United States
Environmental Protection
Agency
Office of
Radiation Programs
Washington, D.C. 20460
ORP/EAD-76-2
January 1976
Radiation
Technical Note
A Measurement of
RF Field Intensities
in the Immediate Vicinity of
an FM Broadcast
Station Antenna
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Technical Note
ORP/EAD-76-2
"A MEASUREMENT OF RF FIELD INTENSITIES IN THE IMMEDIATE
VICINITY OF AN FM BROADCAST STATION ANTENNA
R. A. Tell
January 8, 1976
U.S. Environmental Protection Agency
Office of Radiation Programs
Electromagnetic Radiation Analysis Branch
9100 Brookville Road
Silver Spring, Maryland 20910 .
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PREFACE
The Office of Radiation Programs of the Environmental Protection Agency
carries out a national program designed to evaluate population exposure to
ionizing and noniohizing radiation, and to promote the controls necessary to
protect the public health and safety;*. This report gives the results of a
study of electric-field energy density on the antenna tower of a frequency-
modulated (FM) broadcast station. Readers of this report are encouraged to
inform the Office of Radiation Programs of any omissions or errors.
?loyd L. Galpin, Director
Environmental Analysis
Division (AW-461)
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A MEASUREMENT OF RF FIELD INTENSITIES IN THE IMMEDIATE
VICINITY OF AN FM BROADCAST STATION ANTENNA
INTRODUCTION
In a recent study of,-broadcast radiation levels, measured values .of ,the
radiation intensity on an FM broadcast tower were obtained. The measured
values could lead to exposures in excess of established standards and suggest
the need for corrective action to protect operating and maintenance personnel
who must climb these-towers. • ' '
As of January 1975, there were 4,434 AM radio stations, 3,373 FM radio
stations, and 953 TV stations operating in the United States [1]. A number ,
of .activities require work on broadcast towers including painting, beacon
replacement, repairs to de-icing equipment, antenna adjustment, and tower
rigging and replacement. It is common practice for this tower work to be
done while the broadcast station is operating at full power. No attempt has
been made to quantitate the duration of exposure associated with the tasks
enumerated above, but exposure times are significant for some of them, i.e.,
greater than one hour. The size of the exposed group is also unknown. In
some cases maintenance is performed by station personnel, on others the work
Is performed by a contractor whose organization may service many towers. ,
Though experimental and theoretical values for radiofrequency exposure
levels in the general vicinity and at the tower base of FM and TV .broadcast
antennas have been published [2-4], measured values of radiofrequency levels
in and on the towers supporting broadcast antennas are not available. But
undocumented reports by tower maintenance personnel of the sensation of
warmth when climbing energized broadcast towers indicate the possibility, of
intense radiofrequency fields near the antennas radiating structures. The
published values for heating sensation [5] exceed the current Occupational
Safety and Health Administration (OSHA) radiofrequency exposure standard of
10 mW/cm2 [6]. This is to say that current standards applicable to this
situation are established at levels below the threshold of heat sensation.
The threshold data for heating are for higher frequencies than the FM band,
i.e., for 3,000 and 10,000 MHz, but are pertinent at the lower frequencies
since there is recent evidence that man's resonant absorbtion frequency may
occur in the vicinity of 80 MHz [7,8].
SOURCE DESCRIPTION
The measurements reported here were obtained as part of a larger study
of ground level environmental radiofrequency radiation levels on Mt. Wilson
conducted in cooperation with the Los Angeles County Department of Health
Services. The results of the ground level measurements are reported else-;
where [9]. Located in close proximity on Mt. Wilson are 27 broadcast .-
stations (12 FM radio stations and 15 television stations) serving the
greater Los Angeles area. The measurements reported here were made November
20, 1975, on a tower supporting a single FM station. The station transmits
24 hours a day with 105 kilowatts (kW) of effective radiated power (ERP) in
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both the horizontal and vertical planes using circular polarization. FM
broadcast stations are authorized a maximum ERP of 100 kW in each plane by
the FCC. Certain stations, such as this one which were licensed before
September 10, 1972, may continue operation at their originally authorized
power. A total transmitter power of 40 kW is fed to a Jampro model JSCP-6,
6 bay, circularly polarized antenna which is mounted to the side of a pole
which is in turn mounted to the side of the tower. The tower is 120 feet
high and the antenna center of radiation is about 80 feet above ground. The
tower is of the self standing type with an interior ladder for convenience
in climbing. The Jampro antenna provides a gain of 7.17 dB relative to an
isotropic antenna. Figure 1 is a photograph of several FM and TV broadcast
towers on Mt. Wilson and shows the measured exposure data for the tower under
investigation on the left. Three other FM transmitting antennas are shown
with each bay circled for clarity.
MEASUREMENTS
Measurements were made with an electric field energy density meter,
model EDM-3, designed and developed by the National Bureau of Standards in
Boulder, Colorado. This instrument consists of a small, orthogonally
arranged group of three dipolar elements, with a detecting diode at each
element's center. The detected signal, consisting of a dc voltage, is fed
to the electronic readout circuitry thru very high resistance, semi-
conducting lines. The active probe is at the end of a 1.2 meter long wand
which is used as a handle and the overall probe unit has the desirable
properties of not perturbing the field in which it is immersed and is
isotropic in response, i.e. , it measures energy density independent of its
orientation in the field. The meter is calibrated to give electric field
energy density, UE, in units of microjoules per cubic meter (yJ/m3). For
conversion of the meter readings to equivalent, far field power density, S,
the following relation is used [10]:
S (mW/cm2) - 60.0 UE (yJ/m3) (1)
Alternatively, the near-field electric field strength squared E2 in units of
volts squared per meter squared (V2/m2) is expressed as:
2xlO~6 UE (yJ/m3)
eo is the permittivity of free space and is equal to 8.854xlO~12 farads/meter
(F/m) . The units of V2/m2 are used as another method of quantifying the
exposure in the near field as provided for in the American National Standards
Institute (ANSI) standard [11] and this unit is related directly to the
measured electric field energy density without assumptions about the relation-
ship between the E and H field components. The values of V2/m2 provide a
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I
.05-1
3
3
3
3
3
2
1
Is 2 BAY
I vl -
Figure 1. FM and TV Broadcast Towers on Mt. Wilson Showing Measured Values of
Radiation Exposure (Energy Density in yJ/m3) on Tower at Left. Three Other
FM Antennas Are Also Shown with the Number of Bays Shown for Each. Equivalent
Far Field Power Density (mW/cm2) Is Obtained By Multiplying Indicated Energy
Density Values By 60.
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measure of exposure only from the electric field and provide no insight to
the magnetic field exposure. Since these measurements were taken in the
near field, the data are presented in the measured units of yJ/m3 and V2/m2.
Computed values of the far-field equivalent power density are also given in
mW/cm2. The EDM-3 has a usable frequency range from 3 to 3,000 MHz with a
response within ±1 dB over the range of 10-1,000 MHz. It has a 50 dB
dynamic range and is completely portable. A discussion of the development
of the EDM-3 can be found in the literature [12].
A quantitative comparison of the measured field intensity by the EDM-3
and a spectrum analyzer with calibrated half wave dipoles was made in the
far-field in the vicinity of the tower's base. In this case values obtained
with the spectrum analyzer were within 0.8 dB of the values obtained with
the NBS probe.
Measurements of the electric field energy density made with the NBS
probe at various locations on the tower were called out to one of the survey
team stationed at the tower base who recorded the data. The duration of the
measurements, climb and descent, was about 20 minutes.
The approximate location of the measurement points are shown in Figure 1
together with the measured values of electric field energy density, Ug, in
units of yJ/m3. The equivalent values of far-field power density, S, and
the square of the electric field strength, E2, as obtained from Equations
(1) and (2) are given in Table 1. The highest value of energy density
measured directly under the towers on the ground was 0.032 yJ/m3.
Table 1. Equivalent Energy Density, Power Density, and Electric
Field Strength Measured on FM Tower
Energy Density Far Field Electric Field
(yJ/m3) Power Density Strength Squared
(mW/cm2) (V/m)*
.032 1.9 7,200
.05 3 . 11,300
1 60 678,000
1.5 90 339,000
2 120 452,000
3 180 678,000
i
These results, though not a detailed mapping of the tower radiation
levels, do show that the intensities can be very high. In addition there
are locations on the tower where the field intensity was well beyond the
instrument's full scale reading of 3 yJ/m3 (equivalent to 180 mW/cm2 in the
far field) due to field intensification between two conducting structures.
The values reported here are fields typically encountered when climbing the
tower1 Higher levels might be encountered depending on the maintenance task
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to be performed. For example in many locations the hands are placed on
ladder parts where exposures were in excess of the meter's capability.
Since this particular tower was in the immediate area of a number of
other towers on Mt. Wilson, it is necessary to examine the possible con-
tribution of nearby stations to the measured exposure values. The radio-
frequency fields of other sources within approximately 500 feet of the FM
station were calculated assuming that the contributing exposure was due to
main beam radiation. This is a conservative approach in that it will over-
estimate any possible contribution. The total maximum exposure due to all
sources other than the FM station in question was determined to be 27 mW/cm2.
In all probability the field would be substantially (at least a factor of 10)
below this value since most of the tower is below the main beam of radiation
of the other sources. Therefore, it seems reasonable to conclude that the
fields measured on the tower are due principally to the antenna mounted on
the tower.
DISCUSSION
From an examination of the data it is clear that exposure intensities
on FM broadcast towers can exceed the OSHA recommended safety level of 10
mW/em2 by more than a factor of 10. The upper limit of exposure was not
established since the radiation fields exceeded the range of the instrument.
While these measurements were made on one of the more powerful FM stations
in this country the results can probably be applied to all but the most
minimally powered FM stations. This is because the localized field intensity
near any given radiator (antenna bay) is dependent upon the fraction of the
transmitter output power fed to it; this means that the local field near the
elements of a single bay antenna may approximate the fields near a multiple
bay antenna which is fed with significantly more power; i.e., input power
per bay is more significant than total station EKP. Thus, a relatively low
ERP station with few antenna bays may produce local fields near its antenna
elements nearly as intense as a higher ERP station using more antenna bays.
While these results apply strictly to FM broadcast stations, they raise very
serious questions about the fields on television broadcast towers where the
antenna input powers can be several, orders of magnitude greater than that
used in the FM broadcast service. The AM standard broadcast service ^operates
at frequencies (0.54-1.6 MHz) below the 10 MHz lower frequency limit of the
OSHA standard. However, there are very intense surface field gradients on
AM towers [4] and fields on AM antenna towers may also be of concern.
Additional studies are heeded to determine if the exposure levels found on
FM towers are also present on AM and TV broadcast towers.
Simple methods for controlling exposure of workers on broadcast towers
are not immediately obvious. The OSHA standard permits higher exposures for
periods less than 6 minutes; in these cases an exposure energy density value
of 1 mW-hr/cm2 shall not be exceeded. Thus, no upper limit ,1s placed on
short time exposure except that the higher level exposure may not occur more
than once in each successive six minute period. Table 2 summarizes exposures
and their associated durations permitted by the OSHA standard.
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Table 2. Exposure Power Density and Duration Permitted by
OSHA Standard for Whole or Partial Body Exposure
Exposure Power Density Exposure Duration
(mW/cm2) (min. or sec.)
10 6 min. or longer
20 3 min.
30 2 min.
50 1.2 min.
100 36 sec.
150 24 sec.
200 18 sec.
300 12 sec.
500 7.2 sec.
As an example the OSHA standard would technically permit an individual to be
exposed to 100 mW/cm2 for a period of 36 seconds if there is a "cooling off"
period of 5 minutes and 24 seconds following the 100 mW/cm2 exposure before
it was commenced again. These OSHA limitations apply to partial body as well
as whole body exposure. The variability of the fields, the lack of instru-
mentation, and variable exposure durations of workers on broadcast towers
would seem to suggest difficulty in compliance with this standard. Commonly
mentioned durations for tower work reach several hours. Just the time it
takes to safely climb up a modest height tower (200 feet) and climb down
again implies relatively high exposures which will exceed minutes in duration.
The U.S. Army and Air Force have already recognized the need to limit
the maximum possible exposure and have set an absolute upper limit of 100
mW/cra2 regardless of how short the exposure time [13]. In actual practice,
the Army uses an upper limit of 55 mW/cm2 which corresponds, for the Army-
Air Force standard, to a 2 minute exposure. Exposures of less than two
minutes duration are deemed impractical to control. If a minimum exposure
duration of 2 minutes is used with the OSHA standard, the maximum allowable
exposure level is 30 mW/cm2. In .this context it is interesting to note that
the American Conference of Governmental Industrial Hygienists (ACGIH)
recommends a maximum level of 25 mW/cm2 for any exposure duration [14].
Since exposure times of less than 2 minutes are not simply controlled, it
is felt that exposure to levels exceeding the equivalent of 25-30 mW/cm2,
or 94,000 to 113,000 V2/m2 would be difficult to control in terms of the
OSHA standard and exposure to levels exceeding these values should not be
permitted. Because of the relatively continuous nature of exposure on
broadcast towers and.the requirement for close attention to the work, this
conclusion seems consistent with the philosophy expressed in the OSHA
regulations. A further complicating factor is that portable, inexpensive
instruments for measuring fields in this frequency range are not available.
It does not seem practical to develop an instrument which would track the
OSHA standard and indicate how long an individual might remain in one
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location, tell him where to move to recover, and still be portable enough
not t6 interfere with whatever task is being performed. Nor does it seem
feasible to train personnel to make their own evaluation using presently
available instrumentation. In our limited experience tower workers are not
aware of the significance of thermalizing exposures and cumbersome safety
considerations are apt to be discarded or ignored under the pressures of
getting the job done. The most effective method of controlling excessive
exposure would be to turn off or drastically limit the power fed to the
antenna while the necessary work is done. This would probably require a
rule making procedure by the FCC or promulgation of specific work procedures
for broadcast towers by OSHA.
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SUMMARY
High intensity radiofrequency fields on an FM broadcast tower have been
found' which require careful attention to prevent personnel exposures' from .
exceeding OSHA regulations. These exposure levels are probably common to
localized areas near antennas of all but the very lowest powered FM stations.
The simplest method of control appears to be to turn off the transmitter
while work which requires people to be on the tower is done. Alternative
means to determine if compliance with safe practice is being met while working
on energized towers will be cumbersome and probably impracticable. Television
towers are also suspected of having very high levels near the antenna since
they have significantly higher powers than FM broadcast stations. Standard
broadcast AM towers may also be of concern because of high surface fields on
the tower. Additional studies are needed to determine if the exposure levels
on FM broadcast towers are also present on AM and TV broadcast towers.
REFERENCES
1. Broadcasting Yearbook 1975. Published by Broadcasting Publications, Inc.,
Washington, DC 20036.
* " • . •
2. Report to the National Association of Broadcasters on the Measurement of
Power Density Relative to OSHA Radiation Hazard Standards, Prepared by Smith
and Powstenko, Broadcasting and Telecommunications Consultants, Washington,
DC, March 1975.
3. Safety Level of Electromagnetic Radiation with Respect to Personnel at
One Shell Plaza at Houston, Texas. Engineering Report by Silliman, Moffet,
and Kowalski, Consulting Radio Engineers, Washington, DC, May 21, 1975.
4. Tell, R.A. and D.E. Janes. Broadcast Radiation: A Second Look.
Presented at USNC/URSI-IEEE meeting, Boulder, Colorado, October 20-23, 1975.
Text available from author, U.S. Environmental Protection Agency, Washington,
DC 20460.
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8
5. Michaelson, S.M. Human Exposure to Nonionizing Radiation Energy -
Potential Hazards and Safety Standards. Proc, IEEE 60:389-421 (1972).
6. Department of Labor. Occupational Safety and Health Administration.
Title 29 Code of Federal Regulations 1926.54 and Title 29 Code of Federal
Regulations 1910.97.
7. Gandhi, O.P., K. Sedigh, G.S. Beck, and E.L. Hunt. Distribution of
Electromagnetic Energy Deposition in Models of Man with Frequencies Near
Resonance. Presented at USNC/URSI-IEEE meeting, Boulder, Colorado, October
20-23, 1975. Text available from author, University of Utah, Salt Lake
City, Utah 84112.
8. Barker, P.W. Numerical Study of Electromagnetic Power Deposition in
Biological Tissue Bodies. Presented at USNC/URSI-IEEE meeting, Boulder,
Colorado, October 20-23, 1975. Text available from author, University of
Utah, Salt Lake City, Utah 84112.
9. Tell, R.A. and P.J. O'Brien. An Investigation of Broadcast Radiation
Intensities at Mt. Wilson, California. In preparation for EPA technical
report.
10. Bowman, R.R. Quantifying Hazardous Electromagnetic Fields: Practical
Considerations. National Bureau of Standards Technical Note 389, 15 pages,
April 1970.
11. American National Standards Institute. Safety Level of Electromagnetic
Radiation with Respect to Personnel. Report ANSI-C95.1, 1974.
12. Bowman, R.R. Some Recent Developments in the Characterization and
Measurement of Hazardous Electromagnetic Fields. In Biologic Effects and
Health Hazards of Microwave Radiation, proceedings for an international
symposium, Warsaw, October 15-18, 1973, published by Polish Medical Publish-
ers, Warsaw, 1974.
13. U.S. Departments of the Army and the Air Force. Control of Hazards to
Health from Microwave Radiation. Report TB MED 270/AFM 161-7, December 1965.
14. American Conference of Governmental Industrial Hygienists. Threshold
Limit Values of Physical Agents with Intended Changes Adopted by ACGIH for
1971 (American Conf. of Governmental Industrial Hygienists Publ.), Cincinnati,
Ohio; 1971.
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