Jammers function by broadcasting a signal at the same frequency at a high enough power level to disrupt and neutralize the original signal. Cell phones are built to enhance their power when they experience low interference, which means the jammer must be capable of detecting and matching this increased power.
Cell phones function as full-duplex systems, employing two different frequencies concurrently—one for voice transmission and the other for reception. Certain signal jammers may only disrupt one of these frequencies, but this action effectively blocks both. As a result, the phone is deceived into thinking it has no service, since it can only pick up one frequency.
Less advanced jammers tend to block a single frequency set, whereas more complex devices can disrupt multiple network types at the same time. This capability is essential for preventing dual-mode or tri-mode phones from automatically switching between networks in search of an open signal. Some high-end jammers can block all frequencies simultaneously, while others can be fine-tuned to specific frequency ranges.
To interfere with a cell phone, a device that transmits on the appropriate frequency is necessary. Although cellular systems differ in their signal management, all mobile networks utilize radio signals that can be interrupted. In Europe and Asia, GSM operates within the 900 MHz and 1800 MHz bands, while in the United States, it operates in the 1900 MHz (or 1.9 GHz) band. Jammers can transmit on any frequency and are effective against various systems, including AMPS, CDMA, TDMA, GSM, PCS, DCS, iDEN, and Nextel. Both older analog phones and modern digital devices are equally prone to jamming.
The range of a jammer is contingent upon its power and the environmental factors present, such as hills or building walls that may hinder the jamming signal. Low-power jammers are effective at blocking calls within roughly 30 feet (9 meters), while high-power jammers can generate a no-signal area equivalent to a football field. Law enforcement jamming devices can effectively cut off service within a mile (1.6 kilometers) of their position.
The development of cell phone jammer was primarily aimed at law enforcement and military applications, intended to disrupt the communications of criminals and terrorists. The March 2004 commuter train bombings in Spain, along with the attacks in Bali in October 2002 and Jakarta in August 2003, were all orchestrated through cell phones. Reports suggest that jammers effectively prevented an assassination attempt on Pakistani President Pervez Musharraf in December 2003. Additionally, during President Bush's visit to London in November 2004, British police reportedly considered the use of jammers to protect his motorcade.
In cases of hostage-taking, authorities can dictate when and where kidnappers are allowed to make phone calls. Furthermore, during drug-related operations, police can block phone signals to inhibit suspects from reaching out to individuals outside the operation zone.
