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If your RTU's are located where the associated
antenna cannot visually see the SCADA master radio station's antenna then it is
not clear if the radio signals will make the journey reliably. When the two radio stations antennas are
within optical view of each other the radio path is are considered "Line
Of Sight". When they are not they
are referred to as "Path Critical".
Path critical is not a matter to be taken lightly, as guessing
incorrectly as to the RF signal quality can cause errors in cost estimations,
taller antenna support structures, cost more, and potential delays in turn up
time. It can also cost frustration with
the contractors, and a sense that the engineer really did not know what they
needed to know about this aspect of the project. It takes time to build a taller antenna
support structure, especially when you created an initial structure that was
too low, and now has to be removed.
If the SCADA radio network designer chooses to
perform a "Radio Study" in effect they can enjoy a report that allows
the end user to try the actual SCADA Radio Network design before they buy it so
to say. The designed antenna heights
will be tested to see if they facilitate a reliable (20 dB Fade Margin)
communications. A radio study will place
the actual SCADA Radio Network hardware specific out in the field. The actual Radio models will be used, it is
often practical to use slightly smaller antennas when performing these Radio
Studies the differences between as studied antennas and as built are negated
with a math formula to compensate for any possible differences. The Master Radio Station and its associated antenna
system located at the initially designed height, along with the RTU's sites to
be studied. If a site has a measured low
SCADA radio communications performance issue, the RTU's antenna height can be
adjusted upwards and if necessary the master radio site can be adjusted upwards
as well. The Radio Surveyors will travel
to all of the RTU's sites of interest for this SCADA Radio Network and actually
take field measurement with an actual RTU version of the SCADA Radio. Receiver signal strength (RSSI) along with
Signal to Noise Radios and possibly "Data Error Rate" information as
well will be documented. GPS coordinates
along with Distance and Bearing data will be logged and valuable for future
installation crews as this gives the installers a bearing to aim what is
usually a directional antenna at each RTU.
Photos of the test equipment being deployed can be useful for future
reference.
I have attached a PDF copy of a report we
performed recently in Cape Coral, Florida.
It is representative in of a SCADA radio network radio study in many
ways, yet different in a couple of ways too.
This radio study was performed in the 900MHz multiple address system
(MAS), MAS requires FCC licensing for operation. 900MHz RF signals can be highly unpredictable
when non line of sight conditions exist as they did in Cape Coral. This radio study took advantage of the fact
that there was an existing 300' tall antenna tower located at the water plants
property that was available for use for this SCADA radio network. The SCADA designer, myself choose to locate
the SCADA master's antenna at the 100' above ground level height on the
tower. This was likely 25% higher than necessary,
but there was no additional cost associated in using a tower that was already
constructed and available to the city. Also this mounting location was unused
antenna tower real estate, as the other antennas on the tower were located
above the 100' height. The RF signal
strengths to the RTU's measured were very strong due to the high placement of
the master SCADA radio station's antenna.
The farthest RTU's away from the master were ~ 3 miles. Click this link to read the radio study.
If your SCADA radio network master and associated
RTU's are relatively close, with in about 1/2 mile and the antennas are able to
see each other, then the need for a radio study is ambiguous. However if the antennas are not with in
visual sight of each other, plan on a radio survey. A case in point was in Eustis Florida, I once performed a radio study there. The master radio had its antenna located atop
a 100' plus tall water tank. It needed
to communicate through a pine forest to an RTU less than a mile away. Zero signal was available until the RTU
antenna cleared the 80' tall slash pine trees.
The City abandoned their interest in 900 MHz spread spectrum for 450MHz
licensed to mitigate this problem. The
initial design would have failed miserably and caused cost overruns and delays
that would have made both sides frustrated, if a radio study was not performed
as part of the project. Learn from the
mistakes of others, perform a radio study when in a path critical environment.
A radio study can save provide significant cost
savings when it comes to antenna structure costs. It can be wasteful to use a 40' tall antenna
tower when a 20' tall one will supply the necessary RF fade margin and cost 1/4
as much. Multiply this idea in a 50 or
100 RTU system and the cost savings can be compelling. Almost all SCADA radio network projects require
the performance of a radio study.
If you have any questions on a specific project or
an application you would like to discuss, please feel free to call on me, Mark
Lavallee. I enjoy sharing my 30 years of
experience and knowledge. My company is
often used to sub consult with consultants, assist controls system integrators,
and work with the end users directly.