RFI conductive coating FAQ’s

Summary

What is EMI/RFI shielding?

Electromagnetic waves are attenuated, i.e. their energy is reduced, when they pass through a grounded conductor.

The most straight forward form of EMI/RFI shielding is a metal enclosure. A good example of this is shielded twisted pair wiring, where a twisted –pair wire is put inside a metal tube. The EMI is first reduced through the twisted pair technique, and then the energy of the remaining EMI generated by the wires is then eliminated almost completely as it passes through the metal enclosure.

Most all electronic devices to require some level of shielding for the overall device, however, they also commonly require shielding at the circuit level. When EMC has not been optimized by proper circuit design, individual components and areas of a circuit must be shielded from each other.

What is EMC?

Electromagnetic compliance is the condition of an electronic device being designed so that it is both protected from EMI/RFI, and does not emit significant amounts of EMI/RFI that might cause other devices to malfunction. EMC is a subset of electrical engineering and a critical part of any electronic design.

EMI was first noticed when the North American electrical grid was begun, and electrical wires started to cause interference in existing telegraph wires. Ever since then, the amount of EMI/RFI generated by the increasing number of electronic and electrical devices in our society has increased exponentially, and as many of these devices get smaller, operating at lower voltage and higher frequencies.

Throughout the 20th centuries, various government and industrial organizations passed laws and guidelines for EMC that modern devices must meet. The most prominent organization in this regard in the United States is the FCC, the Federal Communications Commission. In Europe, devices must pass rigorous EMC tests in order to comply with mandatory EC labeling. A prominent industrial organization in this regard is the Society of Automotive Engineers.

How is EMC achieved?

Most EMC is achieved through good circuit design. The EMI created by one part of a circuit is nullified by engineering another part of the circuit to make an opposing EMI, which when combined cancels the EMI out. Such engineering not only eliminates the creation of EMI, but it also makes the circuit resistant to external sources of EMI.

One of the most basic engineering techniques demonstrating this wave cancelling technique is twisted pair wiring, where one wire is run in opposite direction to another, and they are twisted together, so that the EMI produced by one wire is cancelled by the EMI produced by the other.

Understanding the wave nature of EMI

It is important to understand that waves are two dimensional objects. Picture an EM wave as a piece of paper flying lengthwise through the air. The width of the paper is the amplitude of the wave, and the length of the paper is the wavelength. For purposes of understanding this example, assume the paper is actually infinitely thin.

Now try to fit this paper through your window. If you open your window just the tiniest amount possible, you will see that you can slip this piece of paper through that little crack. However, if you open the window wide, but you have a screen in it to prevent bugs from flying through; you will see that you cannot fit the paper through the holes in the screen. Despite the fact that the screen is not solid, you can’t get the paper through, yet in the case of the tiny wide crack, the crack in the window might be less than the distance between the wires in the screen, but you can still get the paper through it.

The point of this example is to help you understand that EM waves can easily escape through seems in electronic enclosures, but they cannot pass through holes, as long as the hole is significantly smaller than the wavelength of the wave. So one can have a metal box with an EMI generating device in it, and you can have rather large holes in your box, and the device will still be perfectly shielded as long as the box does not have a seem.

If the box is hinged, so that it opens and closes, likely the two sides of the box are not in perfect contact at all points, and EMI will easily pass through the seam, just like the paper passed through the tiniest crack in the window.

This leads to the concept of a Faraday Cage, where a mesh box is used to shield a device from EMI/RFI.

What is EMI and RFI?

EMI stands for Electromagnetic Interference. It refers to the phenomenum where electromagnetic radiation interferes with the operation of an electronic or electrical device or system.

RFI (Radio Frequency Interference) is a subset of EMI, where the electromagnetic radiation in question is confined to the radio wave portion of the electromagnetic spectrum. EMI tends to be a shorter range phenomena, often occurring when the distance between circuits of devices is less than one wavelength, whereas RFI tends to be a longer range phenomena.

EMI and RFI are created by nearly every electronic device, and the operation of nearly every electronic circuit can be negatively affected by external sources of EMI/RFI. RFI is everywhere in today’s environment with radio communication devices filling the atmosphere with radio signals. Solar activity is also a very large generator of RFI and there are many sources outside of our solar system and galaxy that create significant radio signals, not the least of which is the cosmic background radiation left over from the big bang.

EMI / RFI together refer to the phenomena where electronic and electrical devices and wires both create and are affected by electromagnetic waves.