Mobile phone jammer

A mobile phone jammer is an instrument used to prevent cellular phonesbase stations.
When used, the jammer effectively disables cellular phones. These
devices can be used in practically any location, but are found
primarily in places where a phone call would be particularly disruptive
because silence is expected. from receiving signals from or transmitting signals to

History
The rapid proliferation of cell phones at the beginning of the 21stcentury to near ubiquitous status eventually raised problems such astheir potential use to invade privacy, contribute to academic cheating,

or even aid in industrial espionage.
In addition public backlash was growing against the intrusive
disruption cell phones introduced in daily life. While older analog
cell phones often suffered from chronically poor reception and could
even be disconnected by simple interference such as high frequency
noise, increasingly sophisticated digital phones have led to more
elaborate counters. Cell phone jamming devices are an alternative to
more expensive measures against cell phones, such as Faraday cages,
which are mostly suitable as built in protection for structures. They
were originally developed for law enforcement and the military to
interrupt communications by criminals and terrorists. Some were also
designed to foil the use of certain remotely detonated explosives. The
civilian applications were apparent, so over time many companies
originally contracted to design jammers for government use switched
over to sell these devices to private entities. Since then, there has
been a slow but steady increase in their purchase and use, especially
in major metropolitan areas.
Operation
As with other radio jamming, cell phone jammers block cell phone use by sending out radio wavesalong the same frequencies that cellular phones use. This causes enough interference with the communication between cell phones and towers to
render the phones unusable. On most retail phones, the network would
simply appear out of range. Most cell phones use different bands to
send and receive communications from towers (called full duplexing).
Jammers can work by either disrupting phone to tower frequencies or
tower to phone frequencies. Smaller handheld models block all bands
from 800MHz to 1900MHz within a 30 foot range (9 meters). Small devices
tend to use the former method, while larger more expensive models may
interfere directly with the tower. 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 block cellular communications
for a 5-mile (8 km) radius.

Older jammers sometimes were limited to working on phones using only
analog or older digital mobile phone standards. Newer models such as
the double and triple band jammers can block all widely used systems (AMPS, iDEN, GSM,
et al) 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
worldwide, some work only in specific regions such as Europe or North
America.
The jammer's effect can vary widely based on factors such as

proximity to towers, indoor and outdoor settings, presence of buildings
and landscape, even temperature and humidity play a role.

There are concerns that crudely designed jammers may disrupt the functioning of medical devices such as pacemakers.
However, like cell phones, most of the devices in common use operate at
low enough power output (<1W) to avoid causing any problems.