What is the optimal thickness for a Faraday cage to attenuate radio signals?

[schristy]

Greetings,

I have a question regarding the attenuation properties of Faraday cages. More specifically, I'm looking for a generalization I can use to better understand how thick they should be for a given frequency. I did a small experiment by taking a small enclosed aluminum box and placed an AM radio inside and to my surprise the radio kept playing (with some attenuation). I was tuned to 560kHz, the box was 1.2mm thick. According to an online calculator, the cage is over 10 skin depths. Unfortunately, I am illiterate when it comes to calculus and all the formulas for this exceed my ability, but I think it should be simple enough to have a generalization here. How many skin depths would I have needed to stop the radio signal, 50, 100? I realize the exact answer may depend on how strong the signal is, but I'm not really looking for an exact answer, more of a rule of thumb or generalization I can apply maybe something along the lines of how many skin depths to achieve a certain level of signal attenuation.

Thanks in advance!

 

[trurle]

Greetings,

I have a question regarding the attenuation properties of Faraday cages. More specifically, I'm looking for a generalization I can use to better understand how thick they should be for a given frequency. I did a small experiment by taking a small enclosed aluminum box and placed an AM radio inside and to my surprise the radio kept playing (with some attenuation). I was tuned to 560kHz, the box was 1.2mm thick. According to an online calculator, the cage is over 10 skin depths. Unfortunately, I am illiterate when it comes to calculus and all the formulas for this exceed my ability, but I think it should be simple enough to have a generalization here. How many skin depths would I have needed to stop the radio signal, 50, 100? I realize the exact answer may depend on how strong the signal is, but I'm not really looking for an exact answer, more of a rule of thumb or generalization I can apply maybe something along the lines of how many skin depths to achieve a certain level of signal attenuation.

Thanks in advance!

electrically small Faraday cages do not effectively (exponentially) shield sources confined within them, or attenuate low-frequency incoming wave. Thickness of cage is actually has small effect, because your entire cage is capacitively coupled to radio source and act as antenna, although poorly matched (due low resistance of walls). In some cases you actually need to make lighter cage to reduce RF emission or reception (this is called "defected ground plane" method of EMI control). In general, you nee full-3D EM calculation to stop emission/reception at specific wavelength of small Faraday`s cage, or use Faraday cage which is significantly larger than your wavelength.

 

[schristy]

Thank you Trurle for taking the time to respond. If I change my question to only consider the AM band - does your answer remain unchanged? I really would like to keep this simple. If I wanted to block my 500kHz signal, how thick would my cage need to be? If you say, I couldn't block the signal even if it was 100cm thick that's still an answer, but I think that such a thing is very unlikely and there should be an answer even if it is impractical.

 

[trurle]

Thank you Trurle for taking the time to respond. If I change my question to only consider the AM band - does your answer remain unchanged? I really would like to keep this simple. If I wanted to block my 500kHz signal, how thick would my cage need to be? If you say, I couldn't block the signal even if it was 100cm thick that's still an answer, but I think that such a thing is very unlikely and there should be an answer even if it is impractical.

You can make (small) cage-like structure to antiresonance at 560 kHz. Simplest way may be to construst 2 concentric cages of 2 perpendicular wire loops each. Connect one end of your radio antenna to concentric loops through pair of 10nF capacitors. Size of loops must be ~20% smaller than 10cm and 20% larger than 10cm in perimeter, respectively. Or use larger loops and smaller capacitors, keeping length*capacitance product the same. Some tweaking will be needed. Idea is what the phase of parasitic current induced in longer and shorter loops must be opposite, cancelling undesirable transmission. It is not a Faraday cage in classical sense though (as i said before, Faraday cage do not work for your application).

 

[schristy]

That's not the answer to my question, but thank you for the effort.

 

[Tom.G]

The usual reason for leakage thru a Faraday cage is that the seams are not in continuous electrical contact. It's not terribly surprising that a casually assembled Aluminium cage leaks. Aluminium forms an insulating oxide layer on its surface almost instantly when exposed to the air.

That's why Faraday cages are made with a metal that you can readily make conductive contact at the seams. Copper is the common choice because it is both easy to work with and is easy to solder. There are metal gaskets available you can use but they require some clamping force to keep them in electrical contact with the overlapping surfaces, they are usually installed around access doors just as you would put weather stripping around the door of your house.

Commercial Screen Rooms are often made using Copper screen, like window screening, with all the joints soldered.

For more background and details see:
https://learnemc.com/practical-em-shielding
(above link, and many others, found with:
https://www.google.com/search?&q=attenuation+of+faraday+cage+versus+frequency)

Cheers,
Tom

 

[sophiecentaur]

I did a small experiment by taking a small enclosed aluminum box and placed an AM radio inside

If you could see or even hear that the radio was receiving then the box was not perfect. Light and/or sound could get out so RF could get in. It is fiendishly hard to make a perfect Faraday enclosure. The skin needs to be continuous, with no door and no holes to let power and other signals get in and out. The sides would be, ideally, welded together. The thin slot round a door, despite beryllium copper spring fingers etc., can allow some induced currents to flow over the surface of the metal and into the insides of the box. Depending on how much you are prepared to pay and how much trouble it is to use, combined with the level of screening you need (always a finite amount) you will use an appropriate construction (as @Tom. describes).

 

[Dale]

electrically small Faraday cages do not effectively (exponentially) shield sources confined within them, or attenuate low-frequency incoming wave. Thickness of cage is actually has small effect,

That is surprising, but it makes sense! I didn’t think about small cages because the ones I am familiar with are room sized.

 

[trurle]

The usual reason for leakage thru a Faraday cage is that the seams are not in continuous electrical contact. It's not terribly surprising that a casually assembled Aluminium cage leaks. Aluminium forms an insulating oxide layer on its surface almost instantly when exposed to the air.

That's why Faraday cages are made with a metal that you can readily make conductive contact at the seams. Copper is the common choice because it is both easy to work with and is easy to solder. There are metal gaskets available you can use but they require some clamping force to keep them in electrical contact with the overlapping surfaces, they are usually installed around access doors just as you would put weather stripping around the door of your house.

Commercial Screen Rooms are often made using Copper screen, like window screening, with all the joints soldered.

For more background and details see:
https://learnemc.com/practical-em-shielding
(above link, and many others, found with:
https://www.google.com/search?&q=attenuation+of+faraday+cage+versus+frequency)

Cheers,
Tom

Leakage through sometimes an issue. At low frequencies, skin depth typically more important (~0.11 mm at 560 kHz in aluminum. To say, even perfect infinitely wide 1.5 mm Al sheet will attenuate just 73 dB at 0.56 MHz if receiver is in near-field zone (which is 120m large) close to sheet. Resistive seams excitation modes are dominant if contact resistance between halfs of box are larger than 10 milliOhm (which is likely not the case unless sheet is anodized)

 

[schristy]

At low frequencies, skin depth typically more important (~0.11 mm at 560 kHz in aluminum. To say, even perfect infinitely wide 1.5 mm Al sheet will attenuate just 73 dB

Thank you Trurle, that is closer to the type of answer I was hoping for. Would you be able to share with me a link (or a description) of how you arrived at 73 dB for 1.5mm thickness? I'm hoping you can point me to the equations you used or perhaps even better a link to a site which I could learn them. I found the one for skin depth yesterday, but didn't really know how to translate that into dB.

I decided last night, I could simply test and determine a good thickness by adding layers of heavy duty aluminum aluminum foil to my aluminum box. If I arrive at a reasonable thickness then I could construct a better cage (by doubling that thickness) or if I run out of foil I might simply need to face reality that it is not practical for those frequencies. I also considered that perhaps this similar to how polycarbonate passes visible light but blocks UV, but then I wonder at what frequency this becomes a significant factor. Should I expect my cage to easily block an AM radio transmitting at 1.2MHz, but fail to block 560kHz at any reasonable thickness? If so then at what point does this shift occur and do we know why?

 

[schristy]

For more background and details see:
https://learnemc.com/practical-em-shielding

Thank you Tom, it looks like that page has some of the answers I was looking for.

 

[trurle]

https://www.pasternack.com/t-calculator-skin-depth.aspx
Then attenuation is exp(-t/t_skin)

 

[spareine]

Try wrapping your radio in a single layer of aluminium foil.

In my experience placing an AM radio in a metal box doesn't work as a Faraday cage, but wrapping the radio in a aluminium foil does. The difference is that a box has seams, whereas foil doesn't. (Obviously the radio should be battery powered, because a power cord requires a hole in the box). The 0.02 mm thickness of household aluminium foil is sufficient, and the wrap doesn't need to be a larger space than the radio. The aluminium foil is an effective Faraday cage for FM radio and mobile telephony as well.