Machine transmitter
A machine transmitter is a transmission system that generates the carrier frequency to be emitted with the help of an alternating current generator.
story
In the early days of radio technology , there was no way to generate undamped vibrations purely electronically. In addition to the arc transmitter , vibrations could be generated with the help of a motor, i.e. electromechanically . This required high-speed generators whose rotor and stator were very finely divided. Around 1904 such machine transmitters were used in the long-wave range . Typical values were e.g. B. 50 kW transmission power at a transmission frequency of 50 kHz, which corresponds to a wavelength of 6000 m.
When the electron tube appeared in the 1920s, arc and machine transmitters quickly lost their importance. The discovery made by radio amateurs at the time that the shortwaves , which had been considered worthless up until then, were better and more economical for intercontinental connections than longwaves also contributed to this. With the help of tube technology, these higher frequencies and also more transmission power could be achieved. Around 1928 the last large machine transmitter station was set up by the German company Telefunken in Japan . The longest wave transmitter Grimeton is the last still functional machine transmitter . It dates from 1924 and is located in Sweden . The transmitter is a UNESCO World Heritage Site and uses the SAQ call sign.
Types
Alexanderson alternator
right: drive motor; top left in the background: magnetic amplifier for modulation
The Alexanderson alternator, developed in 1904 by Ernst Fredrik Werner Alexanderson , is a reluctance machine used as a generator . It represents the basic form of the machine transmitter and is an electrical generator that is specially designed to generate high-frequency alternating voltages of up to 100 kHz . The machine transmitters of this design were used worldwide for the operation of long-wave and long-wave transmitters.
The stator consists of windings that are supplied with direct current and generate a static magnetic field. The rotor is a rapidly rotating iron wheel with several hundred to over 1000 slots that form magnetic poles from the iron in between . The slots are filled with a non- ferromagnetic material to reduce air resistance . As a result, the magnetic flux is periodically changed by the opposing windings of the stator at the air gap and a high-frequency alternating voltage is generated by means of electromagnetic induction due to the highest possible number of pole pairs . Generators for up to 100 kHz and 200 kW power have been developed for long-wave transmitters.
Goldschmidt alternator
One of the disadvantages of the Alexanderson alternator is the fact that a high number of pole pairs on the rotor is necessary to achieve the highest possible frequency. Associated with this is the need for a rotor that is sufficiently large in scope, because the circumferential speed of the rotor can not be increased at will for mechanical reasons due to the centrifugal forces and the associated tensile stresses in the rotor, and the poles need a sufficient size to prevent magnetic leakage flux to grow big. As a result, the Alexanderson alternator is limited by an upper, technologically determined limit frequency.
In 1908, Rudolf Goldschmidt developed the Goldschmidt alternator named after him, which is an earlier form of the mixer stages commonly used in radio technology today to achieve higher frequencies. When used in the same frequency ranges as the Alexanderson alternator, the Goldschmidt alternator allows reduced rotational speeds on the rotor and a smaller number of pole pairs that are technically easier to control. The upper limit frequency for the Goldschmidt alternator is approx. 200 kHz.
The structure of the rotor consists of two separate windings on the rotor, each of which delivers the basic frequency f at a certain speed . The mutual magnetic coupling creates the two mixed products and , i.e. a constant component and a double frequency . As a result of feedback, higher mixed products are formed with decreasing amplitude, which represent integer multiples of the fundamental oscillation. Due to the decreasing amplitudes towards higher frequencies, there are also limits to this method; frequency multiples of up to were common . The desired mixed product, for example the frequency , is decoupled by filters that are matched to this frequency and consist of capacitors and coils . These filters, adjusted for resonance, are located in the immediate vicinity outside the electrical generator and are a fixed component of the machine transmitter.
Large systems
The most powerful machine transmitters were intended for longitudinal wave transmitters; General Electric produced 20 pieces, see table. They were able to cover a frequency range from 12.5 kHz to 28.8 kHz (operation in power grids with 60 Hz) or 10.4 kHz to 24 kHz (in 50 Hz grids in Europe). The engine speed varied between 720 and 864 / min depending on the line frequency. There were also rotors with different numbers of poles and gear ratios of 40: 107, 37: 110 and 34: 113. The rotors of these machines had a circumference of 7.5 cm and a diameter of 160 cm. At up to 2500 / min, the outer peripheral speed reached around 800 km / h, i.e. about two thirds the speed of sound in air. A very important component of the generators was their speed regulator to keep the frequencies constant. 0.25% deviation of the number of revolutions of the rotor from the optimal number of revolutions led to a reduction of the power that can be coupled into the antenna by more than 50%.
Deployment stations
| City (state), state |
blown sound character |
wave length |
frequency frequency |
installed lation |
off circuit |
Comparable scrapping |
comment |
|---|---|---|---|---|---|---|---|
|
New Brunswick ( New Jersey ), USA |
WII | 13761 m | 21786 Hz | 1918 | 1948 | 1953 | initially 50 kW generator |
| WRT | 13274 m | 22585 Hz | 1920 | ||||
|
Marion ( Massachusetts ), USA |
WQR | 13423 m | 22334 Hz | 1920 | 1932 | ||
| WSO | 11623 m | 25793 Hz | 1922 | 1942 to Haiku | |||
|
Bolinas ( California ), USA |
KET | 13100 m | 22885 Hz | 1920 | 1930 | 1946 | |
| KET | 15600 m | 19217 Hz | 1921 | 1942 to Haiku | |||
|
Radio Central ( Long Island ), USA |
WQK | 16484 m | 18187 Hz | 1921 | 1948 | 1951 | |
| WSS | 15957 m | 18788 Hz | 1949 to Marion | ||||
|
Kahuku ( Hawaii ), USA |
KGI | 16120 m | 18598 Hz | 1920 | 1930 | 1938 | |
| KIE | 16667 m | 17987 Hz | 1921 | ||||
|
Tuckerton, New Jersey, USA |
WCI | 16304 m | 18388 Hz | 1921 | 1948 | 1955 | |
| WGG | 13575 m | 22084 Hz | 1922 | ||||
|
Caernarfon , UK |
MUU | 14111 m | 21245 Hz | 1921 | 1939 | ||
| GLC | 9592 m | 31254 Hz | |||||
|
Warsaw , Poland |
AXO | 21127 m | 14190 Hz | 1923 | destroyed in World War II | ||
| AXL | 18293 m | 16388 Hz | |||||
|
Grimeton , Sweden |
SAQ | 17442 m | 17188 Hz | 1924 | still operational | initially 18600 m (16118 Hz) | |
| 1960 | 1960 | for parallel connection | |||||
|
Recife ( Pernambuco ), Brazil |
never | Delivered in 1924 | |||||
- comment
From 1942 four stations were operated by the US Navy: the newly built Haiku station in Hawaii and the stations in Bolinas (both until 1946), Marion and Tuckerton (both until 1948). The Marion station was taken over by the US Air Force in 1949 and used to transmit weather reports to the Arctic and to the bases in Greenland, Labrador and Iceland until 1957. One of the generators was scrapped in 1961 and the other turned over to the US Bureau of Standards.
The two machines in Brazil could never be used there due to organizational problems. They were returned to Radio Central after 1946.
Send and receive operation
The long-wave transmitters were each equipped with at least one Alexanderson antenna , only one of which is still preserved in Grimeton. In Radio Central on Long Island (USA) twelve star-shaped Alexanderson antennas were planned for broadcasting with Denmark (1), Sweden (2), Germany (3), France (4), Great Britain (5), South America (6, 7, 8), the Pacific and telephony with Europe (9, 10, 11) and Poland (12). Telegrams were transmitted in Morse code on punched tape in a central office, located in Gothenburg , for example , and then transmitted in quick succession by wire to the transmitting station (Sweden: Grimeton) as direct current impulses.
In the transmitting station, the transmitter was modulated by so-called magnetic amplifiers ( transducers ), which controlled power relays in Morse code using the direct current pulses transmitted over the long-distance line .
The receiving antennas were at some distance from the transmitters and consisted of about 13 km long wires that were suspended from wooden masts. Not a single one of these systems has survived. However, some of the reception buildings have still been preserved, for example in Kungsbacka , Sweden.
There are many simple options for receiving long-wave transmitters such as the Grimeton transmitter, which is operated once a year. It can audio circuits , but also the WebSDR, a SDR receiver , which in the Internet is freely accessible, or even more modern reception circuits that the NF signal the sound card of a PC , can be used out.
literature
- Johne Brittain: Alexanderson. Pioneer in American Electrical Engineering . Baltimore et al. 1992.
Web links
- The time of the machine transmitters . Article by A. Meißner from the anniversary publication "50 Years of Telefunken" (May 1953) on seefunknetz.de .
- The Alexanderson transmitterson the homepage of the Alexander Freundeskreis for the Swedish transmitter Grimeton SAQ
Individual evidence
- ^ Russell Burns: An International History of the Formative Years . In: IEE History of Technology . Institution of Engineering and Technology, 2003, ISBN 978-0-86341-327-8 , pp. 365-369 .
- ↑ freely accessible SDR radio receiver maintained and continuously developed by Pieter-Tjerk de Boer websdr.ewi.utwente.nl