spark gap transmitters. After careful calculations, the station was located in Grimeton, on the southwest coast of Sweden, which allowed good radio wave propagation conditions over the North Atlantic to North America. To achieve daytime communication over such long distances, transoceanic stations took advantage of an earth-ionosphere waveguide mechanism which required them to transmit at frequencies in the very low frequency (VLF) range below 30 kHz. Radio transmitters required extremely large antennas to radiate these long waves efficiently. The Grimeton station had a huge multiple-tuned antenna flattop antenna 1.9 km (1.2 miles) long consisting of twelve (later reduced to eight) wires supported on six 127 m (380 foot) high steel towers together with six vertical radiator wires. The station started operation in 1924, transmitting radiotelegraphy traffic with the callsign SAQ at 200 kW on a wavelength of about , later changed to , to RCA's Radio Central receivers on Long Island, New York. The Alexanderson alternator technology was becoming obsolete even as it was installed. Vacuum tube electronic oscillator transmitters, which used the triode vacuum tube invented by Lee De Forest in 1907, replaced most pre-electronic transmitters in the early 1920s. However the large capital investment in an alternator transmitter caused owners to keep these huge behemoths in use long after they were technologically obsolete. By the mid-1930s transatlantic communication had switched to short waves, and vacuum tube shortwave transmitters were installed in the main building, using dipole and rhombic antennas in a neighbouring field. The Alexanderson alternator found a second use as a naval transmitter to communicate with submarines, as VLF frequencies can penetrate a short distance into seawater. During the Second World War 1939–1945, the station experienced a heyday, when it was one of Scandinavia's gateways to the outside world. Underwater communication cable connections had once again been quickly severed by nations at war and the radiotelegraphy transmissions were a link to the outside world. The alternator continued to be used for naval transmissions until the mid-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,