Without any doubt Jan Hendrik Oort was the stimulator of and the driving force behind radio-astronomy in the Netherlands for the first three decades following the end of World War II.
Thanks to him, the development of radio astronomy in the Netherlands was driven by astronomical goals from the very beginning. He was strongly convinced that extremely important advances in astronomy were to be made by observing the galaxy at radio wavelengths.
He did realise, though, that advanced techniques had to be used to achieve this; so he was glad to leave the development of the techniques and building the necessary equipment to technical specialists.
In other parts of the world, notably in Australia and the USA, radio astronomy was developed by physicists and engineers who found it a natural field to apply their war-time radar experience to. In the early days astronomy was not always the prime driver there.
Although radio waves from our galaxy were first discovered by Karl Jansky in the early thirties, not very much progress was made in the field of radio astronomy until almost a decade later (1940) when Reber published his radio-observations in the Astrophysical Journal.
In 1942 Hey and Southworth independently discovered radio-emission from the sun, but the publication of their observations was held up (until 1946 and 1945, respectively) for reasons of military secrecy during World War II. As early as the spring of 1944, a year before the end of the German occupation, Oort started making real plans to observe the interstellar radio radiation.
Despite the war the most important American scientific publications had reached the Netherlands and other European countries thanks to the efforts of Bart Bok, who had set up an informal circulation scheme. Thus Oort had seen Grote Reber's publication (1940) of his observations of radio-radiation from the galaxy.
A letter written on April 19, 1944 to Dr. C.J. Bakker of the Physical Laboratory of the Philips factories contains a request to equip a radio telescope with a mirror of 10 to 20 metres - which, Oort then believed, could be built by the mechanical workshop of the Leiden Observatory - with a receiver produced by the Philips factories. Bakker replies that they may have to wait until after the war, but that this would certainly be possible then.
In the exchange of letters between Oort and Bakker the problems of the poor angular resolution of such a telescope, operating at a wavelength of 1 metre or 50 cm, are discussed. Oort's obser vation that the radiation from the direction of the galactic centre would require a resolution of half a degree or better triggered the suggestion by Bakker that it might be worthwhile to measure the width of the radiating part of the Milky Way using the radio-equivalent of a Michelson interferometer.
Later in 1944 Henk van de Hulst did the theoretical work which would lead to the prediction of the spectral line of interstellar hydrogen at 21 cm wavelength. So, well before astronomy at radio-wavelengths took off in the Netherlands, the basic thoughts were born which would ultimately lead to the construction of the Westerbork Synthesis Radio Telescope two decades later. In the following paragraphs it will become clear that for the next twenty or twenty-five years Oort was always planning ahead while the astronomical results were being reaped as a result of having pushed for a shorter-term, less ambitious plan.