Radio Frequency (RF) Jammers
The Basics of Cell Phone Jamming
Cell phones work by communicating with a service network through the utilization of cellular towers or base stations. Individual towers partition cities into small sections called cells. As a cell phone user traverses the cells in an area, the signal is passed from tower to tower.
Jamming devices take advantage of this fact by transmitting on the spectrum of radio frequencies used by cellular devices. Through its concurrent transmission, the jamming device is able to disrupt the two-way communication between the phone and the base station. This form of a denial-of-service attack inhibits all cellular communication within range of the device.
How It's Done
Through the transmission of a high power signal on the same frequency of a cell phone, the jamming device creates a competing signal that collides with, and, in effect, cancels out the cellular signal. Cell phones, which are designed to increase power in the case of low levels of interference, react to this interference. Consequently, jamming devices must be aware of any increases in power by the cellular device and match that power level accordingly.
As cellular telephones are full-duplex devices utilizing two separate frequencies (one for talking, one for listening, where all parties to a call can talk at the same time as opposed to half-duplex walkie-talkies and CBs), any removal of one of these frequencies tricks the phone into thinking there is no cellular service. Consequently, the jammer need only block one of the frequencies.
The less complex jammers can only block a specific frequency group while the more complex jammers can block several different networks thus preventing dual- or tri-mode phones from switching to a different network with an open signal. Jammers are able to broadcast on any frequency and can interrupt AMPS, CDMA, TDMA, GSM, PCS, DCS, iDEN and Nextel systems. The effective range of a jammer is dependent upon the strength of its power source and the immediate pysical environment (hills or walls which may block the jamming signal). Lower powered jammers have a call-block range of about 30 feet while higher power units can create a cellular signal-free zone about the size of a football field. In addition, certain units applied by law enforcement have been known to shut down cellular service approximately 1 mile from the jamming device.
What's Inside a Cell Phone Jammer
RF jammers are fairly simplistic devices. The most simple jammers have only an on/off switch and an LED signifying power to the unit. The more complex devices contain switches or dials that allow the user to select the frequencies desired for jamming. The components which can be located in every jammer include:
Antenna -- The antenna transmits the jammers interrupting signal. Certain jammers contain an internal antenna while others have external antennas which give the user a longer range to broadcast the signal and more control over frequency tuning.
Voltage-controlled oscillator -- The oscillator is responsible for generating the competing radio signal.
Tuning circuit -- In those units the enable user-specified frequency tuning, the tuning circuit controls the broadcast frequency of the circuit by sending a specific voltage to the oscillator.
Noise generator -- The noise generator, which is part of the tuning circuit, actually creates radomized electric output within a specific frequency range which is used to disrupt a cellular network signal.
RF amplification (gain stage) -- This amplifier controls the level of power to the tuning circuit. It is responsible for boosting the power as necessary to jam signals.
Power Supply -- Smaller jamming devices may use batteries while larger and more power-intensive devices can be plugged into a standard outlet or connected through the electrical system of a vehicle.
In Which Legal Situations are RF Jammers Used?
Originally developed for law enforcement and military, RF jammers have been mainly used to disrupt the communications of criminals and terrorists. One of the most useful applications has been to use RF jammers to prevent the use of cell-phone activated bombs in subways, airports, and the routes of presidential motorcades. Furthermore, in hostage situations, police can use RF jammers to place controls on when and where the criminal can make a call. In drug raids, jammers can be used to prevent outside communication. Finally, corporations have also been known to use RF jammers to prevent corporate espionage by blocking voice and image transmissions from camera-enabled phones.
Legal Issues of Using RF Jammers
The United States, United Kingdom, Australia, and many other countries have determined that cell-phone blocking is illegal. Explicitly, the United States calls upon the Communications Act of 1934 to enforce such a law as it prevents people from "willfully or maliciously interfering with the radio communications of any station licensed or authorized." Furthermore, manufacturing, importing, and advertising RF jamming devices.