ABB AWIN GW120, User Manual

Page: 25 / 76

WIRELESS HART SYSTEM PLANNING ACCESS ING THE SITE
4JNO0 0030 8 A 21
4 WirelessHART System Planning
4.1 Accessing the Site
To achieve the best radio performance possible, the installation sites have to be given careful
consideration. The primary requirements for a reliable installation include:
– Antenna placement that allows for line-of-sight or adequate signal strength
– Primary power source that provides required current
– Protection of radio equipment from exposure to weather or temperature extremes
– Suitable entrances for antenna, lightning arrestor, interface or other required cables, if us-
ing remote antennas.
These requirements can be quickly assessed in most applications. A possible exception is the
first item, verifying that a clear line-of-sight exists. A non-obstructed path is ideal; however,
minor obstructions in the signal path will not always block communication. In general, the
need for a clear path becomes greater as the transmission distance increases.
4.2 Path Quality Analysis
With the exception of short-range applications, a path loss study is generally recommended
for new installations. The exceptions include distances of less than 50 m, where no test is re-
quired in 90% of applications. If a test is to be conducted a path loss study is practical. It pre-
dicts the signal strength reliability and estimates the fade margin of a proposed radio link.
While terrain, elevation and distance are the major factors in this process, a path loss study
also considers antenna gain, coaxial cable loss, transmitter power and receiver sensitivity to
arrive at a final prediction.
Path loss studies are normally performed by a communications consultant, wireless hardware
vendor or system integrator who uses topographic maps or a software path analysis to eval-
uate a proposed path.
Although path studies provide valuable assistance in system planning, they are not perfect in
their predictions. It is difficult, for example, to consider the effects of man-made obstruc-
tions or foliage growth without performing an actual on-air test. Such tests can be done us-
ing temporarily installed equipment. ABB can provide radio survey service (AWIN Survey Ser-
vice) to assess radio environment.
4.3 Signal Strength
The strength of radio signals in a well-designed radio network must exceed the minimum
level needed to establish basic communication. The excess signal is known as the fade mar-
gin, and it compensates for variations in signal level which may occur from time to time due
to foliage growth, minor antenna misalignment or changing atmospheric losses.
While the required amount of fade margin differs from one system to another, experience
has shown that a level of 20 dB above the receiver sensitivity threshold is sufficient in most
systems. AWIN GW120 provide the means for direct measurement of received signal strength
using “RSSI (Received Signal Strength Indicator)” readings available in the gateway’s network
statistics webpage.