The “electric listening device” (1936 – 1941)
In 1936, during propagation tests carried out to determine the operational range of the portable Ultra-High Frequency wave (240 MHz/1.25 m) telephony devices between the beach and the Waalsdorpervlakte, for a short time the very weak signal of the transmitter quite quickly periodically changed in strength as if interference was occurring. A crow flying over seemed to be the cause. Some moments later, the phenomenon reoccurred. This time a herring gull flew over. So these birds reflected the waves. The next day a military aircraft was ordered to fly back and forth from Noordwijk to Hoek van Holland over Waalsdorp. The interference effect and the reflections were very clear. Von Weiler and Van Soest who performed these tests soon understood their scope: aircraft could be detected by active detection methods.
Von Weiler quickly drew a few conclusions:
- If the reflection works with a remote transmitter and receiver, detection must also be possible if the transmitter and receiver are set up in one place, provided that the necessary mutual shielding is possible.
- Although electromagnetic waves are fast, finite time may be measured if a pulse shape is used for the transmitted signal. A measured time interval times 300 km/s gives the double distance to the target (duration from channel to target + duration from target to receiver).
Under the direction of Von Weiler research was carried out with a large part of the staff. A typical example is a discovery by one of the technicians, Tabbernée. To meet certain requirements of capacitive coupling in the electrical circuit, it used a method called the Tabbernée capacitor.
Particularly good inventions were made in the fields of pulse technology and shielding the transmission signal in the receiver. Other countries (e.g. England) used a continuous transmission signal as well as separate antennas tens of meters apart. With continuous transmission, it is difficult to determine the time difference between sending and receiving. Therefore, determining the distance to the radar contact is difficult.
Von Weiler’s invention was the use of a single antenna for transmitting and receiving. The antenna had to be large because the energy received back from a plane at a considerable distance is extremely small. The higher the frequency of the electromagnetic wave, the smaller the antenna could become. Unfortunately, the wave tubes by Philips at that time could not produce very high frequencies. Therefore, they were a limiting factor. No available usable technology abroad could be identified.
For that reason, 425 MHz (wavelength 70 cm) was the limit in those days. The transmitted pulse had a length of about 3 μsecs. The pulse repetition rate was high, reaching 15,000 and 7,500 pulses per second resulting in maximum detection distances of 10 and 20 km, respectively. The peak power was 1 kW. The transmit pulse was acquired from a tuned cavity using a coupling loop; this loop was coupled to two parallel wires (a Lecher line) leading to the antenna and then transmitted. For the transmitter part, four Western Electric TB 04/8 “doorknob” valves were used; the ones produced by Philips did not work well. The final model of the pulse generator comprised only two valves in a balanced circuit.
The receiver was a single superheterodyne equipped with a self-generating “mixer” (acorn pin method) at the input. The intermediate frequency was 6 MHz; the medium frequency amplifier was equipped with the steep Telefunken pentode AF100. Many precautions such as shielding and HF decoupling were required to avoid unpleasant effects due to the strong transmit pulse (such as “ringing”, etc.) from the receiver.
A single antenna was used for transmission and receiving. During the transmission pulse, the receiver input chain normally remained connected to the antenna cable; there was, therefore, no switch at all. The mixer tube (acorn pin method) was provided with a limiting resistor in the grating chain, whereby the grating was automatically brought to a high negative voltage during the transmission pulse. The space charge in the tube also changed, as a result of which the receiver input circuit was also automatically tuned.
The minimum detection distance -due to the pulse transmission period- was approximately 400 to 500 metres.
The mattress (or ‘ billboard’) antenna consisted of 64 parallel coupled ½ƛ-dipoles behind it and a reflective metal surface (copper mesh) of approximately three by three meters. The result was a cone-shaped beam with an opening angle of about 15 degrees. The antenna gain was approx. 23 dB compared to an isotropic radiator.
The transmitted beam had an opening angle of some 15 degrees. Connected to bicycle pedals with a chain, the antenna could be baked with the feet and elevated with the aid of a handle. The received signal was visualised on a cathode-ray tube (a so-called J-scope) with a circular time base. But it could also be made audible in headphones; in general, hearing detection was found to be less tiring than the tense observation of the cathode ray tube. For generating the sound, the emitted pulse was amplitude-modulated with a 1.000 Hz signal to later make the received echo audible with the headphones. Hence the device’s name is “Electric listening device“. This name also fits with previously performed research. During the Second World War, this became RAdio Detection And Ranging (“radar“). In 1938, the first laboratory version was ready. Simple mechanics were used: the operator could turn the antenna in azimuth by pedals like a bicycle while for moving the antenna in elevation a handle had to be used.
Versions of parts quickly followed each other over the next two years. Unaware of the fact that the problem was also worked in other countries, the great value of this development was realised in Waalsdorp. Because the ordinary listening devices could hardly follow the fast aircraft anymore, the electric listening device came as called. Yet it took some time before the Ministry of Defence, and more specifically the Royal Netherlands Army, recognised the military value of this new technology.
A general who came to see the electric listening device asked: “If I throw some buckets of water over it, will it still work?” “No,” said Von Weiler. Then the general wanted to throw a bucket of sand in the device. That was not allowed either. Next, the general wanted to know if every farmer could operate the device. No, a short training would be required. “Then it is of no military use”, was the answer.
A professor looked at the electric listening device: “Nice, but too big. Now reduce the size” and he pointed out a few cubic decimetres with his hands.
The Ministry of Defence was requested to be allowed to develop a small series while the research was still ongoing. The Department, however, did not permit the development: Waalsdorp had to do research, not development.
Finally, in 1939, the department decided that the development series of the M39 by the NSF (Nederlandse Seintoestellen Fabriek) would be made very secretly, that Waalsdorp would deliver the drawings and that M. Staal would be stationed in Waalsdorp as a conscripted engineer. He was responsible for the coordination of the production of the components that, because of secrecy, had been distributed over three companies (including J. B. van Heijst and Zonen NV, The Hague) and two laboratories (Leiden University (transmitter), Technische Hoogeschool Delft (receiver)).
In late 1939, a demonstration of the electric listening device was given to Prince Bernhard. Unfortunately, there was a thick fog on the day of the demonstration. No aircraft could take off. Von Weiler, therefore, showed the radar echoes of the church towers in the surroundings of Waalsdorp. “You can certainly see the Catholic churches better than the Protestant ones,” asked the Prince. “Why do you think that Royal Highness?” “Well, the Catholic churches have a large metal cross on top of their towers.”
His Royal Highness was right. The electrical listening device detects a Fokker C.V reconnaissance plane at about 15 kilometres (at 15 kHz). Larger aircraft and especially groups of aircraft could be detected at a distance of 30 kilometres (with transmissions at 7.5 kHz). When an aeroplane made a turn, it gave rise to typical phenomena in the echoes. The influence of the propeller (or propellers) was very noticeable.
The Navy also became interested. Installation of a prototype on the HNLMS Sumatra did occur due to the outbreak of the war. Plans for further development existed. A wish was to use even shorter wavelengths so that the large antenna could be reduced in size. One was thought of 15 cm wavelength, but the electron valves that existed at that time could not produce any reasonable power output at the high frequency. Moreover, the build of a ‘ladder antenna’ was considered: A directional antenna with the dipoles one above the other to produce a sharp bundle in the vertical plane and a fairly wide bundle in the azimuthal direction. By making a pumpjack motion, a height radar could be produced. The development of a combination of the electric listening device with searchlights as well as with anti-aircraft artillery was also considered.
After a demonstration of the device, a former minister stated: Your laboratory, Mr van Soest, is suffering from the curse of perfection.
At the outbreak of the war, four electric listening devices were completed (* about the number of devices available for use; saved documents contradict each other). On 10 May 1940, the day of the German invasion, an electric listening device was installed on the roof of the Meetgebouw for a planned demonstration to military authorities of a prototype recognition friend or foe (later called Identification Friend or Foe (IFF) transponder. The IFF device responded to the energy emitted by an electric listening device, with a code. Previous experiments with the IFF system on the tower of the Grote Kerk in The Hague and the tower of the Nieuwe Kerk in Delft with the electric listening device set up at Waalsdorp showed that the IFF system worked well. In this case, too, the friend or enemy device was installed in the tower of the Grote Kerk in The Hague (to save an aeroplane).
The electric listening device on the roof of the Measurement Building was quickly moved to the Hertenkamp (Maliebaan) in The Hague next to a pair of machine guns. German planes were tracked with the device. To prevent the electric listening device from falling into German hands, the device was destroyed with hand grenades.
It was the Royal Netherlands Navy that ensured that Von Weiler and Staal could make the crossing from Hoek van Holland to England with the English destroyer Wessex in the company of the English naval attaché Admiral Sir Gerald Dickens in the morning of May 14, 1940. According to the book “WEG” by Danny Verbaan, the gentlemen were first brought with the Scheveningen motor lifeboat Zeemanshoop from the port of Scheveningen to the HMS Malcolm. The HMS Malcolm brought them then to the HMS Wessex in the harbour of Hoek van Holland (ref: Roering jg 33, nr 1, p.72, 1996).
The drawings of the “electric listening device” were in their luggage. Essential parts of an electric listening device were transported through the German parachute lines to Zandvoort and from there via Ijmuiden shipped to England. All other components and information, including notes, drawings, and correspondence had been destroyed and set on fire.
A few years ago, however, the museum discovered the complete package of drawings of the electrical listening device at the Depot Defence Archives as part of the correspondence between the Commission for Physical Arms and the Genie. Those copies were made at J.B. van Heijst and Zn, The Hague. The complete package of drawings currently resides at the Dutch National Archives.
After arrival in England, it appeared that Von Weiler and Staal had interesting knowledge for the Brits (Admiralty’s HM Signal School): they were surprised about the compact device which needed only one antenna. The development at Waalsdorp was ahead of the English in the field of pulse technology and on the single combined transmitting and receiving antenna. On the other hand, the English could reach lower wavelengths with their magnetron. Besides, they put more energy into each pulse. That was forgotten in Waalsdorp: a short pulse could have higher power peaks than a constant load of the transmitter tube allowed! Incidentally, the existence of the British radar activities for the two Dutch was a surprise.
Von Weiler went to work at the Admiralty Signal Establishment (ASE) at Portsmouth. ASE was renamed Admiralty Radar Establishment, and later it became Admiralty Surface Weapon Establishment (ASWE). Using the components of the aforementioned third listening device, Von Weiler built a complete electrical listening device that was equipped with an English Yagi antenna (‘fishbone aerials’). The device was adapted to serve as a radar distance meter for the modern Hazemeyer fire control installed on HNLMS Isaac Sweers. The (radar) device was coupled to a stabilised 40 mm anti-aircraft (AA) Bofors machine gun, a unique combination and the first in the world. The device was named Range & Detection Finder 289 (RDF 289), also known as the ‘Dutch system’. The device worked perfectly: with the arrival of Italian or German attackers, all guns were already aimed in the right direction. On 13 November 1942, northwest of Algiers, the HNLMS Isaac Sweers was hit by two torpedoes launched by the German submarine U-431 under the command of Wilhelm Dommes.
Royal Netherlands Navy officers who had to operate the RDF system received a top-secret manual that explained the basic principle of radar technology understandably. The manual also contained instructions for resolving system malfunctions. Neither system parts nor the manual was allowed to be taken ashore.
To a large part, this text is a translation of Van Soest’s contribution to the book Physisch Laboratorium 1927 – 1977. Moreover, some details were clarified by Ir. Max Staal’s “Hoe de radar naar Hengelo kwam” set of articles in Roering 1996-1998.
Incidentally, from 1933 onwards, the Philips Physics Laboratory worked on the development of a split anode microwave at 1 GHz, where the transmitter and receiver with parabolic antennas were close to each other. It was possible to create a microwave connection between Eindhoven and Breda and Eindhoven and Venlo with wavelengths of 15 cm. In 1939, tests were carried out with a pulsed transmitter on the island Texel where the transmission and reception parabola were placed adjacent to each other and a flat plate (ship) served as a reflector; the tests, however, failed because of sea clutter.