10tips for cheaper emi shielding

10 EMI Shielding tips and tricks

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Example of the different levels in shielding in an electronics enclosure

Principle of shielding

1?The principle of shielding is?creating a conductive layer completely surrounding the object?you want to shield. This was invented by Michael Faraday and this system is known as a Faraday Cage.

2?Ideally, the shielding layer will be made up of conductive sheets or layers of metal?that are connected by means of welding or soldering, without any interruptions. The shielding is perfect when there is no difference in conductivity between the used materials. When dealing with frequencies below 30 MHz, the metal thickness affects shielding effectiveness. We also offer a range of shielding methods for plastic enclosures. A complete absence of interruptions is not a realistic goal since the Faraday cage will have to be opened from time to time so electronics, equipment, or people can be moved in or out. Openings are also needed for displays, ventilation, cooling, power supply, signals, etc.

3?Shielding works in both directions,?items inside the shielded room are shielded from outside influences. (Fig. 3.1)

Figure 3.1: Shielding works in both directions

4?The quality of the cage?is expressed as the ratio of the field strength in Volts/meter (V/m) inside the cage and outside the cage.

5?It is common practice to?present field strength Figures in a logarithmic scale (in dB).

6?The reduction depends on the frequency?in Hz. Each frequency has a wavelength in meters. For example 100 MHz = 100.000 kHz = 3 meter. For a better explanation, see the table below.

40 dB100 times reduction of the field strength60 dB1.000 times80 dB10.000 times100 dB100.000 times120 dB1 million times140 dBVery difficult to measure and only used in scientific applications

Waves

7?A wave is a combination of electric field and magnetic fields.

An electromagnetic wave is composed of a magnetic part depending on the electric current (ampere), and an electrical section, depending on the electrical voltage (volts). Near the source (near-field) the magnetic part is dominant. At a greater distance, the electrical part and the magnetic part are present in a fixed ratio (far-field). (Fig. 7.1)

Figure 7.1: Wavelength vs. Frequency

8?The material thickness determines which frequencies?are blocked from penetrating into or out of the cage. For low frequencies like 10 kHz (generally the near-field/magnetic fields), a mild steel layer of 6 mm is needed to achieve a reduction of 80 dB, but a frequency of 30 MHz can be shielded by a copper foil that is only 0.03 mm thick. For higher frequencies in the GHz area, the mechanical strength of the used shielding material will generally specify the thickness of the shield.

9?For very low frequencies and DC,?where the magnetic field is dominant, besides thick layers also special materials like Mu-metal and Mu-ferro alloys are needed. In addition, combinations of multiple layers are required to get sufficient shielding performance. Please consult our engineers.

10?When a wire penetrates a shield?that is not completely connected to the shield, it will work as an antenna and this reduces the shielding performance of the cage. This is especially the case at higher frequencies. (Fig. 10.1)