The Würzburg Riese radar antenna.
During the Second World War, the German army created the Atlantic Wall along the entire European coast, all the way from Norway, through Denmark, the Netherlands, Belgium and France. Part of this Kammhuberlinie were overlapping radarstations, equipped with the so-called Würzburg Riese radar antennas, 7.5 meter in diameter. The radars worked at a wavelength of about 53 cm (560 MHz).

After the War several of the antennas were confiscated by the Dutch Telecommunication service PTT. Some were brought to the radio station at Kootwijt, in the province of Gelderland (East Netherlands). From Kootwijk the communication with the Dutch colonies was maintainted. Ir. de Voogt from PTT gave one of the antennas to the Dutch Association for Radio Emission from the Sun and the Milky Way (Stichting Radio Straling van Zon en Melkweg, now ASTRON). Other antennas were used by the PTT for research on radio emission from the Sun and how that emission affected radio communication.

On may 11 1951 Ir. Lex Muller detected the predicted 21cm emission from atomic hydrogen. That was some six weeks after Harold Ewen, a student at the Harvard University in the USA, had detected the emission first. The radar antenna with which the first research and surveys of the Galaxy were done does not exist any more. Journalist George Beekman tried to reconstruct what happened to the different antennas and wrote it in the Dutch popular astronomy journal Zenit in 1999. [see Background for pdf of the article, in Dutch only].

There are several websites with more information on the antennas. Here is one of them. [website Wikipedia]