NEW YORK — A cafe customer fed up with cell phone chatter sits in a bubble of blissful silence as nearby patrons puzzle over dead handsets.

A man tries to take a secret snapshot with his camera phone, but gets only a blank screen.

A priest imbues his church with a new energy — the electromagnetic kind — to keep his sermons serene and free from beeps, chirps and rockin' ring tones.

These are glimpses at a war of gadgets quietly playing out around the world.

As millions embrace the freedoms of mobile communications, some people and companies are pushing back against the tide. They are fighting technology with technology, using detectors, jammers and other gizmos to defend privacy, security and sometimes sanity.

Jamming cell phones is illegal in the United States, but with pocket-sized jammers sold online by foreign companies and even on eBay, and the military and government already using such devices, the wireless fight is already here.

"It's like the battle between the radar detector and radar guns. It keeps on escalating," said Jeff Kagan, an independent telecommunications analyst based in Atlanta. He said the need for such devices is prompted by the "double-edged sword of technology."

"The inventor of the cell phone never thought about the fact that people would be using them constantly and impeding on other people's privacy," he said. "The inventor of the camera phone never thought about the fact that they would be used in locker rooms and other inappropriate places."

Jamming a cell phone, essentially a two-way radio, is relatively straightforward.

Jammers typically disrupt the communication between handsets and cellular towers by flooding an area with interference or selectively blocking signals by broadcasting on the same frequencies that phones use.

Some jammers may have to be as smart as cell phones, which try to increase power or hop to other radio channels to avoid interference.

Depending on their power, jammers can disrupt communications in an area spanning a few yards or across several miles.

Commercial jammers have been sold overseas for years, and some Internet postings even offer instructions on building homemade models.

The Federal Communications Commission prohibits people in the United States from building, selling, operating or importing radio jamming devices.

People who violate this provision of a 70-year-old communications law face up to a year in prison and fines of $11,000 for each violation.

However, FCC officials say they have received very few complaints about jammed cell phones and have never taken action against anyone for that violation.

Those in the jammer industry say individuals use low-powered devices with little fear of reprisals, because it's difficult, if not impossible, for a caller to distinguish between a jammed signal and a normal cell phone dead zone.

Despite that, the U.S. law does deter jammer use and limits its spread among consumers, said Kagan, the analyst.

The wireless industry says jamming devices endanger the public.

"One hundred and fifty million Americans rely on wireless phones. If those phones are jammed, doctors might miss calls from hospitals or parents could miss emergency calls from baby sitters," said Travis Larson, spokesman for the Cellular Telecommunications and Internet Association.

Larson said that in order to cut down on the annoyances of cell phone use, customers should use their "mute button, volume control, vibrate mode, voice mail and an on-off button" when appropriate.

He said jamming a specific area where silence is expected, such as a movie theater, is still a risk even with warning signs.

"Jammers may leak into other adjacent frequency bands, blocking public safety radio signals used by police officers and fire fighters," he said.

But safety concerns, courtesy suggestions and the law haven't stopped people from buying jammers.

British firm Global Gadget UK sells to people in other countries an array of jamming and detection products, including a portable jammer disguised as a cell phone that can disrupt cellular communication up to 45 feet away from the user.

"You will be able to silence those anti-social types who insist on using their mobile phones in the most indiscreet way," an Internet ad for the product says. "The beauty is that they will not know it is you that has switched them off! All they will see is that their signal has dropped."

Michael Menage, the company's director, said he sells hundreds of pocket jammers to people in the United States, which is his biggest market. Each costs about $320.

"They're illegal to use over there. People are still quite keen to buy them," Menage said. He said he has sold to the U.S. military, but most customers are individuals or small businesses tired of cellular distractions.

He said some businessmen also use jammers to keep meetings quiet or to disable potential eavesdropping devices, particularly cell phones rigged to be bugs.

While also illegal in England, jammers are more widely used in other parts of the world, including the Middle East, Africa, Asia and some parts of Europe. They are blocking phone calls in theaters, restaurants, libraries, prisons and religious buildings like mosques and churches.

Jamming also has long had a military and security function.

Law enforcement officers have used jammers to block mobile communications in dangerous situations, such as isolating hostage takers or protecting government officials on the move.

Jamming is used by U.S. troop convoys in Iraq to protect against remotely detonated roadside bombs. Some bombs may use cell phones as wireless triggers, and a jamming signal can stop or delay an explosion.

Pakistani intelligence officials say jamming devices in the motorcade of Pakistan's President Pervez Musharraf delayed the detonation of a huge bomb that blew up moments after his car passed over a bridge on Dec. 14.

For consumers and businesses, the latest social and technical battlefield is the world of picture phones. The ability to covertly snap and transmit candid images almost anywhere at any time has seemingly overnight blurred the lines between public and private space.

About 6 million picture phones were sold to U.S. consumers in 2003, a number expected to double this year. Overseas sales are far higher.

With revealing or embarrassing photos of people appearing on the Internet without their knowledge, locations around the nation have banned camera phones, including many health clubs and schools. Sensitive government and corporate buildings also are trying keep the phones out, and a handful of states are considering new laws to limit their use.

A technological counter-measure is on the way. Iceberg Systems, a British firm, is rolling out a system called Safe Haven that disables the camera portion of a phone while leaving its other functions alone. Locations using such "privacy zones" would broadcast a shutdown signal to specially equipped picture phones or digital cameras.(from chron.com)