EMI Shielding: Principle Mechanisms Of Shielding and Kinds Of EM Fields
EMI shielding is the process of reducing or preventing the transmission of electromagnetic waves or noise signals from the source to the receiver. The purpose of using EMI shielding material on cables is to shield the internal conductors from the electrical field, thus reducing the impact of the electromagnetic field. When a conductor comes in contact with these fields, it produces a voltage that manifests as an annoying noise onto a system of circuits. These circuits perform a special function in an electronic device. Therefore, when a shielding material is applied around the conductor, the energy from the electrical field is drained away, thus lessening or eliminating the noise without affecting the performance of the conductors.
Principle Mechanisms of Shielding
Do you know what happens when an electromagnetic wave moves through space and comes in contact with a shield? When an EMI shielding material is used, most of the energy is reflected and the remaining portion is absorbed by the shield. The dual effects of absorption and reflection are independent of each other. However, when combined, they result in the true effectiveness of EMI shields. Re-reflection is yet another factor that needs to be considered if you’re using extremely thin shields. It happens at the boundary of the shield material.
Most of the high-frequency problems associated with shielding are caused due to openings in the material, and not necessarily by the material you are using. Make sure that you use copper, aluminum or mild steel as they provide considerable shielding effectiveness. You can consider aluminum foil that offers 85dB of shielding effectiveness at frequencies ranging from 30 to 100 MHz. However, aluminum foil is not a good material to use in lower frequencies. Use highly permeable ferrite material or thick steel to provide shielding against low frequency electromagnetic fields.
Kinds of EM Fields
When considering EMI shielding, it’s also important to know about the three kinds of EM fields that occur. Due to the presence of these fields, the same shielding material can behave in a different manner under diverse operating situations.
Electric Fields: When the energy field is less than one-sixth of the wavelength from a high impedance source, the wave impedance is called ‘near field source’. And, when a capacitive energy has an influence on the field during the near field effect, the loss of EM is likely to be more. That’s why it’s easy to do effective shielding from the damaging electrical fields.
Plane Waves: As far as plane waves are concerned; they exist in about one-sixth of a wavelength from where they emanate. Under this condition, the electric field ratio in comparison to the magnetic field remains constant. This is also called for field radiation. Radio wave is a suitable example of this radiation type where at 30 MHz a wavelength is conveyed as 10 meters. If there is a transmitter that’s 1.6m away the source, it’s is known as far field.
Magnetic Fields: If the magnetic field lies in close proximity to a low impedance source, a near field energy source is developed. But, there is a difference in this situation as the inductive energy prevails. As far as reflection losses are concerned, they are pretty less effective because of lower wave impedances. And, the effect will continue when there is a dip in the frequency level. This is the reason why EMI shielding is less effective when applied in low-frequency magnetic fields.
Use the right EMI shielding material to protect your electrical devices and appliances from electromagnetic interference or noise disturbances. Consult with an experienced EMI shielding product supplier to understand your precise requirements.