7/29/2019 Causes of Low Signal Levels in Point to Point Radio
Relay Hops
2/2
Page 1 of 1 Harris Stratex Networks Division Doc. No. 261 R. U.
Laine 8/ 2005
Network Engineering Paper
Causes of Low Signal Levels in Point-to-Point Radio-Relay
Hops
GeneralReceive signal levels (RSLs) in point-to-point
radio-relay hops fluctuate rapidly over short-term periods
withmultipath fade activity, and slowly over the long term due to
power fades caused by geoclimatic variations,infrastructure
instability, antenna feeder system misalignment and contamination,
transmitter output powerdegradation, and other reasons discussed in
this technical brief.
Short-Term RSL FluctuationsShort-term variations in receive
signal level, unless contributed by unstable antennas vibrating in
a strong wind,are always caused by the reception of multiple
signals (rays) at the receive antenna. Known as multipath
fading,the voltage addition of the direct path and one or more
reflected or refracted rays cause short-term RSL
fluctuations. If the multipath signal is low, typically 20 dB
below the direct paths signal, the signal fluctuations areless than
+/-1 dB. But if the reflected multipath signal is high, it may
cause the RSL to drop 5-10 dB or more, andeven remain low over a
long period of time (hours to days) if the reflection is specular
(mirror-like) and the pathgeometry and atmosphere are stable.
Signals from many reflective and refractive multipath routes are
easilyidentified by their very fast, frantic short-term fade
activity.
Excessive
Path
Clearance
Refle
cted
Multipa
th
DirectPa
thRe
fracte
d
Mult
ipath
ElevatedSu
perrefractiv
e
Layer
\\\\\\\\\\\\\\
\\\\
Specular Reflection Long-Term RSL Fluctuation
The long-term reduction in receiver signal level may be the
result of one or a combination of the following:
Stable multipath reflection, as described above, usually more
severe in one direction but could depressthe path both ways.
Long-term reduction in a transmitters power output, usually only
one-way, verified at the transmitterspower output test port.
Antenna misalignment Power fading, e.g. ray defocusing (beam
spreading) or obstruction due to a ducting atmosphere in longer
(>20km) paths Waveguide feeder contamination (water or
moisture in the waveguide or antenna feed) Power fading, e.g.
ray defocusing (beam spreading) or obstruction due to a ducting
atmosphere in a longer (>20 km) path. The k-factor decoupling of
the ray (RF signal) off the antennas main lobe
