I
BASIC MANUAL
INSTALLATION NOTES
For amateur base station installations it is
recommended that the forward clearance in front of
the antenna array is calculated relative to the EIRP
(Effective Isotropic Radiated Power). The clearance
height below the antenna array can be determined in
most cases from the RF power at the antenna input
terminals.
As different exposure limits have been recommended
for different frequencies, a relative table shows a
guideline for installation considerations.
Below 30 MHz, the recommended limits are specified
in terms of V/m or A/m fields as they are likely to fall
within the near-field region. Similarly, the antennas
may be physically short in terms of electrical length
and that the installation will require some antenna
matching device which can create local, high intensity
magnetic fields. Analysis of such MF installations
is best considered in association with published
guidance notes such as the FCC OET Bulletin 65
Edition 97-01 and its annexes relative to amateur
transmitter installations. The EC recommended limits
are almost identical to the FCC specified ‘uncontrolled’
limits and tables exist that show pre-calculated safe
distances for different antenna types for different
frequency bands. Further information can be found at
http://www.arrl.org/.
•
Typical amateur radio installation
Exposure distance assumes that the predominant
radiation pattern is forward and that radiation vertically
downwards is at unity gain (sidelobe suppression is
equal to main lobe gain). This is true of almost every
gain antenna today.
Exposed persons are assumed to be beneath the
antenna array and have a typical height of 1.8 m.
The figures assume the worst case emission of a
constant carrier.
For the bands 10 MHz and higher the following power
density limits have been recommended:
10–400 MHz
2 W/sq m
435 MHz
2.2 W/sq m
EIRP clearance heights by frequency band
Watts 10–2 m
70 cm
23 cm 13 cm and beyond
1
2.1 m
2 m
2 m
2 m
10
2.8 m
2.7 m
2.5 m
2.3 m
25
3.4 m
3.3 m
2.7 m
2.5 m
100
5 m
4.7 m
3.6 m
3.2 m
1000
12 m
11.5 m
7.3 m
6.3 m
Forward clearance, EIRP by frequency band
Watts 10–2 m
70 cm
23 cm 13 cm and beyond
100
2 m
2 m
1.1 m
0.7 m
1,000
6.5 m
6 m
3.5 m
3 m
10,000
20 m
18 m
11 m
7 m
100,000
65 m
60 m
35 m
29 m
In all cases any possible risk depends on the
transmitter being activated for long periods. (actual
recommendation limits are specified as an average
during 6 minutes)
Normally the transmitter is not active for long periods
of time. Some radio licenses will require that a timer
circuit automatically cuts the transmitter after 1–2
minutes etc.
Similarly some types of emission, i.e., SSB, CW, AM
etc. have a lower ‘average’ output power and the
assessed risk is even lower.
