Radiating cables can solve tricky wireless communication
Leaky feeders can improve communication with moving devices and complex environments.
Wireless communication depends on antennas to transmit and receive RF (radio frequency) signals. There are many shapes and sizes depending on the nature of the type of transmission and frequency. For this discussion, one useful analogy can be trying to water your garden with a sprinkler.
A central nozzle that sends equal amounts of spray in every direction is like an omni-directional antenna. The signal goes in every direction from a central point. A directional antenna concentrates the signal in a specific direction like a sprinkler that sends out a stream. It can carry a longer distance but covers a smaller area.
A third irrigation technology uses a porous hose that “weeps” water over its length, such that anywhere it runs, water will soak into the ground. The application can be highly controlled by placing the hose in the most critical areas. The same exists in wireless communication where special cable can serve as a continuous antenna over a specific distance, receiving the RF signal as well as transmitting.
Radiating cable is an alternative to traditional RF antenna systems. It solves RF design challenges in certain situations involving moving equipment such as assembly carriers, overhead cranes, and automatic guided vehicles (AGVs). Applications where radiating cable is a consideration include communication in confined spaces, tunnels that snake throughout a process, and large monorail systems requiring consistent RF signal thresholds.
Radiating cable technology, also known as a leaky feeder, has been applied for decades in bi-directional low-frequency radio applications. Highway and railway tunnels commonly use it for emergency radio communication and, more recently, cellular phone connectivity. The technology has also been installed in mines for underground voice radio communications. Now high-speed wireless technologies such as 802.11n with fast roaming may benefit from radiating cable systems, thus extending automation networks to moving equipment.
Radiating cable is similar to standard discrete antennas in many ways. RF energy is sent through the cable and the signal propagates out. Radiating cable, however, does not have gain. As the cable gets longer, the signal gets weaker. Slots in the shield part of the coax, underneath the insulation, allow the RF signal to propagate out and be received along the length of the cable. This effectively creates a long, flexible RF antenna at over 100 m in length.
There are limitations when compared with more conventional antennas. Radiating cable reduces available bandwidth, so 802.11n performance is slower as compared to a multi-stream MIMO antenna system. The cable itself and all the necessary mounting hardware can make for higher material costs and more complex installation. So, in most industrial wireless applications, traditional discrete antennas are recommended unless they are not capable of reliable radio links due to confined spaces or line-of-sight challenges. But where traditional antennas are not practical, radiating cable is a good alternative to investigate.
Jim Ralston and Gary Enstad are product strategy managers for ProSoft Technology.
Read more on industrial wireless coverage at www.controleng.com/wireless
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