A transmitter (short channels , engl. Transmitter ) is a means for generating and radiating electromagnetic waves , for example, with speech or music modulated are. Nowadays it usually consists of an oscillator followed by an amplifier and a transmitting antenna . In addition, a device for modulating the vibration is always necessary so that messages can be transmitted.
In a narrower sense, a device such as a mobile phone that contains these necessary components is called a transmitter.
High frequency generator
In the early days of radio technology , transmission systems were built in which vibrations were generated with sparks or arcs or machines (e.g. long-wave transmitter Grimeton ). But as early as the 1920s , electronics with vacuum tubes prevailed in this area , and from 1960 with semiconductor components .
Structure and components
In principle, an oscillator oscillation can be sent directly to the antenna . However, since the oscillator generally generates too little power , there are usually several amplifier stages between the oscillator and antenna in order to increase the transmission power. Often it is not the frequency generated by the oscillator that is used as the transmission frequency, but rather a harmonic whose frequency is a multiple of the oscillator frequency, especially at very high transmission frequencies such as VHF . This is filtered out of the oscillation behind a distortion circuit in the form of an overdriven amplifier with LC elements and then amplified. In systems for frequency modulation or several frequencies, voltage-controlled oscillators (VCO) or oscillators based on the synthesizer principle are used. With normal frequency transmitters such as DCF77 , the carrier frequency of the transmitter is generated by an atomic clock and brought to the desired value by frequency division or multiplication. Since this process is very complex, it is not used for most systems - mostly quartz oscillators have sufficient frequency stability.
Electron tubes are also used as amplifier elements, especially in high power output stages . Special semiconductor components or time -of- flight tubes - such as klystrons or magnetrons - are used for microwave transmission systems .
Resulting from the generation and amplification harmonics . These should not be emitted via the antenna and must be suppressed with low-pass filters. Induction transmitters are occasionally used for small ranges of up to 20 m (remote control) .
The output stages of very low power transmission systems do not require any special cooling equipment. Tube power amplifiers are often only radiation-cooled up to medium outputs (a few 100 watts). Convection air cooling (semiconductor transmitter) or forced air cooling is used for medium transmission power. Water cooling of the output stages (tubes or transistors) as shown in the picture on the right has been used for high outputs since 1930. Since high electrical voltages are required in tube output stages, distilled deionized water is often required in the cooling circuit. In a heat exchanger, this water transfers its heat to a second circuit, in which the water does not have to meet any special purity requirements, as it does not come into contact with live components.
Boiling condensation cooling is also used today in high-performance tubes. With this technology, evaporative cooling and condensation are spatially close to one another. The coolant flows through the cooling channel, which is equipped with grooves oriented towards the inside of the anode. The steam produced in these grooves gets into the main cooling channel, where it is swirled and condensed again. Since this process takes place in the case of water at temperatures of over 100 degrees Celsius, only tubes can be cooled with this cooling method. The process works on the principle of a heat pipe . If heat carriers with a lower boiling point or at lower pressure are used in evaporative cooling or a heat pipe, semiconductor transmitters can also be cooled in this way. Heat pipes allow maintenance-free heat dissipation in a small space with high heat outputs. One application is therefore the cooling of transmitters on board satellites and aircraft.
Tube transmitters require high anode voltages (of the order of 1 to 20 kilovolts), which are generated with mains transformers and rectifiers. There are often generator sets in order to be independent of the power grid.
Transmitting systems are often fed from a higher network voltage level to increase security of supply or due to the high output. The large French broadcasters Allouis and Roumoules , as well as Konstantynow in Poland, are fed from the high-voltage network (110 kV in Allouis and Konstantynow, 150 kV in Roumoules), although a medium-voltage power supply (approx. 20 kV) would also have the power requirement can cover.
The type of antenna depends on the frequency range, the power and the desired directional characteristic . Shield antennas are mostly used for long waves, self-radiating masts for long and medium waves , dipole antennas and group antennas consisting of them are used for short and ultra-short waves (VHF) . Parabolic antennas are often used for microwaves , which are usually radiated in a directional manner .
Antenna supports for VHF and TV transmitters are earthed. Both guyed steel truss masts and free-standing steel and reinforced concrete towers are used, with the transmitting antennas being at or near the top. Some transmission towers for VHF have high-altitude operating rooms and / or tourist facilities such as restaurants and viewing platforms that are accessible via an elevator. Such towers are mostly called TV towers . Parabolic antennas are often used for microwaves. For radio relay applications, these can be set up on transmission towers for VHF on special platforms. Large parabolic antennas with a diameter of 3 to 100 meters are required for program feed from television satellites and radio contact with spacecraft. Such systems, which can also be used as a radio telescope , are often designed to be movable.
As transmitting antennas for long and medium wave transmitters are mostly self-radiating masts used which are either isolated against ground and are fed at the base or are designed as grounded constructions over the backstays are fed associated auxiliary ropes. Trap antennas and long wire antennas on earthed towers and masts are also used. Occasionally, T, L and triangular planar antennas are also used. Transmitting antennas for long and medium waves are usually designed as guyed masts. Similar antennas with smaller dimensions are also used for shortwave transmitters when they transmit in omnidirectional mode.
See transmitting antenna for more information .
Since large currents at high voltages (up to 20 kV) can flow in transmission systems with higher outputs and transmission systems are exposed to an increased risk of overvoltage due to the antenna carrier, which is usually exposed to thunderstorms , extensive protective circuits must be provided to ensure the operation of the system and to Protect system components from destruction as much as possible.
The transmitter must always be operated with the load (antenna) connected. If this is not the case due to a fault, it must be switched off, otherwise the output stage can be destroyed. This also includes monitoring the standing wave ratio , the ratio of U max / U min along the high-frequency line. It must be as close as possible to 1 in order to keep cable losses low. The cause of a deviating standing wave ratio can be defects in connectors, on the antenna or its matching network .
In the case of tube-equipped transmitters, the heating voltage must first be applied to the tubes, the anode voltage is only switched on after heating. Otherwise the tubes are subject to increased wear.
Protection against lightning strikes is particularly important for isolated self-radiating transmission masts or transmission antennas. A spark gap is connected between the antenna and the earth as coarse protection , which ignites in the event of a lightning strike. A fine protection grant additional gas-filled surge. A monitoring device for the standing wave ratio switches the transmitter off briefly if the standing wave ratio is no longer correct after a lightning strike due to an ignited arrester. If the standing wave ratio remains outside the norm even after several attempts at switching on, the transmitter remains switched off. In some transmitter systems there are also ultraviolet detectors at critical points , which react to pre-discharges or arcs and switch off the transmitter so that the arc is extinguished.
With water-cooled power amplifiers, the electrical conductivity of the water must be monitored and kept low. If it exceeds a certain value, the transmitter must be switched off and the water replaced.
The degree of modulation, the operating voltage, the transmission frequency and other operating parameters are also monitored. The evaluation takes place either on site or from a remote control center to which these values are transmitted (wired or based on the transmission signal).
In the case of large stationary systems, an appropriate building, the transmitter building , is required to accommodate the technical equipment . This structure, which is mostly designed as a purely functional building, is located directly next to the antenna carrier in transmission systems for VHF and TV, and often 30 to 600 meters away from the transmission antenna in transmission systems for long, medium and short wave for radiation reasons. The transmission power is then transmitted using a trap line or a coaxial cable .
Among other things, in television towers there are rooms for the reception of the transmitters. Such structures are used for directional radio and VHF transmitters.
Under or near self-radiating transmission masts there is often a building to accommodate the impedance matching (the matching network) of the antennas, the tuning house .
Since radio waves spread across borders, international coordination, such as that laid down in the Geneva Wave Plan , is necessary for transmission systems in frequency ranges where large ranges are possible .
In Germany, the Federal Network Agency for Electricity, Gas, Telecommunications, Post and Railways controls this area.
An illegal transmission system is also known as a black transmitter .
Transmitters that are legally operated without a permit work in the ISM bands and must have type approval. As a radio amateur, you are allowed to build your own transmitter and antenna system or modify purchased transmitters. The amateur radio service is the only radio service that is allowed to do so. In the radio service regulations, it is internationally stipulated that radio amateurs must, in accordance with the ITU recommendation ITU-R M.1544, have a minimum theoretical knowledge of technology, legal knowledge, the handling of radio connections (so-called operating technology ) as well as electromagnetic environmental compatibility ( EMC ) and electromagnetic Compatibility ( EMC ) must have. A prospective radio amateur must prove this knowledge during an examination at his national telecommunications administration.
The CB radio , on the other hand, can be used privately to a limited extent up to certain maximum powers; changes to the device, as in amateur radio, are not permitted.
Since February 2006, the operation of transmitters (so-called FM transmitters) in the VHF band (Band II) with a short range of a few meters with up to 50 nW ERP for private purposes has been permitted in Germany, e.g. B. to transmit the signals of an MP3 player to the car radio.
Planning a transmitter system begins with the choice of location. For reasons of electromagnetic environmental compatibility , a minimum distance to residential buildings must be maintained, which depends on the transmission frequency and the design of the transmission antenna.
Long and medium wave transmitters are installed at a location with high electrical conductivity in the ground to ensure good grounding. Locations by the sea or in river plains are ideal for this. It is best to build transmitter systems for VHF elevated, as these waves propagate quasi-optically. You can therefore supply larger areas from there.
It is common practice to specify the geographic coordinates of the location of transmission systems. The location of the transmitting antenna (used in regular operation) is specified. This is particularly important with radio beacons. When specifying a place name, the place where the transmitter is located is mentioned. For reasons of disguise, this was mostly not done in the former Soviet Union and the states of the former Eastern Bloc - the next largest place was named. In the case of transmitter systems on mountain peaks, the name of the peak is usually given, but sometimes the area where this peak is located is also given. For this reason, there are often several location names for numerous transmitters.
Some cities, such as Mühlacker, Ismaning , Langenberg, Rothsürben , Kalundborg, Hörby and Allouis, became widely known as locations for high-performance transmission systems. Some transmission towers such as the Berlin TV tower or the Stuttgart TV tower have become landmarks of cities. Many transmission systems have very high antenna supports, the implementation of which was often a structural achievement. That is why they are also listed under the heading Tall buildings .
- Highest transmission mast
- Highest transmission power
In addition to the station and the program (see radio ), the term "radio transmitter " also refers to a special transmitter system that modulates music, speech or data to a transmission frequency using a modulator , amplifies it and feeds it to an antenna (see radio transmitter ). This allows signals to be transmitted over large distances and received with a radio receiver .
As FM transmitter is called very weak as FM station frequency modulated transmitter. They are used for the transmission of music and speech over short distances and can be received with FM receivers. Such radio stations are used for. B. in the drive- in cinema or inside your own car to feed an MP3 player into your own car radio .
- Information and photos on German transmitters
- DCF77 website
- Picture collection UHF / VHF and VHF transmitters - transmitter technology, antennas, processing and emergency power supply
- Order 07/2006: General allocation of frequencies in the frequency ranges 87.5-108 MHz, 863-865 MHz and 1795-1800 MHz for wireless audio radio applications. (PDF; 19 kB) (No longer available online.) Federal Network Agency, February 8, 2006, archived from the original on September 27, 2007 ; Retrieved February 14, 2009 .