Jammers operate by transmitting a signal at the same frequency with sufficient power to create interference that cancels out the original signal. Cell phones are engineered to boost their power in response to low interference levels, necessitating that cell phone jammer detects and adjusts to the phone's increased power output.
Cell phones operate as full-duplex devices, meaning they utilize two distinct frequencies simultaneously—one for transmitting voice and another for receiving. Certain jammers are designed to disrupt only one of these frequencies, which inadvertently results in the blockage of both. This interference causes the phone to perceive that it has no service, as it can only access one frequency.
Simpler devices are designed to obstruct a single frequency range, whereas more advanced signal jammers have the capability to disrupt multiple network types simultaneously. This prevents dual-mode or tri-mode phones from seamlessly transitioning between various networks in their quest for an available signal. Certain high-end models can block all frequencies concurrently, while others allow for tuning to particular frequencies.
Disrupting a cell phone signal requires a device that transmits on the appropriate frequency. Although various cellular systems manage signals in distinct ways, all mobile networks utilize radio signals that can be disrupted. In Europe and Asia, GSM technology operates within the 900 MHz and 1800 MHz frequency bands, while in the United States, it functions in the 1900 MHz (or 1.9 GHz) band. Jammers can emit signals across any frequency and are capable of interfering with AMPS, CDMA, TDMA, GSM, PCS, DCS, iDEN, and Nextel networks. Both older analog phones and modern digital devices are equally vulnerable to jamming.
The effective range of a jammer is influenced by its power output and the surrounding environment, which may consist of obstacles such as hills or building structures that can obstruct the jamming signal. Low-power jammers typically disrupt calls within a radius of approximately 30 feet (9 meters), while high-power units can establish a no-signal area comparable to the dimensions of a football field. Law enforcement devices are capable of disabling service within a distance of up to one mile (1.6 kilometers) from the device.
Cell phone jammers were initially created for use by law enforcement and military agencies to interfere with the communications of criminals and terrorists. Notably, the commuter train bombings in Spain in March 2004, along with attacks in Bali in October 2002 and Jakarta in August 2003, were all initiated via cell phones. Reports indicated that cell phone jammers successfully prevented an assassination attempt on Pakistani President Pervez Musharraf in December 2003. Additionally, British police contemplated the deployment of jammers to safeguard President Bush's motorcade during his visit to London in November 2004.
In a hostage scenario, law enforcement has the authority to regulate the timing and location of calls made by kidnappers. During drug enforcement operations, police can disable phones to prevent suspects from contacting individuals outside the vicinity.
