Around 1924 the news of a rumoured death ray expanded throughout the press. This caught the attention of many including the Dutch government. Whilst this did not prompt any significant research in a death ray it did highlight an apparent lack of investment into physics. Consequently this formed the birth of the Commissie Physische Strijdmidellen, which soon had its own laboratorium at Waalsdorp.
Around 1933 the acoustic listening devices proved insufficient for the rapidly advancing technology and speeds of aircraft. An attempt was made to use passive electromagnetic detection. A superregenerative receiver for wavelengths 3 - 10 m was built which could detect aircraft at 10 km. However this swiftly ended as engines were shielded and a shift took place to diesel fuel.
An attempt to develop an IFF transmitter was also made. The project started in 1934 and took off in 1938 but trial and fabrication did not proceed. A different tangent with the radar did start in 1939 and would be tested on the 10th of May 1940, the day of the German invasion.
During trials with ultra short wavelength radio an interesting phenomenon occurred. Interference took place and this was attributed to the crows and gulls near the testing site. Von Weiler, who led these tests, swiftly realised the potential in detection. Furthermore by using pulses a method of rangefinding could be developed. For logistical reasons the wavelength 425 MHz = 70 cm was used. The transmitter had a max power of 1kW. The antenna was rather large as it was a 3 by 3 meter square mattress containing 64 parallel coupled dipoles. The transmitted beam had a conical shape of 15 degrees and the received signal was both audible and along with a cathode-ray (J-scope) display.
Image: The antenna matress, from Friedman, N., Naval Radar
Fabrication of the “Electric Listening Device” M39 started in 1939 with 10 prototypes being ordered. Tests showed that a Fokker C. V could be detected at 15 km at 15 KHz and aircraft groups at 30 km at 7,5 KHz. The Navy also seemed interested in a model for the cruiser Sumatra, preferably with a 15 cm wavelength model. A heightfinder could be produced by creating a vertically stacked model.
Of the 4 models ready one was mounted at the Maliebaan, The Hague. Another was mounted at the deer camp. One atop the laboratorium was destroyed. Max Staal [note 1], who was in north Drenthe at the time, rushed back on his motorcycle through the afsluitdijk. After his brief stay in The Hague he departed along with Von Weiler to Scheveningen where they left aboard HMS Malcolm. Engineer Piket managed to sneak two models with him through IJmuiden and then arrived at Portsmouth.[note 2]
Von Weiler and Staal reached HM Signaal school. During debriefings the English were surprised that the Dutch developed radar, not the least one which was more modern especially in the field of pulse technology and the use of a signal antenna. However, the Dutch model came with a significantly weaker magnetron. Another oversight by the Dutch was that the maximum power output of a short pulse could exceed the constant maximum of the transmitter tube allowed.
Von Weiler and Staal cannibalised one of the two models and placed the other aboard Hr. Ms. Isaac sweers. The ship received the 40mm Bofors Hazemeijer mounts from Willem van der Zaan and together with British Yagi-Antennae she received what became known as RDF 289.
[note 1]: Engineer M. Staal placed at the Commissie Physische Strijdmiddelen in 1939 was different from Engineer C.H.J.A. Staal at philips.
[note 2]: Only 4 were produced but I found 5 different ‘sightings’ of the radar and an known ultimate fate of 3 models.
Sources:
Cruijff, H., Hr. Ms. Isaac Sweers en de ‘Hollandse Radar’
Staal, M., Hoe de Radar naar Hengelo Kwam
1927 - 1977 Physisch Laboratorium TNO
1975 LEOK Jubileum boek
The Electric listening device (1936 – 1941), Museum Waalsdorp at https://www.museumwaalsdorp.nl/en/museum-waalsdorp-2/5547-2/radar-electric-listening-device-1936-1941/
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