Installation Notes; Typical Amateur Radio Installation; Vertical Clearance By Eirp Output; Forward Clearance By Eirp Output - Icom IC-705 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:
10–50 MHz 2 W/sq m

Vertical clearance by EIRP output

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 by EIRP output

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
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.
I