Faraday Cage Effectiveness versus Frequency

[Matt1]

If inside of a metal (for ex. Aluminum) enclosure, could it potentially create more harm than good if frequencies were able to penetrate due to an imperfectly sealed cage or due to a relatively high power level of frequencies that allowed them to penetrate the metal? If the power level was high enough, could it exceed the ablility of absorptive materials used on the interior to try to capture what gets in?

 

[jedishrfu]

Here's a discussion on Faraday cages where they talk about what it can and can't block and how well its done.

It did mention that cell phone signals are attenuated meaning a powerful enough signal could get through.

https://en.wikipedia.org/wiki/Faraday_cage

Exterior fields[edit]

Skin depth vs. frequency for some materials at room temperature, red vertical line denotes 50 Hz frequency:

• Mn-Zn – magnetically soft ferrite
• Al – metallic aluminum
• Cu – metallic copper
• steel 410 – magnetic stainless steel
• Fe-Si – grain-oriented electrical steel
• Fe-Ni – high-permeability permalloy (80%Ni-20%Fe)

Effectiveness of shielding of a static electric field is largely independent of the geometry of the conductive material, however, static magnetic fields can penetrate the shield completely.

In the case of a varying electromagnetic fields, the faster the variations are (i.e., the higher the frequencies), the better the material resists magnetic field penetration. In this case the shielding also depends on the electrical conductivity, the magnetic properties of the conductive materials used in the cages, as well as their thicknesses.

A good idea of the effectiveness of a Faraday shield can be obtained from considerations of skin depth. With skin depth, the current flowing is mostly in the surface, and decays exponentially with depth through the material. Because a Faraday shield has finite thickness, this determines how well the shield works; a thicker shield can attenuate electromagnetic fields better, and to a lower frequency.

 

[sophiecentaur]

If the inside of the cage were to resonate at a particular frequency, and the coupling from outside to inside were just right, I think the field inside it could exceed the field outside. I am thinking along the lines of a tuned circuit connected to an antenna. But a small hole in a cage would not admit more than a very small fraction of the incident power flux so I think the answer would have to be No. Any dissipative element inside the cage would damp the resonator and produce a very low Q factor.

 

[Matt1]

I have to review the study you've referenced.
I agree about the dissipative element being capable of absorbing penetration due to small leaks, but what about if of the power level of the frequency was high enough to result in a lot of frequency penetrating to the interior, where it can bounce around continuously - if the power level was high enough could this exceed the ability of damping materials to reduce frequencies to a safe level?

 

[sophiecentaur]

I have to review the study you've referenced.
I agree about the dissipative element being capable of absorbing penetration due to small leaks, but what about if of the power level of the frequency was high enough to result in a lot of frequency penetrating to the interior, where it can bounce around continuously - if the power level was high enough could this exceed the ability of damping materials to reduce frequencies to a safe level?

Are you suggesting an absorbing layer? Not a bad idea but you'd need to look up details of actual designs and calculate whether it would be worth it. What isolation do you need? 60dB or 120dB - which? What do you mean by 'Safe'? It's a bit 'how long is a piece of string' at the moment.
Perhaps more important than the material and thickness of the box, the seams, door and in/out connectors should be considered.

 

[Matt1]

What I meant to say was if the power level was high enough to penetrate the shielding material itself, not just leaking through holes. In this case could it create conditions inside the cage that would exceed the ability of absorptive materials to reduce the frequencies?

 

[davenn]

What I meant to say was if the power level was high enough to penetrate the shielding material itself, not just leaking through holes.

Reread the quoted bits of that article above ... skin depth is related to frequency, not power

Effectiveness of shielding of a static electric field is largely independent of the geometry of the conductive material, however, static magnetic fields can penetrate the shield completely.

In the case of a varying electromagnetic fields, the faster the variations are (i.e., the higher the frequencies), the better the material resists magnetic field penetration. In this case the shielding also depends on the electrical conductivity, the magnetic properties of the conductive materials used in the cages, as well as their thicknesses.

A good idea of the effectiveness of a Faraday shield can be obtained from considerations of skin depth. With skin depth, the current flowing is mostly in the surface, and decays exponentially with depth through the material. Because a Faraday shield has finite thickness, this determines how well the shield works; a thicker shield can attenuate electromagnetic fields better, and to a lower frequency.

you have to get up to X-rays and higher for any significant ( but still small) depth penetration ... they use X-Rays for looking for metal defects

As far as I'm aware of, for a solid continuous shield, there is no penetration. EM enters/exits via cracks and holes
This is why RF waveguides work so well for carrying very high power ( 10's of kW's) from transmitters to antennas for TV stations etc
there isn't any leakage through the WG, only from joints between sections and they use methods to reduce thatDave

 

[Matt1]

I need to read that article. Interesting that thickness of shield material apparently does not matter, it is the continuity of the enclosure that determines leakage. Therefore, absorptive materials on the inside should prevent any leaked frequencies from continuously reflecting back + forth on the inner surfaces. And joint leakage reduction techniques are important.

 

[davenn]

Interesting that thickness of shield material apparently does not matter,

No, the thickness is important it has to be at least greater than the skin depth for a given frequency

it is the continuity of the enclosure that determines leakage.

primarily, yes

Therefore, absorptive materials on the inside should prevent any leaked frequencies from continuously reflecting back + forth on the inner surfaces

yes that will help

but absorptive materials on the inside are primarily used for dampening oscillations inside a box that are radiated by the circuitry inside the box.
These radiated oscillations can cause circuit instability. The higher the frequency, the more these oscillations can be a problemDave

 

[Matt1]

Actually, the primary concern is if people are inside of the shielded room. It seems it will be ok if properly designed.

 

[davenn]

you might find this of interest:
RF shielding
http://www.idc-online.com/technical...eering/Designing_an_RF_Shielded_Enclosure.pdf

 

[sophiecentaur]

Actually, the primary concern is if people are inside of the shielded room. It seems it will be ok if properly designed.

You need to get things in proportion here. A shielding of only 30dB would reduce the power exposure by 1/1000. (1kW arriving outside produces 1W inside) What exactly are you trying to protect humans (?) from? You want an Engineering answer so your question needs to specify the requirements with some accuracy. to make it worth while. Comments about a foot bridge may not apply to a Motorway bridge.
The requirements for a good measurement facility is entirely different from the requirement for personal safety.