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JUL-AUG 2017

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INTECH JULY/AUGUST 2017 41 AUTOMATION BASICS top of the pole, so it can extend into free space, or to extend the antenna mounting horizon - tally, so there is at least a 1-meter gap between the antenna and the pole. Understanding the Fresnel zone Elevated antenna placement is important, be- cause radio communication does not move in a tight beam like a laser. To send the signal from one point to another efficiently, some area in the shape of an ellipse is required. This area is called the Fresnel zone (figure 2). The amount of room available for the signal to spread has a huge effect on signal strength and the distance it can carry, since the longer the distance, the fat- ter the zone needs to be in the center. Anything violating the zone, which could even be the ground itself, attenuates the strength. There- fore, trying to squeeze a signal through a narrow space, even though it may allow direct LOS, can result in signal attenuation. For example, where the LOS side clearance has an open space with a radius of 4 m, the communication range can be 500 m. However, when trying to send the signal through a more constricted area where the open space radius is only 2 m, the effective distance will be cut by 75 percent to 128 m. Having open, unobstructed space makes a huge difference, but this is typi- cally a problem in most congested plant envi- ronments. This is why mounting devices and antennas as high as possible is so important. Meshing vs. routing devices ISA-100.11a has mechanisms for device-to-device meshing, but the more desired network topol- ogy is one where a field device can communicate directly with the gateway, or directly to a router connected to the gateway (figure 3). The goal is to avoid the need for meshing device-to-device, because sending signals between multiple field devices slows down data movement and taxes the devices' batteries. To facilitate these transmissions, gateways and routers should be mounted as high as practical to clear any surrounding equipment and permit clear LOS connections. My company calls this practice of having a mesh of routers communi- cating above the plant equipment a sky mesh, and it takes advantage of more powerful trans- mitters than are practical for individual wireless field devices. Placement of individual field devices is not as simple. Most native wireless devices, such as a differential pressure instrument, have an integral wireless transmitter and antenna (figure 4). This is very convenient, but can complicate signal propa - gation. Placement in the process piping or vessel often dictates where the device must be mount - ed, the antenna orientation, and the surround- ing obstructions. Using an antenna extension can address these issues. Another alternative is I/O: Field instrument RT: Router Fresnel zone Gateway I/O or RT Figure 2. Radio waves tend to propagate through an elliptical space formed between the two antennas. The longer the distance, the larger the required diameter at the center. This space should be as unobstructed as possible to avoid signal attenuation. RT: Router I/O: Field instrument Gateway Figure 3. The gateway is the end point of the network, and is connected to the control and monitoring system via hardwiring. Routers serve as relay points, gathering information from the field devices and passing it to the gateway. Figure 4. Having an antenna mounted on the field device is common, but placement of the field device may put it in a location prone to interference. An external add-on antenna may be needed to improve communication.

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