Understanding RFI and EMI’s effects
A troublesome offshoot of radio frequency (RF) technology is radio frequency and electromagnetic interference, or RFI and EMI, respectively. RFI is unique to a particular RF spectrum and is a narrowband phenomenon, while EMI is applicable to the entire electromagnetic spectrum and is a broadband phenomenon.
RFI typically manifests as narrowband interference. AT 2.4 GHz, for example, in the unlicensed band, several ordinary household items can produce RFI and produce interference in this band. In the 5 GHz band, it is Wi-Fi equipment that can cause interference to technology such as Doppler and approach radar systems. This allows for dynamic frequency selection (DFS) and transmits power control (TPC) to allow for peaceful coexistence between systems. Even cordless telephones produce a significant amount of RFI, but newer wireless devices are capable of rejecting much of this RFI.
EMI, meanwhile, is a much larger and more challenging problem for users. EMI can render a wireless local area network (WLAN) completely useless faster than any determined miscreant hacker using a jammer tuned to the target frequency. EMI can come from many sources and while some are systemic and cannot be eradicated most of the time they can be contained and isolated. This is always a challenge for the designer engaged in RF work.
EMI is electrical noise that is uncontained and is propagated over a wide range of frequencies that exists at a base frequency and produces multiple harmonics. For example, the earliest radio transmitters used rotary spark gap devices to generate a carrier. This method was used before the invention of the vacuum tube oscillator. It was found that a receiver placed close by a spark gap transmitter was completely useless. This was because the transmitter generated not only the basic carrier frequency, but also all of the harmonics associated with that frequency—in essence, it blanketed the entire RF spectrum with noise. Using these transmitters is now banned in the U.S.
Most rotating machinery generates a significant amount of electrical noise. DC motors, in particular, such as those in small hand tools and subway traction motors, are very noisy. The slip rings and brushes used tend to "arc and spark" and as a result produce a lot of electrical noise similar to a spark gap transmitter. Variable frequency drives (VFDs), while extremely efficient and useful, can and will disrupt both wired and wireless communication if not properly located and associated with wiring that is isolated through proper containment and routing. Even wired networks will suffer greatly from being in proximity to VFDs. Enclosing or isolating noisy equipment is another way to deal with an intransigent noise problem. Placing VFDs in their own space is one recommended approach, preferably in the form of a Faraday cage, which uses a metallic conduit throughout to effectively eliminate the vast amounts of EMI these devices produce.
Any length of wiring will act as an antenna. This includes power wiring, audio cables, and lamp cord and signal transmission lines. A simple rule of thumb is the longer a wire is, the better an antenna it becomes. It is essential to reduce the "loop length" to reduce the tendency for wiring to pick up noise. Shielding is very effective in reducing the interfering signals, hence the use of shielded cables in RF applications. Placing the wires in a metallic conduit will virtually eliminate the problem of interference. Grounding rarely solves noise problems and in many cases will make it worse; differences in ground potential between two different locations or equipment will play havoc on either system by creating "ground loops" which are very difficult to resolve.
One reason to be cognizant of EMI and its effects is that WLANs are becoming more common in industrial facilities. EMI is a difficult issue to harden a network against. Proper design methods and retrofits of existing wiring infrastructure can make a big difference in the noise background of the facility. This will lead to a much better, cleaner RF propagation environment. If the noise floor can be lowered by 3 dbm, then it has been halved. Noise floor measurements come into play when determining the link budget; a high noise floor and a receiver with low sensitivity will cancel out any useable signal. EMI and RFI can be reduced or eliminated by good design practice. Avoiding the use of improperly applied cables or enclosing them in metallic conduit will go a long way to producing a clean RF environment.
A well-planned and executed site survey will identify most RF energy sources on site. Using this tool in the early stages of a system design will help ensure that a wireless system can be implemented successfully in spite of a less than favorable RF propagation environment. Producing a map that localizes sources of uncontained RF energy, such as noisy machinery or other wireless systems will help the designer to work around these sources of potential interference.
EMI is present everywhere on some frequency or spectrum, and it is always with us like bad weather, as the expression goes. However, with prudent and thoughtful design that takes advantage of the latest tools and materials, the effects of EMI can be substantially reduced or eliminated.
– Daniel E. Capano, owner and president, Diversified Technical Services Inc. of Stamford, Conn., is a certified wireless network administrator (CWNA); firstname.lastname@example.org. Edited by Chris Vavra, production editor, CFE Media, Control Engineering, email@example.com.
www.controleng.com/blogs has other wireless tutorials from Capano on the following topics:
WLAN troubleshooting best practices
Wireless intrusion detection and protection systems
Integrating a wireless LAN into an existing wired LAN
www.controleng.com/webcasts has wireless webcasts, some for PDH credit.
Control Engineering has a wireless page.