CFR Title 47 Part 97.13(C) (as of Apr. 1, 2020) requires the amateur operator to be in
compliance with the radio frequency exposure requirements. The chart Table 5 above is
a calculation of distances in meters for several different antenna gains using the
formulae in the bulletin. To calculate the distances in feet multiply the distance in meters
by 3.28. The chart Table 6 below is a calculation of some common antenna gains. This
will give you a rough estimation of the safe distance needed from the antenna in the
direction of maximum gain. The chart is worst-case conditions of a continuous carrier for
the full time allowed for exposure. The charts do not take into account the antenna
patterns or elevation above the ground. In practice, most antennas have the maximum
gain elevated above the ground. For a better estimate, the actual antenna would need
to be analyzed for direction of maximum gain and gain in the direction of access.
Dipole
@30 ft
G5RV
@30 ft
43'
Vertical
3
Element
Triband
Beam
6
Element
6M Yagi
Multiband
Vertical
Freq
Gain dBI
Gain dBI
Gain dBI
Gain dBI
Gain dBI
Gain dBI
3.800
8
6.1
0.6
7.200
6.9
6.3
-0.1
3
10.100
5.4
7.6
0
3
14.150
6.2
6.6
1.9
10.1
3
18.150
8
9.6
4.3
3
21.250
8
9.8
3.8
10.1
3
24.950
7
9.9
4.4
3
28.500
7
9.7
6.1
10.1
3
50.125
8
10.5
7.1
13.7
3
Table 6 Approximate Antenna Gain for Some Common Antennas
Controlled Distance exposure in Table 5 is that of the operator, family, and other
members in the residence, given proper training, and allowing exposure for no more
than 6 minutes at a time. Uncontrolled Distance is the neighbors and general public with
no expectation of any training and the exposure is no more than 30 minutes at a time.
The actual exposure level is time and power averaged which will usually reduce the
total exposure. For example, the average SSB power is around 20 to 30% of the peak
power and will be averaged for the time of the actual transmission. For CW the power
will be 100% but the transmitter will be transmitting for only about 40% of the time.
RTTY (digital modes), AM, and FM will be 100% duty cycle and last for the length of the
transmission.
These calculations are for the direction of maximum gain. The FCC Office of
Engineering and Technology (OET) Bulletin 65 Supplement B has more information on
meeting the requirements.
Electronic Code of Federal Regulations (eCFR)
https://www.fcc.gov/bureaus/oet/info/documents/bulletins/oet65/oet65.pdf
https://www.fcc.gov/bureaus/oet/info/documents/bulletins/oet65/oet65b.pdf
https://www.fcc.gov/bureaus/oet/info/documents/bulletins/oet56/oet56e4.pdf
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