A cell-phone jammer transmits radio signals to cut off communications between cell-phones and their networks.  If you activate a phone jammer, all idle phones nearby will indicate “NO NETWORK.” Incoming calls are blocked, as if the cellular phone were off. When the phone jammer is turned off, all cell-phones will automatically re-establish communications and provide full service.

Cell phones are basically handheld two-way radios. A cell phone works by communicating with its service network through a cell tower, or base station. Cell towers divide a city into small areas, or cells. As a cell-phone user drives down the street, the signal is handed from tower to tower.

Like any radio, the signal can be interfered with, or jammed. Disrupting a cell phone is the same as jamming any other type of radio communication. The jamming device transmits on the same radio frequencies as the cell phone, disrupting the communication between the phone and the cell-phone base-station in the tower.

Jamming devices overpower the signal from the cell phone. Cell phones are designed to add power if they experience low-level interference, so the jammer’s power must be more than a match for the power increase from the phone.

Cell phones are full-duplex devices, which means they use two separate frequencies simultaneously, one for talking and one for listening. Some jammers block only one of the frequencies used by cell phones, which can have the same effect as if blocking both. The phone is tricked into thinking there is no service because it can receive only one of the frequencies.

Although different cellular networks process signals differently, all cell-phone networks use radio signals that can be interrupted. GSM, used in digital cellular and PCS-based systems, operates in the 900-MHz and 1800-MHz bands in Europe and Asia, and in the 1900-MHz (sometimes referred to as 1.9-GHz) band in the United States. Jammers that can broadcast on any frequency are effective against AMPS, CDMA, TDMA, GSM, PCS, DCS, iDEN and Nextel systems. Old-fashioned analog cell phones and today’s digital devices are equally susceptible to jamming.

To jam a cell phone, all you need is a device that broadcasts on the correct frequencies. Less complex devices block only one group of frequencies, corresponding to a particular network; while sophisticated jammers can block several types of networks at once to head off dual-mode or tri-mode phones that automatically switch among different network types to find an open signal. Some of the high-end devices block all frequencies at once, and others can be tuned to specific frequencies.

Some cell-phone jammers are made to look like actual phones. Others are briefcase-sized or larger. The biggest jammers used by police and the military can be mounted in vehicles for convoy security.

The actual range of the jammer depends on its power and the local environment, which may include hills or walls of a building that block the jamming signal. Low-powered jammers block calls in a range of about thirty feet. Higher-powered units can create a cell-free zone as large as a football field. Units used by law enforcement can shut down service up to one mile from the device.

This entry was posted on Wednesday, July 2nd, 2008 at 2:41 am and is filed under Technical Info. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.