Electromagnetic shielding foiled again

The ineffectiveness of tinfoil hats against government mind control rays received a bit of media and internet attention this year, despite the fact the study in question is 7 years old and that mind control doesn’t appear to exist.

In 2005, four electrical engineers at MIT tested the shielding of three different helmets made of ‘tinfoil’ – actually aluminium foil – over radio frequencies from 10 kHz to 3 GHz (Rahimi A, Recht B, Taylor J & Vawter N 2005, “On the effectiveness of aluminium foil helmets: an empirical study”, published online 17 Feb 2005).

The principle is based on the Faraday cage, invented by English physicist Michael Faraday in 1836. This is a box made of conducting material: when an electric field is applied to the outside, the charges in the conductor realign and cancel out the electric field inside. For a more detailed explanation, see The Feynman lectures on physics vol. 2, 1964, section 5.10.

A homemade Faraday cage, made of a box covered in aluminium foil. In an external electric field, represented by arrows running from positive to negative, the charges in the conductor move accordingly. Negative charges are attracted towards the external positive charges, and positive to the external negative.
My homemade Faraday cage. When an external electric field is applied (traditionally represented by arrows running from positive to negative), the charges in the conductor move accordingly. Negative charges are attracted towards the external positive charges, and positive to the external negative. This redistribution of charges sets up its own electric field, which is equal and opposite to that from outside. The two fields cancel each other out, so inside the cage there is no field (Photo own work)

Faraday cages work really well at low frequencies, and they’re the reason you’re not affected if you’re in an airplane that’s struck by lightning: the metal fuselage shields you from the high voltages outside.

At higher frequencies, you need to make sure you’re using a good conductor (refer to this table of electrical conductivity, by TIBTECH).

You also need to make sure that any holes in the enclosure are small enough so that the electromagnetic waves don’t fit through. The basic rule is that the holes need to be less than about half a wavelength (with the wavelength equal to the speed of light divided by the frequency).

A microwave oven is a good example. Microwaves have a frequency of about 2.45 GHz, i.e. a wavelength of 12 cm. So the holes in the metal mesh in the oven door are too small for the microwaves to fit through, but big enough for you to see in (light is also electromagnetic radiation, but with wavelength between 390 and 750 nm). For more technical details, see ‘Practical electromagnetic shielding’, by Learn EMC.

Incidentally, the microwave oven’s shielding is meant to block radiation from the inside, not the outside as in a basic Faraday cage. For this purpose the principle is pretty much the same, except that the enclosure needs to be grounded. Without the ability to bring in more charge from the ground, the conductor is basically transparent to any charges inside (thanks to Gauss’s law).

It is fairly easy to build your own Faraday cage using aluminium foil and test it by placing a mobile phone inside. We did this on air, risking violation of the rule of not using a mobile phone in the radio studio. Fortunately, the shielding worked and the phone didn’t ring when we called it.

Aluminium works well because it’s a fairly good conductor, not far behind gold (which itself ranks below silver and copper). But you really need to make sure you crimp all the seams, as these are the main weak points. Even though they’re not very wide, they can easily be long enough to let in the electromagnetic waves (in Australia, 3G mobile networks use 0.85-2.1 GHz, with 4G or LTE on 1.8 GHz, or a wavelength of 17 cm).

This is the big problem with the foil hats. Because they have to fit a head in, they’re never totally enclosed. So right away, basic electromagnetic theory tells us they won’t work as Faraday cages.

However, although they won’t totally block all radiation, we should at least see some attenuation of the signal. Which is what the MIT researchers looked for, and indeed what they found.

There was a 10 decibel (dB) reduction in signal strength at most frequencies, with the greatest attenuation being 20 dB at 1.5 GHz.

But the biggest surprise was amplification of the signal at 2.6 GHz and 1.2 GHz! Those frequencies saw increases of 30 dB and 20 dB respectively.

Now, I don’t have specific data on the dimensions of the hats involved, but considering those frequencies correspond to wavelengths of 11 cm and 25 cm, I suspect they’ve hit on resonant frequencies of the cavities they’ve created. Effectively, the radiation is bouncing around inside, echoing and building up in strength.

The engineers go on to point out that frequencies in the range of 1.2-1.4 GHz are reserved by the Federal Communications Commission, nominally for GPS and other satellite use. And in the US, 2.6 GHz is used by mobile phone companies, i.e. multi-national corporations.

This leads to the ‘conclusion’ that perhaps the very idea of foil hats was seeded by the government and their corporate cohorts as a bluff to get people to wear them and so amplify the mind control rays. Which is really doubling-down on the conspiracy theory.

So if you’re paranoid, this may be enough to amplify your paranoia.

For the rest of us, what it does show is that although electromagnetic shielding is quite possible, it requires a bit more care and crimping than you might have thought.

(This story aired on 20 December 2012 – you can listen to the podcast.)

2 thoughts on “Electromagnetic shielding foiled again

  1. Please forgive and indulge my total ignorance – when you write radiation is ‘echoing, bouncing around, building up in strength’, how can that be? How can energy increase in ‘strength’ of itself with no more energy being put in?

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