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OD set
WWII clandestine transmitter and receiver

OD Set was a collection of devices that together formed a clandestine radio station, developed during World War II by the Dutch resistance organisation Ordedienst (OD) 1 for communication at national level between the various regions and groups of the OD. It was used aside British and American spy radio sets, that had been supplied for communication with the Dutch Government in exile in London, and that were in short supply. Some OD sets were made by radio amateurs.

Early in the war, the OD had been established as a sleeping law enforcement service, that would activate itself in case of a sudden retreat by the German occupant. It was thought that when this happened, a power vacuum that might arise.

When this turned out to be unrealistic, the OD transformed itself into an intelligence gathering organisation, and became arguably the most important link between occupied Netherlands and the Dutch Government in London (UK). For this, the OD obtained spy radio sets from the UK, such as the Type 3 Mk II (B2), Paraset and A3.
Parts of a clandestine OD radio set

The OD was also given the task to build a national (radio) network, that could be used between the anticipated capitulation of the Germans, and the arrival (and re-establishment) of the official Dutch law enforcement services. As the British-supplied radio sets were in short supply and were needed for the intelligence exchange with London, the OD decided to develop its own radio sets.

Within the OD, engineer Jan Thijssen (1908-1945) was charged with building the national network. Anton van Schendel, an employee of the Radio Monitoring Service (RCD) of the PTT, would be responsible for training the operators, which he recruted from radio amateurs (HAMs).

As secrecy of the messages was of the utmost importance, B.J. Suermondt was tasked with the establishment of an encryption bureau. He produced the coding instructions and trained the crypto-officers of the region commanders. Each radio station was given common code material.
Cover removed (rear view)

After the reorganisation of the OD in 1942, the OD transmitters and receivers were developed by ir. J.P. Heyboer (1912-1945), who had been made available by Philips in Eindhoven, and built in the workshop of Jan Hendrik (Henk) Op den Velde 2 in Zaandam, and probably also somewhere in Eindhoven. Philips also supplied the required components, under control of ir. G.H. Thal Larsen (1899-1963) and radio technician H.A. Hoekstra [2]. The design of the OD Receiver was later improved by Jan Lourens (PA0BN) in Oosterbeek (Netherlands), who built at least 34 of them.

After Op den Velde was arrested on 2 March 1944, Hoekstra took over his work on the roll-out of the national radio network. In the south of the country, Heyboer had meanwhile completed the south part of the network and had taken over the activities of Jan Thijssen on 31 December 1943, after the latter had been expelled from the OD, due to a conflict with the OD management. 3

  1. Literally translated: Order Service. During WWII, the OD was preparing an interim law-enforcement service, that would activate itself immediately after the (expected) liberation of the Netherlands. In practice, it became an internal intelligence service that passed information to the Dutch Government in the UK.
  2. During the war, Henk op den Velde was known by the codename HEIN [3][5].
  3. Thijssen found the OD too passive and wanted to increase the use of the radio links with the UK. After he left the OD, he formed the new resistance organisation Raad van Verzet (RVV) — the Resistance Council.

OD transmitter
OD receiver
OD Wavemeter
Morse key
Poem Code used by the Ordedienst (OD)
OD Transmitter
For the secret national radio network, the OD used a simple transmitter, tht was built with two PE06/40 valves in self-excited balanced (push-pull) configuration. Due to stability issues, the design was improved and altered several times.

The transmitter covered a small frequency range around 3 MHz, which was based on pre-war experiences of radio amateurs (HAMs).

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OD Receiver
The receiver covered the same frequency range as the transmitter, and was built with three EF6 valves. The initial version was developed by an engineer from Philips, but several varants and modifications are known.

A late variant had removable coils, which made it suitable for reception of the Medium Wave broadcast bands as well.

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OD Receiver

OD Wavemeter
As both the OD Transmitter and the OD Receiver were free-running and did not have a calibrated frequency scale, they had to be checked against a known good frequency standard from time to time, such as a crystal-controlled transmitter.

Later in the war, the wavemeter shown in the image on the right was used to tune the OD Transmitter to the desired frequency.

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xFrequency meter for OD transmitter

OD Morse key
This NSF morse key was used during the war in combination with the OD Transmitter of Region 13 (The Hague). It is mounted on a wooden base, which is padded with felt at the bottom.

The latter was done to suppress the acoustic clicks when operating the key. In large buildings, the acoustic clicks could sometimes give away the position of a clandestine transmitter.

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Morse key of the OD transmitter

  1. Cor Moerman, OD Receiver, Frequency Meter and Morse Key - THANKS !
    Received November 2020.

  2. Louis Meulstee, Wireless for the Warrior, volume 4
    ISBN 0952063-36-0, September 2004

  3. A.S.M. van Schendel, Mijn werkzaamheden als chef-marconist van de OD en mijn belevenissen in de gevangenis
    Organisation of the Interal Radio Service (BR) of the OD and the radio links with the UK.
    Post-war report, in Dutch language. Date unknown.

  4. Jan Schulten, De radiopost van de Ordedienst in Rijsbergen
    - achtergronden van het drama op de Vloeiweide - (in Dutch language).
    Jaarboek De Oranjeboom 47, 1994.

  5. Wikipedia (Netherlands), Henk op den Velde
    Retrieved December 2020.
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Crypto Museum. Created: Monday 30 November 2020. Last changed: Thursday, 01 April 2021 - 09:56 CET.
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