Epidemiological principles for EMF and EMR studies - Lincoln ...

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0.155, or relative gains of 1.0, 0.00027 and 0.2 respectively. The side-lobes have
elevation angles of 8, 15, 40, 57 and 72 degrees. For an antenna at 500m above
ground level, the ground level side-lobe peaks occur at 160, 390, 600, 1870 and
3560m from the base of the tower, with troughs of significantly low exposure
between them.
The amplitudes of the peaks and troughs are very large because of the
logarithmic nature of the dB units. It is common to tilt the antenna pattern slightly
downwards so that the main beam is directed towards major population centres
in the listening and viewing area rather than towards the far horizon.
RF/MW antennas, including cell phone antennas have complex horizontal and
vertical radiation patters, for example Figure 17.
Figure 17: The antenna pattern for a 900 MHz cell phone base station
transmission, Bernardi et al. (2000).
In Figure 17 note that the radial scale is in decibels (dB) which is a logarithmic
scale. From the main beam peak at 90° to the minimum on either side at about 75°
and 105°, the signal is 30 dB lower, i.e. 1000 times weaker. The other smaller
peaks are called side-lobes. They are typically 12 to 16 dB smaller or 16 to 40
times less intense than the main beam. All antennas have main beams and sidelobes
producing a complex spatial pattern that needs to be understood when
carrying out health effects studies around radio/TV broadcast or cell phone
transmission stations.
Figure 18 shows the dominant strength of the main beam, which is actually
stronger because of the square of the relative field factor. The elevation angle of
the antenna is usually slightly tilted downwards by 0.5° to point the main beam at
the more remote listening or viewing audience.
The Relative Field peaks in Figure 18 are at 0.5, 3.5, 5.7, 7.9, 10.1 and 12.3°. With
the assumption of the mean height of the antennae at 460m, these peaks
correspond to ground level positions at 52.7, 7.5, 4.6, 3.3, 2.6 and 2.1km from the