On April 15, 1944, the Netherlands Astronomers Club held a colloquium organized by Prof. Oort on "Radio Waves from Space". Henk van de Hulst, then a student in Utrecht, had been asked by Oort to speak on the origins of the radio emission which had been detected by Jansky and Reber. It was during his talk that Van de Hulst (here shown in a restaging of the colloquium in the mid-1950s) suggested that a hyperfine transition in the ground state of atomic hydrogen, corresponding to a wavelength of 21 cm, might produce detectable emission. He cautiously concluded, "The matter does not look hopeless, although the existence of the line remains speculative." This can be said to mark the birth of radio astronomy in the Netherlands. (Hydrogen line emission from the Milky Way was detected 7 years later at Harvard and Kootwijk.)
For more details, see "The beginnings of radio astronomy in the Netherlands," by H. van Woerden and R.G. Strom, Journal of Astronomical History and Heritage, 9, 3-20 (2006).
Related images may be gleaned by clicking on "Archive" at the top of this webpage, and searching with the keyword "van de Hulst". Or, alternatively, by means of the following link.
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