User's Manual

SEL-3031 Instruction Manual Date Code 20091009
Installation
Overview
2.4
feed line with enough loss to remain compliant with the
maximum 36 dBm EIRP or reduce the radio transmitted
power accordingly.
For example, if you wanted to design a radio system to achieve a 32-km (20-mile) radio
path and run at the highest allowed transmitted power, one way would be to use the
SEL-3031 with 15 meters (50 feet) of LMR 400, lightning arrestor, and the 5-Element
Yagi with 11.1 dBi of gain. The radio would transmit 30 dBm, the feedline loss is 1.95
dB, lightning arrestor loss is 0.15 dB, and the antenna gain is 11.1 dBi. The result
would be 39 dBm, which is 3 dBm higher than allowed by the FCC. You must reduce
the radio’s transmit power setting TXPWR to 27 dBm. This would make your EIRP
equal to 36 dBm. After setting the transmit power, measure the actual power.
NOTE: To reduce potential radio interference to other
users, the antenna type and its gain should be so chosen
that the equivalent isotropically radiated power (EIRP) is
not more than that permitted for successful
communication.
Fade Margin
The fade margin determines the allowable signal loss between the transmitter and
receiver. The fade margin is a function of system gains (transmitter power, receiver
sensitivity, and antenna gain) and system losses (free space loss, losses due to Earth’s
curvature, and coaxial cable loss). Variations in temperature and humidity of the
atmosphere with elevation causes the signals to bend more or bend less, resulting in
fading at the receiver. The longer the path, the more likely deep fades will occur,
requiring a greater fade margin. The formula to calculate free-space loss is shown
below.
Free Space Loss = 92.4 + 20log(f) + 20log(d) dB
where:
f = frequency in GHz
d = distance in km
Regional conditions also impact the probability of signal fade. Generally, mountainous
terrain is more favorable, while tropical areas and those near large bodies of water are
less favorable. One of the results from a site survey is the fade margin. A fade margin
of 20 dB will yield good radio performance.
Site Survey
A line-of-sight path provides the most reliable transmission in all cases. As the distance
increases the need for a clear path becomes more critical in creating a reliable,
available radio link. If the radio path is short then a path study may not be needed. If
you have a clear line-of-site to the other location without obstructions to the Fresnel
zone (see Radio Path) then a path study is generally not needed. Longer distances may