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1.1 Health & Safety

Exposure to Non Ionising (RF) Radiation/Safe Working Distances

The safe working distance from a transmitting antenna may be calculated from the

relationship:

D = √

in which D = safe working distance (metres)

PT = transmitter or combiner power output (watts)

GR = antenna gain ratio = anti log (gain dBi ÷10)

w = power density (watts/square metre)

The RF power density value is determined by reference to safety guidelines for exposure

of the human body to non-ionising radiation. It is important to note that the guidelines

adopted differ throughout the world and are from time-to-time re-issued with revised

guidelines. For RF Central use, a maximum power density limit of 1w/m² is to be applied

when calculating minimum safe working distances. Appendix A refers.

Important Note: It must be remembered that any transmitting equipment radiating power

at frequencies of 100 KHz and higher, has the potential to produce thermal and athermal

effects upon the human body.

To be safe:

a) Operators should not stand or walk in front of any antenna, nor should they allow

anyone else to do so.

b) Operators should not operate any RF transmitter or power amplifier with any of its

covers removed, nor should they allow anyone else to do so.

Worked examples

Antenna Transmitter Power

Type Gain (dBi) Gain Ratio 2W 4W 10W 30W

OMNI

4 2.5 1 1 1.5 2.5

HELIX

20 100 4 5.6 9 15.5

PARABOLIC

DISH

35 3,162 22.5 32 50 87

MINIMUM SAFE DISTANCE (METRES)

PT. GR

4π.w

1 2 3 4 5 6 7 8 9 ... 18 19

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