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| + | [[Afbeelding:CrystalPalaceMast(large).jpg|100px|right|thumb|De zendmast van de [[Crystal Palace Transmitter]], Londen]] | ||
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| + | Een '''radiozender''' (in het Engels ''transmitter'', soms afgekort tot XMTR) is een elektronisch apparaat dat een radiofrequent signaal opwekt en met behulp van een antenne een elektromagnetische golf uitzendt, zoals voor radio, televisie, radar en telecommunicatie. | ||
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| + | Een radiozender bestaat uit een voeding, een oscillator, een modulator en versterkers voor geluidsgolven (''audio frequency'', AF) en radiogolven (''radio frequency'', RF). De modulator is het elektronische deel dat de over te brengen informatie moduleert op een draaggolf van een bepaalde frequentie. De meest gebruikte typen modulatie zijn amplitudemodulatie en frequentiemodulatie. De gemoduleerde draaggolf wordt vervolgens door middel van de antenne uitgezonden. Weglaten van de draaggolf kan ook (zie het artikel over enkelzijbandmodulatie). | ||
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| + | Een apparaat, zoals een mobiele telefoon, kan naast een zender (transmitter) ook een ontvanger (receiver) bevatten. Deze combinatie wordt wel een transceiver genoemd. | ||
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| + | ==Geschiedenis== | ||
| + | In de eerste dagen van de radio-elektronica werden radio-frequenties opgewekt door vonken van mechanische HF-generatoren (bijvoorbeeld met een Alexanderson alternator, hiervan is nog een zeldzaam exemplaar te vinden in het Radiostation Varberg in Grimeton, Zweden). Men noemt dit "machinezenders". Radio Kootwijk maakte in het begin ook gebruik van deze zogenaamde machinezenders. In de jaren '20 van de vorige eeuw begon men elektronische zenders gebaseerd op elektronenbuizen te gebruiken. | ||
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| + | ==Elektromagnetisch principe== | ||
| + | In principe kan iedere [[geleider]] ([[Elektriciteitsleiding|draad]]) die een [[wisselstroom]] kan geleiden, een radiosignaal uitzenden. Dus een simpele zender is gewoon een oscillator die direct aan een draadantenne is gekoppeld. | ||
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| + | [[Afbeelding:Emetteur.jpg|right|thumb|333px||Voorbeeld van een [[Elektrisch netwerk|elektrisch transmitter-circuit]]]] | ||
| + | Omdat radiozenders een goede frequentiestabiliteit vereisen, zijn er meestal enkele [[versterker (elektronica)|versterkertrappen]] tussen de oscillator en de antenne. De [[tussenversterker]]s voorkomen dat de frequentie van de oscillator beïnvloed wordt door het antenne-circuit. Vaak is de frequentie van de zender niet gelijk aan die van de oscillator, maar een van de [[harmonische]]n. Dit wordt gegenereerd door het signaal van de oscillator door niet-lineaire elektronische onderdelen te leiden (bijv. een [[diode]]) en te filteren door combinaties van [[spoel]]en en [[condensator]]en, en vervolgens te versterken. | ||
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| + | ==Radio-omroep== | ||
| + | Met radiozender bedoelt men in bepaalde gevallen een [[radio-omroep]] die radio-uitzendingen verzorgt. | ||
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| + | <!-- HIER VERDER VERTALEN: | ||
| + | Special standard frequency transmitters use [[Frequency synthesiser|frequency synthesis]] referenced to a very stable [[atomic clock]]. Since this procedure, which gives the most precise carrier frequencies, is very complex, it is not used in most transmitters. Typically a [[quartz]] crystal is used as a frequency reference, which provides adequate stability for nearly all purposes. Historically mechanically tuned [[VFO|variable-frequency oscillators]] were used, and are still found in classic [[amateur radio]] and antique equipment. | ||
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| + | During the generation and amplification, harmonics are created. These normally are filtered out by low pass filters before reaching the antenna. | ||
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| + | Vacuum tubes are still occasionally used as amplifier elements in high-power stages, for more than a few kilowatts of radio-frequency power. At high transmitting powers these tubes are water-cooled. For microwave transmitters, special semiconductor components or vacuum tubes (such as the [[klystron]], [[cavity magnetron]] or [[TWT]]) are needed, because signals of these frequencies and power levels 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. | ||
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| + | === Skin effect and waveguides === | ||
| + | A varying magnetic field will generate an electric field in a conductor. Conversely, a varying electric field in a conductor will generate a magnetic field. At high frequencies, inside a conductor this reciprocal effect creates essentially a "dead zone" in the center of the conductor, which substantially reduces the ''effective'' cross-sectional area of the conductor. In other words, if a cable is one inch (2.5 cm) in diameter, half an inch in the center may carry essentially no signal. This phenomenon is known as the [[skin effect]]. At microwave frequencies, the effect is so severe that there is no point in having a center, so cables are replaced with hollow [[waveguides]], which can be thought of as metal pipes. | ||
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| + | ==Cooling of final stages== | ||
| + | Low-power transmitters do not require special cooling equipment. Modern transmitters can be incredibly efficient, with efficiencies exceeding 98 percent. However, a broadcast transmitter with a megawatt power stage transferring 98% of that into the antenna can also be viewed as a 20 kilowatt electric heater. | ||
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| + | For medium-power transmitters, up to a few hundred watts, air cooling with fans is used. At power levels over a few kilowatts, the output stage is cooled by a forced liquid cooling system analogous to an automobile cooling system. Since the coolant directly touches the high-voltage [[anode]]s of the [[vacuum tube|tube]]s, only distilled, deionised water or a special dielectric coolant can be used in the cooling circuit. This high-purity coolant is in turn cooled by a heat exchanger, where the second cooling circuit can use water of ordinary quality because it is not in contact with energized parts. Very-high-power tubes of small physical size may use evaporative cooling by water in contact with the anode. The production of steam allows a high heat flow in a small space. | ||
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| + | == Power supply == | ||
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| + | Transmitters are sometimes fed from a higher voltage level of the power supply grid than necessary in order to improve security of supply. For example, the [[Longwave transmitter Allouis|Allouis]], [[Warsaw Radio Mast|Konstantynow]] and [[Transmitter Roumoules|Roumoules]] transmitters are fed from the high-voltage network (110 kV in Alouis and Konstantynow, 150 kV in Roumoules) even though a power supply from the medium-voltage level of the power grid (about 20 kV) would be able to deliver enough power. | ||
| + | [http://perso.orange.fr/monte-carlo-radiodiffusion/anglais/olan.htm], [http://perso.orange.fr/tvignaud/am/allouis/allouis4.htm] | ||
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| + | ==Protection equipment== | ||
| + | The high voltages used in high power transmitters (up to 40 kV) require extensive protection equipment. Also, transmitters are exposed to damage from [[lightning]]. Transmitters may be damaged if operated without an antenna, so protection circuits must detect the loss of the antenna and switch off the transmitter immediately. Tube-based transmitters must have power applied in the proper sequence, with the filament voltage applied before the anode voltage, otherwise the tubes can be damaged. The output stage must be monitored for [[VSWR|standing waves]], which indicate that generated power is not being radiated but instead is being reflected back into the transmitter. | ||
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| + | Lightning protection is required between the transmitter and antenna. This consists of [[spark gap]]s and gas-filled surge arresters to limit the voltage that appears on the transmitter terminals. The control instrument that measures the voltage standing-wave ratio switches the transmitter off briefly if a higher voltage standing-wave ratio is detected after a lightning strike, as the reflections are probably due to lightning damage. If this does not succeed after several attempts, the antenna may be damaged and the transmitter should remain switched off. In some transmitting plants [[UV]] detectors are fitted in critical places, to switch off the transmitter if an [[electric arc|arc]] is detected. The operating voltages, modulation factor, frequency and other transmitter parameters are monitored for protection and diagnostic purposes, and may be displayed locally and/or at a remote control room. | ||
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| + | ==Building== | ||
| + | A transmitter site will have a control building to shelter the transmitter components and control devices. This is usually a purely functional building, which may contain apparatus for both radio and television transmitters. To reduce transmission line loss the transmitter building is usually immediately adjacent to the antenna for [[VHF]] and [[Ultra high frequency|UHF]] sites, but for lower frequencies it may be desirable to have a distance of a few score or several hundred metres between the building and the antenna. Some transmitting towers have enclosures built into the tower to house radio relay link transmitters or other, relatively low-power transmitters. | ||
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| + | ==Legal and regulatory aspects== | ||
| + | Since radio waves go over borders, international agreements control radio transmissions. In European countries like [[Germany]] often the national Post Office is the regulating authority. In the [[United States]] broadcast and industrial transmitters are regulated by the [[Federal Communications Commission|FCC]]. In [[Canada]] technical aspects of broadcast and radio transmitters are controlled by Industry Canada, but broadcast content is regulated separately by the [[CRTC]]. | ||
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| + | ==Planning== | ||
| + | As in any costly project, the planning of a high power transmitter site requires great care. This begins with the location. A minimum distance, which depends on the transmitter frequency, transmitter power, and the design of the transmitting antennas, is required to protect people from the radio frequency energy. Antenna towers are often very tall and therefore flight paths must be evaluated. Sufficient electric power must be available for high power transmitters. Transmitters for long and medium wave require good grounding and soil of high electrical conductivity. Locations at the sea or in river valleys are ideal, but the flood danger must be considered. Transmitters for [[Ultra high frequency|UHF]] are best on high mountains to improve the range (see [[radio propagation]]). The antenna pattern must be considered because it is costly to change the pattern of a long-wave or medium-wave antenna. | ||
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| + | 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 UHF and TV transmitter can be implemented in principle as grounded constructions. Towers may be steel lattice masts or reinforced concrete towers with antennas mounted at the top. Some transmitting towers for UHF have high-altitude operating rooms and/or facilities such as restaurants and observation platforms, which are accessible by 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 FM 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 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. | ||
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| + | Just as important as the planning of the construction and location of the transmitter is how its output fits in with existing transmissions. Two transmitters cannot broadcast on the same frequency in the same area as this would cause co-channel interference. For a good example of how the channel planners have dovetailed different transmitters' outputs see [http://www.aerialsandtv.com/crystalpalacetx.html#crystalpalaceschannels Crystal Palace UHF TV channel allocations]. This reference also provides a good example of a grouped transmitter, in this case an A group. That is, all of its output is within the bottom third of the UK UHF television broadcast band. The other two groups (B and C/D) utilise the middle and top third of the band. By replicating this grouping across the country (using different groups for adjacent transmitters), co-channel interference can be minimised, and in addition, those in marginal reception areas can use more efficient grouped receiving antennas. Unfortunately, in the UK, this carefully planned system has had to be compromised with the advent of digital broadcasting which (during the changeover period at least) requires yet more channel space, and consequently the additional digital broadcast channels cannot always be fitted within the transmitter's existing group. Thus many UK transmitters have become "wideband" with the consequent need for replacement of receiving antennas (see external links). Further complication arises when adjacent transmitters have to transmit on the same frequency and under these circumstances the broadcast radiation patterns are attenuated in the relevant direction(s). A good example of this is [[Waltham transmitting station|Waltham]] which broadcasts digital MUXES 5 & 6 on the same frequencies as Sandy Heath, another transmitter 50 miles SSE of it. Thus [http://www.aerialsandtv.com/walthamtx.html#WalthamsTransmittingArray Waltham's transmitter array] does not broadcast these two channels in the direction of Sandy Heath and vice versa. All of the above provides a perfect case study in transmission frequency planning. | ||
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| + | ==Transmitters in culture== | ||
| + | Some cities in Europe, like [[Muehlacker]], [[Ismaning]], [[Langenberg]], [[Kalundborg]], [[Hoerby]] and [[Allouis]] became famous as sites 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. | ||
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| + | Having the tallest building in the world, the nation, the state/province/prefecture, city, etc., has often been considered something to brag about. Often, builders of high-rise buildings have used transmitter antennas to lay claim to having the tallest building. A historic example was the "tallest building" feud between the [[Chrysler Building]] and the [[Empire State Building]] in [[New York City| New York, New York]]. | ||
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| + | ==Records== | ||
| + | *Tallest radio mast | ||
| + | **1974-1991: Konstantyno for 2000 kilowatt longwave transmitter, 646.38 metres (2120 ft 8 in) | ||
| + | **1963-1974 and since 1991: KVLY Tower, 2,063 ft (628.8 m) | ||
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| + | *Highest power | ||
| + | **Longwave, Taldom transmitter, 2500 kW | ||
| + | **Medium wave, transmitter Bolshakovo, 2500 kW | ||
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| + | *Highest transmission sites (Europe) | ||
| + | **FM Pic du Aigu in Chamonix | ||
| + | **MW Pic Blanc in Andorra | ||
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| + | ==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. | ||
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| + | 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 HF the ERP is typically 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 quarter 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. | ||
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| + | Where a particular service needs to have wide coverage, this is usually achieved by using multiple transmitters at different locations. Usually, these transmitters will operate at different frequencies to avoid interference where coverage overlaps. Examples include national broadcasting networks and cellular networks. In the latter, frequency switching is automatically done by the receiver as necessary, in the former, manual retuning is more common (though the Radio Data System is an example of automatic frequency switching in broadcast networks). Another system for extending coverage using multiple transmitters is quasi-synchronous transmission, but this is rarely used nowadays. | ||
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| + | ===Main and relay (repeater) transmitters=== | ||
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| + | Transmitting stations are usually either classified as a main station or a relay station (also known as a repeater or translator). | ||
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| + | Main stations are defined as those that generate their own modulated output signal from a baseband (unmodulated) input. Usually main stations operate at high power and cover large areas. | ||
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| + | Relay stations take an already modulated input signal (usually by direct reception of a parent station (off-air)) and simply shift (translate) its frequency before rebroadcasting. Usually relay stations operate at medium or low power, and are used to fill in pockets of poor reception within, or at the fringe of, the service area of a parent main station. | ||
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| + | Note that a main station may also take its input signal directly off-air from another station, however this signal would be fully demodulated to baseband first, processed, and then remodulated for transmission. | ||
| + | --> | ||
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| + | ==Externe links== | ||
| + | *[http://www.radioluisteren.nu Radio luisteren via internet] | ||
| + | *[http://hawkins.pair.com/radio.html Jim Hawkins' Radio and Broadcast Technology Page] | ||
| + | *[http://www.wcov.com/technical/transmitter.html WCOV-TV's Transmitter Technical Website] | ||
| + | *[http://www.aerialsandtv.com/digitalnationwide.html Major UK television transmitters including change of group information, see Transmitter Planning section.] | ||
| + | *[http://www.rfident.org/ A general overview of RFID Transmitters and Readers - discussing attributes relating to the uses and possibilities of rfid]. | ||
Versie van 26 mei 2011 om 14:58
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Een radiozender (in het Engels transmitter, soms afgekort tot XMTR) is een elektronisch apparaat dat een radiofrequent signaal opwekt en met behulp van een antenne een elektromagnetische golf uitzendt, zoals voor radio, televisie, radar en telecommunicatie.
Een radiozender bestaat uit een voeding, een oscillator, een modulator en versterkers voor geluidsgolven (audio frequency, AF) en radiogolven (radio frequency, RF). De modulator is het elektronische deel dat de over te brengen informatie moduleert op een draaggolf van een bepaalde frequentie. De meest gebruikte typen modulatie zijn amplitudemodulatie en frequentiemodulatie. De gemoduleerde draaggolf wordt vervolgens door middel van de antenne uitgezonden. Weglaten van de draaggolf kan ook (zie het artikel over enkelzijbandmodulatie).
Een apparaat, zoals een mobiele telefoon, kan naast een zender (transmitter) ook een ontvanger (receiver) bevatten. Deze combinatie wordt wel een transceiver genoemd.
Inhoud |
Geschiedenis
In de eerste dagen van de radio-elektronica werden radio-frequenties opgewekt door vonken van mechanische HF-generatoren (bijvoorbeeld met een Alexanderson alternator, hiervan is nog een zeldzaam exemplaar te vinden in het Radiostation Varberg in Grimeton, Zweden). Men noemt dit "machinezenders". Radio Kootwijk maakte in het begin ook gebruik van deze zogenaamde machinezenders. In de jaren '20 van de vorige eeuw begon men elektronische zenders gebaseerd op elektronenbuizen te gebruiken.
Elektromagnetisch principe
In principe kan iedere geleider (draad) die een wisselstroom kan geleiden, een radiosignaal uitzenden. Dus een simpele zender is gewoon een oscillator die direct aan een draadantenne is gekoppeld.
Omdat radiozenders een goede frequentiestabiliteit vereisen, zijn er meestal enkele versterkertrappen tussen de oscillator en de antenne. De tussenversterkers voorkomen dat de frequentie van de oscillator beïnvloed wordt door het antenne-circuit. Vaak is de frequentie van de zender niet gelijk aan die van de oscillator, maar een van de harmonischen. Dit wordt gegenereerd door het signaal van de oscillator door niet-lineaire elektronische onderdelen te leiden (bijv. een diode) en te filteren door combinaties van spoelen en condensatoren, en vervolgens te versterken.
Radio-omroep
Met radiozender bedoelt men in bepaalde gevallen een radio-omroep die radio-uitzendingen verzorgt.
Externe links
- Radio luisteren via internet
- Jim Hawkins' Radio and Broadcast Technology Page
- WCOV-TV's Transmitter Technical Website
- Major UK television transmitters including change of group information, see Transmitter Planning section.
- A general overview of RFID Transmitters and Readers - discussing attributes relating to the uses and possibilities of rfid.
