NTIA REPORT 79-16

Estimates of

Maximum Electric Field Strengths

in the Automobile Environment

H.J. Liebe

D.D. Crombie

u.s. DEPARTMENT OF COMMERCE

Juanita M. Kreps, Secretary

Henry Geller, Assistant Secretary

for Communications and Information'

February 1979

TABLE OF CONTENTS

LIST OF FIGURES

LIST OF TABLES

ABSTRACT

1. INTRODUCTION

2. GENERAL CONSIDERATIONS

3. BROADCAST RADIATION

3.1. AM Standard Radio

3.2. VLF-LF Transmission

3.3. FM and TV Stations

3.4. Mobile Radio

4. MICROWAVE SATELLITE COMMUNICATIONS ANO ENERGY BEAMING

5. MICROWAVE RADARS

6. OVERHEAD POWER TRANSMISSION LINE

7. LIGHTNING AND NUCLEAR EM RADIATION

8. CONCLUSIONS

9. REFERENCES

iii

Page

iv

iv

2

5

5

6

6

11

12

15

20

22

25

28
LIST OF 'FIGURES

Page

Figure 1. Ground-level electric field strength estimates. 4

Figure 2. Vertical radiation pattern of UHF TV antenna. 8

Figure 3. Electric field strength of UHF TV station. 9

Figure 4. Max. free-space field strength from FM and TV

broadcasting. 9

Figure 5. Measured sidelobes of 12 m reflector antenna at 4 GHz. 13

Figure 6. Measured electric field strength from 100 to 5 m. 16

Figure 7. Computed near-field gain reduction factor. 17

Figure 8. Measured on-axis electric near field strength from

300 to 3 m. 17

Figure 9. Calculated electric field strength for 765 kV power

line. 21

Figure 10. Peak field strength for radiated lightning emission. 24

Figure 11. Change in field strength for a lightning flash. 24

Figure 12. Overview of possible max. electric field strengths. 26

LIST OF TABLES

Page

Table 1. Measured Electric Near Field Strength at VHF and UHF 10

Table 2. Microwave Radar Specifications 18

Table 3. Maximum Electric Field Strength Data in the Vicinity

of Ground-Based Radars 19

iv
ESTIMATES O~ MAXIMUM ELECTRIC FIELD STRENGTHS

IN THE AUTOMOBILE ENVIRONMENT

Hans J. Liebe and Douglass D. Crombie*

Strong electromagnetic (EM) energy sources

up into the microwave range are examined and

estimates are made of maximum EM field conditions

to which automobiles could be exposed. The

results are meant to alert automotive engineers

to potentially hazardous EM radiation that might

upset on-board electronic control devices.

1. INTRODUCTION

The electromagnetic (EM) radiation environment of modern vehicles has

become of concern as the number and complexity of on-board electronic devices

is growing (Jurgen, 1978). There has been a tremendous proliferation of EM

emitters permeating the air space. While ~pical electric field strengths

E rarely exceed levels of one volt per meter, there are "brute-force" situations where increases into the kilovolt per meter range might endanger

electronic control devices (fuel injection, ignition, anti-skid brakes, etc.).

For example, metal oxide semiconductor (MOS) high-density circuits used as

processors in those functions are susceptible to disruptions in the logic

when E > 2 Vim and can be destroyed by fields E > 200 Vim (Dicken, 1978).

To enable engineering predictions of the ambient fi,elds, we have surveyed

strong sources of EM energy across the radio frequency portion of the spectrum

(from dc up to 306Hz) and estimated the order of magnitude for the maximum

field strength levels to be expected, in particular when approaching at road

level the radiation source as close as possible (R > 3 m).

Strong EM radio sources are legion and fall broadly into five categories:

1. Broadcast radiation (AM, FM, TV and Mobile).

2. Microwave satellite communication and energy beaming.

3. Microwave radar (pulsed fields).

4. High-Voltage overhead power transmission line .

.5. Transients (radio flash) due to lightning and EMP from nuclear

explosions.

* The authors are with the u.s. Department of Commerce, National Telecommunications and Information Administration, Institute for Telecommunication

Sciences, Boulder, Colorado 80303.
Each category has its own set of specific variables. However, the

more intriguing question: I'how does the ambient field couple into on-board

devices?lI, is not addressed.

2. GENERAL CONSIDERATIONS

AOn electric field E ;s generated and moves outward from an emitter

where charges are accelerated. The field diminishes inversely proportional

to distance (lJR) outside the sphere of direct influence from the charges.

The fields examined are assumed "free" meaning they are not modified by

reflecting objects, nearby scatters, or the vehicle itself. Knowing the

radiated power P (kW) emitted from a point source in a given direction

yields at a distance R (m) that (note the mixed units, kW and m)

E = 173 P/R (V 1m). (1)

The ~adiated power P in (1) is equal to the transmitter power Pt if energy

is radiated uniformly in all directions. An antenna always has directivity

in its energy distribution. A power factor G is introduced to indicate the

gain over the isotropic radiator. The effective isotropic radiated power

(EIRP) in main beam direction is

P = G Pt ( kW) · (2)

Microwave transmitters with EIRP's as high as 3-107 kW are in operation

(Hankin, 1974).

The electric field intensity IEI 2can be converted into an equivalent

plane wave power density S(W/m2) or electric field energy density U(nJ/m3) by

E2 = 377 S = 226 U (3)

All three exposure units [(V/m)2, (W/m2)=O.1 (mW/cm2), or (nJ/m3)] are in

use.

Routine environmental field monotoring at a point x, y, z in space

is generally done with an isotropic probe yielding the RMS magnitude,

E

2 = E2 + E2 + E2 (4) x y z

·which includes the contribution of all wave polarization components (Larsen

and Shafer, 1977).

2
Motor vehicles are moving in and out of different EM environments.

The problem in using the simple equation (1) lies in finding the maximum

EIRP in the direction of a point located approximately 1.5 m above ground

and approaching the origin of the radiation (e.g., antenna base) as close

as Rz 3 m. The EM field considerations fall broadly into two ranges, the

far and the near field. Far-field conditions are relatively easy to analyze

and measure; the field quantities E, S, U have the simple interrelationship

(3). Experience is needed to recognize reflections from other objects inc~uding the ground that can add to or subtract from the primary signal.

The near field is most important in terms of IIhazardous" consequences.

It is characterized by interactions with the emitter and displays a complicated

structure including reactive (stored) and real (radiated) energies, multipath

reflections, irregular phase surfaces and unknown polarizations. The waves

have not formed into a pattern making measurements exceedingly difficult.

Far-field instruments do not achieve meaningful results. Hence, the measured

electric fie,ld strength E is not a measure of the energy density (i .e., (3) is

not valid). If far-field value~ of the EIRP are used at close range, the

field strength of E is generally overestimated. In some cases (see later)

approximations are available to assess the near-field strength Eo. Depending

upon antenna dimensions i'n relationship to the wavelength, the near-field

domain can range from less than one meter from the antenna (mobile radio)

up to several thousand meters (satellite microwave earth terminal).

Several Federal agencies are currently involved in assessing and meas~

uring the EM radiation environment {e.g., EPA, DoT, 000, DoC-NBS, FCC). In

particular, the u.s. Environmental Protection Agency is working to develop

a data bas,e on EM radi ati on exposure for the hi gh densi ty popul ati on centers

(e.g., Tell and Janes, 1976; Tell, 1977; Tell et al~, 1977); and the National

Bureau of Standards has pioneered the development of EM measurement instrumentation, methodology, and refined measurements of EM radiation (e.g., Adams

et al., 1977; Larson and Shafer, 1977). We have drawn heavily upon their

resu'lts fo'r the purpose of thi's overview. In the following, the conditions

for high electric field strength~ are examined more closeJY~ additional

variables are introduced that govern the EIRP in ground level direction; and

the findings are, whenever available, backed up by verified or verifiable

evi.dence.

3
AM

P = 50kW

single monopole

Near Field

0.5MHz

h= 135 m

'\': (7"=30 m mho/m

,

,

,

Monopole over perf.ect /

d · d----' , con uctlng groun "

1.6MHz

h=30m ' u=1mmhalm 1'-------'------...a----+--------...--_.a....-.._...... , 3 10 30 100 300 1000

10 u

'-

+-

u

Q)

W

E

"'

>

W

..c

-+-

(J)

c

Q)

'-

+-

(f)

-0

Q)

i..L

Distance from Monopole R, m

Figure 1. Ground-level electric field strength estimates for a

50 kW AM broadcast station employing a single monopole

antenna. + Measured (Adams et al.,1977); near field

calculations (Tell and Janes, 1976).far field (Berry,

1978). '

4
3. BROADCAST RADIATION

On a per area average, most of the EM radiation results from broadcasting. There qre roughly 104 commercial stations in the U.S. plus an additional 2.107 licensed mobile (CB) transmitters. Maximum broadcast power is

regulated in terms of either the transmitter power P

t (AM, Mobile) or the

EIRP (Po) (FM, TV).

3.1. AM Standard Radio

In the U.S., the following specifications apply: Frequency, f = 0.5 to

1.6 MHz, wavelength, = 600 to 188 m; maximum permissible transmitter power,

Pt = 50 kW - amplitude modulated; number of stations is 4500 of which 135 use

50 kW (Tell and Janes, 1976).

The antenna is a vert,ical monopole but sometimes a phased-array of

several towers is employed to reduce far-range coverage in a particular

direction (Barlett, 1977). A vertically polarized ground-wave signal is

propagated in an initially omni-directional pattern. The earth acts as

ground plane for the antenna. Factors, besides the transmitter power Pt ,

that influence the emitted field strength are the monopole height h (up to

5MB) and the soil conductivity o{A) (10-3 toO.03 mhojm). A program that

uses these three variables to compute the ground-level field strength E for

ranges R > 1 km is. described by Berry (1978). A field strength estimate in

the near field (R < 0.1 km) is obtained from the solution for the hypothetical case of a monopole over perfectly conducting ground.

Figure 1 combines far- and near-field results for a 50 kW station. The

two far-field curves represent the limits for variations of a(f) and h(~)

typi:cal for AM radi'o broadcast condi'ti.ons. A value, E := 23 VIm, is predicted

at ground level for the distance, R = 100 m, from the antenna base decreasing

with, the i:nvers.e of the distance. The near-field IIhalfll of Figure 1 represents electric field strengths for two antenna heights. Equations for field

calculations at the surface of the monopole arfe given by Jordan (1950). The

electric field strength at the antenna base can reach very high values (2 kVjm

for P

t 1 kW).·

5
Based on measurements carried out when R > 1 km, the following gain

factors are typical (Sams, 1977),

G = 1.7 when h = (0.15 to 0.25)A

G = 1.9 when h = (0.25 to 0.40)A (5)

G = 2.3 (max) when h = (0.40 to 0.65)A

to be inserted in (2) to calculate field strengths using (1). A station with

an optimum antenna height(h = 5A/8) and an excellent ground system might

slightly exceed the limits shown in Figure 1. A measurement close to a

50 kW AM antenna (h = 5A/8) yielded at ground level E = 800 Vim when R = 3 m

(Adams et al., 1977). Measurements around 8 and 12 MHz avionic communication

transmitters with 15 kW power showed field hot spots, E = 830 Vim, close

(~'O.5 m) to the feeder point of a rhombic antenna system about 2 m above

ground (Larsen and Shafer, 1977).

3.2. VLF-LF Transmission

Transmissions in the 10 to 300 kHz range are used for special long-range

ground-wave communication and long-range navigation (e.g., pulsed fields of

LORAN C at 100 kHz). The' influence of soil conductivity on propagation is

pronounced. The EM wave penetration into the ground varies between 3 and

150 meters (at 10 kHz). When the tranSmitter power Pt is available, field

strength estimates maybe obtained as discussed under 3.1. and 5.

3.3. FM and TV Stations

Broadcast FM and TV stations operate in the VHF and UHF portions of the

spectrum. They usually employ antennas which radiate in a uniform azimuthal

pattern and concentrate the power into a vertical plane beam. A small degree

of tilt might be used to optimize the coverage. One tower sometimes supports

antenna systems for several TV and/or FM transmitters. The following table

summarizes pertinent station data:

6
174 - 216 470 - 890

1.7 - 1.4 0.63 - 0.34

Frequency Range f, MHz

Wavelength A, m

Max. Permissible

Radiated Power Po' kW

VHF-TV FM-Radio

54 - 88 88 - 108

5.5 - 3.4 3.4 - 2.7

100 vertic.

100 +100 horiz.

VHF-TV

316

UHF-TV

5000

Common

Polarization

No. of Stations in U.S.

horiz.

610

circular

3400

horiz. horiz.

350

Propagation at VHF and UHF is along a line-of-sight path. Antenna

systems are mounted typically between 50 and 350 m high. Soil conductivity

plays little part (except for possible reflections) in determining the field

strength at a ground location. A relative field factor FA ~l is introduced

to account for angle deviations from the horizontal main beam axis due to

the directional pattern of the antenna. A plot of FA is shown in Figure 2

for a typical UHF dipole array. The effective radiated power to be inserted

in (1) becomes

(6)

The influence of the field factor is elucidated in Figure 3. Variations of

field strength with distance are depicted for an UHF transmitter assuming

several vertical levels below the beam center. In this case, main beam

exposure at close range (R < 1 km) is only possible on upper levels of tall,

nearby buildings. FM stations using dual polarization have the largest field

strengths in the vertical plane (up to +5 dB relative to horizontal) (Tell,

1974) .

Worst case field strengths in the main beam of FM and TV broadcasting

stations operating with maximum authorized effective radiated power Po

are calculated with (1) (see Fig. 4). An even more conservative way of

estimating the maximum field strength is to include in (6) a factor two to

account for reflections of the incident wave which lead to a standing wave

field.

Broadcast antennas for FM and VHF-TV have wide beamwidth in the vertical

plane when compared to UHF and in some instances less well controlled

7
'-

0

....

0

,;:J

(1)

iL 0.5

Cl)

>

....

c

Cl)

a::

UHF - TV

0.5 0 beam tilt

powergain G=24

Vertical Pattern

o 5 10

Degrees Down From Horizontal Plane

Figure 2. Vertical radiation pattern of typical UHF TV antenna

(Tell and Janes, 1976).

8
E

100 ... W

£:.

c::

10

(jj

]2

Q)

iL

u

·C

U

Q)

W

0.02 0.1

UHF TV

EIRP 1MW

Antenna -Height 305m

10

Distance from Tower R, km

Figure 3. Electric field strength of UHF TV station (Tell and

Janes, 1976) .

.. W

.s=.

+-

C"

c:

102 +-

CJ)

"'0

Q)

i.i:

u

~.

+-

10

w

10

EIRP:

0.1 MW

0.2

0.32

5.0

100

Distance from Tower R, m

500

Figure 4. Maximum free-space electric field strength from FM

and TV broadcasting (equations 1, 6) for main beam

exposure.

9
Table 1. Measured Electric Near Field Strength at VHF and UHF

Electr. Distance to Effective Transm. Frequency Antenna Service Reference

Field Str. Tower Base Power Power Height

E R Po Pt f

VIm m kW W MHz m

823 a (at dipole) -- 25 123 -- F~1 a

184 a (at dipole) -- 5 278 -- TV a

124 a 100 -- 90.7 28 FM b

80 160 5000 -- 600 -- TV b

38 a 100 -- 103 96 FM b

0 25 1 -- 150 110 -- FM a

10 160 100 -- -- -- FM b

8 200 105 -- 104 36 FM b

6 200 2300 -- 735 54 TV b

4 200 186 -- 199 72 TV b

1 6600 4000 -- UHF 247 TV c

a) Larsen and Shafer, 1977

b) Tell and Janes, 1976

c) Te11, 1974
illumination of the array. The result is that power is lost in grating lobes

directing radiation straight up and down the vertical antenna plane. In

extreme cases, the electric field strength at the base of such antenna tower

can therefore be as intense ,as in the main lobe at a distance equal to the

tower height (Tell and Janes, 1976). Examples of field strength levels in

the immediate vicinity of transmitting antennas are in Table 1.

3.4. Mobile Radio (HF, VHF, UHF)

The following frequency bands are reserved for mobile radio communication:

25-50 MHz, 150-175 MHz, and 406-512 MHz. All three are used with vertically

polarized radiation and relatively low transmitter power (Pt 110 W). As a

short-range emitter, a mobile radio can generate potentially hazardous fields.

There ha vie been reported inc idences where the fue1 i nj ecti on of a passer-by

car was blocked or, even more serious, where blasting caps for dynamite

were triggered at highway construction sites or munition was detonated accidentally by stray EM radiation. Since the antenna is located on the vehicle

the concept of a "free" EM field is useless; the interaction with the carls

body determines the EM radiation environment. Such a complicated near-field

structure calls for measurements.

Results for the electric field distribution around cars are reported by

Adams et ale (1977). The measurements were made with all common combinations

of mobile transmitters and antennas. The maximum legal transmitter power

(110 W) was used at the test frequencies 40, 162, 416 MHz; and at the citizen

band frequency of 27 MHz, the power was boosted to 100 Wwith a special

authorization .. Electric field strengths at about 0.3 mdistance from vehicles

with on-board transmitters ranged mostly between 10 and 300 Vim with an

occasional hot spot reaching 500 Vim. The field strengths around cars parked

adjacent to a mobile transmitter were, as expected, somewhat lower in the

1 to 100 Vim range. In about 2m distance from a free-standing car, the

values had dropped to roughly one tenth of the 0.3 m value. The field

strengths were measured on concrete or asphalt roads and som~ values doubled

over metal surfaces due to reflections (e.g., on bridges).

11
4. MICROWAVE SATELLITE COMMUNICATION AND ENERGY BEAMING

Under this heading fall the hig~ gain (G > 104) antennas which, in

combination with kilowatt transmitter powers Pt , are the strongest EM continuous wave (CW) field sources. In 1976, there were about 250 stations on record

in the U.S. emitting in main beam direction EIRP's of larger than one megawatt

and operating in several frequency bands between 0.2 and> 30 GHz as satellite

communication earth terminals. High ground-level field strengths become a

possibility when low elevation angles are used in tracking satellites. Although,

international regulations limit the minimum elevation angle to 3 degrees and

the maximum EIRP in the direction of the horizon to 2.5 mW/Hz bandwidth,

only to be exceeded in special cases (e.g., deep space research) by no more

than a factor of 30 (ITU Radio Regulations 470). The bandwidth for maximum

power distribution is limited to 4 kHz for frequencies 1 to 15 GHz and to a

1 MHz band above 15 GHz.

Specifications for three typical earth terminals operating in the 7.9 to

8.4 GHz band and employing circular parabolic reflector antennas are as

follows (Hankin, 1974):

Transm. Power Main Dish Diameter

P

t (kW) D (m)

2~5 4.5

4 5.5

4 18

Gain

G

6.3-104

1.6-105

1.3-106

Beam Width Aperture Efficiency

(degree) n (%)

0.6 50

0.5 75

0.1 50

The near field range,

R < R z 0.2D2/A

- 0 (m) , (7)

is considerably spread out for large-aperture antennas. The maximum on-axis

electric field strength is approximately given by

Eoz 1.4e 1Q3"nP

t (kW)'/D(m) (V/m), (8)

a constant across the diameter 0 and over the range Ro. This assumes

(i) that the near-field power flux density in front of a highly directional

antenna is obtained by dividing the available power at the feed point by onefourth of the cross-sectional area of the circular antenna (Hankin, 1974) and

(ii) that (3) holds. Various illumination problems force a reduction of the

12
1

l.L«

'-- 10-1 o = 12m

0

+-

u

-c 10-2

Q)

l.L

Q)

> "

"'- ..... "-

0 10-3 j"-- Q) ....... -- 0::: CCIR Rec 465

90 45 o

Elevation Angle ,8, degree

Figure 5. Measured sidelobe structure of a D = 12 m reflector

antenna operating around 4GHz.

13
theoretical optimum described by the aperture efficiency n (typically between

0.5 and 0.9). Between the near-field limit Ro and the far-field limit,

2 R Rl z 0.350 /A (m) , (9)

one assumes an intermediate range with a linear drop of the power density

with distance yielding

(10)

In the far field, the wave form is well defined (plane wave behaviur perpen-

---dicular to direct;'on of propagation) and the normal l/R -dependence holds

expressed as

(11 )

Predictions for the near field based upon (7) and (8) have been confirmed

by measurements in the vicinity of the three satellite earth terminals given

as examples above. For the D = 18 m system~ the calculated on-axis field

strength is Eo = 110 Vim and the near-field range extends up to Ro 1600 m.

The measured values at R = 18 and 100 mwere E = 91 and 106 Vim, respectively.

The relative field factor FA for a D = 12 m reflector antenna is depicted

in Figure 5. The sidelobes are well above CCIR recommended levels causing

higher field strengths in the horizontal plane even at more common elevation

angles (8 > 10°).

Solar power satellites (SPS) are emerging as a seriously considered

source for IIcleanll energy. There may be many (> 100) in orbit within the

next thirty years. It is, proposed to capture in geostationary orbit

electrical energy from the sun and, after conversion, to beam it at 2.5 GHz

to a dipole array (Dz 3km) located in an area of exclusion. The array is

projected to be capable of extracting as much as five gigawatts of power

from an on-axis power flux, S = 230 W/m2 (Nalos, 1978). At the perimeter,

the power density would fall-off to "safe" levels (~2 W/m2). The maximum

power density is about twice the IIsafe ll U.S. biological limit for microwave

exposure (100 W/m2). One might consider driving service cars for short time

periods under full exposure within the antenna field.

14
5. MICROWAVE RADARS

In the vicinity of airports and military installations, high-power

(Pt > 5 kW) radars are a major source of EM radiation. The radar field is

I') 4 pulsed, concentrated by a high-gain (G = 10~ - 10 ) antenna into a narrow

beam, and on ~he average reduced by the antenna rotation. In field strength

calculation two cases need to be distinguished, (i) the peak value E

pk

present for the duration T of the radar pulse and (ii) the average value E

av

being the time-averaged mean over pulse duty cycle and antenna rotation.

The relationship between average and peak power density is defined by a

duty factor

·22 d = Sav/Spk = Eav/Epk = Pav/Ppk = fpT (12)

where f p is the pulse repetition rate and T the pulse width. A rotational

reduction factor p relates the point averagel

to on-axis power density; e.g.,

p = {3 dB horizontal beam width)/360° (13)

for a fully rotating antenna. Hence, the radiated power to be inserted in (1)

is given by

P

pk = 2 P

av

2 P

t FA G Pt/d or = FA pG (14 a,b)

Experimental high field strength data measured in the vicinity of eight

radars (technical specifications, see Table 2) are enumerated in Table 3.

The values are selected from a larger body of data obtained from surveys at

radar sites (Tell and Nelson, 1974; Larsen and Shafer, 1977). The on-axis

(FA = 1) radiated peak pulse power can ~ach levels as high as Ppk =7 GW

(radar #1 in Table 2) causing at the far-field limit R, = 230 meters a field

strength of E z 2 kV/m. The example shown in Figure 6 is for a radar with

a re~ativelY small aperture (D = 0.5m - slot array) and the l/R-dependence

of E (Eqs. 1 and 14a) holds to close range (R > 7m). Under normal conditions

the measured off-axis behavior is of concern~ The significant (~ -10 dB)

spectral field components of a radar pulse train are spread typically over

a ± 10 to 30 MHz interval around the center frequency (Tell and Nelson,

1974} .

As pointed out in the previous section, the calculation of field strengths

in the near-zone range (7) is complicated especially for the case of largeaperture antennas. A more rigorous approach to the near-field problem has

15
.04 .08 ~12 .J6 .20

Inverse Distance I/R,l/m

E

>

"

.::s::.

a.

W

.c tOO -+-

Ol

C

(1)

,...

en

-0

(1)

LL 50

u

.~

-+-

u

(1)

W

o

OJ

Q..

Radar #5

/

Ie

Main Beam

,// at 2.6 m height

Off-Axis

at 1.7m height

Figure 6. Measured on-axis and off-axis electric field strength from lOa to 5 m approaching radar #5 (Table 2) (Larsen

and Shafer, 1977).

16
0::: Radar **1 """-"

(/)

\-

0.....

u

Lf

c

0

+=U

:J

'"'0

OJ

cr:

00 100 200 300

Distance R, m

Figure 7. Computer near-field gain reduction factor for uniform

aperture illumination from 300 to 1 m approaching radar

#1 (Table 2) (Larsen and Shafer, 1977)

Radar #8 2r----~----------.

300

\

\ I

\

10 30 100

Distance R, m

Ro=30m

A=0.06m

>

c

W

.c.

.....

Ol

c:

Ci5

0.5

LL

OJ

g 7]=0.8

OJ

0.2 3

Figure 8. Measured on-axis electric near field strength from

300 to 3 m approaching radar #8 (Table 2) (Glaser

and Heimer, 1971) ..

17
Table 2. Microwave Radar Specification

1-----------------------Transmi tter-------------------------j- ---------------------Antenna------------ - -- --- -..--

FreQ. Power Pulse Width Rep. Rate Duty Factor Rotation Bealll t~ i dth Center

10# Type Rate Gain Diameter 3dB Height

(See Table 3) f p Ppk Pt L f d G D horizontal h L

GHz kW W llS Hz x10-4rpm dB m degrees m

Air Route ARSR-IO 1.34 4000 2880 2 360 7.2 6 34 12 1.4 24

Surveillance

{ARS-4B 2.72 425\ 403 0.83 1140 9.5 15 34 5.5 1.4 22.5 Ai r Port

Survei1- ASR-7 2.82 425 336 0.83 950 7.9 13 34 5.5 1.4 8

lance

4 ASR-8 2.7 1160 725 1.4 34 5.5 1.4 15

00

0.5 5 (Fig.6) Aircraft {VQ-1O 5.4 75 60 2 400 8 Fixed 2.6

6 Nose-Cone ROR-IB 9.35 40 24 1.5 400 6 Fixed 0.5 3.9

Ai rport ASDE-2 23.9 49 14 0.02 14000 2.9 60 45 1 0.25 30

Q(Fig.8) Mil i tary AN-SPG-99 4-6 - 6000 - - - - - 3 - 10

(C-band)
Table 3. Selected maximum electric field strength data measured in

the vicinity of ground-based radars (ID # see Table 2).

Elect. Field Distance Test point location Antenna Radar See Reference

Strength to center above ground Rotating 1D Figure

Epk E R # av

Vim Vim m

1200 20 on-axis stopped 8 8 a

9700 238 2.3 II

- 6 b

2740 67 9.1 1.8 m, off-axis - 6 b

1700 52 84 on-axis yes 2 b

1060 1 II yes 2,3,4 b

549 0.05 II stopped 7 b

412 4 II

J-

\I~,oJI 2 b

250 7.8 260 1.7 m, on road stopped 3 c'

194 4.6 1.7 m - 5 6 b

194 3 1.7 m - 6 b

173 0.5 on-axis stopped 7 b

83 2.1 260 1.7 m, on road yes 2 c

12 0.6 260 II II , yes 1- c

a) Glaser and Heimer, 1971

b) Larsen and Shafer, 1977

c) Tell and Nelson, 1974
been taken by postulating that the full gain G is only realized in the far

field (9). ,A theoretical reduction factor s(R) for (14) is formulated based

upon the Ilknown ll aperture illumination. The derivation involves inserting

the illumination dist~ibution into the power-densi~ field equation and

solving for the on-axis field intensity (Larsen and Shafer, 1977). Figure 7

gives an example for s(R); however, the difficulty, besides considerable

mathematical effort, lies in the uncertain knowledge of the illumination

function. The measured electric field strength close to a powerful radar

(see Figure 8) provides some quantitative data for on-axis - versus - distance

response. The near-field calculation using (7) to (11) seems to provide

reasonable estimates.

6. OVERHEAD POWER TRANSMISSION LINE

Extreme high-voltage (EHV) power lines above 300 kV are a source of high

field strength when approaching their right-of-way zone. The principal EM

environmental effects are (i) the electric field intensity near the ground

causing induced voltages on objects positioned underneath the conductors,

and (ii) high conductor surface fields producing weather-dependent corona

phenomena such as radio noise. Some specifications for three-phase high

voltage transmission lines in the u.S. are (Tell et al., 1977):

Transmission Voltage

Frequency

Min. Conductor Height Above Ground

V(kV)

f(Hz)

ho(m)

765

60

21-12

500

60

18-10 .

When the complete electrical and geometric characteristics of a line

are provided, the ground-level electric field can be calculated with the

fundamental field equation,

E = 1.80 • 107 CV/r (VIm) , (15)

where C is the capacitance per unit length to ground in farads per meter, V

is the voltage impressed on conductor in kilovolts, and r is the distance

from the charge CV in meters. An example of computed ground-level field

strength is plotted in Figure 9 versus the distance R from the center of the

transmission tower. The peak values of E for the 765 kV case vary from 12 to

5 kV/m underneath the outer conductor depending on the clearance height hoe

20
Measurement, ho=14m

765 kV Line

3 Phases

E

:>

.:::c:.

.. W

..t::.

+-

0'

C

<V

'-+-

Cf)

-c

-

<V

iJ...

u

'-

.....

U

<V

W

•• • ..

10

10

5

• • • •

o

Ground

.¥ Conductor

..,J( Loci --r • • •

h..1"o

I j I

10 20 30 40 50

Distance from Tower Center R, m

Figure 9. Calculated electric field strength profile at ground

level (1 m) for 765 kV line (Te11 eta1., 1977 ).

21
The field strength has dropped to about 1 kV/m when R = 50 m and falls off

from this point as l/R reaching 0.1 kV/m at R 100 m. More details and

further calculations for other line types are found in two publications

(Tell et al., 1977; Stewart and Wilson, 1977).

Measurements agreed well with calculations based upon (15) (Tell at al.,

1977). The maximum field strength for a 765 kV line was measured to be

9.5 kV/m at one meter aboveground occurring just outside the outer conductor

at R = 16 meters (ho = 14 m). That field s~rength induced short-circuit

currents of 0.5 to 2 rnA from limousine-type cars to ground. The vehicle's

presence increased the field strength at 0.3 m above the car's roof from

10 to 16 kV/m.

Secondary variables influencing E are conductor motion due to wind, high

load surges (> 5 GW), and shedding of ice (Stewart and Wilson, 1977). The

radio noise due to conductor corona effects is broadband (~30 Hz to 50 MHz)

at much lower field strengths « 0.1 VIm) and is a complex function of conductor size, conductor bundling, surface conditions, insulator aging, and

meteorological parameters.

7. LIGHTNING AND NUCLEAR EM RADIATION (EMP)

Lightning discharges and nuclear explosions emit strong transitory

fields over a broad frequency band although little energy is contained in

them compared to CW or pulsed fields. Transients are best described bY Joule

energy content since average power flow has no meaning for a single pulse.

The electric field can be calculated if the temporal and spatial distributions

of the charges (i.e., currents) at the source are known. A lightning flash

involves a multiplicity of sparks; a nuclear electromagnetic pulse (EMP) is

one single event producing all EM field emissions almost simultaneously. The

time hi.story typical to both transi:ents is roughly described by the following

pulse:

1.0

E/E · o ..

o.~~

o t l t 2

Time t

t

1 (]ls)

t

2 (]ls)

Eo (kV1m)

22

Lightning

0.1

200-1000

17 a)

EMP

0.01 - 0.02

0.05 - 0.5

20 - 80 b)

(16 )
a) Measured maximum field strength Eo at the center of a return stroke from a

cloud-to-ground lightning flash (Cianos and Pierce, 1972).

b) Estimated maximum electric field strength in the source region of a nearsurface burst (60-80 kV/m), as well as maximum horizontal component of a

high altitude EMP (20-50 kV/m) at ground level (Longmire, 1978).

The different time scales for lightning and EMP place the peaks of the occurring broadband emission spectra around 5 kHz (Cianos and Pierce, 1972) and

10 MHz (Ricketts, 1976), respectively. The EMP spectrum occupies AM radio

to UHF TV frequency bands (see Figure 12).

The EM fields due to lightning are a common threat to vehicular

electronics. There are about 105 lightning flashes per day in the U.S. A

typical levent consists of a fast upward-going; leader and several powerful

return strokes (peak current 50 - 200kA). The total charge exchange is on

the order of 20 to 50 coulombs. Lightning is initiated when the fair weather

electrostatic field of about 100 Vim is raised by charged clouds to the

breakdown point of air, which is given approximately by (Sams, 1977)

E z 3.l06(p/1010)(298/T) 00 (Vim) , (17 )

wherep is the pressure in millibars and T the temperature in degrees Kelvin.

The fields generated by lightning are different for each of .the various

stages in a flash and very complex in many respects (Levine and Meneghini,

1978; Cianos and Pierce, 1972). In general, near- and far-field components

have to be separated. Far fields are radiated from the conducting channel

between a cloud and ground (average length is 5 km), and their spectral distribution was measured as shown in Figure 10. Scaling of the envelope

inversely with distance R from the center is appropriate when R 10 km.

The static and inductive fields at close distance to a flash in general

are very large when compared with the radiated component. These fields

are a function of the interplay between the charge drawn from the cloud, the

charge deposited along the channel (on the order of 1 C/km), and the redistribution of charge during various current stages in the flash. Electrostatic

fields of 200 kV/m have been observed close (R < 100 m) to flashes to occur

transiently just before the leader makes contact with the ground. The steady

fi.eld be.tween ground and cloud is limited by screening space charges due to

23
E

"'-

> 10

0..

W

.c.

+-0\

c

+0-

10 (f) 1

"0

Q)

ii:

.~ 102 \-

+-.

U

Q)

w

103

0

Q)

0...

10 10 2

Frequency,kHz

R= 10 km

• I • I

Figure 10. Peak field strength for radiated lightning emission

received with 1 kHz bandwidth (Cianos and Pierce, 1972).

>..:::&:. 15 ,..

W

<l

Q)

C'

C

0

.c.

U

''"0

Q)

i.L

u

+-u

Q)

W

10 20

Distance R, km

Figure 11. Change in static field strength versus distance for a

lightning flash (charge exchange 39C) to ground (cloud

height 6.4 km) (Cianos and Pierce, 1972).

24
corona to 10 to 20 kV/m (Cianos and Pierce, 1972). The measured electrostatic field change for a typical flash is depicted as a function of d;-stance

i n Figure' 11.

The nuclear pulse (EMP). fields within a surface burst of a nuclear

explosion are probably not of great interest because of other strong destructive forces. The source region is hemispherically-shaped several kilometers

in diameter. Beyond this burst center potentially degrading EMP fields are

radiated several tens of kilometers decreasing as l/R.

A high-altitude burst presents a different picture with the resulting

EMP the only prompt effect on the ground covering a large area. When the

height of the burst is taken as 300 km, the distance to the horizon is

2000 km and the large-amplitude fields cover geographical regions of that

tangential radius. The source region for the EMP lies in the altitude range

20 to 40 km where the gamma rays emitted from the burst begin to interact

with the atmosphere thus generating the EMP. Representative pulse 'shapes

similar to the one given in (16) are given by Ricketts (1976) with peak

amplitudes ranging between 20 and 50 kV/m.

7. CONCLUSIONS

Auto makers are fearful that future investment in electronic control

systems could be compromised by ambient EM fields. In the preceeding

sections, order of magnitude estimates of maximum electric field strengths

have been made that might be expected from d.c. up to the microwave range

in the motor vehicle environment. Various expressions relating field strength

to distance from the source, transmitter power, antenna characteristics, etc.

are given; more elaborate modelling in general is available (see references);

and a few supporting measurements have been reported. Extremes, important

with respect to assessing possible electronic component degradations, are

summarized in Figure 12. For all common failure modes (e.g., thermal overload, burn out, dielectric breakdown, malfunction, bit error and dropout,

etc.), the strength E should be a realistic indicator of direct and indirect

degradation energies. However, the conversion of the ambient E field into

meaningful currents and voltages at the on-board electronics level has been

a major difficulty and more research is needed to assure safe operation and,

at the same time, avoid costly overdesign.

25
\-

t3

10

DC (M)

104 106

Frequency f, Hz

0.1

a) Static Electric Field

1. Thunder cloud-to-ground, Fig. 11

2. Fair weather

b) EHV Power Line

Fig. 9, R = 16 m

c) Radiated Lightning Spectrum

1. R -+ 0, center

2• R = 1 km , Fig. 10

d) EMP Spectrum

1. Surface

2. High altitude

e) AM Radio

Fig. 1,R = 3 m

f) Mobile

g) FM Radio

Fig. 3, Table 1

h) TV

Fig. 4, Table

i) Radar

1. Table 3

2. Fig. 8

k) Satellite Earth Station

1) Safe U.S. Biological Exposure

Level for Microwaves

m) Dustfree Dry Air Breakdown6 V'oltage at Sea Level (3-10 Vim)

Figure 12. Schematic overview of possible maximum electric field strengths

E (power density S) from 0 to 100 GHz in the automobile environmente

26
The highest field strengths (see Figure 12) are produced by transients

due to the EMP from nuclear explosions or from a very close lightning

stroke. In both cases, the peak values come near the breakdown field

strength (17) of the surrounding air. Semiconductor devices (diodes, Ie·s)

especially are targets for disruptions since they display a relative

"softness" to transients. Matters will get worse if, as planned, further

miniaturization reduces the activation current for a flip-flop from 10-12

to 10-16A. Magnetic field components at low frequencies ( zlO Hz) are

impossible to shield, and they induce currents of that order in even the

smallest wire loop. These fields probably set a natural limit to what degree

miniturization can be pushed. The present degradation energy falls in the

1 to 10 microjoule per microsecond range. Most other electronic components

can tolerate transient fields far above their rated maximum allowable CW

level~ a fact to be considered in cost-effective EM ruggedization measures.

The next worst condition is under an EHV power transmission line. The quasistatic 60 Hz electric field is effectively shielded by the vehicle1s body

from penetrating the inside, but the strong magnetic field under full load

of the line can induce internal currents. The maximum electric field

strengths produced by AM, FM, TV transmitters, high-power radars and satellite

earth terminals are, at the most, on the order of kilovolts per meter near

the ground. These fields are essentially continuous. Should they degrade

the performance of the electronics without destroying it, it would mean that

the vehicle malfunctions until removed from the field.

Instances in which vehicles will be exposed to the highest field

strengths will be extremely rare. Nevertheless, if modern automobiles with

elaborate electronic controls are to be as reliable as their non-electronic

predecessors they should be designed to operate under the field conditions

displayed in Figure 12. Less conservative design, which might cause a

vehicle to become inoperative from EMP and near lightning strokes, requires

hardening (i.e., shielding) measures to withstand ambient fields on the

order of 10 kV/m at 60 Hz and about 5 kV/m at frequencies above 0.5 MHz. It

is worth conjecturing that if public policy dictates that vehicles should be

able to withstand the fields from a high-altitude nuclear explosion (on the

order of 20 kV/m), they will probably be able to tolerate any man-made EM

field that is likely to occur betweendc and microwave frequencies.

27
9. REFERENCES

Adams, J.W., M. Kanda, and J. Shafer (1977), Near field electric field

strength levels of EM environments applicable to automotive systems,

Proc, IEEE Internat. Symp. on EMC, seattle, Wash., 336-343, August.

Barlett, G. W., editor (1977), NAB engineering handbook (Chapter 7), Nat.

Assoc, Broadcasters. 1771 N. Street, N. W., Washington, D. C. 20036.

Berry, L. A. (1978), User1s guide to low frequency radio coverage programs,

OT Technical Memorandum 78-247, January.

CCIR (1975), "propagation in nonionised media; Vol. V," XIII1th Plenary

Assembly, ITU Geneva.

Cianos, N., and E. T. Pierce (1972), A ground-lightning environment for

engineering use," Technical Report l-SIR 1834, Stanford Res. Inst.

August.

Dicken, H. K. (1978), Integrated circuit failure modes, EW/DE, 66-68,

October.

Glaser, z. R~, and G. M. Heimen (1971), Determination and elimination of

hazardous microwave fields aboard naval ships, IEEE Trans. MTT-19(2),

232-237.

Hankin, N. N. (1974), An evaluation of selected satellite communication

systems as sources of environmental microwave radiation, EPA Report

520/2-4-008, December.

Jordan, E. C. (1950), Electromagnetic waves and radiating systems, PrenticeHall Inc., Englewood Cliffs, N. J.

Jurgen, R. K. (1978), For Detroit: smaller and smarter ships, IEEE Spectrum,

33-35, November.

Larsen, E. B., andJ. E. Shafer (1977), "Surveys of EM field intensities

near high-power FAA transmitting antennas," FAA Report RD-77-179,

December.

Levine, D. M., and R. Meneghini (1978), Simulation of radiation from

lightning return strokes, Radio Science 1l(5), 801-809.

Longmire, C. L. (1978), On the EMP produced by nuclear expolsions, IEEE

Trans. AP-26(1), 3-13, January.

Nalos, E. J. (1978), New developments in EM energy beaming, Proc. IEEE

66(3), 276-288.

28
Ricketts, L. W., J. E. Bridges, and J. Miletta (1976), EMP rarliation and

protective techniques, J. Wiley &Sons, N. Y.

Sams, H. S.~ editor (1977)~ Reference data for radio engineer3, Sams &Co.,

Inc., ITT, Indianapolis.

Stewart, J. R., and D. D. Wilson (1977), EM environmental aspects of new

high voltage electric power transmission technologies, Proc, IEEE

Internat. Symp. on EMC, seattle, Wash., 249-253, August.

Tell, R. A., and J. C. Nelson (1974), RF pulse spectral measurements in the

vicinity of several air traffic control radars, EPA Report 520/1-74-005,

May.

Tell, R. A. (1974), Signal polarizations in the VHF and UHF broadcast

spectrum, IEEE Trans. BC-20(4), 73-76.

Tell, R. A., and D. E. Janes (1976), Broadcast radiation: A second look, in

Biological Effects of EM Waves (C. Johnson and M. Shore, editors),

USDHEW Publication 77-8011, Rockville, MD; II, 363-388, December.

Tell, R. A. (1977), An analysis of radar exposure in the San Francisco area,

EPA Technical Note ORD/EAD 77-3, March.

Tell, R. A., J. C. Nelson, D. 1. Lambdin (1977), An examination of electric

fields under EHV overhead power transmission lines, EPA Report

520/2-76-008, April.

29

FORM OT-29 (3-73)

u.s. DEPARTMENT OF COMMERCE OFFICE OF TELECOMMUNtCATIONS

BIBLIOGRAPHIC DATA SHEET

1. PUBLICATION OR REPORT NO.2. Gov't Accession No.3.. Recipient's Accession No.

NTIA Report 79-16

4. TITLE AND SUBTITLE

ESTIMATES OF MINIMUM ELECTRIC FIELD STRENGTHS

IN THE AUTOMOBILE ENVIRONMENT

5. Publication Date

February, 1979

6. Performing Organization Code

NTIA/ITS

7. AUTHOR(S) 9. Project/Task/Work Unit No.

H. J. Li.ebe and D. D. Crombie

8. PERFORMING ORGANIZATION NAME AND ADDRESS 9101900

U.S. Department of Commerce

National Telecommunications & Information Administration 10~ Contract/Grant No.

Institute for Telecommunication Sciences

Boulder~ CO 80303

11. Sponsoring Organization Name and Address

14. SUPPLEMENTARY NOTES

12. Type of Report and Period Covered

13.

15. A!3S!RACT (A 2?O-word or less factual summary of most significant information. If document includes a significant

bIbIzography O't lIterature survey, mention it here.)

Strong electromagnetic (EM) energy sources up into the microwave range

are examined and estimates are made of maximum EM: field conditions to which

automobiles could be exposed. The results are meant to alert automotive

engineers to potentially hazardous EM radiation that might. upset on-board

electronic control devices.

16. Key Words (Alphabetical order, sepa.rated by semicolons)

Ambiellt free field conditions, electromagnetic environment, EM degradation,

field strength maxima, strong EM field sources

17. AVAILABILITY STATEMENT

[Xl UNLIMITED.

o FOR OfFICIAL DISTRIBUTION.

18. Security Class (This report)

None

19. Security Class (This paj5e)

None

20. Number of pages

29

21. Price:

*,u.s. GOVERNMENT PRINTING OFFICE: 1979-0-679-128/370 L'SCOMM-OC 29716-P73