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Commuters shouting into their mobiles? Just jam 'em

There's just one problem. It's illegal

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Column When a columnist starts off "Silent, but deadly..." you know he's trying to be funny; and Matt Rudd's recent praise for phone radio jammers is, clearly, not based on the fact he doesn't know what SBD actually means.

Anti-social behaviour on trains these days isn't limited to producing methane; it is more commonly believed to be a problem with telephony. Rudd wrote his chucklesome piece saying that actually, you don't have to put up with people talking at the top of their voices behind you. We have the technology.

Technically, he's quite right. Phones use a very weak signal (which is why most electronics engineers don't believe they cause cancer, duh) and anybody who can transmit a slightly stronger signal on the same channel will block it. For quite amazingly large sums, if you want to impress people, you can buy a little black box that will do this, and if you want a list, try Google and ask for "phone jammer" and watch the hits pour in.

There's a small problem, which the column does mention: it's illegal. Not everywhere in the world, admittedly, and so actually making the devices is hard to prohibit; and there aren't likely to be any laws in Russia (say) to stop you from selling phone jamming equipment to Taiwan... or vice versa, come to that. And so you can get them; and if you just want utility, then you can actually get them very cheap.

Beware, because the devil is in the detail.

If you are looking for a clue, have a look at the very erudite review of phone jammers at Spy Review.

The reviewer gives the technical background first: "The jammer has three aerials to match the three frequency bands it's designed to block out. If an aerial does not match the wavelength (and thus frequency) of the transmitted radio signal, the strength of the signal can be severely limited," the article explains.

All well and good, but then it notes, casually enough: "The jammer is intended to block out the following frequencies" and lists them:

  • 851 - 960Mhz
  • 1805 - 1990Mhz
  • 2100 - 2170Mhz

The first thing you'll note is that this should, in theory, block all UK based cellphone signals. But if you're an expert (and the reviewer is) you'll notice that the widely used European band, GSM1800 is not completely knocked out. "The first set of frequencies on GSM1800 do not fall within the specified range on the jammer. However, GSM900 and GSM1900 are completely covered by the jammer. I wonder if the jammer does completely knock out service providers using GSM1800?"

Yes, of course he would wonder because a genuine review would require a genuine test which would be genuinely illegal activity.

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