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 Design, Test, Then Build It Radio Study Why? How to Build it Reliably Photo Gallery


If your RTUs are located where the associated antenna cannot visually see the SCADA master radio station's antenna, it is not clear whether the radio signals will make the journey reliably. When the two radio station antennas are within optical view of each other, the radio path is 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. Guessing incorrectly about RF signal quality can cause errors in cost estimations, taller antenna support structures that cost more, and potential delays in turn-up time. It can also cost frustration with 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, they can enjoy a report that lets the end user try the actual SCADA radio network design before they buy it, so to speak. The designed antenna heights get tested to see if they facilitate reliable communications at 20 dB fade margin. A radio study places the actual SCADA radio network hardware out in the field. The actual radio models are used — it is often practical to use slightly smaller antennas during the study, and the differences between as-studied and as-built antennas are negated with a math formula to compensate. The master radio station and its associated antenna system are located at the initially designed height, along with the RTU sites to be studied. If a site has a measured low SCADA radio communications performance issue, the RTU's antenna height can be adjusted upward, and if necessary the master radio site can be adjusted upward as well. The radio surveyors travel to all the RTU sites of interest and take field measurements with an actual RTU version of the SCADA radio. Receiver signal strength — RSSI — along with signal-to-noise ratio and possibly data error rate information are documented. GPS coordinates along with distance and bearing data are logged, which is valuable for future installation crews because it gives them a bearing to aim what is usually a directional antenna at each RTU. Photos of the test equipment being deployed are useful for future reference.

A radio study can provide significant cost savings on antenna structure costs. It can be wasteful to use a 40-foot tall antenna tower when a 20-foot one will supply the necessary RF fade margin and costs a quarter as much. Multiply that across a 50 or 100 RTU system and the savings are compelling. Almost all SCADA radio network projects require a radio study.

Cape Coral case study: I have attached a PDF copy of a report we performed recently in Cape Coral, Florida. It is representative 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 900 megahertz multiple address system, which requires FCC licensing for operation. 900 megahertz 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-foot tall antenna tower located at the water plant's property that was available for use. The SCADA designer — myself — chose to locate the SCADA master's antenna at the 100-foot above-ground-level height on the tower. This was likely 25 percent higher than necessary, but there was no additional cost associated with using a tower that was already constructed and available to the city. This mounting location was also unused antenna tower real estate, as the other antennas on the tower were located above the 100-foot height. The RF signal strengths to the RTUs measured were very strong due to the high placement of the master SCADA radio station's antenna. The farthest RTUs away from the master were about 3 miles.

Eustis case study: If your SCADA radio network master and associated RTUs are relatively close — within about half a 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 within visual sight of each other, plan on a radio survey. A case in point was in Eustis, Florida, where I once performed a radio study. The master radio had its antenna located atop a 100-foot-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-foot tall slash pine trees. The city abandoned their interest in 900 MHz spread spectrum for 450 MHz licensed to mitigate this problem. The initial design would have failed and caused cost overruns and delays that would have made both sides frustrated if a radio study had not been performed as part of the project. Learn from the mistakes of others — perform a radio study when in a path-critical environment.

If you have questions on a specific project or application, call Mark Lavallee at 954-961-2642 or email mark.lavallee@advantage-com.com. I enjoy sharing my thirty years of experience. My company is often called upon to subcontract with design-build consultants, assist control system integrators, and work with end users directly. For more on the design process before a radio study, see Design, Test, Then Build It.
954 961 2642 or email : mark.lavallee@advantage-com.com
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