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Radio jamming is the (usually deliberate) transmission of radio signals that disrupt communications by decreasing the signal to noise ratio. 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 TV plant that accidentally emits on an aircraft emergency frequency. 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.
Distinction between "jamming" and "interference" 
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 usually is 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 signal to enable individual transmitters to be identified in order to assess their effectiveness. For example, China, which used jamming extensively and still does, plays a loop of traditional Chinese music while it is jamming channels (c.f. Attempted jamming of number 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.
Subtle jamming is jamming during which no sound is heard on the receiving equipment. The radio does not receive incoming signals yet everything seems superficially normal to the operator. These are often technical attacks on modern equipment, such as "squelch capture". Thanks to the FM capture effect, frequency modulated broadcasts may be jammed, unnoticed, by a simple unmodulated carrier. The receiver locks onto the larger carrier signal and hence will ignore the FM signal with information.
Digital signals use complex modulation techniques such as QPRS. These signals are very robust in the presence of interfering signals. However the signal relies on hand shaking between the transmitter and receiver to identify and determine security settings and method of high level transmission. If the jamming device sends initiation data packets the receiver will begin its state machine to establish two way data transmission. A jammer will loop back to the beginning instead of completing the handshake. 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.
Bluetooth and other consumer radio protocols have built in detectors so that they only transmit when the channel is free. Simple continuous transmission on a given channel will continuously stop a transmitter transmitting, hence jamming the receiver from ever hearing from its intended transmitter.
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 WW2), Israel, Cuba, Iraq, Iran (Iraq and Iran war, 1980–1988), 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.
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" (English: "kraut sieve").
Cold War era 
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, utilised highly directional antennas and added extra frequencies 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.
A further method used was operating transmitters for domestic radio stations on the same or nearby frequencies. For example, for many years East Germany operated at Wiederau a transmitter on the same mediumwave frequency (575 kHz) that Mühlacker radio transmitter used with an output power of 100 kW, which made it difficult to receive the AFN Mühlacker radio transmitter in much of the East Germany.
Other stations targeted by the Soviet jammers (but not to the same extent as RFE/RL/VOA/BBC) included Deutsche Welle and occasionally Radio Vaticana, Kol Yisrael and Radio Canada International. The jamming usually only took place during programming in languages widely spoken in Eastern Bloc countries (e.g., Russian, Polish, Czech, Lithuanian, etc.). Programmes in English or other major Western languages were rarely (if ever) jammed intentionally.
Some parts of the world were more impacted 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 signal radio propagation on shortwave can be difficult to predict reliably listeners sometimes found that there were days/times when the jamming was particularly ineffective because radio fading (due to atmospheric conditions) was affecting the jamming signals but favouring the broadcasts. On other days of course 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 only used 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 in the USSR didn't cover the higher bands. Skywave jamming was usually accompanied by morse signals in order to enable (coded) identification of the jamming station in order 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 USSR itself Bulgaria was one of the most prolific operators of jamming transmitters in the Eastern bloc with East Germany 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 USSR 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 communist bloc.
However 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.
Post Cold War (1989-present) 
People's Republic of 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.
- It is assumed that China is using ALLISS technology for jamming foreign radio stations broadcasting into China.
- Thales jamming technology only operates 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 companies 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.
The government of Iran has frequently used jamming of satellite TV (as well as filtering the Internet and restricting Internet connectivity speed and other methods) as a strategy to prevent the consequences of freedom of expression in the last decade. Most of the jamming took place in the year 2009 after the controversial presidential election in Iran to control the flow of information and updates about the protests. Although most of the jamming is done on news channels or political ones, another famous series of jamming started around July 2010 on a non-political, Persian language satellite TV channel called "Farsi1" which airs Persian-dubbed popular TV series.
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 an stop to this practice. The government has generally denied any involvement in jamming and claimed they are sent from unknown sources. According to some sources IRGC is the organization behind satellite jamming in Iran.
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 VOA) but also radio stations owned and/or operated by (or selling airtime to) Cuban exile groups transmitting from Miami, Florida, such as La Cubanisima, Radio Mambi, WWFE La Poderosa and Cadena Azul. The same practice has been applied to Radio Marti and TV Marti, 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. (See: Radio jamming in Korea)
- Several middle eastern countries (particularly Iran) jam shortwave broadcasts (and even occasionally attempt to jam satellite TV signals ) targeted at their countries.
- Pakistan has recently stated its intention to begin jamming clandestine radio stations operated by the Taliban
- Ethiopia has jammed the DW and VOA transmissions as well as ESAT Ethiopian Satellite Television and also as well Eritrean radio stations.
- Vietnam jams the Vietnamese service of Radio Free Asia with a "siren" jammer.
See also 
- Radio jamming in China
- Radio jamming in Korea
- Mobile phone jammer
- Association of Old Crows
- Wireless signal jammer
- Electronic warfare
- Eastern Bloc information dissemination
- Culture jamming
- Jerome S. Berg (2008). Broadcasting on the Short Waves, 1945 to Today. McFarland. pp. 46–. ISBN 978-0-7864-5198-2. Retrieved 17 February 2013.
- NIST 800-97 Establishing Wireless Robust Security Networks: A Guide to IEEE 802.11i
- "'Kraut sieve'". Museum: Second World War. Dutch Resistance Museum. Retrieved 22 May 2012.
- Col. David Eshel. "Hezbollah's Intelligence War". Defense Update. Defense Update. Retrieved 23 May 2012.
- "Radio Northsea International". http://www.bobleroi.co.uk. Bob Le-Roi. Retrieved 23 May 2012.
- "Cooperate?". Second World War: Kingdom of the Netherlands. Ducth Resistance Museum. Retrieved 22 May 2012.
- Office of Research, USIA (1983), Jamming of Western Radio Broadcasts to the Soviet Union and Eastern Europe, United States Information Agency Text "" ignored (help)
- "ARCHIVE DOCUMENTS". www.radiojamming.info. Radio Baltic Waves. Retrieved 22 May 2012.
- "Russian Jammers". The Laughing Policeman Wireless Society. The Laughing Policeman Wireless Society. Retrieved 22 May 2012.
- Office of Research, USIA (various), Radio Free Europe archive documents, United States Information Agency Text "" ignored (help)
- Rimantas Pleikys. "JAMMING BY THE FREE WORLD". JAMMING by Rimantas Pleikys. R. Pleikys. p. 11. Retrieved 22 May 2012.
- BBC (in Persian) http://www.bbc.co.uk/persian/iran/2012/08/120821_l39_jamming_satellite_health.shtml
|url=missing title (help).
- "IRGC is Behind Satellite Jamming" (in Persian).
- "Global media accuse Iran over signal jamming". Iran Focus. AP. 7. Retrieved 22 May 2012.
- Stephanie Nebehay (26). "UN tells Iran to end Eutelsat satellite jamming". Iran Focus. Reuters. Retrieved 22 May 2012.
- "Pakistani army to jam Taliban radio in Swat Valley". Media Network. Radio Netherlands Worldwide. Retrieved 22 May 2012.
- Ludger Schadomsky (28). "Jamming of DW is part of Ethiopia's campaign against press freedom". Deutsche Welle: Press Freedom. Deutsche Welle. Retrieved 22 May 2012.
- "Ethiopia admits jamming VOA radio broadcasts in Amharic". BBC News. BBC. Retrieved 22 May 2012.
- "US Criticizes Ethiopia for Jamming VOA Signals". VOA: News / Africa. VOA. 19. Retrieved 22 May 2012.
- Various articles on Soviet jammers 1 2 3 4
- extensive site on radio jamming
- Article on recent jammers with audio samples
- Audio sample of jamming (c1982) at start of BBC World service (Russian) programme includes jamming station morse ID
- Ethiopia jams VOA (2010)
- Ultra Fast Folloow Jammer Description (2007)
- R. Pleikys, D. Vildžiūnas. Empire of Noise (video).