What is electromagnetic shielding?
- What is electromagnetic waves?
- Principle of electromagnetic shielding
- How to measure electromagnetic shielding
- Description of shield performance
What is electromagnetic waves?
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
absorption loss
Multiple reflection loss
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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