Faraday cage

Faraday Cage was discovered and tested in 1836 by the chemist and physicist of the same name, Micheal Faraday. This is a current-carrying metal enclosure that electrically shields the internal environment.

The scientist observed, that if you electrically charge a wire conductor, the charges arrange themselves on the outer surface without affecting the inner environment of the conductor. To prove his point, he constructed a room covered with a metal sheet to which he applied a voltage, demonstrating by measurement with an electroscope that no electrical charge was present inside the chamber.

Using the principles of physics, Gauss's theorem allows us to describe the distribution of charge on the conductor. Equal charges repel each other, they are concentrated on the outside of the conductor so imagining the shielded chamber as an ideal conductor on this creates an equipotential surface while inside it the field will be zero.

If the surface is uniform the external charge distribution will be constant otherwise there will be different charge densities according to the geometry of the envelope.

A key feature of the Faraday cage is the electrical continuity of its surface; This must be ensured to prevent the passage of charges. Therefore, during construction it is very important to ensure contact between the chamber panels using EMI gaskets, fingers, etc., otherwise a hole is left in the shielding (draft).  

The Faraday cage principle is used in airplanes, cars, lightning rod systems or, for example, microwave ovens.

The main applications in radiofrequency are:

• Magnetic Resonance Imaging (to prevent external signals from affecting the measurement and thus the examination result)
• Anechoic chamber (to prevent external signals from changing the quiet zone and thus varying the known signal generated)
• Military shelters (to avoid sensitive communication apparatus from being interfered with by external signals)

To test the Faraday cage, and thus its proper operation for RF applications, we test the shielding effectiveness. this describes how a signal, as frequency changes, is attenuated by the shielding.