Grimeton Radio Station, located in southern Sweden near Varberg in Halland, is an early long-distance radio communication station built between 1922 and 1924. It has been kept as a historical site to remember its importance.
From the 1920s through the 1940s, the station sent telegrams using Morse code to North America and other countries. During World War II, it was Sweden’s only way to communicate with the rest of the world. The station is the only place left that uses an early radio technology called an Alexanderson alternator. It was added to the UNESCO World Heritage List in 2004, with the note: "Grimeton Radio Station, Varberg is a very well-preserved example of a telecommunication center, showing the technological progress of the early 1920s and how it changed over the next 30 years."
The radio station is also an important part of the European Route of Industrial Heritage. The transmitter still works and is turned on every year on Alexanderson Day to send short Morse code test messages. These messages can be received across Europe.
History
Around 1910, industrial countries began building networks of powerful radio stations to send telegraph messages across oceans to other countries. During World War I, radio became an important tool because a country without long-distance radio could be cut off from the world if an enemy damaged its underwater telegraph cables. Sweden relied on other countries' underwater cables, and losing these connections during the war led the Swedish Parliament to decide in 1920 that the Royal Telegraph Agency should build a large radio station in Sweden to send telegrams across the Atlantic.
At that time, several companies had different technologies for sending high-power radio signals. Bids were requested from companies in Berlin, London, New York, and Paris. The chosen transmitter was the 200 kilowatt version of the Alexanderson alternator, invented by Swedish-American Ernst Alexanderson, made by General Electric, and sold by their subsidiary RCA. This device used a large rotating generator turned by an electric motor to create radio frequency electricity, which was sent to the antenna.
This was one of the first transmitters to send smooth, continuous radio waves, which could travel farther than the short, broken waves used earlier. The alternator was chosen because it was already used in other transatlantic stations, reducing problems with compatibility. Its design by a Swede may have also influenced the decision.
After calculations, the station was built in Grimeton, on Sweden’s southwest coast near North America, where radio waves could travel well over the Atlantic to the U.S., Norway, Denmark, and Scotland. The site was bought in 1922, construction began by year’s end, and the station was completed in 1924. Two 200 kilowatt Alexanderson alternators were installed so one could be repaired without stopping radio traffic.
To send messages during the day over long distances, transoceanic stations used a method that required very low frequency (VLF) radio waves below 30 kHz. These waves needed extremely large antennas to send signals efficiently. The Grimeton station had a long, flat antenna 1.9 kilometers (1.2 miles) long, made of wires supported on six 127-meter (380-foot) steel towers. The station began operating in 1924, using the callsign SAQ on a wavelength of about 18,000 meters (16.7 kHz), later changed to 17,442 meters (17.2 kHz), to send messages to RCA’s station in New York. It quickly handled 95% of Sweden’s telegrams to the U.S.
The Alexanderson alternator technology was becoming outdated even as it was used. Vacuum tube transmitters, which used triode tubes invented by Lee De Forest in 1907, replaced older systems in the 1920s. However, the high cost of alternators kept them in use for many years. By the 1930s, most transatlantic communication shifted to short waves, and vacuum tube shortwave transmitters were added in 1938, using dipole and rhombic antennas. The Alexanderson alternator was later used for naval communication with submarines, as VLF waves can travel through seawater.
During World War II (1939–1945), the station became a key link for Scandinavia to the outside world. Underwater cables were again cut during the war, and radio was the only way to send messages. New transmitters were added because the station was used by the Swedish Foreign Ministry and embassies, making its signals a target for intelligence operations like the British Y service.
After the war, more transmitters were added, and the number of countries receiving messages grew. By the 1950s, the station operated twelve shortwave transmitters and one longwave transmitter, sending messages to twenty countries. Telegraph messages shifted from Morse code to radioteletype, and the station also offered radiofax and radiotelephony services. In the 1950s, experimental FM and TV transmitters were tested using the towers.
By the 1960s, many transmitters were outdated and replaced. A new facility was built in 1966 to house modern equipment, preserving the older systems. New antennas were added in the 1960s–1970s, but their use declined as satellites and cables replaced fixed radio circuits. Focus shifted to maritime radio, and modern FM and TV transmitters were added to the new facility, which has a 330-meter antenna tower.
The new facility, built in two stages during the 1960s and 1970s, included eight 30 kW Telefunken SV2470 HF transmitters and one 100 kW HF transmitter, plus one 40 kW LF transmitter. All HF transmitters shared antennas like rhombic and log-periodic types. These were used for international telegraph and telephone circuits but fell out of favor by the 1970s, with only a few circuits remaining until the 1980s.
The system was later used as backup and for maritime radio, air-ground communication, and SSB broadcasting tests. In the 1980s, the MARITEX maritime radiotelex system needed more capacity, so smaller HF and MF transmitters were added. In the 1990s, the station briefly supported an experimental aeronautical HF digital datalink system.
MARITEX was shut down in 2000, freeing transmitters and antennas, which were leased by Globe Wireless for maritime digital communications. When Globe Wireless ended operations in 2012, the HF system was reused for air-ground voice services after moving from its original Karlsborg site, which claims to be the last civilian user of the facility. The Swedish Maritime Administration also uses the site for MF telephony and NAVTEX transmitters.
Technical description
The electromechanical transmitter in Grimeton sent signals at a frequency of 17.2 kHz, which is in the very low frequency (VLF) range. This allowed the signals to travel across the Atlantic Ocean to America.
To create this signal, an electric generator (A) was used. A motor with 500 horsepower and 711.3 revolutions per minute turned the generator through a gearbox with a 2.97 ratio. This movement produced a smooth, repeating electrical current (B) of 17.2 kHz, or 17,200 cycles per second.
In comparison, generators used in public electricity networks typically create alternating current (AC) at 50 or 60 cycles per second, depending on the country. To make such high frequencies, a generator must spin very fast—2,115 revolutions per minute—and have a special design.
In Grimeton, Morse code was used to send messages. Texts were converted into short and long pulses using a Morse key (D). A switch system (C) used these pulses to control the electrical current sent to the antenna (F). When the key was pressed, the current was sent to the antenna and transmitted. When the key was not pressed, the current was blocked, and no signal was sent. For example, the letter "A" was sent as a short pulse followed by a long pulse, as shown in (E).
The electrical current generated had a voltage of 2,000 volts and a power of 200 kilowatts (though now it is often limited to about 80 kilowatts). Such strong signals could not be turned on or off with a simple switch, as this would cause sparks. In Grimeton, a different method was used to control the signal.
As in old radios, the antenna and nearby coils and capacitors formed a resonant circuit. This circuit had to be tuned to the correct frequency for efficient signal transmission. In Grimeton, the switch system (C) changed the tuning of this circuit when the Morse key was not pressed, stopping the signal. This allowed a small amount of power (3 kilowatts of direct current) to control a large signal (200 kilowatts of alternating current).
The antenna resonant circuit included the antenna (I), a transformer (D), and a magnetic amplifier (G).
In electric generators, rotating magnetic fields create alternating current in nearby coils (B) inside the generator (A). In Grimeton, these coils were attached to the stator, which was divided into 2×32 sections on both sides of the rotor. Each section’s windings were connected to primary windings (C) of the transformer (D). When the primary voltages were sent to the transformer’s secondary winding (E), they combined to form a strong, smooth signal sent to the antenna.
The control winding (F) and magnetic amplifier (G) controlled the signal based on the operator’s Morse key (H). The magnetic amplifier used coils and capacitors whose resistance to alternating current was indirectly controlled by the Morse key and a direct current source. When the Morse key was open, the amplifier short-circuited the control winding (F), disrupting the signal. This reduced the antenna current to about 9% of normal, as described in [2, page 53]. This method allowed the signal to be either fully transmitted or completely stopped, which was sufficient for practical use.
To achieve the required frequency, the generator used in Grimeton had to spin quickly and have a special design with many magnetic poles. The rotor disk (A), made of magnetizable steel, had 488 slots (B) at its edge filled with non-magnetic material. Direct current from coils (D) created a magnetic field (E) in the stator, where coils (C) were also located. As the rotor turned, the magnetic field alternated between being strengthened by the steel disk and weakened by the non-magnetic slots. This changing magnetic field caused a smooth electrical current to be produced in the coils (C).
The drawing is not to scale. The gap between the rotor and stator frame was only 1 millimeter wide. The rotor was a steel disk 1.6 meters in diameter and about 7.5 centimeters thick at the edges.
Antenna system
To achieve the farthest possible distance, like other long-distance radio stations from this time, it used the very low frequency (VLF) band, which is 17.2 kilohertz. At this frequency, the wavelength is about 17,442 meters. The antenna is about 2 kilometers long (1.2 miles), but it is much shorter than the wavelength, so it does not work very well.
The antenna system includes wires held up by tall poles, similar to those used for power lines. Six poles support the antenna, each with a 46-meter crossbar at the top and standing 127 meters high. Today, these poles carry 8 antenna wires, though they originally had 12.
The antenna at Grimeton uses a special design called a multiple-tuned antenna, invented before World War I by E. F. W. Alexanderson. This design connects several vertical wires together using flat wires at the top. These flat wires help store electrical charge and carry high voltage.
Each vertical wire ends with a coil placed on the ground. This coil helps balance the electrical resistance of the wire and ensures the current flows correctly through the wires.
By splitting the electrical current into several points where it connects to the ground, the overall resistance is much lower than if all the current went through a single vertical wire. This makes the antenna about ten times more efficient.
Gallery
- A 1900-meter (1.2-mile) flattop antenna
- The inside of the Grimeton radio station
- A log-periodic shortwave antenna next to the transmitter building
- The inside of the transmitter hall showing a control panel for the alternator
- The inside of the transmitter hall showing the Alexanderson alternator
- The entrance hall of the Grimeton World Heritage site
- A warning sign at the entrance
- The VLF masts at Grimeton