What is Electromagnetic Wave Shield ?

• What is an electromagnetic wave ?
• Principle of electromagnetic wave shield
• Measuring method of electromagnetic wave shielding performance
• Shielding performance index

What is an electromagnetic wave ?

An electromagnetic wave consists of two components: an electric field and a magnetic field. Generally, a wave refers to a phenemenon where vibrations are transmitted. In the case of an electromagnetic wave, an electric field and a magnetic field are transmitted through the space while vibrating alternately. At a point that is well away from the radiation source, the electric field and the magnetic field vibrate on a surface that is vertical to the traveling direction, and the directions of the vibrations of the electric field and the magnetic field are vertical to each other. Generally, the intensity of the electromagnetic wave per unit area is indicated with V/m and mG, which apply to the intensity of the electric field and that of the magnetic field, respectively.

Principle of electromagnetic wave shield

An electromagnetic wave shield reduces the energy of electromagnetic waves by means of the reflection, absorption, and multiple reflection of the waves. By attenuating the electromagnetic waves, the shield avoids disruptions to human body and precision equipment.
In addition, electromagnetic waves not only go straight but may also be diffracted just as light is, depending on the frequency. Accordingly, when using a shielding material, it should be kept in mind that the electromagnetic wave may be diffracted around the shield and reach the part to be shielded.

Reflection loss

This loss occurs at the boundary between the shielding material surface and the air. The level of the loss is determined by the conductivity of the shielding material, irrespective of the thickness of the shield.

Absorption loss

This loss occurs due to the excess current that is generated when the electromagnetic wave passes through the shielding material. The level of the loss is determined by the thickness, magnetic permeability, and conductivity of the shielding material.

Multiple reflection loss

This loss occurs where the electromagnetic wave is reflected repeatedly in the shielding material.

Measuring method of electromagnetic wave shielding performance

Advantest method

This is a standard measurement method for assessing the properties of an electromagnetic wave shield.
The measurement is made by transmitting an electromagnetic wave of a specific frequency inside the shield box, receiving the electric field and magnetic field that pass through the sample, and measuring the level of attenuation caused by passing through the sample. The measurement can be made within the range of 10 to 10,000 MHz, and the shielding ratios of the electric field and magnetic field corresponding to the frequency are to be clearly stated.

Resistivity

With this method, resistivity is used as an alternative to the electromagnetic shield property. In other words, the lower the resistivity, the higher the electromagnetic shield property. There are three standards: insulation resistance, volume resistance, and surface resistance.

Shielding performance index

Shielding performance is expressed in decibels (dB). The value expresses the degree to which the electromagnetic wave was attenuated. The ratio between the intensity of the electric field before shielding and that after shielding (attenuation) is expressed in log.

Shielding performance (dB) = 20 x log(intensity of electric field after shielding / intensity of electric field before shielding)

For example, if shielding has reduced the intensity of the electromagnetic wave to;

• 1/10,the value is -20dB (shielding ratio: 90%)
• 1/100, the value is -40dB (shielding ratio: 99%)
• 1/1000, the value is -60dB (shielding ratio: 99.9%)
• 1/10000, the value is -80dB (shielding ratio: 99.99%)

It is said that shielding performance of 30dB or above is generally desirable to prevent the malfunction of electronic equipment such as computers.
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