Middle Frequency Electric Field Probe (MEP) 4 2.3 High Frequency Electric Field Probe (HEP) — — 5

3. MAGNETIC FIELD PROBE PERFORMANCE 6 3.1 The Transfer Function of an Electrically Small Loop Antenna

with a Loading Resistance — .—— . —— .— 7 3.2 Low Frequency Magnetic Field Probe (LMP) • — — 8 3.3 Middle Frequency Magnetic Field Probe (MMP )————————— 9

4. CONCLUSION —— . ________ lo

4.1 Electric Field Probe Performance —— — • — — 10

4.2 Magnetic Field Probes —— 11

5. REFERENCES — 12

LIST OF FIGURES

e Page

Figure 1. Schematic diagram for low frequency electric field probe (LEP) 13

Figure 2. Transfer function of an electrically-short dipole with FET load .-— 14

Figure 3. Schematic diagram for middle frequency electric field probe (MEP) —— 15

Figure 4. Photograph of resistively loaded antenna- - • • —•—~ 16

Figure 5. Transfer function of resistively loaded antenna——— ————— 17 Figure 6. Far-field radiation patterns: at 100 MHz 18

Figure 7. Far-field radiation patterns: at 1 GHz ———— 19

Figure 8. Far-field radiation patterns: at 2.5 GHz-—• ——— 20

Figure 9. Schematic diagram for high frequency electric field probe (HEP)- —— 21

Figure 10. Photograph of dipole with resistive-capacitive loading —— — 22

Figure 11. Radiation pattern of dipole with resistive-capacitive

loading at 2 GHz — 23

Figure 12. Radiation pattern of dipole with resistive-capacitive

loading at 8 GHz — — 24

Figure 13. Radiation pattern of dipole with resistive-capacitive

loading at 12 GHz - • 25

Figure 14. Transfer function of dipole with resistive-capacitive loadingTM 26

Figure 15. Schematic diagram of magnetic field probe— ———— — 27 Figure 16. Equivalent circuit of electrically-small loop antenna — — 28

Figure 17. Response of electrically-small loop antenna 29

Figure 18. Normalized transfer function of electrically-small loop antenna 30

iii

THE CHARA.CTERISTICS OF BROADBAND, ISOTROPIC ELECTRIC FIELD

AXD MAGNT:TIC FIELD PROBES

Motohisa Kanda

A feasibility study and a preliminary engineering test program have been

conducted to establish performance specification limits and preliminary engineering design for broadband, isotropic, receiving, electric field and magnetic

field probes for electromagnetic emission measurements. Three electric field

probes cover the frequency range from 20 Hz to 12 GHz. Two magnetic field

probes cover the frequency range from 20 Hz to 32 MHz. The tangential sensitivities and the dynamic range of these broadband isotropic electric field and

magnetic field probes are specified. Preliminary engineering design and

supporting test data are also included.

Key words: Broadband probe; dipole antenna; dynamic range; electric field probe;

isotropic probe; loop antenna; magnetic field probe; tangential sensitivity.

1. INTRODUCTION

The purpose of this report is to provide results of a feasibility study and an engineering

test program to establish performance specification limits and preliminary electrical design

for broadband, isotropic, receiving probes for measurements of electric and Tiiagnetic field

emissions. Here, a broadband isotropic probe consists of a dipole antenna for an electric

field probe, and a loop antenna for a magnetic field probe. These antenna elements are then

fed into a device that performs a balance-to-unbalance transformation (e.g.s a balun, a differential amplifier, etc.) and an amplifier if necessary. Three electric field probes cover the frequency range from 20 Hz to 12 'GHz. Two magnetic field probes cover the frequency range from 20 Hz to 32 MHz. The frequency ranges, the tangential sensitivities based on a signal to noise ratio of

3 dB for various bandwidths, and the djniamic ranges based on 1 dB compression point are specified for each electric field and magnetic field probe. Engineering design and

supporting test data are also included in this report.

2. ELECTRIC FIELD PROBE PERFORMANCE

This section defines the performance specification limits and operating characteristics

of the electric field probes. The receiving characteristics of three types of linear an- tennas with three different loading mechanisms are investigated theoretically and experimentally to cover the frequency range from 20 Hz to 12 GHz. The received voltage V, (f ) of a linear antenna across a load impedance Z (f) is given

by

-\(f) E.^,(f) Z,(f)

=

Z (f) ^ 7 (f) ' o L

where h^(f) is an effective length, Z^(f) is an input impedance of a linear antenna,

E^^^(f) is an incident electric field to an antenna and f is frequency. Then, the transfer

function of a linear antenna, S(f), is defined to be

V. (f) -h^(f)Z^(f)

S(i) = —= = — = ^^"^ (2)

- E. (f) Z (f)+Z, (f)

• mc o

In each case, i.e., low frequency electric field probe (LEP), middle frequency electric

field probe (MEP), and high frequency electric field probe (HEP), the transfer functions of

specific antennas are first calculated theoretically and verified experimentally. Then, the

antenna power patterns are investigated experimentally to ensure that these specific linear
antennas can be made into isotropic probes. Finally, the tangential sensitivities and the

dynamic ranges of each electric probe are estimated theoretically.

2.1 Low Frequency Electric Field Probe (LEP)

The technical approach to be used for a low frequency electric field probe (LEP) is an electrically-short dipole in order to achieve a broad bandwidth. The LEP covers the frequency range between 20 Hz and 10 MHz. A field effect transistor (FET) amplifier is used

for the input stage with differential mode operation, in order to reject common mode.

The schematic diagram for the LEP is shown in figure 1.

For an electrically-short dipole antenna (i.e., Bh<l), the effective length, hg(f), and

the input impedance, Z (f), of the antenna are given by [1] o

, h(n-l)

K^^^ = 2(^^-2-Hiln 4) "'^^^^^

and

Z (f) = -j (Q.-2-ln 4) ohm . (4) o 47;6h ^

'

The symbols have the following meanings: h is a half physical length of a dipole antenna in

meters, 3 is a wave number in meters ^, is the free space impedance in ohms, and Q. is an antenna fatness factor (i.e., Q. = 2 2,n(2h/a) where a is an antenna radius in meters).

With an FET input stage, the antenna is considered to be terminated with a purely

capacitive load. Since the load impedance of the antenna is given by

^L^^) = -j 2^°^ ' (5)

where C is the capacitance of an FET amplifier in farads, the load voltage, Vj^(f), across the

antenna terminal is

- h/2 E (f)

, V^> = 1^ c/c

where

C = —^ .^^T —TT- farad , (7) a V c (,Q.-l-ln 4) ^

' o o

- 1 /ox - a - 2 + in 4

'

and v^ is the speed of light in free space. Thus, the transfer function of an LEP is given

by

c rf^=A!£i_ . ct- h/2

^LEP*- E. (f) 1 + C/C ' ^

' mc a

which is independent of frequency. In practice, the input impedance of an FET amplifier is

not pure capacitive, but has a resistive component as well as a capacitive component. This

resistive component of the input impedance will cause a 6 dB per octave roll-off at the low

end of the frequency range.

2
Preliminary experiments were performed using the 15-cm long dipole, i.e., h = 7.5 cm with 0.6 cm radius. The experimental result for the transfer function of the electricallyshort dipole is shown in figure 2. Please note that the transfer function of the electricallyshort dipole with an FET amplifier is very flat to ± 3 dB from 2 kHz to 400 MHz. When the

capacitance of the FET amplifier used for our experiment is assumed to be 5.9 pF, the

transfer function of the antenna is given in dB by eq. (9) as

20 log|Sj^^(f) 1= -41.1 dB. (10)

This theoretical result agrees very well with our experimental data for the frequency range

from 2 kHz to 400 MHz. When the input resistance of the FET amplifier is assumed to be

100 kil, the transfer function of the electrically-short dipole rolls off at -6 dB per octave

due to the input resistance of the FET amplifier below 2 kHz, as shown in figure 2. Above

400 MHz, the 15 cm long dipole becomes electrically long so that the theoretical expressions

for the effective length and the input impedance given in eqs. (3) and (4), respectively,

will not be valid.

It should be remembered, however, that, in practice it is not possible to build a high

input impedance FET differential amplifier with high common mode rejection above 10 MHz with

present state of the art. For this reason the highest frequency of LEP is limited to 10 MHz as specified previously.

Consider an electrically short dipole with a length of 30 cm and a radius of 1.5 cm with

FET amplifiers as the input stage for an LEP. When the capacitance of the input stage FET amplifiers is assumed to be 6 pF, the transfer function of an LEP is

S^^(f) = -3.59 X 10~^ or -28.9 dB.

The tangential sensitivity (i.e., S/N = 1, or, a signal to noise ratio of 3 dB) can be

estimated as ,

S ^?/^L

N k{T n + (l-n)T + T }B ant o amp

(11)

-23 -1

The symbols have the following meanings: k is Boltzmanns' constant (= 1.38 x 10 JK )

,

is an antenna temperature in kelvins, ri is an antenna radiation efficiency, T^ is an ambient temperature in kelvins, T is the effective input noise temperature of an

amp

FET amplifier in kelvins, and B is a bandwidth in Hertz. Here T n + (l-n)T is the noise

ant 0 contribution due to the antenna and T is the noise contribution due to the amplifier. If amp

the antenna is surrounded by 290 K environment, the noise temperature contributed by the

antenna would be 290 K regardless of its radiation efficiency, n. Assuming that the noise

figure and the resistive input impedance of an FET amplifier are, respectively, 10 dB and

1 Mii, the tangential sensitivities of the active antenna for various bandwidths for the

frequency range from 20 Hz to 10 MHz are listed below.

• 17.6 uV/m, or, 24.9 dByV/m with 10 Hz bandwidth (BW)

•55.7 uV/m, or, 34.9 dBuV/m with 100 HZ BW

• 176 yV/m, or, 44.9 dBuV/m with 1 kHz BW

The djnnamic range of an LEP is limited by the FET differential amplifier characteristics. It is estimated that the maximum input rf voltage before 1 dB gain compression is

about 0.05 volt rms for a typical FET amplifier. Therefore, the maximum electric field to produce 0.05 volt rms at the antenna terminal is 1.39 V/m or 122.9 dByV/m. Thus, the

predicted dynamic range for a LEP is 98 dB for 10 Hz bandwidth.

3
Similarly, the dynamic ranges of the LEP with various bandwidths are also calculated

and are summarized below for the frequency range from 20 Hz to 10 MHz.

• 98 dB with 10 Hz BW

• 88 dB with 100 Hz BW

• 78 dB with 1 kHz BW

2.2 Middle Frequency Electric Field Probe (MEP)

The technical approach to be used for a middle frequency electric field probe (MEP) is a resistively loaded antenna. The MEP covers the frequency range between 10 MHz and 1 GHz.

The schematic diagram of an MEP is shown in figure 3.

When an antenna is resistively loaded such that its internal impedance along the antenna

is given as

Z\z) =^ObLL , (12)

' -- h - |z|

where

11^ = 2[sinh"^^- C(2Sa,26h) - jS(23a,2eh)] +^ (l-e"j^^ , (13)

and C(a,x) and S(a,x) are the generalized cosine and sine integrals, the effective length

hg(f) and the driving-point impedance Z^(f) of the resistively loaded antenna are then given

by

h^(f) = (l-jBh-e"j^^ , (14)

and

Z^(f) = 60i|;(l - (15)

The more detailed discussions on the characteristics of a resistively loaded antenna were given by solving the wave equation using the method of moments and are given elsewhere by

the author [2,3,4]. Then, using the effective length, h^{f), and the driving-point impedance,

Z^(f), the transfer function of the resistively loaded antennas is calculated by use of

eq. (2).

The preliminary experiments were performed using a 15 cm long resistively loaded dipole,

i.e., h = 7.5 cm with a = 0.127 cm. The photograph of the resistively loaded dipole is

shown in figure 4. A commercially available balun (200 Q. balance to 50 Q, unbalance) was

used. The theoretical and experimental results of the transfer function for the resistively

loaded antenna are given in figure 5. The agreement is very good. It is found for figure

5 that the transfer function is very flat above 200 MHz, whereas it rolls off at -6 dB

per octave below 200 MHz. In order for the response of the MEP to be flat across the

frequency range from 10 MHz to 1 GHz, the gain of the broadband rf amplifier is tailored to have 6 dB gain increase per octave below 200 MHz and to have a constant gain above 200 MHz.

The far-field radiation patterns of the resistively loaded antenna were investigated

theoretically and experimentally at 100 MHz, 1 GHz, and 2.5 GHz. The results are shown in

figures 6, 7, and 8. It is found from the figure that an isotropic MEP can be fabricated

up to at least 3 GHz using three orthogonal resistively loaded dipoles.

4
The tangential sensitivity (i.e., S/N = 1, or, a signal to noise ratio of 3 d3) is estimated in the same way as described in section 2.2. It is assumed that the broadband amplifier used for the MEP has a noise figure (NF) of 5 dB including the loss due to the balun.

Using eq. (11) the tangential sensitivities at various frequencies with various bandwidths

are calculated and are summarized below.

•89.5 uV or 39 dBuV/m at 10 MHz with 1 kHz BW

• 40.0 uV or 32 dBuV/m at 25 MHz with 1 kHz BW

• 31.7 uV or 30 dByV/m at 400 MHz with 100 kHz BW

•22.5 vV or 27 dBuV/m at 1 GHz with 100 kHz BW

The dynamic range of a MEP is limited by the low noise broadband rf amplifier. A typical

output power of a broadband amplifier at 1 dB gain compression is -10 dBm with a gain of

30 dB at 10 MHz. Thus, the maximum rf voltage that can be applied to the amplifier is

-4

7 . 07 X 10 V. To produce this maximum rf voltage at the antenna terminal, the maximum

electric field to the antenna is 1.26 V/m, or 122 dByV/m. The dynamic range, is, thus 83 dB

with 1 kHz BW.

Similarly, the dynamic ranges of the MEP with various bandwidths are also calculated

and are summarized below for the frequency range from 10 MHz to 1 GHz.

• 83 dB with 1 kHz BW

• 73 dB with 10 kHz BW

• 63 dB with 100 kHz BW

2.3 High Frequency Electric Field Probe (HEP)

The technical approach to be applied for a high frequency electric field probe (HEP)

is a relatively short linear antenna with resistive-capacitive loading. The schematic

diagram for an HEP is shown in figure 9. The technical reason why resistive-capacity

loading is chosen for the HEP is as follows. If a linear antenna with pure resistive-loading

is to be used, the maximum dipole physical length before antenna patterns degrade is found

to be on the order of three (3) times the half wavelength at the highest frequency of interest

as indicated in figure 8. Thus, if the highest frequency of interest is 18 GHz, the

1 66 maximum dipole length allowed must be less then 3 x '

cm = 2.5 cm. In practice, it is

very difficult to fabricate a 2.5 cm long linear dipole antenna with the proper resistive

loading. On the other hand, when a linear dipole antenna with resistive-capacitive

loading is to be used for the HEP, the maximum dipole physical length before antenna

power patterns degrades can be about six (6) times the half wavelength at the highest

frequency of interest. This means that, if the highest frequency of interest is 18 GHz,

1 66

the maximum allowed dipole physical length is 6 x —

^— cm = 5 cm. If the highest frequency

of interest could be lowered to 12 GHz from 18 GHz, then the maximum allowed dipole physical

length becomes 6 x -^y^ cm = 7.5 cm. It is feasible to fabricate a 7.5 cm long dipole with

resistance-capacitance loading without much difficulty.

There is another severe restriction which limits the highest frequency of the HEP to 12 GHz instead of 18 GHz. In order to assure equal currents in the two arms of a dipole for

a symmetrical power pattern, the arms should have the same impedance to ground. Such a load

should be fed by a transmission line as a two-wire line, which in itself is balanced to ground. However, at high frequencies, particularly above 1 GHz, unbalanced coaxial lines are always used in practice, so that we encounter the problem of transforming from an

5
unbalanced to a balanced system. The device that accomplishes the balance-to-unbalance

transformations is a balun. After our extensive search for a high frequency broadband

balun, we are not able to find a balun to cover the frequency range from 1 GHz to 18 GHz.

The only possibility which comes close to our requirement is a 1-12 GHz broadband balun,

which may be obtained from a commercially available flat spiral antenna. Because of the

time schedule, it was not possible at this time to perform a more extensive research effort

for both an antenna and a balun for the purpose of extending the frequency from 12 GHz to 18 GHz. Therefore, all engineering design and supporting test data for the HEP included in

this section cover the frequency range between 1 GHz and 12 GHz.

A linear antenna with capacitive-resistive loading was fabricated and is shown in

figure 10. The present linear dipole antenna with capacitive-resistive loading is- 7.5 cm long with 0.127 cm radius. The far-field radiation patterns of the antenna are investigated

experimentally at 2, 8, and 12 GHz. The results are shown in figures 11, 12, and 13. These

figures show that an isotropic HEP can be fabricated up to 12 GHz using three orthagonal

linear dipoles with capacitive-resistive loading.

The preliminary experimental results for the transfer function of the antenna are shown

in figure 14. Typically, the transfer function of a linear antenna with a resistive-capacitive

loading is -51 dB at 1 GHz and -38 dB at 10 GHz.

The tangential sensitivities are calculated in the same way as before. Assuming that

the noise figure of a pre-amplif ier is 7 dB including the loss due to a balun, the

tangential sensitivities at various frequencies with various bandwidths are summarized

below.

• 71 viV/m, or 37 dByV/m at 1 GHz with 10 kHz BW

• 40 uV/m, or 32 dB V/m at 5 GHz with 100 kHz BW

• 159 pV/m, or 44 dB V/m at 10 GHz with 1 MHz BW

The dynamic range of an HEP is limited by the pre-amplif ier . A typical amplifier with

a gain of about 20 dB has an output power of 0 dBm at 1 dB gain compression. Thus, the

-2

maximum rf input voltage before 1 dB gain compression is 2.24 x 10 volt rms. The

-2

maximum electric field at 1 GHz to produce 2.24 x 10 volt rms at the antenna terminal

is 7.93 V/m, or 138 dBuV/m. Therefore, the dynamic range of the HEP for 10 MHz BW is 101 dB.

Similarly the dynamic ranges of the HEP for various bandwidths are calculated and

summarized below for the frequency range between 1 and 12 GHz.

• 101 dB with 10 kHz BW

• 91 dB with 100 kHz BW

• 81 dB with 1 MHz BW

3. MAGNETIC FIELD PROBE PERFORMANCE

A magnetic field probe consists of an electrically-small, balanced loop antenna. The

response of a loop antenna is directly proportional to frequency. To make the response of

a loop antenna flat over the frequency range of interest, the Q of a loop antenna is reduced

through a loading resistance at the antenna terminal. The induced voltage at the antenna

terminal is then fed into a conventional differential amplifier. The schematic diagram of

a magnetic field probe using a loop antenna is shown in figure 15. In this section, the

transfer function of an electrically small loop antenna with a loading resistance is first

discussed by considering the response and the resonance effect of the loop antenna. Then,

using the transfer function of the loop antenna with a loading resistance, the preliminary

6
designs and the performance specification limits, i.e., tangential sensitivities and dynamic

ranges for broadband magnetic field probes for the various frequency ranges of interest, are discussed.

3.1. The Transfer Function of an Electrically Small Loop Antenna with a Loading

Resistance

The induced voltage of an electrically small loop antenna is determined from Maxwell's

equations with Stoke 's theorem, and is given by

V. = / E. -di = -j'jjuH. NS (16)

1 mc mc o

where ^^^^ is the tangential component of an electric field strength, I is the circumference

of the loop, la is the angular frequency, u is the permeability, H^^^ is the normal component of a magnetic field, N is the number of loop turns, and S is the area of a loop. It should

be noted that the induced voltage of an electrically small loop antenna is proportional

to frequency, the number of loop turns, and to the area of a loop. To make the response

of a loop antenna flat over the frequency range of interest, the Q of the loop antenna is to be reduced through a loading resistance.

The resonance is the result of the combined effect of the distributed capacitance of a loop, the gap capacitance, and the capacitance of then amplifier along with the inductance of

a loop. The equivalant circuit for an electrically small loop antenna is shown in figure 16.

Here V. is the induced voltage, L is the loop inductance, C is the capacitance, R is the

loading resistance, and is the voltage across a load resistance. Then, the response of an electrically small loop antenna is given by

V. 1 ^ . Ix (17)

where

„ R „ . T 1 - oj A O IUjC (jJ o o

and

the resonance angular frequency. The response of an electrically-small loop antenna in dB

(i.e., 20 log|Vj^/V^!) as a function of normalized frequency, ^/'-Uq> is shown in figure 17 for

various Q <_ 1.

Combining eq. (16) and eq. (17), the transfer factor of an electrically small loop

antenna is, then, given by

S(f) =i

=V--"'^ 1 ' 1, •. (18) xnc - + J (0 - -)

The normalized transfer factor, S^(f), of a loop with a loading resistor, i.e..

7
in dB as a function of normalized frequency, oj/oj^, is given in figure 18 for various Q <^ 1.

It is clearly found from figure 18 that the upper frequency end of the 3 dB roll-off

point, (jj^, is given by S^^Q = 1 and, similarly, the corresponding lower frequency end of

the 3 dB roll-off point, w^^, is given by "S^/Q = 1. Thus, from these conditions, it is found

that

o or

0) = /cj, 0). . o h £, (20)

The self -resonance frequency of a loop should be determined as the geometrical mean of its

highest and lowest cut-off frequencies.

3 . 2 Low Frequency Magnetic Field Probe (IMP)

The technical approach for a low frequency magnetic field probe (LMP) is to use a loop

antenna with a loading resistance. The LMP covers the frequency range between 20 Hz and

50 kHz. As discussed in section 3.1, by selecting 400 Hz and 50 kHz as the upper and the

lower 3 dB roll-off frequencies, the self -resonant frequency of a loop antenna, which is

chosen as a geometrical mean of these frequencies, becomes f =4.47 kHz. Then, the required

Q for the loop is Q = 0.0894. The transfer function of the 2oop antenna, which is flat

"from 400 Hz to 50 kHz is given by eq. (18),

\ -3

S(f) = -jp^ = 3.157 X 10 ^ SN. (21)

inc

When an electrically small, twenty- turn, balanced loop antenna of 0.3 m in diameter is

chosen for the LMP, the transfer function is given by

S^(f) = jj-^ = 4.463 X 10"^ or -47 dB. (22)

inc

The tangential sensitivities of the LMP are determined in the same way as described

in section 2. Assuming that the noise figure and the input impedance of a typical differential amplifier used for LMP are 10 dB and 5 kO, respectively, the tangential sensitivities

for the frequency range between 400 Hz and 50 kHz are given below.

• 10 uA/m or 20 dByA/m (22 dBpT) with 10 Hz BW

• 31.7 uA/m or 30 dByA/m (32 dBpT) with 100 Hz BW

• 100 yA/m or 40 dBpA/m (42 dBpT) with 1 kHz BW

Since the transfer function of the loop antenna with a loading resistance rolls off

at -6 dB per octave below 400 Hz, the tangential sensitivities for 10 Hz bandwidth are estimated to be, for example, 35 dByA/m (or 37 dBpT) at 100 Hz and 41 dByA/m (or 43 dBpT)

at 50 Hz, provided that the noise figure of the differential amplifier stays 10 dB at these

low frequencies.

The dynamic range is limited by the differential amplifier. Assuming that maximum input

rf voltage to a differential amplifier before 1 dB gain compression is about 0.01 volt rms.

8
the dynamic ranges of the LMP for the frequency range between 20 Hz and 50 kHz are given

below.

• 107 dB with 10 Hz BW

• 97 dB with 100 Hz BW

• 87 dB with 1 kHz BW

3 . 3 Middle Frequency Magnetic Field Probe (MMP)

The technical approach for a middle frequency magnetic field probe (MMP) is to use a loop antenna with a proper loading resistance. The MMP covers the frequency change between

50 kHz and 32 MHz. As discussed in section 3.1, the self-resonance frequency is to be chosen

as a geometrical mean of the lowest and highest cut-off frequencies, i.e., f = 1.265 MHz.

Then the required Q of the loop antenna should be Q = 0.0395. The transfer ?unction of the

loop antenna, which is flat across the frequency range from 50 kHz to 32 MHz, is

S(f) = = 0.3945 SN. (23)

inc

^Jhen an electrically small, six-turn, balanced loop antenna with 10 cm in diameter is

chosen for MMP, the transfer function of the MMP is given by

\ -2 S^(f) = -g-^ = 1.859 X 10 ^ or -34.6 dB . (24)

inc

Since it was observed experimentally that the inductance of six-turn loop antenna with 10 cm

in diameter is about 6.86 uH, the required loading resistance is 2.15 Q.

The tangential sensitivity of an MMP is determined in tae same way as described in

section 2. It is assumed that the noise figure and the input impedance of a typical differential amplifier used for an MMP is 6 dB and 1 kTi, respectively c Then, the tangential

sensitivities for the frequency range from 50 kHz to 32 MHz for various bandwidths are given below.

• 21.5 uA/m or 26.6 dBuA/m (28.6 dBpT) , with 10 kHz BW

• 67.9 Wm or 36.6 dBuA/m (38.6 dBpT) with 100 kHz BW

• 215 ^^:A/m or 46.6 dBuA/m (48.6 dBpT) for 1 MHz BW

The dynamic range is limited by the differential amplifier. Assuming that the maximum

rf input voltage to the differential amplifier before 1 dB gain compression is about 0.01

volt rms, the dynamic ranges for the frequency range between 50 kHz and 32 MHz for various

bandwidths are given below.

• 88 dB, with 10 kHz BW

• 78 dB, with 100 kHz BW

• 68 dB, with 1 MHz BW

9
4. CONCLUSION

A feasibility study and a preliminary engineering test program have been conducted to establish the preliminary designs and performance specification limits for broadband,

isotropic, receiving electric field and magnetic field probes for electromagnetic emission

measurements

.

From the feasibility study and preliminary engineering test program, it is concluded

that the following electric field and magnetic field probes performance requirements are

feasible.

4.1 Electric Field Probe Performance

Low Frequency Electric Field Probe (LEP)

Frequency range: 20 Hz to 10 MHz

Tangential sensitivities for the frequency range specified above:

24.9 dByV/m with 10 Hz BW

34.9 dBuV/m with 100 Hz BW

^ .

' 44.9 dBvjV/m with 1 kHz BW

Dynamic range: 98 dB with 10 Hz BW

88 dB with 100 Hz BW

78 dB with 1 kHz BW

Middle Frequency Electric Field Probe (MEP)

Frequency range: 10 MHz to 1 GHz

Tangential sensitivities for the frequency range specified above:

39 dByV/m at 10 MHz with 1 kHz BW

V 32 dBuV/m at 25 MHz with 1 kHz BW

• ^ 30 dByV/m at 400 MHz with 100 kHz BW

27 dByV/m at 1 GHz with 100 kHz BW

Dynamic range: 83 dB with 1 kHz BW

73 dB with 10 kHz BW

63 dB with 100 kHz BW

High Frequency Electric Field Probe (HEP)

Frequency range: 1 GHz to 12 GHz

Tangential sensitivities for the frequency range specified above:

37 dBuV/m at 1 GHz with 10 kHz BW

32 dBuV/m at 5 GHz with 100 kHz BW

44 dByV/m at 12 GHz with 1 MHz BW

10
Dynamic range: 101 dB with 10 kHz BW

91 dB with 100 kHz BW

81 dB with 1 MHz BW

4.2 Magnetic Field Probes

Low Frequency Magnetic Field Probe (LMP)

Frequency range: 20 Hz to 50 kHz

Tangential sensitivity:

For the frequency range from 20 Hz to 400 Hz:41 dBuA/m (43 dBpT) at 50 Hz with 10 Hz

35 dB'tiA/m (37 dBpT) at 100 Hz with 10 Hz For the frequency range from 400 Hz to 50 kHz:

20 dBuA/m (22 dBpT) with 10 Hz BW

30 dBuA/m (32 dBpT) with 100 Hz 3W

40 dBuA/m (42 dBpT) with 1 kHz BW

Dynamic range: 107 dB with 10 Hz BW

97 dB with 100 Hz BW

87 dB with 1 kHz BW

Middle Frequency Magnetic Field Probe (MMP)

Frequency range: 50 kHz to 32 MHz

Tangential sensitivity for the frequency range specified above:

26.6 dByA/m (28.6 dBpT) with 10 kHz BW

36.6 dBvA/m (38.6 dBpT) with 100 kHz BW

46.6 dBvA/m (48.6 dBpT) with 1 MHz BW

88 dB with 10 kHz BW

78 dB with 100 kHz BW

68 dB with 1 MHz BW

Dynamic range:

11
5. REFERENCES

[1] King, R. W. P., Theory of Linear Antennas (Harvard University Press, 1956).

[2] Kanda, M. , The characteristics of a relatively short broadband linear antenna with

tapered resistive loading, 1977 AP-S International Symposium Digest, 230-233 (June 1977).

[3] Kanda, M. , A relatively short cylindrical broadband antenna with tapered resistive

loading for picosecond pulse measurement, Nat. Bur. Stand. (U.S.), Internal

Report 77-861 (Aug. 1977).

[4] Kanda, M. , A broadband antenna with tapered resistive loading for EMI measurements,.

1977 IEEE International Symposium on Electromagnetic Compatibility Digest, 13-18

(Aug. 1977).

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19
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NBS-1 i4A (REV. /-/3)

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO.

NBSIR 77-868

2. Gov't Accession

No.

3. Recipient's Accession No.

4. TITLE AND SUBTITLE

The Characteristics of Broadband. Isotropic Electrical

Field and Magnetic Field Probes

5. Publication Date November 19 77

6. Performing Organization Code

276.03

7. AUTHOR(S)

Motohisa Kanda

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No. 2763495

11. Contract/Grant No.

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

same as #9.

13. Type of Report & Period

Covered

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less factual summary ol most si^iticant information. If document includes a significant

bibliography or literature survey, mention it here.)

A feasibility study and a preliminary engineering test program have been

conducted to establish performance specification limits and preliminary engineering

design for broadband, isotropic, receiving, electric field and magnetic field

probes for electromagnetic emission measurements » Three electric field probes

cover the frequency range from 20 Hz to 12 GHz. Two magnetic field probes cover

the frequency range from 20 Hz to 32 MHz. The tangential sensitivities and the

dynamic range of these broadband isotropic electric field and magnetic field

probes are specified. Preliminary engineering design and supporting test data

are also included.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons)

_ Broadband probe; dipole antenna; dynamic range; electric field probe;

isotropic probe; loop antenna; magnetic field probe; tangential sensitivity.

18. AVAILABILITY [7^ Unlimited 19. SECURITY CLASS

(THIS REPORT)

21. NO. OF PAGES

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Office 20. SECURITY CLASS

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22. Price

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