The principle behind jammers is to emit a signal at the same frequency with enough intensity to interfere with and cancel out the original signal. Cell phones are designed to boost their power in response to low interference, which requires signal jammers to detect and correspond to the phone's increased power levels.
Cell phones are designed as full-duplex devices, meaning they can transmit and receive simultaneously on two different frequencies—one for talking and another for listening. Certain jammers may block just one of these frequencies, resulting in the unintended effect of disrupting both. This causes the phone to be tricked into thinking it has no service, as it can only access one frequency.
Less complex devices are capable of blocking only a single frequency range, while more advanced jammers can simultaneously disrupt multiple network types. This feature is particularly useful for preventing dual-mode or tri-mode phones from switching between different networks in search of a signal. Some high-end devices can block all frequencies at once, while others can be adjusted to focus on specific frequencies.
To jam a cell phone, you just need a device that broadcasts on the right frequency. While different cellular systems handle signals differently, all cell phone networks use radio signals that can be interrupted. GSM is used for digital cellular and PCS-based systems, operating in the 900 MHz and 1800 MHz bands in Europe and Asia, and in the 1900 MHz (sometimes called 1.9 GHz) band in the United States. Jammers can broadcast on any frequency and are effective against AMPS, CDMA, TDMA, GSM, PCS, DCS, iDEN, and Nextel systems. Older analog phones are just as susceptible to jamming as today's digital devices.
A jammer's effective range is shaped by its power and the surrounding environment, which may feature hills or building walls that can impede the jamming signal. Low-power jammers usually block calls within about 30 feet (9 meters), while high-power devices can establish a no-signal zone comparable to a football field. Devices employed by law enforcement can interrupt service within a mile (1.6 kilometers) of the device.
Cell phone jammers were originally developed for law enforcement and the military to disrupt the communications of criminals and terrorists. The March 2004 commuter train bombings in Spain, as well as those in Bali in October 2002 and Jakarta in August 2003, were all triggered by cell phones. It was widely reported that cell phone jammer thwarted an assassination attempt on Pakistani President Pervez Musharraf in December 2003. British police reportedly considered using jammers to protect President Bush's motorcade through London when he visited the city in November 2004.
In hostage situations, police possess the power to control the timing and location of kidnappers' phone calls. Moreover, during drug raids, they can disable phones to ensure that suspects are unable to communicate with anyone outside the immediate environment.
