Comparison of 900 MHz vs 2.4 GHz for Industrial Wireless Connectivity

IOSelect provides equipment to establish wireless
process signal connections.
Image courtesy IOSelect

Wireless transmission of measurement and control signals are the future, and present, of process control. WiFi is already prevalent in higher density environments and providing benefits of reduced cabling and more. Wireless communications also can be used to connect devices over substantial distances, even globally. This article will focus on applications of moderate to long distance that will employ point to point communications of dedicated devices.

In establishing a wireless process signal connection between two points, an initial consideration will be whether to employ 900 MHz or 2.4 GHz as the radio band. There are some general implications associated with the selection.

• Signal attenuation over any distance is greater for 2.4 GHz than 900 MHz. This generally means that 900 MHz can cover a greater distance and provide a signal of sufficient strength to properly communicate.
• Atmospheric attenuation for either frequency band is about the same, with a very slight advantage to 900 MHz.
• Both frequencies require "line-of-sight" to provide predictable and reliable operation. Obstructions within that zone can degrade the signal. Any obstructions with dimensions approximating the wavelength of the signal tend to have a greater impact. The wavelength of a 2.4 GHz signal is 12.5 cm (4.52 inches), 900 MHz is 33.3 cm (84.6 inches). 2.4 GHz signals are susceptible to interference by smaller objects in the transmission path than are 900 MHz signals.
• Without getting too technical, the height of a 900 MHz antenna will need more elevation than that of a 2.4 GHz antenna in order to provide what is known as "free space propagation". This is related to the Fresnel Zone and has greater impact as transmission distance increases.
• FCC rules allow larger transmit power ratings for 2.4 GHz radio signals than 900 MHz, increasing the potential range for 2.4 GHz.

Having a general understanding of the factors that vary between 900 MHz and 2.4 GHz and how they might impact your installation can lead to a better project outcome. Evaluate your potential installations with the above points in mind. Their impact on any particular application can vary depending upon the distance, topography, and potential obstructions. Share your wireless communications challenges with application specialists. Combining your site and process knowledge with their product application expertise will produce an effective solution.

Industrial Wireless Communications - Process Signals from Hile Controls of Alabama, Inc.

Wireless Transmission of Industrial Process Control Signals - Practical Considerations

Establishing wireless process signal connections
requires consideration of a different set of factors
than a wired installation.
Image courtesy IOSelect

Establishing wireless connections for the 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.

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.

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

Share your connectivity challenges with application specialists, leveraging your own knowledge and experience with their product application expertise to develop an effective solution.

Programmable Automation Controller Flexes into Many Applications

One of several programmable automation controller
backplane configurations for the T2750.
Image courtesy Eurotherm

The Eurotherm T2750 is a high performance modular control unit that provides redundancy and capability that are unmatched in a consolidated single unit. The controller backplane, of which there are several variants to accommodate the scope of I/O needed for a wide range of process applications, can be populated from an array of I/O and function modules providing a customized setup that closely matches project requirements.

Capabilities of the programmable automation controller include:

• I/O Block
• Communications
• Signal Conditioning
• Control
• Timing
• Logic
• Math
• Valve and Motor Control
• Diagnostics
• Recorder
• More

There is much more to learn about the highly capable T2750 Programmable Automation Controller. For more information, share your process automation and control challenges with a product application specialist, leveraging your own process knowledge and experience with their product application expertise to develop an effective solution.

Programmable Automation Controller T2750 from Hile Controls of Alabama, Inc.