Shielding from electric/magnetic field using a metal box

[manivas]

Can electric fields can be completely shielded using a metal box or metal sphere,
please clarify the concept.

 

[mind]

Yes. The name of the concept is Faraday cage.

 

[Uglybb]

A faraday cage can give huge attenuation but it can't completely block an EM field it is by definition a shield effect.

The only way to completely block a signal is a anechoic chamber (http://en.wikipedia.org/wiki/Anechoic_chamber)

Look in the RF section.

 

[AlephZero]

The only way to completely block a signal is a anechoic chamber

The walls of an anechoic chamber absorb any radiation produced inside the chamber, so that measurements are not confused by reflections (echoes) from the walls

A Faraday cage stops radiation EM produced outside from getting inside.

These are two different issues. An anechoic chamber for EM radiation would normally be built inside a Faraday cage, as the Wiki link said.

 

[f95toli]

Can electric fields can be completely shielded using a metal box or metal sphere,
please clarify the concept.

Yes, a Faraday cage will shield electric fields and EM fields. However, it won't protect you from inductive pick-up, i.e. it won't help against low-frequency magnetic fields (from say the mains).

 

[Uglybb]

Correct f95 and the better the conductivity the better the faraday cage. I guess a superconductive cage would give ideal shielding.

Most if not all RF anechoic chambers these days are for EMC testing and they are active suppression the same as noise cancellation headphones. If a signal is detected in the setup phase in the area of interest it will be actively canceled whether it is from inside or outside the wiki link is WAY WAY out of date. The cancellation will be to the sensitivity required for the compliance test.

I should say active RF cancelation has got so cheap and integrated that mobile phones are even carrying them on the antena to improve reception (http://www.intersil.com/signalintegrity/NoiseCancellingICs.asp)

 

[f95toli]

I guess a superconductive cage would give ideal shielding.

Unfortunately not, the shielding efficiency of superconductors is frequency dependent. It gets worse as the frequency increases and mu-metal shields are actually better for screening magnetic fields at high frequencies.
Hence, the only way to make a shield that works well at all frequencies is to combine different types of materials (mu-metal, superconductors and good normal conductors).