Magnetic and Electric RF Field strength Limits

Antenna Design Index.

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Safe operating limits

The minimum health and safety requirements are set out in the European Commission Directive 1999/519/EC which gives the limits in terms of Volts/m, Amps/m and mWatts/m2 (Ref.13). The American Federal Communications Commission (FCC) specifies slightly different levels but goes on to give specific advice for amateur stations on the application of these limits (Ref.14). In both cases the exposure limits are divided into two main categories 'Controlled' and 'Uncontrolled'

Occupational/controlled limits

These apply to the amateur station licensees and members of their immediate household. Exposure in this environment is averaged over a 6 minutes period.
The first 3 lines of the table give the general formulae where f = frequency in MHz.

Frequency European Levels American FCC Levels
MHz Vrms/m Arms/m Vrms/m Arms/m
1 - 3 610/f 1.6/f 614 1.63
3 -10 610/f 1.6/f 1842/f 4.89/f
10-30 61 0.16 1842/f 4.89/f
7.0 87 0.23 263 0.70
10.1 61 0.16 182 0.48
14.1 61 0.16 132 0.35
18.1 61 0.16 102 0.27
21.0 61 0.16 88 0.23
24.9 61 0.16 74 0.20
29.0 61 0.16 64 0.17

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General public/uncontrolled limits

These apply to neighbours living near amateur radio stations. Exposure is averaged over a 30 minutes period.
The first 3 lines of the table give the general formulae where f = frequency in MHz.

Frequency European Levels American FCC Levels
MHz Vrms/m Arms/m Vrms/m Arms/m
1-1.34 87/√f 0.73/f 614 1.63
1.34-10 87/√f 0.73/f 842/f 2.19/f
10-30 28 0.073 824/f 2.19/f
7.0 33 0.104 118 0.31
10.1 28 0.073 82 0.22
14.1 28 0.073 59 0.16
18.1 28 0.073 46 0.12
21.0 28 0.073 39 0.10
24.9 28 0.073 33 0.088
29.0 28 0.073 28 0.076

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Application of the limits

It is not a suitable DIY project to build and calibrate equipment capable of measuring the absolute levels of electric and magnetic fields. Repeatability and accuracy of more than a few dB is often difficult to achieve even with the best available instruments and expertise. Also the basic formulae for calculating far field strengths give conservative results that over-predict exposure levels when applied to the near field region i.e. within a wavelength of the antenna. Fortunately the studies carried out on behalf of the American Radio Relay League (ARRL) in Ref.14 has shown that antenna modelling programs such as the Numeric Electromagnetic Code (NEC) and MININEC can yield sufficiently accurate results for safety assessments. The following is an example safety assessment for an ya30 antenna partly installed in an attic carried out for a club member who was having a pace maker fitted ya30.pdf

A feature of the exposure guidelines is that exposures, in terms of power density, (Volts/m)2 or (Amps/m)2, may be averaged over 6 minutes periods for 'controlled' and 30 minutes for 'uncontrolled' environments, with the averages not to exceed the limit for continuous exposure. The concept of power averaging includes both on and off times, and the 'duty factor' of the transmitting mode being used. Various modes of operation have their own duty factor that is representative of the ratio between the average and the peak-envelope-power:-

Mode Duty Factor Notes
Conversational SSB 20% No speech processing
Conversational SSB 50% Heavy speech processor
Voice FM 100% -
FSK or RTTY 100% -
AFSK SSB 100% -
Conversational CW 40% -
Carrier 100% Tune-up purposes

FOR EXAMPLE - If a 500-watt PEP amateur SSB station (with heavy speech processing) transmits ("worst case") two minutes on, two minutes off then two minutes on again in any six-minute period (the averaging time period for controlled exposure), then for controlled exposure situations the effective power would be:-
        500 W x 0.5 (i.e. 50% from above table) x (4/6 minutes) = 167 W

Alternatively the Volts/m and the Amps/m field strength values calculated for the antenna for the steady power condition could be reduced by multiplying by:-
        √(0.5 x 4/6) before comparison with the tabulated field strength limits.

References

Ref.13 -ICNIRP Guidelines for Limiting Exposure to Time-Varying Electric, Magnetic, and Electromagnetic Fields ; accessed on http://www.icnirp.de/PubEMF.htm
Ref.14 - OET Bulletin No.65 : Evaluating Compliance with FCC Guidelines for Human Exposure to Radio Frequency electromagnetic Fields ; accessed on http://transition.fcc.gov/Bureaus/Engineering_Technology/Documents/bulletins/oet65/oet65b.pdf

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