The name Grimeton is well known among radio amateurs and reception specialists. This refers to a transmission system south of Gothenburg, Sweden, which is no longer available anywhere else in the world.
Anyone driving on the motorway from Gothenburg to Malmö in Sweden will notice, after around 70 km, six impressive, T-shaped lattice masts lined up one behind the other. They are each 127 m high and are around 380 m apart from each other. Eight wires are stretched between them. This is the approximately 2.2 km long antenna system of the Grimeton longwave transmitter, which is visible from afar. It was built from 1922 to 1924. The still unofficial commissioning took place on December 1, 1924. Since then, Grimeton has been reporting with the identifier SAQ (… .- –.-).
The system dates back to a time when radio technology began to be introduced on a large scale. In the mid-1920s, many radio stations began operating in Europe. What they had in common was the low transmission power of usually well below 1 kW and therefore only a modest range.
Grimeton played in a completely different league with its 200 kW. But broadcasting was never intended for the system. It wasn’t technically designed for that. It was all about radio communications between Sweden and North America.
Radio technology through the ages
Transcontinental radio communication via long-wave transmitters was only important for a short time. Radio amateurs discovered early on that shortwave was much more suitable for long-distance traffic. In addition, it required far less extensive transmission systems. Transmitters and especially antenna systems could be made more compact and therefore cheaper. The 17 active long-wave transmitters worldwide were put into operation between 1918 and 1924.
About half of it was shut down before the start of World War II. The last ones followed in 1948. Grimeton only survived because the facility was put into military service after 1945. Finally, longitudinal waves can also be used to reach submarines operating at shallow depths. In 1995 Grimeton closed. As a result, an association tried to keep the facility still fully functional and continues to put it into operation again and again. In 2004 the complex was declared a UNESCO World Heritage Site.
What was broadcast technology like 100 years ago?
At Grimeton everything is different, as you would expect from a broadcasting system. No wonder, as this is the state of the art technology from around 1920. When you enter the large transmitter hall at the Grimeton facility, the first thing you notice is a large machine. which you would first like to identify as an emergency generator. In the type that you find here for autonomous power supply in basic network transmitters. Only at second glance do you realize that this machine is not used to provide emergency power, but is the core of the system, namely the transmitter.
In principle, this machine transmitter is an alternating current generator that drives a large rotor with a high number of poles at high speed. The system in Grimeton is a equal-pole machine, called the Alexanderson Alternator after its developer.
The alternator is driven by a 2 kV asynchronous motor and a gearbox with a ratio of approximately 1:3. The rotor consists of a 7.5 cm thick steel disc with a 1.8 m diameter. 488 grooves are milled into them and filled with brass. The air gap between the fixed stator, which has 64 induction windings, and the rotor is just 0.4 mm.
Where was the broadcast frequency?
With a machine transmitter, the transmission frequency is determined by the number of slots and rotor speed. With it the frequency also increases. The Grimeton transmitter has a speed of 2,115 revolutions per minute and thus a frequency of 17.2 kHz.
The transmission operation requires the speed to be kept absolutely constant. The permissible tolerance is just 0.6 per mille.
Incidentally, Grimeton was only the transmitter for transatlantic traffic. The radio messages from America were picked up in a separate receiving station near Kungsbacka, around 25 km south of Gothenburg.
Also interesting:
Source: www.digitalfernsehen.de
