Shortwave
When short-wave (Abbr. KW , engl. SW for shortwave or RF for high frequency ), sometimes Decameter waves are referred to radio waves in a higher frequency range than the long and medium waves . The terms short waves and decameter waves refer to the wavelengths .
Frequency and wavelength
The frequencies f of the short waves are 3 MHz ( megahertz ) to 30 MHz. This corresponds to wavelengths λ (lambda) from 100 m to 10 m. The relationship between the two quantities is λ = c / f . The constant c is the speed of light in a vacuum .
Broadcasting shortwave signals
Shortwave has a special place among radio waves. Due to their long range, shortwave signals can be received worldwide. No other frequency range has such a large range. As with long-wave and medium-wave transmitters , a short-wave transmission antenna emits both a ground wave and a sky wave . The bump spreads along the earth's surface and has a limited range, which is 30 to about 100 km depending on the frequency and transmission power. The sky wave leaves the earth's surface, due to the radiation characteristics of the antenna, mainly diagonally upwards and reaches the ionosphere at a flat angle and is reflected off it under favorable conditions. Compared to radio waves in other frequency ranges, such as long wave (LW), medium wave (MW) and ultra short wave (VHF), short wave is characterized by its very good reflection behavior of its sky waves. As they propagate wirelessly, they are reflected at different layers of the ionosphere and scattered back to the ground. From there they can be reflected back into space, and so the shortwave signal can travel around the whole earth (multi-hop). Shortwave is therefore of great importance for international radio links.
In contrast to radio broadcasts on long wave and medium wave, in which the sky waves are absorbed in the lower layers of the ionosphere during the day, radio broadcasts on short wave can be received worldwide without great effort with a commercially available transistor radio that contains a short wave frequency band ( world receiver ). Long and medium wave transmitters only reliably receive the ground wave. Irrespective of the time of day, their range is a few 100 km for medium waves and up to 1000 km for long waves.
Reflection on layers of the ionosphere
The reflection of short waves on the electrically conductive ionosphere is low-loss, but only works up to a limit frequency that is dependent on the angle of incidence ( Maximum Usable Frequency - MUF). The reflection on the ground is also low-loss for most of the earth, namely the conductive oceans; in the case of the continents it depends on the conductivity of the soil, in particular on the groundwater level. The ionosphere is primarily created by short-wave ultraviolet radiation from the sun .
The electron and ion density is practically zero in the mesosphere up to an altitude of about 60 km. Above this it increases and (during the day) reaches a first maximum in the E-layer. Above this layer it decreases somewhat, but increases again significantly from an altitude of around 200 km. The absolute maximum is reached in the F region, even higher it slowly decreases again. The different zones in this profile are called ionospheric layers . In 1902 Arthur Edwin Kennelly and Oliver Heaviside were the first to independently predict such a shift. Today it is called the E-Layer, the earlier name was the Kennelly-Heaviside-Layer .
A few years before Edward Victor Appleton, the German physicist Hans Lassen discovered a much more strongly ionized layer at a great height, which is now called the F-region and is crucial for the reflection of short-wave signals. The height profile of the layers, especially the strength of the ionization, depends heavily on the time of day, but also on the season. The maximum value of the electron density describes the critical frequency foF2, the worldwide change of which is recorded in ionization maps using the measurement results from many stations. All data are dependent on solar activity , which causes significant changes in the long term. In the course of its (roughly) 11-year cycle, the usable frequency ranges shift quite considerably.
At night there is no solar radiation as a source of ionization. Then different layers dissolve through the recombination of ions and electrons to form uncharged atoms . The D-layer disappears very quickly after sunset, because the high air density causes many collisions. The E layer disappears a few hours after sunset . The F 1 and F 2 layers formed during the day merge to form the F region, the ionization of which decreases during the night, but does not completely disappear.
Shortwave signals must pass through the D and E layers before they can be reflected on the F 2 layer. During the day they are often considerably weakened in these lower layers by collisions of the oscillating electrons with air molecules . At night, when the lower ionospheric layers have dissolved, this attenuation does not occur.
The reflection of electromagnetic waves on the F 2 layer can be explained with Snellius' law of refraction if the refractive index of the plasma is known. According to this law, which is often used in optics , an electromagnetic wave is refracted towards the perpendicular when it enters an optically denser medium. Radio waves below the plasma frequency are reflected by the ionized layers , their trajectories are curved in this area. In the layer, the direction of the beam becomes flatter, then horizontal and finally runs downwards again. The height-dependent plasma frequency has the effect that lower frequencies are reflected in deeper layers than higher frequencies; on the other hand, the former suffer more attenuation in the deep layers during the day. At VHF frequencies above 50 MHz, the refraction in the F 2 layer is never sufficient for total reflection. However, very strongly ionized E-layers can also (rarely) reflect frequencies around 50 MHz at shallow incidence.
The E s layer ( Sporadic E ) occurs sporadically at an altitude of 90 to 120 km ; in Central Europe this usually happens during the day in the summer months. It is believed that long-lived metal ions from meteorite impacts contribute to the formation of this layer. If the ionization of the E s layer is very strong, short waves can be reflected on it and thus no longer reach the F 2 layer (cover). In the VHF range, on the other hand, overreaching can occur if VHF signals are reflected on the E s layer.
The Mögel-Dellinger effect ( sudden ionospheric disturbance SID) is a sudden, massive disturbance of the entire shortwave traffic on the sunlit side of the globe, which lasts a quarter of an hour or a little longer [ dead quarter of an hour ]. It is caused by a hard radiation that the sun at a eruption emits and comes only a few times a year ago.
| layer | height | comment |
|---|---|---|
| D. | approx. 70 ... 90 km | available during the day, ionization according to the position of the sun |
| E. | approx. 110 ... 130 km | available during the day, ionization according to the position of the sun |
| E s | approx. 110 km | thin, in spots, sporadic; preferably in summer |
| F 1 | approx. 200 km | present during the day, goes together with F 2 shift at night |
| F 2 | approx. 250 ... 400 km | Present day and night |
The frequency usage window for radio waves is between the LUF and MUF. Physically, the LUF is determined by damping in the plasma of deeper layers, whereas the MUF is determined by refraction, almost always in the F 2 layer. If a so-called shortwave fadeout occurs, the window closes briefly. High transmission energy shifts the LUF downwards and thus makes the frequency window larger; however, it does not influence the MUF, apart from connections via scattered radiation, which only come about with very high transmission energy (troposcatter connections).
The MUF ( maximum usable frequency ) is significantly higher than the critical frequency foF2, because at an oblique incidence a smaller change in direction is sufficient for total reflection. The minimum cutoff frequency below which the attenuation is too strong is called LUF ( lowest usable frequency ). It depends on the equipment (transmission strength, antennas, sensitivity of the receiver). At certain times, the LUF can be higher than the MUF for certain connections, so that shortwave reception is not possible. For example, in the minimum of the sunspot cycle at midday in Central Europe, no reception from South American channels is possible.
Similar to meteorology, there is a radio weather report for the propagation conditions of the short waves as well as propagation forecasts that are broken down according to frequency, time of day, season and geographical target area.
The reflection behavior depends on the angle of the incoming radiation from the transmitter. Transmitting antennas are also designed and built with this in mind. The lowest radiation angle of a shortwave antenna should not be more than 5 degrees. The F 2 layer is hit at a distance of about 1500 to 2000 km from the transmitter. After the reflection, the signal can be received at a distance of 3000 to 4000 km on the ground. This large jump distance creates an area - on the earth's surface in a ring around the transmitter, in which the signal cannot be received - the so-called dead zone . If the distance between transmitter and receiver is greater than the simple jump distance, several ionospheric reflections are required to cover this distance (multi hop).
History of the short wave
Commercial radio technology began on long wave, followed by medium wave when free frequencies became scarce. All higher frequencies were considered worthless - also because there were no suitable components for powerful transmitters. Shortwave and everything above it was assigned to the radio amateurs as a "playground". It was only when they were able to establish overseas connections with surprisingly low transmission power (only a few watts) that the potential of shortwave was recognized. The activities of radio amateurs were restricted to narrow frequency ranges.
In this context, it is worth mentioning that shortwave was originally used for military purposes, as it was assumed that it could only be received locally to a limited extent. The exact opposite was the case: Nothing was known about the structure of the earth's atmosphere and had not expected the propagation by means of the space wave, which is reflected so well on the F-layer .
In fact, with shortwave it was possible for the first time to have direct radio contact from any point on earth with almost any other point on earth. Morse telegraphy was initially used as a means of communication . With the beginning of the age of broadcasting, radio stations also used shortwave to broadcast their programs. During the Second World War , shortwave was the most important military communication system. Because of the variability of the ionosphere, predictions were urgently needed and at least made on a statistical basis with some success. The method developed by Karl Rawer calculated MUF and LUF for each individual transmission path and took into account the change in the activity of the sun using a method invented by Wolfgang Gleißberg . The French Navy took over the process after the end of the war . Even after the introduction of satellite radio systems, shortwave will continue to be used for wireless international information exchange.
First radio connections
The first wireless connection was made by the Russian Alexander Stepanowitsch Popow , who in January 1896 published an article about a "device for detecting and registering electrical vibrations", which he used on March 24, 1896 for the wireless transmission of signals over a distance of 250 meters demonstrated. Guglielmo Marconi copied the device and had it patented in June 1896. The research and experiments built on the findings of Heinrich Hertz , who had already demonstrated the emission of electromagnetic waves in the laboratory in 1888. Whether electromagnetic waves can travel greater distances was not previously researched. In 1899 Marconi sent from France over the English Channel to England, and on December 12, 1901 he succeeded in establishing a radio link across the Atlantic, from Cornwall over 4000 km to Newfoundland . It is not known what wavelength he used. It was probably a wide range of frequencies.
The first radio broadcast was made by the Canadian Reginald Fessenden in 1906, who had already carried out the first wireless voice transmission on December 23, 1900.
Due to the increase in radio transmissions, the first conference of the International Telecommunication Union (ITU) took place as early as 1906 , at which the principles and rules of conduct for communications were established.
Fixed and mobile communication services
Coastal radio stations and ship transmitters used - in addition to long wave and medium wave - also short wave for message transmission. Marine radio played a central role here, because for the first time in the history of seafaring it was possible to reach a ship on the high seas at any time. In addition, it was possible for the first time to receive precise (to the second) time signals on the high seas with standard radio equipment, which is of fundamental importance for the exact determination of position with sextants . The radio services that use radio teletype RTTY include, for example, press agencies, marine radio, weather radio, military radio services and embassy radio . The use of shortwave reached its peak in World War II . On the German side, the analytical and statistical code developed by Karl Rawer allowed users to estimate the connection probability as a function of time and distance. Since most of them were only given a few (mostly 2) frequencies, which changed daily, this helped them a lot with the frequency choice and made it possible to assess the chances.
Predictions of the solar activity , which is decisive for long-term forecasts, were made with the method of Wolfgang Gleißberg , which is based on a comparison of several previous sunspot cycles.
So-called number transmitters were used mainly during the Cold War and are still occasionally sent spy messages on shortwave frequencies that can be received with almost any portable radio .
Another application are the VOLMET reports of the aeronautical radio service. These are stations that send out weather reports for international air traffic on fixed frequencies at certain times. Aeronautical radio is operated on VHF in the vicinity of airports, shortwave must be used for longer distances such as transatlantic flights. Transmission is in single sideband modulation (SSB). The main broadcast languages are English and Russian. The reports contain information about visibility, cloud cover, ground temperature and air pressure.
Some time signal and normal frequency transmitters also transmit on shortwave , usually on the standard frequencies 2500, 5000, 10,000 and 15,000 kHz. They are used for precise time measurement - mostly for maritime shipping - and the synchronization of clocks. These time signaling services are operated by scientific and technical institutes. Its importance has decreased with the advent of GPS .
Shortwave broadcasting and amateur radio
On November 28, 1923, two radio amateurs, including Léon Deloy , achieved the first two-way radio link on short waves across the Atlantic. That was the hour of birth of shortwave radio / shortwave broadcasting . This made it possible to listen to radio broadcasts from distant countries directly. The first transmission tests took place in 1924, including between Nauen and Buenos Aires. The first regular radio broadcasts began in 1925 on Vatican Radio , the BBC and Radio Moscow, today's voice of Russia .
In the Cold War, the communist states shows the opposite side were frequently part jammer ( jamming disturbed) with intent to unwelcome reception program on prevention. Stations that were often disrupted were, for example, Radio Free Europe / Radio Liberty , Deutsche Welle , BBC World Service and the Voice of America .
Radio amateurs are able to communicate worldwide - often with devices they have built themselves - via shortwave. A license is required to operate amateur radio . In disasters in remote areas, it was mostly radio amateurs who provided the outside world with initial information and contacts.
The HF frequency range is divided into frequency bands that are reserved for various broadcast and radio services. For example, there are special HF radio bands and amateur bands in which no other radio services are allowed to transmit.
Because of the worldwide reflection and propagation conditions, so-called world receivers (all-wave receivers) were developed in the 1960s with a focus on the shortwave bands. In Europe, the world receiver T 1000 from Braun was widespread, which had eight frequency ranges on KW alone, as was the Grundig satellite with its continuous frequency range from 520 kHz to 30 MHz. All-wave receivers were also important for receiving time signals in early satellite and astrogeodesy .
Shortwave and satellite communication
With the introduction of satellite communication , the importance of shortwave for seafaring has decreased significantly . The Internet as a source of information is stiff competition for many shortwave radio stations. For this reason, some large international services have reduced their operations to KW with destinations in Europe, North America and Australia or completely stopped. However, new media analyzes have shown that few radio listeners have switched to satellite reception and the Internet. The associated costs are far higher than the operation of a normal transistor radio. Another advantage is the high portability of a radio that can be used in almost any location.
Shortwave continues to be of great importance in the infrastructurally less developed areas of the world due to the lack of availability and very high costs for other information media. An important source of information is shortwave broadcasting in societies with state censorship of the mass media .
Another advantage is the independence from the power grid, as world receivers can usually be operated with batteries.
Digitized modulation on shortwave
The sound quality of ultra-shortwave broadcasting (VHF) in frequency modulation (FM) is significantly better than in the short, average because of the higher bandwidth of the audio signal used there and the almost complete absence of atmospheric influences (apart from very rare overreaches) - and long wave range. Also, is amplitude modulation (AM) due to the system more susceptible to atmospheric disturbances. So the number of radio stations using FM transmitters in remote areas gradually increased. However, this is not an alternative, since the extensive shortwave cannot be reproduced by a nationwide VHF transmitter network for technical and economic reasons. Digital Radio Mondiale (DRM) was founded in order to introduce better sound quality in AM radio and to reduce the strong distortion of selective carrier shrinkage . This consortium aims to define and introduce a standardized digital transmission system. The DRM consortium now has 80 members, made up of national and international broadcasters, research institutions and manufacturers of broadcast technology and receivers.
At the World Radiocommunication Conference ( English World Radiocommunication Conference , in short WRC) in Geneva in 2003 DRM went into regular operation. A number of radio stations are now broadcasting additional digital signals to the conventional AM radio broadcasts. Since 2003, prototypes of standalone receivers for DRM reception have been presented at the International Consumer Electronics Fair, but until the end of 2006 no such device was commercially available for the consumer sector. The Himalaya DRM-2009 and Morphy-Richard DRM Radio 27024 devices have been available since the beginning of 2007. However, DRM technology does not seem to gain acceptance.
Reception impairments
Various influences can change the propagation conditions for short waves:
1. Natural appearances:
- Activities on the sun (sunspot minimum, sunspot maximum), this changes the ionizing layers (which reflect the shortwave bands).
- the Mögel-Dellinger effect
- Thunderstorms , the discharges of which cause the “crackling” characteristic of shortwave
2. Interference emission from technical devices in the shortwave range, e.g. B .:
- Switching power supplies ,
- computer
- Line output stages in monitors and televisions
- Plasma television
- Fluorescent and energy saving lamps
- generally large, rapidly switched currents
- Powerline Communication, PLC for short , also known as Internet from the socket or LAN from the socket .
On the part of shortwave users, PLC is viewed critically, since it uses the transmission of signals in the shortwave range via unshielded power lines: These unshielded lines behave like antennas and radiate energy in the shortwave range; this radiation disrupts the shortwave reception in the vicinity of PLC applications.
Since the intensity of the radiation can never be precisely predicted, the operating instructions for the in-house PLC devices usually contain an indication of the problem: This device can cause radio interference in living areas; in this case, the operator can be required to take appropriate measures.
Short waves in medicine
The so-called short wave therapy ( diathermy ) is a healing method that belongs to the field of thermotherapy. In contrast to electrical stimulation therapy forms, which act on electrical and electromagnetic currents in the body, the short waves generate warming of the body tissue.
However, no short waves according to a strictly scientific definition are used here, but high-frequency alternating currents, the flow of which short waves are merely a by-product and are completely irrelevant for the desired effect of the treatment method.
A desired healing effect is triggered by this targeted heating. Shortwave therapy has so far shown good results, especially in rheumatic forms, but also in diseases of the musculoskeletal system, muscles and skin and in certain types of tumor formation . Patients with muscle and soft tissue pain, such as tension, can benefit from short-wave therapy. With suitable equipment, deep tissue can also be reached if the power is high enough and the electrodes can be positioned one to two centimeters away from the part of the body that is to be heated. The effect consists in selective deep heating depending on the application technique and dosage. It is also used for diseases of the skin, the eyes and in the ENT area.
The importance of shortwave today
Due to its special propagation conditions, shortwave offers the possibility of receiving radio broadcasts from every country in the world. These programs offer the advantage of receiving news directly from the first source - not quoted or lectured, as is the case in the domestic media. This is a particular attraction of shortwave broadcasting.
Today radio stations from over 30 countries broadcast programs in German. Most of these are information and entertainment programs with a duration of ½ to an hour that are broadcast to Europe in the evening hours. English-language programs can be heard from over 200 countries. In German-speaking countries, many people enjoy shortwave reception as a hobby - more than 4,000 listeners are even organized in shortwave listeners' clubs. Shortwave listeners are also known as SWL, short for Short Wave Listener . Receiving radio stations that are far away is also called DXing . Shortwave listeners, such as radio amateurs, can have QSL cards sent to them , which are also popular collector's items and prove successful receipt.
In remote areas, such as Australia, Africa, Canada, Papua New Guinea, and South America, shortwave connections are still a widespread form of communication today. They are used for normal radio broadcasting, the transmission of messages in emergencies ( emergency radio ) and as a medium for the transmission of training content . But criminals and guerrillas also like to use the easily available and transportable transmitting and receiving devices. A special frequency range is available in the tropical regions of the world, which is also known as the tropical band . This area is less disturbed by thunderstorms, as they often occur in the tropical belt, and is therefore reserved specifically for these transmitters.
While amateur connections on shortwave get by with low power, the operation of radio shortwave transmitters , which have a power of 100 to 500 kilowatts, is very cost-intensive. Since significantly lower transmission powers ( ERP ) are required for digitized broadcasting , shortwave transmitters are currently being digitized. The aim is to keep listeners interested in information in particular through good transmission quality.
In disaster situations , where the local power supply and the telephone lines are destroyed, also help in our time always private radio amateurs in the messaging emergency radio or even in assisting with medical emergencies (MAR).
Even after the end of the Cold War, shortwave over-the-horizon radars are an effective means of locating hostile objects in flight or rocket launches over great distances using this method with relatively little effort .
Short waves are also important for RFID .
See also
literature
- Lothar Wiesner: Teletype and data transmission over shortwave. Basics and networks . Siemens AG, Erlangen 1984 ISBN 3-8009-1391-7 .
- Martin Gerhard Wegener: Modern radio reception technology. Franzis-Verlag, Munich 1985, ISBN 3-7723-7911-7 & Yüce-Group, Istanbul 1989, ISBN 975-411-058-1 .
- Gerd Klawitter: time signal transmitter. Time Signal Stations . Siebel-Verlag, Meckenheim 1992 ISBN 3-922221-61-0 .
- J. Vastenhoud: Shortwave reception practice . Hüthig, Heidelberg ISBN 3-7785-0816-4 .
- Karl Rawer : Wave Propagation in the Ionosphere . Kluwer, Dordrecht 1993. ISBN 0-7923-0775-5 .
- Frequency plan of the Federal Network Agency
Web links
- Small selection of shortwave listeners clubs
- Portals on the subject of world reception
- Amateur radio
- DARC ( German Amateur Radio Club e.V. )
- ÖVSV ( Austrian Association of Experimental Broadcasters )
- USKA ( Union of Swiss Shortwave Amateurs )
- MAR, Medical Assistance Radio ( Memento from June 12, 2008 in the Internet Archive ) (amateur radio as an auxiliary service)
- Intermar amateur marine radio e. V.
- Radio weather
- Digital shortwave
Individual evidence
- ↑ Lassen, H., I. Journal for High Frequency Technology , 1926. Volume 28, pp. 109-113
- ↑ Andreas Lüer, DJ7IK: Short wave propagation , slide 6 ( Memento from September 27, 2007 in the Internet Archive ) (PDF; 1.4 MB)
- ↑ H. Schubothe: Damages and diseases caused by radiating energy. In: Ludwig Heilmeyer (ed.): Textbook of internal medicine. Springer-Verlag, Berlin / Göttingen / Heidelberg 1955; 2nd edition ibid. 1961, pp. 1170–1179, here: pp. 1171 f. ( Damage from short waves ).
- ↑ Frequency plan of the Federal Network Agency ( Memento from January 31, 2016 in the Internet Archive )
