51
Date Code 20210405
Instruction Manual
SEL-FT50/SEL-FR12 Fault Transmitter and Receiver System
Appendix C: Link Budget Analysis
Antenna Polarization Loss
While hanging on the line, the SEL-FT50 has a vertically polarized antenna. The
SEL-FR12 should use a vertically polarized antenna to match. Antenna polariza-
tion refers to the orientation of the e-field in the radiated RF signal. The omnidi-
rectional antennas listed in
Table 20
are all vertically polarized. The Yagi
antennas are polarized in the direction of the short radiating elements of the
antenna (either vertically or horizontally). For proper system operation, the trans-
mitting and receiving antennas should be polarized in the same direction. When a
radio signal propagates over long distances, it is possible for the polarization of
the signal to rotate (because of interactions with the ground or obstructions in the
path). When this occurs, the received signal polarization may not be aligned with
the receiving antenna, which results in polarization mismatch loss. A 45-degree
rotation of signal polarization results in 3 dB loss of received signal power. It is
possible but unlikely for greater polarization loss to occur in an actual radio link.
Fading Loss
Multipath fading occurs when the transmitted signal is reflected off surfaces that
are not on the direct path between the transmitter and receiver. These reflected
signal images combine with the direct path signal and add constructively or
destructively depending on the phase of the reflected signal relative to the direct
path signal. When a direct line-of-sight path exists between the transmitter and
receiver, multi-path fading will generally be approximately 6–10 dB, in the pres-
ence of nearby reflective surfaces. When a direct line-of-sight path does not exist
between the transmitter and receiver, multi-path fading can cause 20 dB or more
of signal loss.
Interference Margin
The SEL-FT50/SEL-FR12 system shares frequency spectrum with other services
and FCC Part 15 (unlicensed) devices in ITU Region 2 (North, Central, and
South America). Signals from other devices and services in the 900 MHz ISM
band can cause interference at the receiver that degrades the receive sensitivity of
the radio. The effective receive sensitivity of the radio is the signal level at which
the radio can properly receive the desired signal in the presence of sustained
interference.
Interference margin should be included in the link budget to account for the effect
of interfering signals. The level of interference at a receiver can vary greatly
depending on the number of other nearby devices and services operating in a
given area. In isolated locations, there may be very little interference (0–6 dB of
interference margin required). In suburban or urban environments, the level of
interference can be substantial (6–20 dB of interference margin required).
Link Margin
The result of a link budget calculation is a determination of the link margin avail-
able for a given radio link. While it is possible for a radio link to work properly
with 0 dB of link margin, it is undesirable to design a system with little or no link
margin. Link budget calculations often rely on estimates of loss factors and inter-
ference levels and may not be accurate. In addition, over time, the conditions of
the link may change, resulting in additional path loss or new sources of interfer-
ence, which could render the link unreliable. In practice, allowing for 15 dB of
link margin should result in a reliable link at installation and provide tolerance
for changes over time. Use
Equation 5
,
Equation 6
, and
Equation 7
to calculate
link margin.
