A jammer makes it difficult or impossible to receive a radio signal (e.g. from radio , television , mobile communications or GPS ) properly . The jamming transmitter, just like the transmitter to be disrupted, sends out electromagnetic waves and completely or partially superimposes the original waves. It can work on the same or a neighboring frequency of the disturbed receiver. The field strength , the modulation of the jammer and the type of the disturbed message are important.
A technical malfunction can generate a jamming transmitter , this must then be repaired or switched off, or the source of the interference must be accepted technically. So-called black transmitters or some legally operated transmitters are also referred to as jammers whose signal interferes with the reception of another transmitter. In particular, in the long , medium and short wave range , such interference often occurs when receiving remote transmitters, since large ranges are possible in these frequency ranges and fewer free channels are available than the transmitters operate. Directional antennas provide a certain remedy , both on the sender and receiver side. Badly shielded or badly suppressed devices can also become jammers.
The problem of machines interfering with radio reception became virulent at the end of the 1920s, when the station density increased dramatically and more and more listeners complained to the Reichsrundfunkanstalten and the postal authorities that they could no longer use anything when the engines were running in the neighborhood Listen to the radio. They went to court arguing that they were getting nothing in return for their license fee. The Braunschweig Regional Court issued its first landmark judgment at the end of 1931 . It condemned the owner of a workshop with malfunctioning engines to refrain from the malfunctions. For a long time the manufacturers defended themselves against this with the argument that the high costs for the interference suppression technology made the operation of the machines unprofitable.
Jammers can be used specifically to make it more difficult for others to use the frequency or an entire band; this is also referred to as jammer or noise jamming . These then send out broadband noise or pulses .
Mobile jammers can, for example, achieve 1 to 2 kW transmission power at 6000 frequencies.
Jammers are also used in the civil sector. For example, German prisons and juvenile detention centers are allowed to operate technical devices that "interfere with or suppress frequencies that are used to establish or maintain unauthorized radio links on the prison premises". The framework conditions of the Federal Network Agency must be observed. If the impairment of use (outside the facility premises) is not significant, no frequency allocation is required ( (1) sentence 5 TKG).
In August 2016 it became known that jammers against mobile telephony had been tested in the St. Pölten correctional facility in Austria for several weeks; According to the prison department in the Ministry of Justice with some success, so that only contact between prisoners is prevented and mobile telephony in the open is not disturbed. A second system is to be checked for its suitability. Equipping the 27 prisons in Austria with such cell phone blockers is estimated at several million euros. The communication of the prison guards via radio equipment is not impaired. Every month around 60 to 70 cell phones are discovered in prison inmates in prisons across Austria.
The interference is directed against various signals:
- In the military sector, especially in the radar , IR and UV spectral ranges
- The Global Positioning System can be blocked with a GPS jammer .
- Jammers are also used to prevent bombs from being detonated remotely. Pakistani opposition leader Benazir Bhutto survived an attack in 2007 unharmed because jammers on her bus prevented a bomb placed in a parked car from being triggered remotely. The bomb only exploded when the car was no longer within range of the jammers on the moving bus.
- Mobile phones
- Disturbance of the trigger signal for the camera of a mobile speed measuring device.
- Jammers against RFID scanners.
- Burglars use jammers to turn off wireless video surveillance systems .
- Thieves use jammers that prevent a vehicle from being locked by the jamming transmitter overlaying the legitimate locking signal of the remote control key , making it inoperable . They can then steal valuables from the supposedly locked but actually open vehicle.
- Jammers to ward off drones ( quadrocopter ), use e.g. B. at airports to avoid collisions with airplanes. Even at major political events, drones can B. could transport explosives pose a security risk.
Interference with the radio
In totalitarian states , too, jammers were and are used to prevent the population from receiving radio or television broadcasts from other people . In order to avoid the accusation of suppressing foreign information, strong transmitters of their own programs are often used on the frequencies to be disturbed.
Jammers were used, for example, during the National Socialist era to interfere with the German-language broadcasts of the BBC from London or in the Eastern Bloc countries to interfere with the broadcasts of Radio Free Europe / Radio Liberty . However, it has never been possible to cover the entire area of the respective state.
The United States interfered with the reception of the satellite on 23 and 24 January 2007 Eutelsat Hotbird 8 because they mistakenly assumed that Islamist transmitter az-Zaura ' would be broadcast via satellite. In addition to numerous radio and television stations, the news agencies Agence France-Presse and the Swiss Dispatch Agency were also affected .
Troubleshooting by radio interference suppression services
Interference caused by unauthorized transmission of a transmission signal is generally prohibited. In Germany, they are determined by the Federal Network Agency , the successor to the former radio interference suppression service, usually at the request of the person concerned. The polluter is obliged to comply with the legal limit values for a fee or the source of the interference is confiscated. If necessary, additional legal measures similar to the pursuit of black markers can then be taken.
Examples of unfavorable frequency coordination
In this legally controversial form of "jamming transmitters" , local, often quite powerful, legal radio transmitters or television transmitters are used to cover and thus interfere with less powerful remote transmitters on the same or a neighboring frequency (see interference ). The actual disturbance occurs with this form of "jamming transmitters" only at the receivers on site, if they can poorly receive the desired remote transmitter due to technical conditions, such as a lack of selectivity or override by local transmitters.
- The medium wave transmitters Mühlacker of the SDR (today SWR ) and Wöbbelin of the former broadcasting station of the GDR used the same frequency. The Wöbbelin transmitter was still in operation after German reunification , most recently for the distribution of megaradio , on the same frequency as the Mühlacker transmitter (576 kHz).
- Obviously for political reasons, the frequencies 557 kHz ( Greifswald transmitter ), 575 kHz ( Leipzig transmitter ) and 1430 kHz ( Dresden transmitter ) were used by the radio stations of the GDR in order to avoid the broadcasts of the SFB (566 kHz), the Saarland radio (1422 kHz) and from Radio Luxemburg (1439 kHz) if there is insufficient selectivity of the medium wave receiver. The internationally agreed and coordinated frequencies were used and the internationally permissible medium wave frequency grid in Europe of 9 kHz was observed.
- The long-wave transmitters Zehlendorf of Deutschlandradio Kultur and Europe 1 in Felsberg-Berus both used to be on the frequency of 180 kHz, but the frequencies were gradually shifted, but are still only around 6 kHz , although they claim a bandwidth of 9 kHz apart (transmission frequency of Deutschlandradio in Zehlendorf 177 kHz and 183 kHz of Europe 1).
- Dieter Görrisch: Jammers - from VHF to microwave. Franzis, 2006, ISBN 3-7723-4127-6 .
- R. Pleikys: Jamming . Vilnius, 1998.
- Schlesische Wellen , Breslau December 18, 1931, p. 1. Signature 4 Ona65 / 66-6, 25 / 52.1931 in the Berlin State Library
- E.g. Section 116 of the Thuringian Penal Code (ThürJVollzGB) of February 27, 2014
- prisoners with jammers orf.at August 11, 2016, accessed August 11, 2016.
- Bloodbath in Karachi Double attack on Benazir Bhutto - 125 dead by Markus Mechnich on tagesspiegel.de on October 19, 2007
- police Dusseldorf 04.04.2013 ( Memento of 7 April 2013, Internet Archive )
- Police Düsseldorf April 12 , 2013 ( Memento from April 15, 2013 in the Internet Archive )
- Anti-UAV Defense System: drone hunt around airports begins - Golem.de . ( golem.de [accessed April 12, 2017]).
- Swiss police: drone defense at the World Economic Forum in Davos - Golem.de . ( golem.de [accessed April 12, 2017]).
- US Army sabotaged the Swiss dispatch agency in 20 minutes , as of March 24, 2007.