RF Safety
By Bob Buus, W2OD
Certification 6)
• Amateur Applicant/Licensee certifies
that they have READ and WILL
COMPLY WITH Section 97.13(c) of
the Commission’s Rules regarding
RADIOFREQUENCY (RF)
RADIATION SAFETY and the
amateur service section of OST/OET
Bulletin Number 65. – FCC Form 605
Section 97.13(c)
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Before causing or allowing an amateur station to
transmit from any place where the operation of
the station could cause human exposure to RF
electromagnetic field levels in excess of those
allowed under § 1.1310 of this chapter, the
licensee is required to take certain actions.
(1) The licensee must perform the routine RF
environmental evaluation prescribed by §
1.1307(b) of this chapter, if the power of the
licensee’s station exceeds the limits given in the
following table:
(2) If the routine environmental evalation
indicates that the RF electromagnetic fields could
exceed the limits contained in § 1.1310 of this
chapter in accessible areas, the licensee must
take action to prevent human exposure to such
RF electromagnetic fields. Further information on
evaluating compliance with these limits can be
found in the FCC’s OET Bulletin Number 65,
“Evaluating Compliance with the FCC Guidelines
for Human Exposure to Radiofrequency
Electromagnetic Fields.”
Wavelength Band
Evaluation required if
power (watts)
exceeds
160 m
500
80 m
500
75 m
500
40 m
500
30 m
425
20 m
225
17 m
125
15 m
100
12 m
75
10 m
50
VHF
50
70 cm
70
33 cm
150
23 cm
200
13 cm
250
SHF
250
EHF
250
Repeater stations
500 W ERP
§ 1.1310 Limits for MPE
Limits for Occupational/Controlled Exposure over 6 minute average
Frequency, Mhz
0.3-3.0
3.0-30
30-300
300-1500
1500-100,000
E Field, V/m
614
1842/f
61.4
H Field, A/m
1.63
4.89/f
0.163
Pwr. Den. mW/cm2
100
900/f2
1.0
f/300
5.0
Limits for General Population/Uncontrolled Exposure over 30 minute average
Frequency, Mhz
0.3-3.0
3.0-30
30-300
300-1500
1500-100,000
E Field, V/m
614
824/f
27.5
H Field, A/m
1.63
2.19/f
0.073
Pwr. Den. mW/cm2
100
180/f2
0.2
f/1500
1.0
The Human Body Heat Engine
• Absorption of RF Power causes heating
• Specific Absorption Rate (SAR) Watts/kg
• A SAR produces same heating regardless
of frequency
• A human at rest produces about 1 Watt/kg
• A SAR of 4 W/kg will raise tissue temp. 1
degree – same as a brisk walk
• Avg. 4 W/kg or PK 20 W/kg in limbs safe.
Living Human Properties
• The human body is a marvel at regulating
its internal temperature through blood
circulation and perspiration.
• Sperm in the testes are very temperature
sensitive.
• The lens of the eye lacks blood flow to
keep it cool. If “cooked” at high
temperature, cataracts will form.
RF Field Strength
• Electric E Field is in Volts per meter
• Magnetic H Field is in Amperes per meter
• Power Density, S is Watts per square
meter
• S=ExH = E2/377 = 377 H2 Watts/sq. meter
• 10 Watts/sq. m = 1 milliwatt/sq. cm
• For a point source, S = Power / 4πd2
Field Strength Examples
• For a point source, S = Power / 4πd2
Eff. Rad. Power
1250 Watts
125 watts
125 watts
125 Watts
Distance, d
1.0 Meter
1.0 meter
0.1 meter
10 Meters
Strength, S
10 mW/sq. cm
1 mW/sq. cm
100 mW/sq. cm
.01 mW/ sq. cm
Conversion from SAR to RF Field Strength
• Exposing a human body
to 10 mW/sq. cm
• Frequency of maximum
absorption (about 70
mHz)
• Localized SAR values
• Average SAR is 1.88
W/kg
• From Proc. IEEE 68:27,
1980
FCC Requirements
• Most stringent between 30 and 300 MHz
• 20 mW/sq. cm deemed safe
• Controlled Environment less than 1 mW/
sq. cm (factor of 20)
• Uncontrolled Environment less than 0.2
mW/sq. cm (factor of 100)
• Evaluation must be done if output power
exceeds 50 Watts at these frequencies.
Must do Evaluation if Power Exceeds
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500 watts below 10 MHz
425 watts on 30 meters
225 watts on 20 meters
125 watts on 17 meters
100 watts on 15 meters
75 watts on 12 meters
50 watts on 10 meters and at VHF
70 watts on 70 cm
150 watts on 33 cm
200 watts on 23 cm
250 watts above 2 gHz
500 watts ERP for repeaters on all bands
Evaluation Procedure
• Limits of 1.1310 vary with frequency so
you must evaluate for each frequency
band you plan to use
• Controlled exposure limits are 5 times the
Uncontrolled exposure limits
• Controlled exposure averaging time is 6
minutes
• Uncontrolled exposure averaging time is
30 minutes
Duty Cycle Advantage
• If we listen as much as we talk, the
transmitter is on only 50% of the time
• For FM, PSK, and TTY, PEP is average
transmitting power
• For CW, because of spaces between dots
and dashes, average xmit power is 0.5
PEP
• For SSB with speech processing, the
average power is about 0.2 PEP
Typical Duty Cycles
Duty Cycle
Modulation
Avg./PEP
Talk/Listen
FM
1.0
0.5
PSK/TTY
1.0
0.5
CW
0.5
0.5
AM
0.3
0.5
SSB
0.2
0.5
Packet
1.0
0.1
Radiated Power Calculation
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Average Radiated Power (6 or 30 min) =
PEP output power (watts)
Reduced by transmission line loss
Times antenna gain
Times ground reflection gain
Reduced by Average/PEP ratio
Reduced by Duty Cycle ratio
Final Calculation
• Must determine how far away you must be
from the antenna to meet the FCC
requirements for both the controlled and
uncontrolled environments
• Using simple 1 / 4πd2 gives conservative
results. More complicated NEC analysis
will permit higher power and/or closer
distances
To Run RF Safety Program
• Run rfsafety.exe
• Input Call
• Change any or all of the defaults by
selecting from drop-down menu or typing a
numeric value followed by “enter”.
• Click “Calculate” and distance results are
given for controlled and uncontrolled
• Clicking “Print” will give hard copy.
THE END