Reality Check – Wireless Lessons from the Real-World
Environmental Effects on Wireless: Electromagnetic Interference Caused by Machinery, Power Distribution or other Nearby Transponders
By John Welch – Apprion Wireless Advisor
Most industrial facilities have various types of electrical machinery, power distribution facilities, sensors and radio transponders within the geographic environment. These systems and devices emit, as an interference, electromagnetic waves. Understanding, locating, identifying, and compensating for electromagnetic interferers is a key element of a successful wireless infrastructure.
In the process of distributing wireless to a geographic area, it is often necessary to place transceivers out of doors. In doing so a prudent designer understands that he cannot afford to overlook the effects of the environment on the life of the equipment and the performance of the wireless system.
Interference in the form of electromagnetic waves from any source is often termed EMI (Electromagnetic Interference). If the source is an intended radio transponder, the term RFI (Radio Frequency Interference) is used. Either type of interference can be a serious problem to a planned or an existing wireless infrastructure causing bandwidth restriction, temporary and repeated wireless connection loss or, in the worst case, wireless equipment failure or damage.
A common misunderstanding is that the majority of interference is caused by other radio transponders, RFI, and therefore it is assumed that "no other transponders" means "no interference" and therefore no need for empirical / measured signal levels. This is not the case.
Although it is true that transponders can and do interfere with each other, all the standards used in the design of equipment have means to predict and compensate for the ordered and logical use of spectra. The result of these standards and practices is that many wireless radios operating within a given area, even radios of different systems, will, with proper planning and configuration, work around each other operating quite happily with only a minimum and usually a negligible amount of degradation of bandwidth.
Spurious signals derived from sparks generated by brushings, commutators, and switching relays are another story. The EMI generated by devices like these are not ordered or predictable, even if operating at low voltages. They can, and often do, emit strong spurious signals over a wide spectra of frequencies and are VERY capable of interrupting or bringing down communications in any system which isn’t optimally designed.
The interference produced by "spark gap" devices is particularly troublesome for base transceivers which are positioned for best reception over a relatively wide geographic area as opposed to hand held devices, which are less sensitive to signal and more shielded by structure and are therefore less affected even when in close proximity of the "offending noisemaker".
The Solution
All too often we hear the "it is what it is" approach to noise within the wireless environment. Though there is no golden arrow, there are many techniques and technologies, which, used together, allow us to deploy in a noisy environment without sacrificing coverage, reliability or throughput.
For wireless infrastructures, the best solution for the problems of magnetic interference starts with the empirical identification of interfering sources. Measurement with a spectrum analyzer is the best method of identifying these interfering sources. By identifying the sources of interference, experienced designers can deploy an arsenal of techniques which not only influence the localized area but the entire geographic area of the wireless infrastructure. The presence, location, and spectral characteristics of the interferers influence and identify the:
- choice of equipment location
- equipment type
- antenna selection
- intended coverage area of the wireless equipment
- configuration of the wireless equipment
- special protections needed for the wired or wireless infrastructure.
Lightning is a serious problem that can at best be guarded against, though not solved, by the careful selection and use of the best protection devices as well as best practices available which will provide a good degree of mitigation. Some suggestions for lightning include:
- For base units higher is not always better - lower the antenna and make use of the structure in the area to "duct" your signal
- Run long Ethernet and power cables in metal conduits
- Use GFI (Ground Fault Interrupt) circuits whenever possible
- Select high quality, industrially rated equipment (e.g., a good metal case with a big backplane offers some protection even against a direct strike)
 John Welch is a Senior Wireless Systems Engineer at Apprion. John has over thirty years experience in technology and engineering with RF, digital and analog devices and various wireless communication and video systems. He brings over 12 years of specialized wireless implementation and design experience with industrial, metropolitan and military wireless infrastructures and systems. Implementations to date include a last mile point to point wireless system under the 802.11 and 802.16 standards, a metropolitan wireless 802.11 deployment in the Auckland, New Zealand central business district and numerous industrial wireless network implementations at oil/gas, chemical and power facilities. In addition, John has developed patents and new inventions for wireless networking systems, antenna design and deployment methodologies.
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