A group of people from MIT investigated the efficacy of three aluminum helmet designs on a sample group of four individuals. Using a $250,000 network analyser, they find that although on average all helmets attenuate invasive radio frequencies in either directions (either emanating from an outside source, or emanating from the cranium of the subject), certain frequencies are in fact greatly amplified.
The helmets amplify frequency bands that coincide with those allocated to the US government between 1.2 Ghz and 1.4 Ghz. According to the FCC, These bands are supposedly reserved for ''radio location'' (ie, GPS), and other communications with satellites.
The paranoid community to develop improved helmet designs to avoid falling prey to these shortcomings.
On the Effectiveness of Aluminium Foil Helmets:
An Empirical Study
http://people.csail.mit.edu/rahimi/helmet/
