Oldham OLCT 200 User Manual page 23

work, the available system operating margin (TX power - RX Sensitivity + Antenna
gains) must exceed the Freespace loss and all other losses in the system. For best RF
line-of-site, the combined height of both antennas must exceed the Fresnel zone
diameter (see below).
Dist. between ant's
1000 ft (300 m)
1 Mile (1.6 km)
5 miles (8 km)
10 miles (16 km)
Example:
The 900MHZ radio modem has the following parameters:

Maximum RF TX power setting = 30 dBm (1 Watt)

RF RX sensitivity = -100 dBm (this is a constant)

Antenna gain (standard equipped dipole) = 2.1dBi x 2 = 4.2dBi
So the system operating margin is 30 - (-100) + 4.2 = 134.2 dBm. This is enough to
transmit 10 miles if freespace was the only loss in the system. For this to be the case,
the antennas must be mounted with a combined height greater than 95ft above all
obstructions (including the ground) to keep the fresnel zone clear. In practice however,
there are many losses in the system besides just freespace and it is recommended
there be at least 20dB extra system operating margin.
RF "Rules of Thumb"

Doubling the range with good RF "Line of Site" (LOS) requires an increase of
6 dB.

Doubling the range without good RF LOS requires an increase of 12 dB.
2.6.4.3 Antenna Selection & Location
A site survey using an RF spectrum analyzer and test radios is highly recommended.
The location of the antenna is very important. Ensure the area surrounding the
proposed location is clear of objects such as other antennas, trees or power lines
which may affect the antenna's performance and efficiency. It is also vital that you
ensure the support structure and mounting arrangement is adequate to support the
antenna under all anticipated environmental conditions. The choice of appropriate
mounting hardware is also important for both minimizing corrosion and maintaining site
performance.
Most installations utilize locally mounted dipole antennas as shown in Figure 2-8. An
option is available for a 6 foot riser to increase the height of the antenna 6 feet above
the OLCT 200. Extreme cases may require special order of directional antennas
mounted in such a way to allow aiming towards the base station antenna. Minimize
obstructions between the OLCT 200 and the base station antenna.
Fresnel zone diameter
16 ft (4.9 m)
32 ft (9.7 m)
68 ft (20.7 m)
95 ft (29 m)
OLCT 200 User Manual | 23
Freespace loss (dB)
81
96
110
116