49
Date Code 20210405
Instruction Manual
SEL-FT50/SEL-FR12 Fault Transmitter and Receiver System
Appendix C: Link Budget Analysis
The SEL-FR12 must be connected to an external antenna, but because the
SEL-FR12 is not a transmitter, you may attach any size antenna to the device.
Keep in mind that larger antennas will amplify the desired signal, but also inter-
ference in the band.
Path Loss
Path loss is attenuation of the transmitted signal as it propagates between the
transmitter and the receiver. There can be multiple contributors to total path loss,
including free-space path loss (FSPL), loss because of obstructions within the
path of the radio signal, polarization mismatch between the transmitting and
receiving antennas, and multi-path fading. Total path loss is calculated by using
Equation 2
. Loss because of obstructions, antenna polarization mismatch, and
fading generally need to be estimated. All potential path loss factors should be
included in the link budget calculation when determining link margin.
Equation 2
Free-Space Path Loss
FSPL is calculated through use of
Equation 3
.
Table 20
shows the 915 MHz
FSPL for some typical path distances.
Equation 3
Obstruction Loss
Path loss caused by obstructions needs to be factored into link budget calcula-
tions when there are obstructions within the first Fresnel zone of the radio link.
The first Fresnel zone is an elliptical space surrounding the direct path between
the transmitter and the receiver antennas, the perimeter of which is described by a
total chord distance (d1 + d2) that is half a wavelength greater than the length of
the direct path (d) between the transmitter and receiver antennas.
The maximum radius of the first Fresnel zone occurs at a point midway between
the transmitting and receiving antennas, as shown in
Figure 32
.
Equation 4
shows how to calculate the radius of the Fresnel zone. For example, at 915 MHz
with a distance of 300 m (1000 ft) between antennas, the Fresnel zone has a
Path Loss (dB)
FSPL (dB)
Obstruction Loss (dB)
Polarization Loss (dB)
Fading Loss (dB)
–
+
–
=
where:
f
= frequency in MHz
d
= distance in km
Table 20
915 MHz
a
Free-Space Path Loss Examples
a
The free-space path loss will decrease slightly for radios in the 860 MHz band.
Distance Between Antennas (d)
Free-Space Loss (dB) (Without
Obstructions)
300.0 m (984.3 ft)
81
1.6 km (1.0 mi)
96
6.4 km (4.0 mi)
108
FSPL (dB)
32.45
20
f
log (MHz)
20
d
(km)
log
+
+
=
