Faraday cage, lower frequencies

[Notinuse]

Faraday cages of the conducting type are not effective for lower frequencies; https://en.wikipedia.org/wiki/Electromagnetic_shielding#Magnetic_shielding mentions a frequency of 100 kHz. The electromagnetic spectrum is composed of both an electrical, and a perpendicular magnetic field. If a faraday cage works by 'earthing' the electrical field, would the cages effectiveness depend on its size? For lower frequencies the wave length is longer, so does the electrical field 'dodge' the cage? hence the need to saturate the magnetic field with Mu-metal.

 

[berkeman]

Faraday cages of the conducting type are not effective for lower frequencies; https://en.wikipedia.org/wiki/Electromagnetic_shielding#Magnetic_shielding mentions a frequency of 100 kHz. The electromagnetic spectrum is composed of both an electrical, and a perpendicular magnetic field. If a faraday cage works by 'earthing' the electrical field, would the cages effectiveness depend on its size? For lower frequencies the wave length is longer, so does the electrical field 'dodge' the cage? hence the need to saturate the magnetic field with Mu-metal.

You are confusing a Faraday cage with Magnetic Shielding. A Faraday Cage is not meant to shield Magnetic fields -- it is generally just for Electric field shielding and shielding RF EM..

 

[davenn]

If a faraday cage works by 'earthing' the electrical field, would the cages effectiveness depend on its size?

and it also doesn't work by earthing the electric field ... a Faraday shield can be completely isolated from earth/ground
and will still shield the inside from the outside fields or visa versaDave

 

[sophiecentaur]

and it also doesn't work by earthing the electric field

Right. It just reduces the PD across the cage by conduction. The more conductive the surface (including seams, doors and signal connectors), the better the screening. There will be currents flowing over the outer surface and these can end up flowing on the inner surface if there is finite resistance across any gaps. It is possible to treat the gaps round doors (as in microwave ovens) over a small range of wavelengths by including a system of quarter wave slots which, cleverly, present a short circuit across the finite door gap.

 

[Notinuse]

Are there any other references as to at what (low) frequency faraday cages become ineffective?

 

[berkeman]

Are there any other references as to at what (low) frequency faraday cages become ineffective?

Faraday cages work down to DC for electric fields.

Can you say what your application is? Maybe we can help you figure out your optimum shielding solution...

 

[sophiecentaur]

Are there any other references as to at what (low) frequency faraday cages become ineffective?

If your knowledge (like mine) is not sufficient to make a good engineering choice of design then you should read as much stuff as you can find. This link seems well informed but there is a lot to read and there is no 'single figure' answer to your question.

 

[tech99]

You are confusing a Faraday cage with Magnetic Shielding. A Faraday Cage is not meant to shield Magnetic fields -- it is generally just for Electric field shielding and shielding RF EM..

But magnetic shielding can be obtained using a non magnetic conducting shield, provided the fields are alternating and the frequency is high enough. For example, radio frequency inductors can be shielded magnetically when placed in an aluminium can. The action is caused by the eddy currents induced in the shielding conductor.

 

[berkeman]

But magnetic shielding can be obtained using a non magnetic conducting shield, provided the fields are alternating and the frequency is high enough.

Sure, I use that all the time. That's why I asked the OP to tell us more about their application. It's hard to know what they really want, and so far their posts haven't really helped us to figure it out...