Ultra-High Frequency Telephony (1935 – 1940)
In early 1935, CDR J.Th. Fürstner, a member of the Commission for Physical Armament, proposed in the interest of the tactical use of the radio equipment of the Naval Radio Service a new research topic: “Making the frequency of ultra-short wave transmitters and receivers constant, so that signals can be received without special operator handling“.
The research results aroused the interest of the Royal Netherlands Army. They tasked the Measurement Building with the development of a radiotelephone device “for the batteries for direct support and forward observers.” The intended use in the field concerned a distance range of more than five to six kilometres over flat terrain with the antenna near the ground level. Because of the still less efficient valves and other electrical parts, this was a difficult task in those days. More sophisticated military requirements and experimental experience led to a series of five subsequent development models in the 1 to 1.5 m wavelength range. However, it turned out that these semi-optical waves were usable for telecommunications. At fairly large distances, the shielding effect of obstacles in the transmission path was not too bad. Even with the signal almost drowning in noise, communication proved to be possible between the Wassenaar dunes and Voorschoten despite the Wassenaar forest in between.
In 1935, the first development model still involved a separate transmitter and receiver. The second development model had a shared transmitting and receiving antenna. That equipment was tested by the 2nd Regiment of Field Artillery. The third development model was developed using the “cell construction” methodology. Moreover, a power source solution was developed. Development was slowed down as a description of the device had to be made at the request of the Head of the Department G. [Material soldier (Colonies)] of the Ministry of Colonies which was allowed to oversee the activities of the Commission of Physical Armament. The fourth model, a pre-production model, was very similar to the third model. Of particular interest was the use of new tube types (TB04-8) in which the electrode system was mounted on sturdy straight pins protruding through the glass wall.
An additional operational requirement was that the equipment had to be portable over relatively short distances (e.g. 2 km), “both on foot, by bicycle and
on horseback“. For this reason, the power source and the device are separated and – supported on a frame of steel tubes – “can be carried on the back“.
Another device was built for our own use, to which some improvements were also made. This showed that with the 199 MHz frequency, a reliable telephone connection could be achieved at much greater distances than the required 5 to 6 km when the devices are placed high. The largest distance covered was 63 km between the lighthouses of Scheveningen and IJmuiden; with one transmitter at ground level, 45 kilometres were bridged.
In order to investigate the influence of metal masses aboard Navy ships, a portable UKG device was used with its antenna at various locations in the ship’s tower of HNLMS De Ruyter to investigate the connection with the shore. Even at a relatively low height on the ship, a distance of 11 km was bridged.
This last prototype model was adapted for industrial manufacturing by the Dutch Signal Equipment Factory (NSF). Both Telefunken valves in the device were replaced by a single American (Western Electric 316a) transmitting valve, which provided the following advantages:
- “one instead of two transmitter lamps (about half the price)”;
- “a slightly better performance”;
- “a simplification, as this lamp no longer requires filament tuning”;
- “less space required, so that the layout of the cabinet (cell construction) could be improved.”
In addition, the modulation was improved by achieving a much better frequency constancy and switching from grid to plate modulation. Moreover, the weight of the battery unit was reduced by 5 kg.
The final (fifth) development model by Von Weiler was completed at the end of 1937. It contained only four single tubes and provided twenty consecutive 0.5 MHz wide telephone channels centred around 199 MHz (wavelength 1.5 m). At that time, 199 MHz was considered to be an ultra-high frequency. In operation, the transmitter/receiver was connected back to back with the power supply block. The combination served as the basis for a single directional antenna for sending and receiving. This prototype was adapted to industrial production and produced as the transportable radio-telephone unit NSF type DR-42 from the end of 1938.
In 1939, the Dutch Army took 240 units into operation. The new communication units were used in the battle for the Grebbeberg in May 1940.
In 1942, the German Army continued the production of the DR-42 during the occupation of the Netherlands. There exists a German-language Philips Emission description, operations manual, and a full schematic plus bill of components called “Tragbare Ultrakurzwellen Sende-Emphanganlage Baumuster DR-42” (Philips 1900D, 1901D, and 1902D). The German version had two ranges, one centred around 200 MHz and one around 185 MHz. Even in 1948, the NSF DR-42 was still considered a viable proposal.
Transportable transceiver NSF type DR-42
The transmission circuit of the Dutch Seintoestellen Fabriek (NSF) DR-42 consisted of an oscillating power triode that supplied 2 Watt to the antenna with anode modulation using a heavy-duty pentode valve. Of special significance was the use of a coaxial tuner with low losses (at the time called “Kolster electrical coupling system” – US2089271) for the oscillator which must have contributed significantly to the frequency stability (“high Q”).
The receiver contained a miniature triode (TB04-8) as a super-regenerative demodulator preceded by a pentode to block the penetration of the oscillation towards the antenna. This was followed by the aforementioned power pentode as a low-frequency amplifier.
The front panel shows tuning scales for sending and receiving. In between was an Ampèremeter with a switch. Underneath that meter was a switch for selecting receiving or sending. This transceiver formed a unit of about 20 kg. The radio-telephone had a range of ten to fifteen kilometres under the specified conditions. The sending unit was equipped with the warning text “Attention: the enemy is listening.”
The second unit (17 kg) contained a Ni-Fe battery that provided 6 V to the filaments and a vibration rectifier for the anode voltage. This provided ten hours of operation based on equal transmission and reception time, although the instruction text on the radio gave a more optimistic battery life of 24/7 operations.
A third unit in the form of a long tube (5.5 kg) contained the foldable three-element Yagi antenna, mast sections for a maximum of 2.2 meters antenna height, a microphone and headphones.
A manual for the NSF DR-42 in German can be found here.
- The German manual of the NSF DR-42 can be found here.
- High-Q Tank Circuits for Ultra-High Frequencies, Frederick A. Kolster, QST, May 1934, Vol. XVIII, pp 69-70.