thumb|[[Alexanderson alternator in the Grimeton VLF transmitter. The drive motor is at the extreme right; the speed-increaser gearbox is just to its left. Note the bronze-colored shaft coupling.]]
Grimeton Radio Station () in southern Sweden, close to Varberg in Halland, is an early longwave transatlantic wireless telegraphy station built in 1922–1924, that has been preserved as a historical site.
From the 1920s through the 1940s it was used to transmit telegram traffic by Morse code to North America and other countries, and during World War II was Sweden's only telecommunication link with the rest of the world. It is the only remaining example of an early pre-electronic radio transmitter technology called an Alexanderson alternator. It was added to the UNESCO World Heritage List in 2004, with the statement: "Grimeton Radio Station, Varberg is an exceptionally well preserved example of a type of telecommunication centre, representing the technological achievements by the early 1920s, as well as documenting the further development over some three decades."
The radio station is also an anchor site for the European Route of Industrial Heritage. The transmitter is still in operational condition, and each year on a day called Alexanderson Day is started up and transmits brief Morse code test transmissions, which can be received all over Europe.
History
Beginning around 1910 industrial countries built networks of powerful transoceanic longwave radiotelegraphy stations to communicate telegraphically with other countries. During the First World War radio became a strategic technology when it was realized that a nation without long-distance radio capability could be isolated from the rest of the world by an enemy cutting its submarine telegraph cables. Sweden's geographical dependence on other countries' underwater cable networks, and the temporary loss of those vital connections during the war, motivated a decision in 1920 by the Swedish Parliament that the Royal Telegraph Agency build a "big radiotelegraphy station" in Sweden to transmit telegram traffic across the Atlantic. later changed to , to RCA's receiving station in Riverhead, New York. It immediately took over 95% of the Swedish telegram traffic to the United States. The Swedish Maritime Administration also leases transmitter and antenna spaces for MF telephony and NAVTEX transmitters at the site. Their NAVTEX antenna is of a special design, using one 127 m high tower as a folded monopole antenna, self-resonant on 518 kHz.
All pre-1980s LF and HF transmitters in the new building have been decommissioned and scrapped. Out of the original system, one of the alternator transmitters had been gradually dismantled and scrapped in the 1950s to free up space in the station building. The remaining alternator continued to be used for naval transmissions until the early 1990s, when a modern solid-state LF transmitter replaced it.
Grimeton Radio Station is now the only station left in the transatlantic network of nine long wave stations that were built during the years 1918–1924, all equipped with Alexanderson alternators. In 2004 it was added to the UNESCO World Heritage List. The Grimeton transmitter is the last surviving example of an Alexanderson alternator, the only radio station left from the pre-vacuum tube era, and is still in working condition. Each year on Alexanderson Day— the closest Sunday to 2 July— the site holds an open house during which the transmitter is started up and transmits test messages on 17.2 kHz using its call sign SAQ, which can be received all over Europe.
Technical description
none|thumb|660x660px|Principle diagram of information transmission
The electromechanical transmitter in Grimeton transmitted at a frequency of 17.2 kHz, i.e. in the VLF range, and was thus able to reach America.
In principle, an electric generator (A) is used for this purpose. This is set in rotation by a motor (500 HP, 711.3 rpm) via a gearbox (setup ratio: 2.97) and thus generates a continuous sinusoidal AC voltage (B) of 17.2 kHz or 17,200 Hz.
For comparison, generators of the public electricity networks produce an alternating voltage of 50 or 60 Hz, depending on the country. To produce such high frequencies with a generator, a fast-rotating generator (2115 revolutions per minute) with a special design is necessary.
In Grimeton, mainly Morse signals were transmitted. To send information with the generated alternating voltage, the texts to be sent are translated into a sequence of short and long pulses according to the Morse code by means of a Morse key (D). The switchgear (C) uses these pulses to control the transmission of the AC voltage to the antenna (F). When the key is pressed, the AC voltage is passed on to the antenna and transmitted from there. If the key is not pressed, the AC voltage is suppressed by the switchgear and no signal is transmitted. Thus, for example, as shown in (E), the letter A can be transmitted by a short and a long wave packet and detected at the receiver.
The AC voltage generated has a voltage of 2000 volts and a power of 200 kW
As usual in electric generators, an alternating voltage is generated in adjacent coils (B) in the generator (A) by means of rotating magnetic fields. In Grimeton, these coils are mounted on the stator, divided into 2x32 sectors, on both sides to the rotor. The individual windings of a sector are connected to corresponding primary windings (C) of the transformer (D). When the primary voltages are transmitted to the secondary winding (E) of the transformer, these voltages are superimposed to form a strong, sinusoidal output signal which is output to the antenna and transmitted.
The control winding (F) and the magnetic amplifier (G) are responsible for controlling the transmission process by the operator's Morse code key (H). The magnetic amplifier is an arrangement of coils and capacitors whose AC resistance is indirectly influenced by the Morse key and a DC source. When the Morse key is open, the solenoid amplifier short-circuits the control winding (F), to put it simply. The short-circuiting of (F) disturbs the transmitting oscillating circuit, so that finally no more than 9 % of the normal antenna current flows [2, page 53]. The situation described above (full transmit or no transmit at all) can therefore only be achieved approximately, but this is sufficient in practice.
none|thumb|660x660px|Principle sketch section through the generator (not to scale)
In order to achieve the necessary transmission frequency, the generator of the Alexanderson alternator type must not only rotate quickly, but it also needs a special construction with many magnetic poles. For this purpose, the magnetizable steel rotor disk (A) is provided at the edge with 488 slots (B), which are filled with a non-magnetic material.