Transmitter

From Academic Kids

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CrystalPalaceMast(large).jpg
Crystal Palace transmitter, London

A transmitter is an electronic device which with the aid of an antenna propagates an electromagnetic signal such as radio, television, or other telecommunications.

A transmitter usually has a power supply, an oscillator, a modulator, and amplifiers for audio (AF), intermediate frequency (IF) and radio frequency (RF). Sometimes a device, for example, a cell phone contains both a transmitter and a radio receiver or transceiver. The modulator is the device which piggybacks (or modulates) the signal information onto the carrier frequency, which is then broadcast.

Contents

History

In the early days of radio engineering, radiofrequency energy was generated in transmitting plants using arcs or mechanical alternators (of which a rare example survives at the SAQ transmitter in Grimeton, Sweden). In the 1920s electronic generators, based on vacuum tubes, began to be used.

Electromagnetic concepts

In principle any conductor (wire) carrying an alternating current will radiate a radio signal. Thus a basic transmitter is just an oscillator connected directly to a wire antenna.

Since transmitters require excellent frequency stability, there are usually several amplifier stages between oscillator and antenna. Often the transmitter frequency is not the frequency produced by the oscillator, but one of its harmonics. This is generated from the oscillator's output by a non-linear device (e.g. a diode or an overdriven amplifier), then filtered with combinations of inductors and capacitors, and then amplified.

Modern standard frequency transmitters use frequency synthesis. The carrier frequency of the transmitter is produced by an atomic clock and brought by frequency division and multiplication to the desired value. Since this procedure, which gives the most precise carrier frequencies, is very complex, it is not used in most transmitters. During the generation and amplification, harmonics are created. These usually have to be filtered out by resonant circuits before reaching the antenna.

Vacuum tubes are still occasionally used as amplifier elements in high-power stages. At high transmitting powers these are frequently water-cooled. For microwave transmitters, special semiconductor components or vacuum tubes, such as the klystron, are needed, because signals of these frequencies cannot be processed with normal semiconductors. The information to be transmitted is then added by modulation of the frequency, amplitude or phase of the carrier.

Cooling of final stages

Small power transmitters do not require special cooling equipments. For middle power transmitters the conventional air cooling is used. For large powers since around 1930 the water cooling of the output stage is already used. Since into this high voltages are used, only distilled deionisised water in the cooling circuit can be used. This water delivers its warmth in a heat exchanger at a second cycle, in which the water does not have to meet special purity requirements, because it comes with no live components into contact.

At modern high power tubes the simmering condensation cooling is used. With this technology steam generation and condensation are spatially not from each other separate. The cooling agent flows through the cooling duct, which is equipped also to the anode inside orientiereten slots. Steam developing in these slots comes into the main cooling duct, where it is swirled and again condenses. Since this procedure takes place at temperatures of over 100 degrees Celsius, large achievements can be produced with this cooling process with relatively small tubes.

Protection equipment

Since into transmitters of larger powers large currents can flow with high voltages (up to 20kV) and transmitting plants are exposed to increased overvoltage risk due to the usually exposed radio tower, extensive protective circuits must be planned, in order to ensure and protect around expensive equipment components as well as possible against destruction the enterprise of the plant. So it must be paid attention for example with nearly all plants very carefully to the fact that the transmitter is always operated with attached antenna. If this is not by a disturbance the case, this must be switched off immediately. Otherwise the output stage can be destroyed. With roar-equipped transmitters it is to be ensured by a Autmatik that first the filament voltage rests against the tubes and only then the anode voltage fishing GET becomes. Otherwise the tubes can take damage. Important is also a monitoring of the voltage standing-wave ratio. It describes whether high frequency achievement is also really radiated. It is to lie if possible close 1. This is not the case can in the circuit of antennas a hidden estimate have taken place. The transmitter is then switched off immediately. The protection from overvoltages is of large importance, in particular on use isolated radiating transmitting poles as transmitting antenna or with transmitting antennas at the point of the radio tower. Here as rough protection a spark gap is switched between antenna and earth, in order to offer possibly hitting lightning a jumping over possibility. Additional gas-filled surge diverters grant a fine protection. The control instrument for the measurement of the voltage standing-wave ratio, mentioned above, switches the transmitter off briefly, if after a thunderbolt the voltage standing-wave ratio does not tune any longer. It undertakes several switching on attempts. Even if it is not correct after these, the antenna is defective and the transmitters remains switched off. In some transmitting plants also UV detectors are in critical places. Footstep in these critical places an arc up, is switched off over these sensors of the transmitters, so that the arc expires. It is restarted after a certain time. With water-cooled output stages the electrical conductivity of the water must be supervised carefully. If it exceeds a certain value, suitable counter measures (Zufuellen of highly pure water or switching of the transmitter off) must be accomplished. Further also the modulation factor, the operating voltage, becomes which supervises transmitter frequency and further operating parameters. The evaluation happens either locally or from a distant directing center, to which these values will transfer wire-bound or wirelessly.

Building

For the accommodation of the technical devices an appropriate building is, the transmitter building necessary. This building is usually designed as pure functional structure is with transmitting plants for UKW and TV directly beside the radio tower, with transmitting plants for superlong -, long -, medium and short wave from radiation-technical reasons frequently 30 to 600 meters from the transmitting antenna far away. There are also transmitting towers, in which areas for the admission of transmitters are present. Such buildings are used for radio relay link and UKW transmitters.

Judical Aspects

Since radio waves go over borders away, is for transmitting plants in frequency ranges, in those large ranges are possible an international coordination, like it for example in Geneva wave plan fixed is necessary. In Germany the adjustment authority for post office and telecommunications controls this range. Illegal equipment is a secret transmitter.

Planning

The planning of a high power transmitter requires large care, because bad plannings can come very expensively. This begins already with the choice of the location. In any case a minimum distance, which depends on the transmitter frequency and the design of the transmitting antenna is, to keep to houses for reasons of the EMVU and the protection from electrical smog. One establishes transmitters for long and medium wave, in order to ensure a good grounding, at a location of high electrical soil conductivity. For this locations at the sea or in river valleys are ideal, whereby the flood danger must be naturally always considered. One builds transmitting plants for UKW best on high mountains. One can supply far areas in such cases with low radio towers. Important it is to be also clarified whether a transmitter with a given radio beam diagram can be operated also in the future. A change of the radio beam diagram can become very expensive with long and medium-wave transmitters in particular, if for this antenna towers must be up and diminished. When transmitting antennas for long and medium-wave transmitters become usually radiating transmitting poles used, which are isolated either against earth and are fed at the toe or also as grounded constructions are implemented, which are fed over with the pardunen connected hilfsseile. Also prism aerials and long wire antennas at grounded towers and masts are used. Occasionally also T are used -, L and triangle plane aerials.

Transmitting antennas for long and medium wave are usually implemented as a mast radiator. Similar antennas with smaller dimensions are used also for short wave transmitters, if these send in the round spray enterprise. For arranging radiation at free standing steel towers fastened planar arrays are used. Radio towers for UKW and TV transmitter can be implemented in principle as grounded constructions. Are used both removed steel framework masts and free standing stealing and reinforced concrete towers, whereby the transmitting antennas on the point are. Some transmitting towers for UKW have over high-altitude operating rooms and/or routistic mechanisms such as restaurants and prospect platforms, which are accessible over an elevator. Such towers are usually called TV tower. For microwaves one uses frequently parabolic antennas. These can be set up for applications of radio relay links on transmitting towers for UKW to special platforms. For the program passing on of television satellites and the funkkontakt to space vehicles large parabolic antennas with diameters of 3 to 100 meters of diameters are necessary. These plants, which can be used if necessary also as radio telescope, are established on free standing constructions, whereby there are also numerous special designs, like the radio telescope in Arecibo.

Indication of transmitter locations

Since transmitting plants lie usually outside of urban areas, it is far common to indicate for the indication of the location of transmitting plants also the geographical coordinates. The location (in the rule enterprise used) of the transmitting antenna is here usually used. During the designation of the place name the place should be always called, on its gemarkung the transmitting plant is. For masking reasons this was usually not made in the former Soviet Union and the states of former Eastern Europe, but was not called that next larger place. With transmitting plants on mountain summits the name of the summit is usually called, sometimes in addition, the gemarkung of the place, on which this summit is. For this reason there are often several location designations for numerous transmitting plants.

Transmitters in Culture

Some cities in Europe, like Muehlacker, Ismaning, Langenberg, Kalundborg, Hoerby and Allouis became famous as site of powerful transmitters. Some transmitting towers like the radio tower Berlin or the TV tower Stuttgart became landmarks of cities. Many transmitting plants have very high radio towers, which are masterpieces of engineering.

Records

  • Tallest radio mast
    • 1974-1991:Konstantynow for 2000kilowatt longwave transmitter, 646.38 metres
    • 1963-1974 and since 1991, KVLY Tower
  • Highest power
    • Longwave, transmitter Taldom, 2500 kW
    • Medium wave, transmitter Bolshakovo, 2500 kW
  • Highest transmission sites (Europe)
    • UKW Pic du Aigu bei Chamonix
    • MW Pic Blanc in Andorra

Broadcasting

In broadcasting, the part which contains the oscillator, modulator, and sometimes audio processor, is called the exciter. Confusingly, the high-power amplifier which the exciter then feeds into is often called the "transmitter" by broadcast engineers. The final output is given as transmitter power output (TPO), although this is not what most stations are rated by.

Effective radiated power (ERP) is used when calculating station coverage, even for most non-broadcast stations. It is the TPO, minus any attenuation or radiated loss in the line to the antenna, multiplied by the gain (magnification) which the antenna provides toward the horizon. This is important, because the electric utility bill for the transmitter would be enormous otherwise, as would the cost of a transmitter. For most large stations in the VHF- and UHF-range, the transmitter power is no more than 20% of the ERP. For VLF, LF, MF and SW the ERP is not determined separately. In most cases the transmission power found in lists of transmitters is the value for the output of the transmitter. This is only correct for omnidirectional aerials with a length of a quater wavelength or shorter. For other aerial types there are gain factors, which can reach values until 50 for shortwave directional beams in the direction of maximum beam intensity. Since some authors take account of gain factors of aerials of transmitters for frequencies below 30 MHz and others not, there are often discrepancies of the values of transmitted powers.

See also


In communications and information processing, a transmitter (sometimes abbreviated XMTR) is an object (source) which sends information to an observer (receiver).de:Sendeanlage fr:Émetteur radio nl:Zender pl:Nadajnik

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