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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 checking that it is 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 is 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 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 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), Israel, 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 and Radio Northsea International off the coast of Britain .
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"
)
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.
.
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 (e.g., Russian, Polish, Czech,
Lithuanian, etc.). Programmes in English or other major Western languages were
rarely (if ever) jammed intentionally.
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
* Eurasia (worst affected, including mediumwave frequencies particularly 720KHz
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 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.
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 ) targeted at their
countries.
* Israeli defense officials jammed Hezbollah's telecommunication and television
broadcasts during five-week campaign
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