RF shielding materials either absorb or reflect EMI radiations, ensuring the device is immune to RFI.
RF shielding effectiveness is the figure of merit that qualifies the ability of an RF shield to attenuate the electromagnetic field.
The higher the RF shielding effectiveness of the designed shield, the less of an impact the RFI has on the electronic device or circuit.
Conductive or magnetic materials are used for RF shielding
The influence of the electromagnetic field can affect the performance of electronic circuits and devices. Because of this, it is critical to provide RF shielding to block or absorb electromagnetic waves from interfering with the functioning of electronics devices.
Usually, conductive or magnetic materials are used to make RF shields. The materials that have high RF shielding effectiveness include metal enclosures and Faraday’s cage. In this article, we will discuss RF shielding, its different types, and the materials used to provide the best RF shielding effectiveness.
The effect of electromagnetic interference (EMI) introduces disturbances in the functioning of nearby devices. When the electromagnetic waves disrupting the normal operation of electronics are radiofrequency waves, then the interference is called radio frequency interference (RFI). RFI can cause devices to malfunction if proper RF shielding techniques are not utilized.
RF shielding protects devices from the exposure of unintended radiofrequency waves or electromagnetic waves. RF shielding materials either absorb or reflect EMI radiations, ensuring the device is immune to RFI. RF shielding also prevents radiated emissions from emanating from devices.
Types of RF Shielding
RF waves can influence the functioning of individual components as well as the entire system. Depending on the severity of the RFI effects, RF shielding can be classified into different types:
Component shielding: A component shield encases a component that is susceptible to RFI or radiating RFI.
Board shielding: A section of a PCB or electronic equipment can be encapsulated inside an RF shield to prevent the detrimental effects of RFI from reaching other circuits.
Cable shielding: The parasitic reactance in high-frequency circuits aggravates the effect of RFI in cables carrying analog or digital signals. Incorporating RF shields on cables can minimize the RFI in cables.
RF Shielding Materials
RF shielding relies on both the reflection and absorption of electromagnetic radiations. Reflections result in RF waves bouncing off the surface of shielding materials. However, sometimes this is not enough; reflections can be extremely harmful to humans, and in some cases, lead to death. Therefore, the absorption of radiation is much safer.
There are a variety of shielding materials, each with unique properties. To ensure safety, it is important to select the appropriate RF shield. Metals are excellent in blocking electromagnetic energy from interfering with a device's operation. RF shielding can be provided in the form of metal sheets, metal screens, or metal foams. Commonly used metals in RF shields are:
Copper: Copper RF shields can be easily manufactured into any shape. The high conductivity property of copper makes it an effective RF shielding material.
Mu-metal: Mu-metal is a nickel-iron ferromagnetic alloy. The metal possesses high permeability and is effective in shielding electronics from static and low-frequency magnetic fields.
Aluminum: Aluminum is used in the form of foil or an enclosure to shield against low-frequency radio fields.
One of the disadvantages of metal-based RF shields is that metals are heavy, rigid, and costly. These drawbacks can be overcome by materials that are flexible, light, and cheap. The RF blocking properties exhibited by certain paints, inks, coatings, resins, and fabrics can be utilized as an alternative for RF shielding in some cases.
RF Shielding Effectiveness
RF shields weaken interference and protect electronic circuits. RF shields establish enclosures that provide a shield between the emitter and susceptor of interference, thus diminishing the electromagnetic field strength.
RF shielding effectiveness is the figure of merit that qualifies the ability of an RF shield to attenuate the electromagnetic field. The shielding effectiveness is mathematically defined as the ratio of electromagnetic field strength before and after the placement of RF shields and is expressed in decibel (dB).
RF shielding effectiveness can also be called electric field shielding effectiveness (ESE) since it involves the ratio of electric field magnitudes. The RF shielding effectiveness can also be determined by taking the ratio of magnetic field amplitudes before and after the RF shield placement, which is why it is also sometimes referred to as magnetic field shielding effectiveness (MSE). ESE is equal to MSE if the medium is air on either side of the RF shields with plane waves as the incident waves.
Factors Influencing RF Shielding Effectiveness
RF shielding effectiveness is dependent on factors such as:
- The frequency of the incident electromagnetic field.
- Properties such as permeability, conductivity, and permittivity of the RF shielding material.
- The thickness of the RF shielding material.
- The type of electromagnetic field sources such as the electric field, magnetic field, or plane wave.
- The distance between the emitter of RF interference and the shield.
- Apertures, seams, or penetrations degrading the RF shield.
- The bonding quality between the RF metal shield surfaces.
It is important to design RF shields with high RF shielding effectiveness. The higher the RF shielding effectiveness of the designed shield, the less of an impact the RFI will have on the electronic device or circuit.
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