Radio jamming

Radio jamming is the deliberate blocking of or interference with wireless communications. In some cases, jammers work by the transmission of radio signals that disrupt telecommunications by decreasing the signal-to-noise ratio.

The concept can be used in wireless data networks to disrupt information flow. It is a common form of censorship in totalitarian countries, in order to prevent foreign radio stations in border areas from reaching the country.

Jamming is usually distinguished from interference that can occur due to device malfunctions or other accidental circumstances. Devices that simply cause interference are regulated differently. Unintentional "jamming" occurs when an operator transmits on a busy frequency without first checking whether it is in use, or without being able to hear stations using the frequency. Another form of unintentional jamming occurs when equipment accidentally radiates a signal, such as a cable television plant that accidentally emits on an aircraft emergency frequency.

Originally the terms were used interchangeably but nowadays most radio users use the term "jamming" to describe the deliberate use of radio noise or signals in an attempt to disrupt communications (or prevent listening to broadcasts) whereas the term "interference" is used to describe unintentional forms of disruption (which are far more common). However, the distinction is still not universally applied. For inadvertent disruptions, see electromagnetic compatibility.

Intentional communications jamming is usually aimed at radio signals to disrupt control of a battle. A transmitter, tuned to the same frequency as the opponents' receiving equipment and with the same type of modulation, can, with enough power, override any signal at the receiver. Digital wireless jamming for signals such as Bluetooth and WiFi is possible with very low power.

The most common types of this form of signal jamming are random noise, random pulse, stepped tones, warbler, random keyed modulated CW, tone, rotary, pulse, spark, recorded sounds, gulls, and sweep-through. These can be divided into two groups: obvious and subtle.

Obvious jamming is easy to detect because it can be heard on the receiving equipment. It is usually some type of noise, such as stepped tones (bagpipes), random-keyed code, pulses, music (often distorted), erratically warbling tones, highly distorted speech, random noise (hiss), and recorded sounds. Various combinations of these methods may be used, often accompanied by regular Morse identification signals to enable individual transmitters to be identified in order to assess their effectiveness. For example, China, which did and does use jamming extensively, plays a loop of traditional Chinese music while it is jamming channels (cf. Attempted jamming of numbers stations).

The purpose of this type of jamming is to block reception of transmitted signals and to cause a nuisance to the receiving operator. One early Soviet attempt at jamming Western broadcasters used the noise from the diesel generator that was powering the jamming transmitter.