Products & Solutions

What is electromagnetic shielding?

What is electromagnetic waves?

Image of electromagnetic waves
Electromagnetic waves are waves consisting of two components: an electric field and a magnetic field. Generally, waves refer to the phenomenon of vibrations being transmitted, but in the case of electromagnetic waves, electric and magnetic fields alternately vibrate as they travel through space. At a point far enough away from the radiation source, the electric and magnetic fields oscillate in a plane perpendicular to the direction of travel, and the directions of oscillation of the electric and magnetic fields are perpendicular to each other. The strength of electromagnetic waves per unit area is generally expressed in V/m for electric field strength and mG for magnetic field strength.

Principle of electromagnetic shielding

The principle of electromagnetic shielding is to attenuate electromagnetic energy by reflecting, absorbing, and multiple reflections of electromagnetic waves. By attenuating electromagnetic waves, danger to the human body and effects on precision equipment are avoided.
Also, depending on the frequency, electromagnetic waves can travel straight or be diffracted, just like light. When installing shielding material, pay attention to the arrival of electromagnetic waves due to wraparound.

reflection loss

Principle of electromagnetic shielding Reflection loss
This is the loss that occurs at the interface between the shielding material surface and the air. It is not related to the thickness of the shield, but is affected by the conductivity of the shielding material.

absorption loss

Principle of electromagnetic shielding Absorption loss
This is loss due to overcurrent that occurs when passing through the shielding material. It is affected by the thickness, magnetic permeability, and conductivity of the shielding material.

Multiple reflection loss

Principle of electromagnetic shielding Multiple reflection loss
This is the loss caused by repeated reflections inside the shielding material.

How to measure electromagnetic shielding

Advantest method

This is a standard measurement method for evaluating electromagnetic shielding characteristics.
Electromagnetic waves of a specific frequency are emitted within a shield box, the electric and magnetic fields passing through the sample are received by the other side, and the attenuation due to passing through the sample is measured. Measurement is possible in the frequency range of 10 to 1,000MHz, and the shielding rate of electric and magnetic fields corresponding to the frequency is specified.

Resistivity

Resistivity is used as a physical property value that represents electromagnetic shielding performance. In other words, the lower the resistivity, the higher electromagnetic shielding properties. There are three types of standards: insulation resistance, volume resistance, and surface resistance.

Description of shield performance

Shielding performance is expressed using dB (decibels). A number that relatively expresses how much electromagnetic waves are attenuated, and is a logarithmic expression of the ratio (attenuation amount) of the electric field strength before shielding to the electric field strength after shielding.

Shielding performance (dB) = 20 × log (electric field strength after shielding / electric field strength before shielding)

For example, a shield can block electromagnetic waves.

  • -20dB when it becomes 1/10 (shield rate: 90%)
  • -40dB when reduced to 1/100 (shield rate: 99%)
  • -60dB when reduced to 1/1,000 (shielding rate: 99.9%)
  • -80dB when reduced to 1/10,000 (shielding rate: 99.99%)

I express it as
In order to prevent malfunctions of electronic devices such as computers, it is generally said that a shielding performance of 30 dB or higher is desired.

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TOYOCHEM CO., LTD. Information and Communication Materials Sales Division

TOYOCHEM CO., LTD.