Electromagnetic shielding
Barriers of conductive or magnetic material reduce or redirect electromagnetic fields.
Electromagnetic shielding is a technique used in electrical engineering to reduce or redirect electromagnetic fields (EMF) within a given space. This is achieved by placing barriers made from conductive or magnetic materials. The method is commonly applied to enclosures, which isolate electrical devices from their surroundings, and to cables, which protect the wires inside from external interference. When the shielding specifically blocks radio frequency (RF) radiation, it is called RF shielding.
To measure local EMF exposure in residential settings, such as bedrooms, consumer-grade EMF meters are often used. The main purpose of shielding is to minimize electromagnetic interference. It can reduce the coupling of radio waves, electromagnetic fields, and electrostatic fields. A conductive enclosure that blocks electrostatic fields is known as a Faraday cage. The effectiveness of the shielding depends on several factors: the material used, its thickness, the size of the shielded volume, the frequency of the fields involved, and the size, shape, and orientation of any holes in the shield relative to the incident electromagnetic field.
Common materials for shielding include thin layers of metal, sheet metal, metal screens, and metal foam. Typical sheet metals are copper, brass, nickel, silver, steel, and tin. How well a shield reflects or absorbs electromagnetic radiation—its shielding effectiveness—is influenced by the metal's physical properties, such as conductivity, solderability, permeability, thickness, and weight. For example, highly conductive metals like copper, silver, and brass reflect electrically dominant waves, while less conductive metals like steel or stainless steel absorb or suppress magnetically dominant waves. Any holes or mesh in the shield must be significantly smaller than the wavelength of the radiation being blocked, or the enclosure will not act like an unbroken conducting surface.
Another common method, especially for electronic devices in plastic enclosures, is to coat the inside of the enclosure with a metallic ink or similar material. This ink contains a carrier loaded with fine metal particulates, usually copper or nickel. It is sprayed on and, once dry, forms a continuous conductive layer that can be connected to the equipment's chassis ground for effective shielding.
- Materials
- copper, brass, nickel, silver, steel, tin
- Shielding methods
- sheet metal, metal screen, metal foam, metallic ink coating, electroless plating
- Applications
- shielded cables, microwave oven doors, RFID chip protection, computer and keyboard shielding, medical and laboratory equipment, broadcast facilities, defense
Lore & Background
Electromagnetic shielding works through the interaction of electromagnetic radiation with conductive materials. When an electric field is applied to the surface of an ideal conductor, it induces a current that displaces charge inside the conductor, canceling the applied field. Varying magnetic fields generate eddy currents that cancel the applied magnetic field. The result is that electromagnetic radiation is reflected from the conductor's surface: internal fields stay inside, and external fields stay outside. Real shields are limited by electrical resistance, ferromagnetic response to low-frequency fields, and holes that force current to flow around them, reducing field-reflecting capability. For high-frequency radiation, energy not reflected is absorbed by the skin of the conductor due to the skin effect.
For static or slowly varying magnetic fields (below about 100 kHz), Faraday shielding is ineffective. Instead, shields made of high magnetic permeability metal alloys such as permalloy and mu-metal, or nanocrystalline grain structure ferromagnetic metal coatings, are used. These materials draw the magnetic field into themselves, providing a path for field lines around the shielded volume. The best shape for magnetic shields is a closed container. Effectiveness depends on material permeability, which drops off at very low and very high field strengths where saturation occurs. Multiple enclosures, one inside another, are often used to achieve low residual fields. Active shielding using electromagnets, solenoids, or Helmholtz coils can also cancel ambient static or low-frequency fields.
Reader's Guide
Electromagnetic shielding is significant for minimizing electromagnetic interference in a wide range of applications. It reduces the coupling of radio waves, electromagnetic fields, and electrostatic fields. Practical uses include shielded cables, where a wire mesh surrounding an inner core conductor prevents signal escape and external signal addition. Microwave oven doors incorporate a screen that completes a Faraday cage for microwaves while passing visible light. RF shielding protects data stored on RFID chips in biometric passports and prevents passive monitoring of keyboard emissions that could capture passwords, though consumer keyboards lack this protection due to prohibitive cost. Medical and laboratory equipment are shielded against interfering signals including AM, FM, TV, emergency services, and cellular bands. Defense applications use grounded conductive barriers to mitigate nefarious electromagnetic interference. The choice of shielding material depends on conductivity, solderability, permeability, thickness, and weight. Highly conductive metals like copper, silver, and brass reflect electrically dominant waves, while less conductive metals like steel absorb magnetically dominant waves. Holes in a shield must be significantly smaller than the wavelength of the radiation to be excluded. New nanocomposites of ferrites, polymers, and 2D materials are being developed as more efficient RF/microwave-absorbing materials.
Did You Know?
- Copper is used for RF shielding because it absorbs radio and other electromagnetic waves.
- The door of a microwave oven has a screen that completes a Faraday cage for microwaves with wavelengths of 12 cm, while visible light passes through.
- NATO specifies electromagnetic shielding for computers and keyboards to prevent passive monitoring of keyboard emissions that would allow passwords to be captured.
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