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A Faraday Cage is a container made of conductor, shields
interior from EMI Noise
But why does it work?
The Cage is named after Faraday, but why it works
begins with Coulomb
Coulomb determined: Electric field E at radius r
from stationary point charge Q is equal to…
ϵ0 is the permittivity of free space and er is the radial unit
Rules that govern “barrier” effect of Faraday Cage
• Coulomb’s Law demands that the charges in a
conductor at equilibrium be as far apart as
possible, and thus the net electric charge of a
conductor resides entirely on its surface.
• Any net electric field inside the conductor would
cause charge to move since it is abundant and
mobile, but equilibrium demands that the net
force within the conductor is equal to zero. Thus,
the electric field inside of the conductor is zero.
When an external field is applied, exterior charges in
conductor redistribute…
…in such a way that the interior fields within the
conductor cancel out to zero
Many Faraday cages have holes, but still act as a barrier.
How big can holes be?
Holes must be sufficiently smaller than the wavelength (λ)
of the signal to block it, which relates to frequency (f)
with this equation:
c = speed of light. You usually want holes smaller than
1/10 of wavelength
For example, a 3G phone transmitts at 2.1 GHz
(that’s 2,100,000,000 cycles per second)
Speed of Light = 300,000,000 meters per second
Thus, the wavelength is 0.14 m, or 14 centimeters
At 1/10, we want holes smaller than 1.4 cm
Question: Would this always work? Is what condition could
3G noise still get through the cage?
One way to improve the EMI barrier capability of a
Faraday cage is to connect it to electrical ground …
…this could help keep cage at equilibrium