The above postcard bears a photograph of the Grossfunkstation at Nauen, Germany as it appeared in 1941. A combination reflecting pool/transmitter cooling pond is located in front of the main radio station. The building was designed by a prominant achitect of the day; Hermann Muthesis. German industrial architecture of this period is still celebrated today.
By the mid 1920's the RF alternator had emerged as the most efficient means of generating high-power VLF energy. By then even the last hold outs had replaced their arc transmitters with alternators.
Telefunken installed 400kW magnetic frequency-multiplied RF alternators at Kootwijk Radio and at Torra Nova near Rome. They installed a 150kW alternator at Prado del Rey (Madrid) and a 100kW station at Funabashi in Japan. Telefunken stations were also erected Malabar and Monte Grande. The last great alternator of this type was the 500kW station built at Nagoya, Japan.
Both the installation and the operating costs for a 200 to 400kW RF alternator represented a huge expenditure. Despite the ever-increasing demand for commercial international wireless services, as the shortwaves gradually became available the primary workload for the VLF alternators shifted from commercial to military operations. In his, History of International Broadcasting, Vol. 1, James Wood remarked
"Paradoxically, the cable telegraphy, whose demise had seemed so clear two decades earlier in 1906, outlived long-wave wireless telegraphy. This was attributable to two factos: the huge costs associated with high power, long wave installations, and the increase in commerical traffic from 1920 onwards, which gave a larger market."
Indeed, the operational cost ratio for several kilowatts at shortwave using vacuum tubes as compared to several hundreds of kilowatts needed at VLF would have spelled the demise of the big alternators by 1930, had it not been for two saving graces.
Firstly, the transmissions from high-powered VLF stations suffer less disturbance due to changes in propagation than those at shorter wavelengths.
Secondly, it had been discovered before the First World War that submarines submerged to a depth of 2 to 3 meters could continue to receive the signals from these VLF "powerhouse" stations.
Following the First World War, the station at Nauen, Germany was run by Transradio A.G. (Radio Trans-Ocean Ltd); with Telefunken as the majority shareholder. Despite the worsening economic situation in Germany, the station was steadily upgraded. A model of the station as it appeared in 1920 may be seen here and here.
In 1931 this charming photograph was taken from the heights of the transmitter towers at Nauen (not a safety belt in sight!). In January of that year the Reichspost took over control of the station. The VLF RF alternators were refurbished in 1937.
In a series of articles titled, Funkgeshichte, Dr. H. Richter described the VLF transmitters that were used for U-Boat communications in the early years of the Second World War
"Eine große Bedeutung für die fernführung der deutschen U- Boote hatten auch die Längstwellen, die von den Booten mit ihren Peilrahmen- Antennen auch noch unter der Wasseroberfläche empfangen werden konnten. Am Anfang des Krieges stand dafür nur der Sender Nauen auf der Hauptbetriebswelle für U- Boote (Wellenlänge 18130 m = 16,55 kHz) mit einer Leistung von 300 kW zur Verfügung. Nach der Besetzung Frankreichs kamen noch drei weitere Längstwellensender dazu, St. Assise mit 300 kW, Croix-d´Hins bei Bordeaux mit 360 kW und Kootwijk (Niederlande) mit 120 kW. Der Sender Nauen und die beiden französischen Sender waren Maschinenfrequenzsender, der holländische ein Röhrensender."
Up till the occupation of France, the RF alternators at Nauen were the main VLF transmitters available to the Kreigsmarine. Later, two French RF alternators as well as the Kootwijk Radio transmitter at Apeldoorn in the Netherlands were pressed into service. Herr Richter states that the later was a vacuum tube transmitter. However, I wonder about this given the Dutch 400kW VLF RF alternator was said to have remained in serviceable condition until the Germans themselves destroyed it upon their retreat.
In his book, Funkführung der U-Boote in der Praxis, Arthur O. Bauer describes the workings of the passive frequency-multiplied Telefunken RF alternators that were used to issue orders to the U-boat fleet
The RF alternators at Nauen are said to have been dismantled by the Soviets at the war's end. Whether they were ever placed on the air again remains a mystery. Fortunately, the Soviets decided not to follow through with their original plan to blow up the grand, old Muthesius building. The structure is used to this day as a radio transmitter.
The previously mentioned Telefunken alternator at Yosami, near Nagoya, Japan was saved from the scrap heap early-on for the same reasons that the German alternators remained in service throughout the Second World War. Originally installed for the purpose of commercial oversea wireless signaling, this largest of all RF alternator stations was nearly obsolete by the time it went on the air in 1929.
The station was eventually taken over by the Japanese Navy and used primarily for communicating with their submarines until the end of the war. I believe the callsign of this station was JNI3. After the war the US Navy took control of the station and changed the callsign to NDT. The US Navy used this station for its own submarine communications from 1950 until 1993.
Happily, after 60 years of on-air operation one of the two alternators - along with its ancillary equipment - has been moved to a permanent museum. In May of 2009, the IEEE commemorated the station with a dedication ceremony.
JA2DJN has posted some wonderful photos of the well-conserved Yosami transmitter here. Be sure not to miss the complete schematic diagram of the station which may be found here. The photo of the RF output feed-through insulator at the rear of the original building reminds me a giant spark-plug!
Another fine photo collection may be found here. I especially like the wrist-diameter Litz wire inductors set into those gorgeous, teak-wood frames.
The Telefunken E-378S "All-Wave" receiver provides a good example of the type of equipment that was used to receive VLF transmissions in this time period. This regenerative receiver was nicknamed "Brotkiste," or "breadbox," for obvious reasons. These receivers were standard issue on Kreigsmarine ships and U-boats up to the end of WW2. For example, the E-378S receiver can be located (#10) above the operating table on the starboard side of this Type-XXI, U-boat radio compartment. The above two links are taken from the web sites that may be found here and here.