Icom IC-7850 Instruction Manual page 22

INSTALLATION NOTES
For amateur base station installations it is recom-
mended that the forwards clearance in front of the an-
tenna 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.
Different exposure limits have been recommended for
different frequencies, a relative table shows a guide-
line 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 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 instal-
lations. The EC recommended limits are almost identi-
cal to the FCC specified 'uncontrolled' limits and tables
exist that show pre-calculated safe distances for differ-
ent antenna types for different frequency bands. Fur-
ther information can be found at http://www.arrl.org/.
• Typical amateur radio installation
Exposure distance assumes that the predominant ra-
diation pattern is forward and that radiation vertically
downward 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 con-
stant carrier.
For the bands 10 MHz and higher the following power
density limits have been recommended:
10–144 MHz 2 W/sq m
EIRP clearance heights by frequency band
1 Watts 2.1 m
10 Watts 2.8 m
25 Watts 3.4 m
100 Watts 5 m
1000 Watts 12 m
Forward clearance, EIRP by frequency band
100 Watts 2 m
1000 Watts 6.5 m
10,000 Watts 20 m
100,000 Watts 65 m
In all cases any possible risk depends on the trans-
mitter being activated for long periods. (actual recom-
mendation 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 as-
sessed risk is even lower.