Alexanderson alternator

A machine transmitter is a transmitter which generates undamped electromagnetic waves without using electronics of an electric generator.

History

In the early days of radio technology, there was no way to produce undamped oscillations purely electronically. In addition to the arc channel vibrations could by means of a motor, that electro-mechanical, are generated. For rapidly rotating generators were required, the rotor and stator were very finely divided. To 1904, working with such machines transmitters in the VLF range. Typical values ​​were 50 kW, for example, transmission power to a transmission frequency of 50 kHz, which corresponds to a wavelength of 6000 m.

Pioneers were among other Reginald Fessenden, Charles P. Steinmetz and Ernst Fredrik Werner Alexanderson.

As in the 1920s arose the electron tube, the lost arc and machine stations are rapidly developing. Also contributed to this at that time made ​​by radio amateurs discovery that up to then as "worthless" considered short waves are better and more economical for intercontinental connections as long waves. By means of tube technology were achieved these higher frequencies and more transmission power. By 1928, the last great machines transmitter station of the German company Telefunken in Japan was built. The VLF transmitter Grimeton is the last functioning machines transmitter. It dates from 1924 and is located in Sweden. The transmitter is a UNESCO world heritage site and uses the call sign SAQ.

Alexanderson alternator

Alexanderson an alternator, an electric generator, especially for producing high-frequency AC voltage with up to 100,000 cycles per second (100 kHz ) is constructed.

The devices were developed by Alexanderson and used worldwide mainly for the operation of long waves and long wave transmitters.

Working principle

The stator of an alternator is Alexanderson on one side of the coil, which are supplied with direct current and produce a magnetic field. The rotor is a fast rotating iron wheel with several hundred to more than 1000 slots. These slots are filled to reduce the air resistance with a non-ferromagnetic material. Wherein the magnetic flux is varied rhythmically on diametrically opposite coils of the other side of the stator and a high-frequency AC voltage is generated ( induction). For long-wave transmitter alternators up to 100 kHz and 200 kW have been developed.

200 kW alternators

The most powerful alternators were designed for long -wave transmitters and were produced by General Electric in a quantity of 20 (see table). You could get a frequency range of 12.5 kHz to 28.8 kHz ( operating in power grids with 60 Hz) and 10.4 kHz to 24 kHz cover (at 50 Hz systems in Europe). The motor speed varied depending on the mains frequency of 720 to 864/min. In addition, there were rotors with different number of poles and gear ratios of 1:2,675, 1:2,973 and 1:3,324. Had the rotors at the periphery of these machines have a thickness of 75 mm and a diameter of 1.6 m. With up to 2500 rpm, the outer circumferential speed reached 800 km / h ( about two-thirds the speed of sound in air). A very important component of the alternators was the speed regulator. For the corresponding matching of the antennas introduced 0.25 % variation of the rotational speed of the rotor to a reduction of the transmitted power to the antenna by more than 50 %.

Use stations

Beginning in 1942, four stations were operated by the U.S. Navy: the newly built station Haiku (Hawaii ) and the stations in Bolinas (both until 1946 ), Marion and Tuckerton (both until 1948 ). The station was taken over in 1949 by Marion the U.S. Air Force and used until 1957 for the transmission of weather reports in the Arctic as well as to bases in Greenland, Labrador and Iceland. One of the alternators was scrapped in 1961 and the other to the U.S. Bureau of passing standards.

The two machines in Brazil could never be used there because of organizational problems. They were returned to the Radio Central after 1946.

Transmission and reception operation

The VLF transmitters were equipped with at least Alexanderson antenna each one of which only one is still preserved in Grimeton. In Radio Central on Long Iceland (USA) twelve star-shaped established Alexanderson antennas were intended for the transmission mode with Denmark ( 1 ), Sweden ( 2), Germany (3 ), France ( 4) United Kingdom ( 5) South America (6, 7, 8), Pacific and telephony with Europe (9, 10, 11 ) and Poland ( 12). Telegrams were transmitted in a central office, which was located in Sweden, for example in Gothenburg in Morse code on paper tape and then in rapid succession by wire to the sending station (Sweden: Grimeton ) transmitted. In the transmitting station via the relay and modulation connected thereto magnetic amplifier ( transducers ) was.

The receiving antennas were located at some distance from the transmitters and consisted of about 13 km long wires, which were suspended on wooden poles. None of these systems is still preserved. However, the reception building are partially still preserved, for example in Kungsbacka, Sweden.

For the transmitter Grimeton operated annually once again welcome, there is now very simple ways. A single receiving circuit, with a short antenna, passes the signal to the sound card of a computer, which processes it.

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