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.
Radar jamming is also important to disrupt use of
radar used to guide an enemy's
missiles or
aircraft. Modern secure communication techniques use such methods as
spread spectrum modulation to resist the deleterious effects of jamming. Jamming of foreign radio
broadcast stations has often been used in wartime (and during periods of tense international relations) to prevent or deter citizens from listening to broadcasts from enemy countries. However, such jamming is usually of limited effectiveness because the affected stations usually change frequencies, put on additional frequencies and/or increase transmission power. Jamming has also occasionally been used by the governments of Germany (during
World War II),
Israel, 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. The United Kingdom government used two coordinated, separately located transmitters to jam the
offshore radio ship,
Radio North Sea International off the coast of Britain in 1970, to enforce its domestic
broadcast licensing law.
World War II In occupied Europe the
Nazis attempted to jam broadcasts to the continent from the
BBC and other allied stations. Along with increasing
transmitter power and adding extra frequencies, attempts were made to counteract the jamming by dropping
leaflets over cities instructing listeners to construct a directional
loop aerial that would enable them to hear the stations through the jamming. In the Netherlands such aerials were nicknamed
moffenzeef (lit. "kraut sieve"). 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. The Soviets tried to trigger the mines by changing frequency; the mines had been set up to be able to be triggered by three different frequencies. The Finns countered this by playing Säkkijärven Polkka on all frequencies. During the
Battle of the Beams, Britain jammed navigation signals used by German aircraft.
Cold War era The Soviet Union started jamming Western radio broadcasts to itself in 1948. The primary targets of jamming have been the
BBC External Broadcasting Services,
Voice of America (VOA) and especially
RFE/RL. Western nations had allowed jamming prior to World War II, but in the post-War era, the Western view has been that jamming violates the principle of
freedom of information, while the Soviet view has been that under the international law principle of
national sovereignty, jamming is an acceptable response to foreign radio broadcasts. During much of the
Cold War,
Soviet (and
Eastern Bloc) jamming of some Western broadcasters led to a "power race" in which broadcasters and jammers alike repeatedly increased their transmission power, utilising highly
directional antennas and adding extra frequencies (known as "barrage" or "frequency diversity" broadcasting) to the already heavily overcrowded
shortwave bands to such an extent that many broadcasters not directly targeted by the jammers (including pro-Soviet stations) suffered from the rising levels of noise and interference. There were also periods when
China and the Soviet Union jammed each other's programmes. The Soviet Union also jammed
Albanian programmes and its own
Warsaw Pact's programmes at times. Some parts of the world were more affected by these broadcasting practices than others •
Eurasia (worst affected, including
mediumwave frequencies particularly 720 kHz used by RFE) •
North Asia, Americas and Sub-Saharan Africa (partly affected) •
Australasia, South America (rarely affected) 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. Because
radio propagation on shortwave can be difficult to predict reliably, listeners sometimes found that there were times when the jamming was particularly ineffective because radio fading (due to
atmospheric conditions) was affecting the jamming signals but favouring the broadcasts (a phenomenon sometimes dubbed "twilight immunity"). At other times, the reverse was the case. There were also times when jamming transmitters were (temporarily) off air due to breakdowns or maintenance. The Soviets (and most of their Eastern Bloc allies) used two types of jamming transmitter.
Skywave jamming covered a large area, but for the reasons described was of limited effectiveness.
Groundwave jamming was more effective, but only over a small area, and was thus used only in/near major cities throughout the Eastern Bloc. Both types of jamming were less effective on higher shortwave frequencies (above 15 MHz); however, many radios sold on the domestic market in the Soviet Union did not tune these higher bands. Skywave jamming was usually accompanied by
morse signals in order to enable (coded) identification of the jamming station so that Soviet monitoring posts could assess the effectiveness of each station. In 1987, after decades of generally refusing to acknowledge that such jamming was even taking place, the Soviets finally stopped jamming western broadcasts with the exception of
RFE/RL which continued to be jammed for several months into 1988. Previously there had been periods when some individual Eastern Bloc countries refrained from jamming Western broadcasts but this varied widely by time and country. 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. Ownership of shortwave radios was less common in western countries than in the Soviet Union where, due to the vast physical size of the country, many
domestic stations were relayed on shortwave as it was the only practical way to cover remote areas. Additionally, western governments were generally less afraid of intellectual competition from the Eastern Bloc. In
Francoist Spain, the dictatorship jammed for decades
Radio España Independiente, the radio station of the
Communist Party of Spain which broadcast from
Moscow (1941–1955),
Bucharest (1955–1977) and East Berlin. It was the most important clandestine broadcaster in Spain and the regime considered it a threat, since it allowed its citizens to bypass the censorship of the local media. Broadcasts from East Germany to South Africa were also jammed. 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. During the early years of the Northern Ireland troubles, the British army regularly jammed broadcasts from both Republican and Loyalist paramilitary groups.
Post Cold War (1989–present) China In 2002, China acquired standard short-wave radio-broadcasting equipment designed for general public radio-broadcasting and technical support from Thales Broadcast Multimedia, a former subsidiary of the French state-owned company
Thales Group. • Thales jamming technology operates only at power levels below 500 kW (for its shortwave jamming products). • Adele Milna (BSEE) of Continental Electronics (in an audio file held at shortwave.org) claims that China has duplicated his company's 100 kW, 250 kW shortwave transmitters. It is unclear if these products were indeed duplicated or if broadcast jamming (as opposed to future product sales) were a reason for the duplication.
Iran Debates have been raised in Iran regarding the possible health hazards of satellite jamming. Iranian officials including the health minister have claimed that jamming has no health risk for humans. However, the minister of communication has recently admitted that satellite jamming has 'serious effects' and has called for identification of jamming stations so they can put a stop to this practice. The government has generally denied any involvement in jamming and claimed they are sent from unknown sources.
Russia The
Russian Armed Forces have, since the summer of 2015, begun using a multi-functional
EW weapon system in
Ukraine, known as
Borisoglebsk 2. It is postulated that this system has defeated communications in parts of that country, including mobile telephony and
GPS systems.
Other countries • Since the early 1960s, the practice of radio jamming has been very common in
Cuba, blocking not only American government funded radio stations (such as
Voice of America) but also
Ham radio signals, and stations owned and/or operated by (or selling airtime to)
Cuban exile groups transmitting from
Miami, such as La Cubanisima, Radio Mambi,
WWFE La Poderosa and Cadena Azul. The same practice has been applied to
Radio y Televisión Martí, operated by the
U.S. Information Agency since 1985. •
North Korea and
South Korea still regularly jam some of each other's radio (and sometimes television) stations. • Several Middle Eastern countries (particularly
Iran) jam shortwave broadcasts (and even occasionally attempt to jam satellite TV signals) targeted at their countries. • Pakistan has contemplated jamming
pirate radio stations operated by the
Taliban in all jails across Pakistan. This decision has led to an outcry from Pakistani cellular operators, who state that most of the jails lie in urban areas with a resultant impact on the cellular service of all operators in the adjacent area of jails. • Ethiopia has jammed the
DW and
VOA transmissions as well as Ethiopian Satellite Television (
ESAT) and
Eritrean radio stations. • Vietnam jams the Vietnamese service of
Radio Free Asia, Radio
Đáp Lời Song Núi, some
FEBC programs, mostly in Vietnamese minority languages as well as
Radio Sweden with a "siren" jammer and "bubble" jammer on
FM frequencies. • In Nigeria, the
Nigerian Broadcasting Commission has claimed it jams the signal of Radio
Biafra. • In
South Africa, the use of wireless signal jammers is illegal. There is a single exception to this rule. South Africa's State Security Cluster may, in certain instances, employ signal jammers. • In
Singapore, a local
Indonesian
Batam Hang FM 106.0 MHz based Islamic radio station of Indonesia radio stations has been jammed. It transmits up to Singapore and Johor Bahru in Malaysia at 10 kW transmission power. Hang FM 106.0 MHz was accused by Singapore authorities of spreading extremist and terrorist propaganda. Four Singaporeans were detained by police under the Internal Security Act (ISA) when they attempted to move to Syria and join ISIS. Radio Hang FM has denied this accusation. Station Manager of Hang FM Radio, Abu Yusuf, rejected any link between Hang FM Radio and radicalism. According to him, the broadcast material is also in accordance with the understanding of Ahlus sunnah wal Jamaah, which apart from rejecting radicalism, terrorism, and especially
ISIS, also rejects other ideas or actions that are contrary to the basic principles of the peace-loving Islamic religion. Meanwhile, Deputy Chair of the Riau Islands
Regional Indonesian Broadcasting Commission (KPID), Suyono, said that while under KPID supervision, in its broadcasts Radio Hang FM actually rejected radicalism and terrorism. Singapore also considered banning Hang FM website. ==In fiction==