User's Manual

RF Exposure Limits
P (W) is the total RMS power fed to the antenna, that is, the sum of all
transmitter channels minus feeder and combiner losses. G is the numerical
far-field antenna gain relative to an isotropic radiator, and d (m) is the distance
from the antenna. PG is the Equivalent Isotropically Radiated Power (EIRP)
of the antenna, expressed in watts.
The far field is commonly considered to start at a distance 2D
2
/λ from the
antenna, where D is the largest dimension of the antenna and λ the wavelength
in meters.
If needed, electric and magnetic field strengths are calculated using the
equation S = E
2
/377 = 377H
2
, where 377 (
 ) is the impedance of free space.
The far-field formula (equation 2) gives relatively accurate estimations (within
± 3 dB) of the far-field power density in free space without significant reflecting
surfaces near the position of exposure prediction. If used in the near field, it
significantly overestimates the exposure.
The near-field formula below is derived from the so-called cylinder formula,
which is used to estimate the average power density closer to the antenna.
The near-field formula estimates the maximum power density in the near-field
region of an antenna. For an antenna with an azimuthal half-power beam width
of
2
degrees, an aperture height of h meters, and a total RMS power feed of P
watts, the power density in the main beam direction at a distance of d meters
from the antenna is as follows:
The far-field formula gives good exposure estimates in the far field, while
overestimating exposure levels close to the antenna. Similarly, the near-field
formula overestimates exposure levels in the far field. An accurate estimation
of the real exposure is obtained by using both formulas and applying the lowest
value.
For RF exposure determination in other directions, that is, above, below,
behind, and at the sides of the antenna, it can be difficult to use analytical
expressions, since the near-field radiation properties are usually unknown.
Other methods are preferred instead, such as those described in the standards
mentioned in section 1.
2.2 Determining Compliance Distances
A compliance distance is a minimum separation allowed between an antenna
and a person to ensure that RF exposure limits are not exceeded. To install an
RBS antenna correctly, it is important to determine RF exposure compliance
distances and ensure that RF exposure levels are below specified limits in
areas accessible to the general public.
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124 46-EN/LZT 720 0399 Uen E 2008-09-29