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JUL-AUG 2017

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40 INTECH JULY/AUGUST 2017 WWW.ISA.ORG Essential wireless network layout concepts By Shuji Yamamoto O ne of the first steps when creating a new wireless instrumentation network using ISA-100 wireless, or any other in- dustrial wireless network, is a site survey. This step is not part of any wireless standard, nor is it likely part of any network management plat- form, so it requires some creativity. Radio prop- agation patterns can be difficult to predict, but following a few basic design guidelines ensures a much higher level of success. Some wireless consultants make the process very complex using simulations and reading test signals, but these often do not ultimately match the real world. Other approaches are sim- pler and involve taking a few distance measure- ments and establishing sight lines, which often works just as well. For this article, we will con- centrate more on the latter, simpler approach. ANSI/ISA-100.11a-2011 (IEC 62734), Wire- less Systems for Industrial Automation: Process Control and Related Applications, networks are designed to support wireless field instrumenta- tion. This protocol specification is part of the larger ISA-100 wireless series. Although network management platforms have an extraordinary capability for self-organization, this feature cannot overcome unreliable radio links. But, the network management platform can use its diagnostic capabilities to measure the health of the communication and the devices. It can identify unreliable links so they can be fixed, and with improved communication, the network manager can reestablish a reliable link. How signals propagate Although it is not a perfect model, thinking of radio in the same way as visible light is accurate much of the time. Wireless networks depend largely on line of sight (LOS). If a wireless flow- meter is trying to transmit to a gateway in its LOS, the likelihood of a good link is very high. More potential obstructions are transparent to radio frequencies than visible light, but this is affected by frequency. A leafy tree is transparent to signals at 90 MHz, but 2.4-GHz signals will suffer some attenuation. Metallic objects are the great enemy of radio propagation, but can also help under the right conditions, which is why refineries and chemi- cal plants provide many challenges for wireless networks. In one case, a steel-shell storage tank can be helpful by reflecting a signal, while other times it is as an obstacle. Like visible light, much depends on the surface angles. General wireless principles say to avoid me- tallic surfaces when placing antennas for field devices, such as process instruments and actua- tors, routers, and gateways. The best situation is to mount the antenna vertically so that it is un - obstructed on all sides (figure 1). If a gateway is mounted next to a metallic pole, the signal will be attenuated, even on the side away from the pole. It is far better to move the antenna to the Install the antenna vertically Keep 1 m horizontally clear from any obstacles Install high enough to clear the line of sight (Recommended height: 20 to 30 m) Figure 1. For the best signal propagation, each antenna should be mounted vertically with at least 1 m of clear space around it horizontally. This normally means mounting the antenna as high on a structure as possible.

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