JAMMER OPERATION

As with other radio jamming, cell phone jammers  prevent cell phone use by sending out radio waves along the same frequencies that cellular phones use. This causes enough interference with the communication between cell phones and antennas to render the phones unusable. On most retail mobile phones, the network would simply appear out of range. Most mobile phones use different bands to send and receive communications from antennas (called FDD). Jammers can work by either disrupting phone to antenna frequencies or antenna to phone frequencies. Smaller handheld models block all bands from 800 MHz to 1900 MHz within a 30-foot range. Small devices tend to use the former method, while larger more expensive models may interfere directly with the antenna. The radius of cell phone jammers can range from a dozen feet for pocket models to kilometers for more dedicated units. The TRJ-89 Jammer can stop cellular communication signal for a 5-mile (8 km) radius.

Less energy is required to disrupt signal from antenna to mobile phone than the signal from mobile phone to the antenna, because the base station is located at long distance from the Jammer than the mobile phone and that is why the phone signal from the antenna is not as strong.

Older jammers sometimes has  limited to working on phones using only analog or older digital signal mobile phone standards. Newer models such as the double and triple frequency band jammers can block all widely used systems (CDMA, iDEN, GSM) and are even very effective against newer phones which hop to different frequencies and systems when interfered with. As the dominant network technology and frequencies used for mobile phones vary world wide, some work only in specific regions such as North America or  Europe. Cell phones that have been sneaked into prison pose a security risk for guards.