A Primer on Electromagnetic Shielding


Electromagnetic radiation passes through the air and can interfere with the proper operation of various types of equipment. This is one of the basic reasons that EMI testing and EMC testing is performed. Circuit should be designed to both minimize the amount of electromagnetic radiation (interference) that it generates and also to tolerate electromagnetic interference from the environment by providing sufficient Electromagnetic Shielding.


Sometimes the circuit design alone is not sufficient to properly protect against excessive levels of electromagnetic radiation emanating from the equipment or affecting the equipment. Electromagnetic Shielding is an option that is commonly used in these cases.


Electromagnetic radiation is a wave. The Electromagnetic Shielding can block the wave if designed properly. This Electromagnetic Shield can work in both directions, blocking emissions from the equipment and blocking electromagnetic radiation that may affect the unit. Thus it is good practice to improve EMC testing and EMI testing alike.


A basic Electromagnetic Shield is to put the equipment in a metal box. Electromagnetic radiation (RF) cannot penetrate metal.


Basic Electromagnetic Shield - Metal Box

A metal box is a great fix since there would be no Emissions from the circuit and no way the circuit would be affected by the outside electromagnetic environment. However, the usefulness of the circuit would be limited if we cannot get to it, electrically, visually, or mechanically.


To access the circuit and give it usefulness we need to penetrate the electromagnetic shield (metal box). This is where the problems begin.


RF can penetrate where there are apertures, penetrations and/or seams in the enclosure. The performance of the electromagnetic shielded enclosure depends on how the apertures, penetrations and seams are treated.


Apertures



  • It is best to keep apertures small, down to a fraction of the smallest wavelength of the frequencies of concern. Typically, this is 1/20th of a wavelength of the highest frequency.

    • The formula for calculating the wavelength is: λ = v/f; where

      • λ = wavelength in meters

      • v = velocity of propagation in meters per second {use 3x 108}

      • f = frequency in Hz


Seams



  • A seam is anywhere on the electromagnetic shielded enclosure where one metal piece meets another.

  • A door or hatch is considered a seam!

  • If the bonding between the parts is not good, then a voltage can develop across the seam. If a voltage can be developed, then RF can go through the seam.

  • The best connection for an electromagnetic shield is metal to metal contact. This will produce a short circuit that no voltage can be developed across.

    • You want to maintain continuous contact between the metal pieces. Flat surfaces that are machined are best, with tight screw spacing. Typically less than 1 inch between screws is best. This can be extended to larger spacing by use of thicker stiffer materials that will not flex and lose contact between the screws. Also, it is possible to use a gasket material such as finger stock to maintain the continuous contact between the metal parts.
  • Anti-corrosion coatings are not always conductive and can cause problems for an electromagnetic shield.

    • Metal plating is better than alodine or iridite. They are not very conductive. These are conversion coatings that convert aluminum to aluminum oxide.

    • Anodizing is non-conductive!

    • Paint is non-conductive. Even so called conductive paints are not very conductive.



Penetrations (Wires and Cables)



  • Every wire carries a voltage (if not, then there is no reason to have them!). If there is voltage, then there is an electric field.

  • These wires and/or cables can carry electromagnetic radiation into the enclosure and disrupt the circuit. They can also carry the electromagnetic radiation generated by the circuit out of the enclosure.

  • Common ways to eliminate this conduction of RF energy are:

    • Filters for wires, such as power and simple signals.

    • Shielded cables with the shields terminated to the enclosure for more complex cables. The best connection is via an EMI type connector that terminates the electromagnetic shield 3600 at the connector backshell.
  • Another typical electromagnetic shield penetration is a fiberoptic cable. Typically there are bulkhead type connections for enclosures that act like a waveguide beyond cutoff to block any RF energy from entering or leaving the enclosure.



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