Radio jamming
[1][2] In some cases, jammers work by the transmission of radio signals that disrupt telecommunications by decreasing the signal-to-noise ratio.
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.
Thanks to the FM capture effect, frequency modulated broadcasts may be jammed, unnoticed, by a simple unmodulated carrier.
But the signal relies on hand shaking between the transmitter and receiver to identify and determine security settings and method of high-level transmission.
This method jams the receiver in an infinite loop where it keeps trying to initiate a connection but never completes it, which effectively blocks all legitimate communication.
During World War II, ground radio operators would attempt to mislead pilots by false instructions in their own language, in what was more precisely a spoofing attack than jamming.
Modern secure communication techniques use such methods as spread spectrum modulation to resist the deleterious effects of jamming.
Jamming has also occasionally been used by the governments of Germany (during World War II),[6] Israel,[7] Cuba, Iraq, Iran (during the Iran-Iraq War), China, North and South Korea and several Latin American countries, as well as by Ireland against pirate radio stations such as Radio Nova.
[9] [10] During the Continuation War, after discovering the fact that the mines that the retreating Soviet forces had scattered throughout the city of Viipuri were radio-triggered rather than timer- or pressure-triggered, the Finnish forces played Vesterinen's recording of Säkkijärven Polkka without any pauses from September 4, 1941 to February 2, 1942, as they, to demine the city, needed to block the Soviets from activating the mines through the correct radio wave.
Some parts of the world were more impacted by these broadcasting practices than others Meanwhile, some listeners in the Soviet Union and Eastern Bloc devised ingenious methods (such as homemade directional loop antennas) to hear the Western stations through the noise.
In general outside of the Soviet Union itself Bulgaria was one of the most prolific operators of jamming transmitters in the Eastern bloc with East Germany and Yugoslavia the least.
While western governments may have occasionally considered jamming broadcasts from Eastern Bloc stations, it was generally accepted that doing so would be a pointless exercise.
It was the most important clandestine broadcaster in Spain and the regime considered it a threat, since it allowed its citizens to skip the censorship of the local media.
In Latin America there were instances of communist radio stations such as Radio Venceremos being jammed, allegedly by the CIA, while there were short lived instances where Britain jammed some Egyptian (during the Suez Crisis), Greek (prior to Cyprus gaining independence) and Rhodesian stations.
[16] During the early years of the Northern Ireland troubles the British army regularly jammed broadcasts from both Republican and Loyalist paramilitary groups.
[21] The Russian Armed Forces have, since the summer of 2015, begun using a multi-functional EW weapon system in Ukraine, known as Borisoglebsk 2.
In the film Star Trek II, after receiving a distress call from the space station Regula I, Captain Kirk attempts to establish communications, but the Enterprise's comm officer Lt. Uhura reports that further transmissions are "jammed at the source".
