Everything about Radio Jamming totally explained
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 checking that it's in use first, or without being able to hear distant stations on the same 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.
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.
Method
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.
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 as 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, erratically warbling tones, random noise (hiss) and recorded sounds. For example, China, which used jamming extensively and still does, plays an infinite loop of traditional Chinese music while it's jamming channels.
The purpose of this type of jamming is to block out 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 that during which no sound is heard on the receiving equipment. The radio doesn't 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 FM
capture effect, Frequency Modulated broadcasts may be jammed, unnoticed, by a simple unmodulated carrier.
History of Jamming
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),
Cuba,
Iraq,
Iran (Iraq and Iran war, 1980-1988),
China, North and South
Korea and several
Latin American countries. as well as by the authorities in several countries against
pirate radio stations including
Radio Nova in Ireland
(External Link
) and
Radio Northsea International off the coast of Britain
(External Link).
World War Two
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" )
(External Link)
Cold War era
During the
Cold War Soviet 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. .
(External Link)
Radio Free Europe and its sister service
Radio Liberty were the main target of Soviet jammers followed by
Voice of America and the
BBC World Service.
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 Israel and
Radio Canada International The jamming usually only took place during programming in languages widely spoken in Eastern Bloc countries (
for example,
Russian,
Polish,
Czech,
Lithuanian, etc.). Programmes in
English or other major Western languages were rarely (if ever) jammed intentionally.
(External Link)
There were also periods when
China and the
USSR jammed each other's programmes. The
USSR also jammed
Albanian programmes at times.
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. Because radio signal
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 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 didnt cover the higher bands.
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.
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.
(External Link)
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
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 500kW (for it shortwave jamming products).
Adele Milna (BSEE) of Continental Electronics (in an audio file held at shortwave.org) claims that China has duplicated his companies 100kw, 250kw 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.
Other Countries
Since the early 1960's, the practice of radio jamming has been very common in Cuba, blocking not only American government funded radio stations (Such as VOA) but radio stations owned and/or operated by (or selling airtime to) Cuban exile groups transmitting from Miami, Florida, such as La Cubanisima, Radio Mambi, 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 others 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 (External Link) (External Link)) targeted at their countries.Further Information
Get more info on 'Radio Jamming'.
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