Overcoming Signal Attenuation in Industrial Wireless Transmission

Wireless signals facilitate mobile and remote connections
of industrial process measurement and control equipment

Wireless connections for process measurement and control devices continue to grow in their application range and adoption. The ease with which remote, mobile, even nearby devices and equipment can be connected to monitoring and control stations keeps wireless connectivity a considered option for many facility modifications and additions.

How do you know if the wireless connection you intend to establish will work? It's not a question of whether the gear will function properly, but one of whether the signal will be able to find its way from point A to point B. Signal attenuation is the reduction in signal strength that occurs along the path between two points. Too much attenuation and the signal is not effective in delivering data to the destination. Two main elements contribute to signal attenuation.

Distance

Radio signals deteriorate over the distance traveled. This attenuating factor can be overcome by boosting transmission power, but regulatory limits are in place that disallow much in the way of increasing transmission power. Keeping in mind that signal attenuation at a level that renders a transmission indecipherable, just caused by distance alone, requires a very substantial distance. That said, there are several effective solutions that can be put in place. One involves increasing the height of the transmission antenna. Another is to install a repeater at a point along the transmission path. A repeater, properly placed, will effectively receive the signal from the transmitter, then transmit a new signal of greater strength that replicates the original received data. The use of repeaters can greatly extend the distance spanned by a wireless transmission.

Obstructions

Many are familiar with the objective of establishing "line of sight" transmission paths between transmitter and receiver. Physical structures and materials of all types should be considered detrimental to the transmission of wireless signals. A basic understanding of Fresnel zones is helpful in overcoming the barriers presented by physical obstructions. As with distance, antenna height or location can be a significant factor in dealing with the challenge of obstructions. Repeaters, described earlier, can be instrumental in getting your signal over or around otherwise impassable obstructions.

Routing wireless signals presents different, but no more complicated, challenges than routing cable. The tools are different, the medium is different, but you are still just trying to find a way from point A to point B. With experience, wireless signal propagation becomes is mundane is routing conduit.

Share your industrial wireless ideas and challenges with the experts at Analynk Wireless. Analynk Wireless manufactures equipment used to establish wireless process connections across the room, across the plant site, across the highway, and around the globe.

Practical Considerations for Wireless Transmission of Industrial Process Control Signals

Industrial process signal transmitter

Rigging up the proper gear to establish wireless transmission of process measurement signals is generally a straight forward task. There are, however, a vastly different set of considerations than those for a wired transmission of the same signal. In order to select the right equipment for the job, some general comprehension of radio signals can be useful.

Radio wave frequencies are below the infrared range on the electromagnetic spectrum, thus their wavelengths are comparatively long. Three things can happen to electromagnetic radiation (radio waves) when encountering a barrier. 

• Reflectance: The wave bounces off the barrier.
• Transmittance: The wave passes through the barrier.
• Absorbance: The wave is stopped.

Which of the three possibilities will occur depends upon a number of factors relating to the signal and the barrier, some of which include:

• The wavelength of the radiation
• The intensity of the radiation hitting the barrier
• The chemical composition of the barrier
• The physical microstructure of the barrier
• The thickness of the barrier

Here is a conglomeration of knowledge items pulled together from a number of public sources that can be applied when considering a wireless installation.

Milliwatts (mW) are the common measurement unit of radio frequency (RF) power. A logarithmic scale of decibels, referencing 1 mW as the zero point, provides a useful way to express the comparative strength of RF signals. Using decibels, a signal strength of 1 mW is registered as 0 dBm. RF power attenuates according to a logarithmic function, so the dBm method of expressing RF power enjoys widespread use.

Industrial wireless communications applications in North America predominantly operate in either the 2.4 GHz or 900 MHz frequency range. Higher frequency will provide more bandwidth, but at the cost of reduced transmission distance and obstacle penetration. Lower frequency can require a larger antenna to attain the same signal gain.

Transmission power is not the only solution for delivering a signal. Low power signals can be successfully received by sensitive radio equipment. Reducing the data transmission rate can increase the functional sensitivity of the receiving equipment, too.

Be mindful of the existence or potential for RF background noise in your communications environment. A higher level of background noise can hamper the effectiveness of your equipment. The "noise floor" varies throughout the frequency spectrum and is generally below the sensitivity level of most equipment. Industrial environments can sometimes provide unusual conditions which may warrant a site survey to determine the actual noise floor throughout the communications area.

Weather conditions can impact signal transmission

Radio transmission is susceptible to environmental elements on a variable basis. Since the environment can change without notice, it is useful to know the fade margin of a wireless installation. Fade margin expresses the difference between the current signal strength and the level at which the installation no longer provides adequate performance. One recommendation is to configure the installation to provide a minimum of 10dB of fade margin in good weather conditions. This level can provide sufficient excess signal strength to overcome the diminishing effects of most weather, solar, and interference conditions.

There are a number of simple methods to determine whether an installation has at least a 10 dB fade margin. Temporarily installing a 10dB attenuator on the system antenna, or installing a length of antenna cable that yields 10dB of attenuation will allow you to determine if the installation can accommodate 10dB of environmental impact on the signal. If the system operates suitably with the attenuation installed, you have at least that much fade margin.

RF signals attenuate with the square of the distance traveled, so if transmission distance is to be doubled, then the signal power must increase fourfold. 

True “line of sight” signal paths are found in a limited number of installations. The number, type, and location of obstacles in the signal path can have a significant impact on the signal and contribute to what is referred to as path loss. Probably the simplest way to reduce the impact of obstacles is to elevate the antennas above them.  Obstacles, in almost every case, are affixed to the earth, so their interference is reduced by elevating antennas to “see” over the obstacles.  

Wooded areas can be a significant barrier

When the signal path extends through an outdoor area, weather conditions have an impact on the path loss, with higher moisture levels increasing the loss. Large plants, most notably heavily wooded areas, can impose substantial reduction on RF signals and may require elevating antennas above the trees or using repeaters to route the signal around a forested area.

Industrial installations routinely present many reflective obstacles in the signal path. The transmitted signal may reflect off several obstacles and still reach the receiving antenna. The received signal strength will be the vector sum of all the paths reaching the antenna. The phase of each signal reaching the antenna can impact the total signal strength in a positive or negative way. Sometimes relocating the antenna by even a small amount can significantly change the strength of the received signal.

Antenna cable 

Antenna cable contributes to signal attenuation. Use high quality cable of the shortest length possible to minimize the impact on performance.

Analynk Wireless has the equipment and expertise to help you deliver wireless process signals across the room, across the street, or across the globe.