User's Manual
21
Date Code 20170317 Instruction Manual SEL-FT50/SEL-FR12 Fault Transmitter and Receiver System
System Installation
Figure 16 Fresnel Zone
The formula used to calculate the widest distance of the Fresnel zone is as
follows:
where:
b = radius of the Fresnel zone in meters
d = distance between transmitter and receiver in kilometers
f = frequency transmitted in GHz
Obstructions in the Fresnel zone may also cause multipath interference
because of reflective or refractive signals that may arrive at the receiver out of
phase with the desired signal. The Fresnel zone should be at least 60 percent
clear of obstructions for reliable radio communications. Large objects outside
the Fresnel zone can also cause reflections that can affect reliable radio
operation.
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 because of Earth’s curvature, and coaxial cable
loss). Variations in temperature and humidity of the atmosphere along with
elevation cause 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. Perform a site survey before installing
the SEL-FT50 Fault Transmitters to obtain the fade margin. A fade margin of
10 dB yields adequate performance for noncritical communications links
Table 9 900-MHz Fresnel Zone Radius
Distance Between
Antennas (d)
Fresnel Zone Radius (b) Free-Space Loss (dB)
304.8 m (1000 ft) 4.9 m (16 ft) 81
1.6 km (1 mi) 11.6 m (38 ft) 96
8 km (5 mi) 25.9 m (85 ft) 110
16 km (10 mi) 36.6 m (120 ft) 116
d
b
b 17.32 d 4 f e=