They are illegal in the US, but a growing market exists.
http://nytimes.com/2007/11/04/techno...excamp=mkt_at1Originally Posted by NYTAs cellphone use has skyrocketed, making it hard to avoid hearing half a conversation in many public places, a small but growing band of rebels is turning to a blunt countermeasure: the cellphone jammer, a gadget that renders nearby mobile devices impotent.
The technology is not new, but overseas exporters of jammers say demand is rising and they are sending hundreds of them a month into the United States — prompting scrutiny from federal regulators and new concern last week from the cellphone industry. The buyers include owners of cafes and hair salons, hoteliers, public speakers, theater operators, bus drivers and, increasingly, commuters on public transportation.
The development is creating a battle for control of the airspace within earshot. And the damage is collateral. Insensitive talkers impose their racket on the defenseless, while jammers punish not just the offender, but also more discreet chatterers.
“If anything characterizes the 21st century, it’s our inability to restrain ourselves for the benefit of other people,” said James Katz, director of the Center for Mobile Communication Studies at Rutgers University. “The cellphone talker thinks his rights go above that of people around him, and the jammer thinks his are the more important rights.”
The jamming technology works by sending out a radio signal so powerful that phones are overwhelmed and cannot communicate with cell towers. The range varies from several feet to several yards, and the devices cost from $50 to several hundred dollars. Larger models can be left on to create a no-call zone.
Using the jammers is illegal in the United States. The radio frequencies used by cellphone carriers are protected, just like those used by television and radio broadcasters.
The Federal Communication Commission says people who use cellphone jammers could be fined up to $11,000 for a first offense. Its enforcement bureau has prosecuted a handful of American companies for distributing the gadgets — and it also pursues their users.