A Faraday cage is a metal enclosure or container that blocks electromagnetic fields (EM fields) or electromagnetic radiation. It acts as a shield to protect the enclosed object from both static and non-static electromagnetic fields such as radio waves and microwaves. The cage operates on the principle of electromagnetic shielding, preventing electromagnetic radiation or EM waves from entering or escaping the cage. This cage is also called a Faraday shield, RF Cage, or EMF (electromagnetic force) cage. The Faraday cage is named after scientist Michael Faraday, who invented it in 1836.

Figure: Faraday cage The Faraday cage can be either a mesh structure or a solid covering of conducting metal. It is typically constructed using electrically conductive materials such as copper, aluminum, and steel. The cage acts as a hollow conductor, ensuring that electrical charges remain on its external surface, thereby effectively shielding the interior from electromagnetic interference (EMI).

The Faraday cages can be in any shape, such as a box, sphere, or cylinder, and in any size, from small to very large. They are commonly used in various sectors like data centers, laboratories, medical facilities, the communication industry, and the defense sectors to protect sensitive electronic equipment from electromagnetic interference (EMI) or radio frequency interference (RFI).  They can be also used to protect people and devices from events such as lightning strikes and electrostatic discharges (ESD).  Also, these cages can help to protect communication and other electronic equipment from Electromagnetic pulses (EMPs).

Figure: A Faraday shield protects a laptop from EMI

How does a Faraday cage work? 

In the absence of an electromagnetic field or electric field, positive charges and negative charges (free electrons) are evenly distributed within the metal enclosure. When an EM field or EM wave strikes the Faraday cage, the electric field component of the EM wave produces forces on free electrons within the cage's conductive material. This causes the free electrons to redistribute or move while the positive charges remain stationary. Consequently, the charges are rearranged or separated in such a way that the electrons collect on the incoming EM wave side while the positive charges accumulate on the opposite side. This redistribution process is called electrostatic induction. The charges only reside on the external surface of the metal cage. The electron movement leads to current flow within the conductive enclosure.

Figure: Faraday cage working principle

The rearrangement of charges (i.e., positive and negative charges separation) creates an opposing electric field that cancels the electric field of the incoming EM wave. As a result, both electric fields cancel each other out, resulting in zero net electric field inside the Faraday cage. With no electric field present, there is no EM wave presence, thereby preventing EM waves from entering or exiting the cage. Thus, the Faraday cage serves as an effective shield against electromagnetic interference.

Note: Stable or slowly varying magnetic fields cannot be blocked by the Faraday cages. For example, the cage cannot block Earth's magnetic field, so a compass inside a cage will still work.

Is grounding necessary for the Faraday cage?

Typically, grounding is not necessary for the Faraday cage to operate. But, for safety purposes and correct use, the cage should be properly grounded. Proper grounding ensures that any excess electric charge that accumulates on the exterior of the cage is safely directed away, reducing the risk of electric shock or other hazards. Without grounding, the charge could potentially remain on the exterior surface, posing a danger to anyone who comes into contact with it. By providing a path for the charge to exit safely, grounding enhances the safety and effectiveness of the Faraday cage. The grounding also provides a path for electrostatic discharge currents and heavy lightning currents.

What are the factors that influence the Shielding effectiveness of the Faraday cage?

The shielding effectiveness of a Faraday cage depends on several factors, including the following:

• Material Conductivity: Materials with higher conductivity, such as copper or aluminum, are more effective at blocking electromagnetic fields compared to materials with lower conductivity.
• Thickness of Material: Thicker materials provide more shielding effectiveness due to the increased attenuation of electromagnetic fields as they pass through the material.
• Frequency: The effectiveness of a Faraday cage generally increases with frequency. Higher-frequency electromagnetic fields, such as microwaves and radio waves, are typically easier to block than lower-frequency fields.
• Gaps and Apertures: Ensuring a tight seal and minimizing gaps is important for maximizing shielding effectiveness. To ensure effective shielding, the size or diameter of holes in the Faraday mesh construction or any opening in the solid Faraday cage should be smaller than the wavelength of the incident EM wave. A general rule of thumb is that the opening or holes in a Faraday cage should be smaller than 1/10th of the wavelength of the incident EM wave.  For example, the hole size of the cage should be smaller than 3 mm to block EM fields with a frequency range of 10 GHz and lower.
• Grounding: While not always necessary, grounding can enhance the shielding effectiveness of a Faraday cage by providing a path for the dissipation of any excess charge buildup on the cage's surface.
• Construction method: Solid Faraday cages generally can block electromagnetic fields over a broader range of frequency ranges than mesh cages.

Applications of Faraday cages:

Faraday cages have various applications in daily life, including:

• Faraday cages are used to shield sensitive electronic devices from EMI or RFI to ensure their proper operation in various applications.
• The scanning room of the Magnetic Resonance Imaging (MRI) machine in hospitals is constructed as a Faraday cage. The cage prevents external EMI signals from affecting patient's diagnostic images or data.
• Faraday cages are used to shield sensitive electronic devices from EMI or RFI to ensure their proper operation in various applications.
• Research laboratories or scientific labs often use Faraday cages to shield sensitive experiments from electromagnetic interference. For example, these cages are commonly used in analytical chemistry to make sensitive measurements without any noise.
• The metal enclosure of a microwave oven acts as a Faraday cage. It keeps microwaves confined within the oven, ensuring efficient cooking while also safeguarding users from exposure to potentially harmful microwave radiation.
• The metal body of a car or airplane can function as a Faraday cage, protecting passengers from external electric charges, such as lightning strikes. 
• The metallic shield of the shielded cable acts as a Faraday cage to protect the internal conductors from external electrical noises and also prevents the escaping of RF signals. The shielded cables are commonly used in applications like cable TV to deliver clear and reliable signals.
• Certain buildings, such as prisons are constructed as a Faraday cage. It blocks both outgoing and incoming cellphone calls by prisoners. Also, some buildings may act as a Faraday cage accidentally due to concrete walls and other metals used in buildings and may block cellphone signals and wireless internet networks.
• Power utility linemen are usually wearing specially made dresses that act as a Faraday cage to protect the linemen working near high-voltage power lines from electric shock.
• Faraday bags or pouches serve as portable versions of Faraday cages, designed to shield electronic devices such as smartphones, tablets, and key fobs from remote hacking or tracking. By placing the devices inside these bags, the electromagnetic signals are effectively blocked, preventing unauthorized access to sensitive information or location tracking.
• Faraday cages can protect critical military equipment and infrastructure from the effects of electromagnetic pulses (EMPs) caused by nuclear explosions or due to man-made EMP attacks.
• Politicians or government officials may use Faraday Cage to discuss sensitive matters and prevent electronic eavesdropping or surveillance.

Faraday cages are available in various sizes and form factors, from small Faraday bags for holding individual electronic devices to larger Faraday tents or boxes designed to protect large equipment and machinery from electromagnetic interference.

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