Faraday cage, lower frequencies

In summary, Faraday cages of the conducting type are not effective for lower frequencies. The frequency of 100 kHz is mentioned as a reference for this type of shielding. The electromagnetic spectrum is composed of both an electrical and a perpendicular magnetic field. A Faraday cage works by 'earthing' the electrical field, but the effectiveness does not depend on its size. However, for lower frequencies with longer wavelengths, the electrical field may 'dodge' the cage, requiring the use of Mu-metal to saturate the magnetic field. There is confusion between Faraday cages and magnetic shielding, as the former is meant for electric field shielding while the latter is for magnetic fields. Faraday cages can work down to DC for electric fields, but for magnetic
  • #1
Notinuse
8
1
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.
 
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  • #2
Notinuse said:
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..
 
  • #3
Notinuse said:
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
 
  • #4
davenn said:
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.
 
  • #5
Are there any other references as to at what (low) frequency faraday cages become ineffective?
 
  • #6
Notinuse said:
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... :smile:
 
  • #7
Notinuse said:
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.
 
  • #8
berkeman said:
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.
 
  • #9
tech99 said:
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... :smile:
 

Related to Faraday cage, lower frequencies

1. What is a Faraday cage?

A Faraday cage is an enclosure made of conductive materials (such as metal) that is designed to block external electromagnetic fields from entering the space inside. It works by redirecting the electric current around the outside of the cage, thus preventing it from passing through and affecting the objects inside.

2. How does a Faraday cage protect against lower frequencies?

A Faraday cage can protect against lower frequencies by creating an electrically conductive barrier that blocks the electromagnetic waves from passing through. This is done by using materials that can easily conduct electricity, such as copper or aluminum, and ensuring that there are no gaps or holes in the enclosure that could allow the waves to enter.

3. What types of objects can be protected by a Faraday cage?

Any object that is sensitive to electromagnetic interference can be protected by a Faraday cage. This includes electronic devices, communication equipment, and even living organisms such as plants and animals. The size and shape of the cage may vary depending on the object being protected, but the principle remains the same.

4. Can a Faraday cage protect against all types of electromagnetic radiation?

No, a Faraday cage is most effective against low and medium frequency electromagnetic radiation. It may not be as effective against high frequency radiation, such as X-rays or gamma rays, as these types of radiation have shorter wavelengths that can pass through the conductive material. However, a Faraday cage can still provide some level of protection against these types of radiation.

5. Are there any limitations to a Faraday cage's effectiveness?

Yes, there are some limitations to a Faraday cage's effectiveness. For example, if the cage is not properly grounded, it may not be able to redirect the electric current and could potentially allow electromagnetic waves to enter. Additionally, if there are any gaps or holes in the cage, it may not be as effective in blocking all frequencies. It is important to properly design and maintain a Faraday cage to ensure its effectiveness.

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