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OD Transmitter
WWII Clandestine transmitter - wanted item

OD Transmitter was a 2-valve shortwave (SW) 1 CW transmitter for the 2.85 - 3.15 MHz range, developed during World War II, with help from Philips, 2 by the Ordedienst (OD) — one of three important resistance organisations in the Netherlands — for a secret national communications network. It was part of a complete improvised radio set that included a receiver and ancillaries.

As far as we know, there were two generations of the transmitter, that differ in circuit design and enclosure. The first generation was built around two PE06/40 valves in a self-excited push-pull configuration. It was built on a metal chassis and was housed in a wooden cabinet, disguised as a commercial LUXOR medical diathermy device [8].

This version delivered ~ 100 Watts of power, but had a rather poor frequency stability. It is known that at least one of them was modified 3 into an Oscillator/PA configuration [1]. This reduced the power output, but greatly improved the stability.
First generation of the OD radio set. Click to see the ful set. Photograph kindly provided by Louis Meulstee [1].

The second generation – also known as the alternative design – was made by radio amateur Piet Neve (PA0PN), and was used by OD Region 15, which comprised the Dutch province of Zeeland with its many islands. It played an important role during the final stages of the war – the Battle of the Scheldt 4 – providing the Allies with detailed information about the German movements [1].

  1. Theoretically it could also be classed as a Medium Wave (MW) device, as its frequency (~3 MHz or 100 m) is right at the crossover from MW to SW.
  2. There are indications that NSF in Hilversum was also involved in the production of clandestine equipment, but this is unconfirmed. It is certain however that the morse key for the transmitter was provided by NSF.
  3. Modification by J.J. (Jan) Zandbergen (PA0ZY) in Zaandam.
  4. The Battle of the Scheldt (Dutch: Slag om de Schelde) is little known, but is nevertheless one of the most important military operations in the Netherlands during WWII, similar in magnitude to Operation Market Garden [3]. The Netflix movie The Forgotten Battle (2021) is about this operation [4].

At present, Crypto Museum does not have an OD Transmitter in its collection, which is why we are unable to show photographs of this device. If you know of an OD transmitter that might be for sale, please contact us.
Related items
Complete OD radio set
OD Receiver
OD Wavemeter
Morse key (made by NSF for use in airplanes)
Poem Code used by the Ordedienst (OD)
Morse key
Although other keys were used as well, the morse key shown in the image on the right was standard issue for use in combination with an OD Transmitter. The key was made by NSF in Hilversum (Netherlands), for use in airplanes.

The key shown here was part of the OD radio post of Region 13 (The Hague), which was dismantled by the Germans on 18 February 1945, shortly before the end of the war.

 Further information
Original morse key that was used with an OD transmitter. It was originally made by NSF for use in airplanes

Within the OD, engineer Jan Thijssen (1908-1945) was charged with building a national radio network. Anton van Schendel, an employee of the Radio Monitoring Service (RCD) of the PTT, became 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.

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 1 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. 2

Click to see more

The image above shows the clandestine radio set of Region 11, when it was located in Zaandam in April 1945 [3]. The receiver is visible at the centre of the picture. It is the first version, which has an output transformer on the chassis and a reaction potentiometer at the front panel. The transmitter is at the left. The one shown here was modified by radio amateur Jan Zandbergen (PA0ZY) to a Master Oscillator/Power Amplifier circuit. At the front left is a rotary DC converter.

  1. During the war, Henk op den Velde was known by the codename HEIN [6][7].
  2. 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.

Circuit diagram
Original design
Below is the circuit diagram of the original design of the OD Transmitter. It is built around two PE06/40 valves in self-excited balanced configuration (push-pull). The circuit delivers approx. 100 Watts into the antenna, but suffers from stability issues. An extra circuit with a thermocross (TC) – here shown at the top left – is used to tune the antenna circuit for maximum power output.

Original design of the OD Transmitter

As the circuit is free-running, it is very difficult to determine the actual transmission frequency, which is why they were regulary calibrated by comparing them with a crystal-based transmitter, such as the British Type 3 Mark II (a.k.a. B2). This means that an OD technician regularly had to visit the (secret) locations of the OD transmitters, carrying a large and heavy British spy radio set.

This problem was later solved by using an external wavemeter, of which the tuned circuit had been calibrated by means of a crystal-based transmitter. A calibration chart then converted the linear scale of the tuning knob to actual frequencies. The wavemeter was much smaller than a suitcase radio and could easily be carried around without attracting the attention of the Germans.

 More about the wavemeter

Alternative design
Below is the circuit diagram of the alternative OD Transmitter, as designed by radio amateur Piet Neve (PA0PN), and used in OD Region 15, the Dutch province of Zeeland, during the final stages of the war. The design is based on a single EL2 valve in an ECO circuit and frequency doubling in the anode circuit [5]. It covers a frequency range of 3.5 to 4 MHz and delivers 15-30 W of power.

Alternative design by Piet Neve (PA0PN)

The circuit was usually built with an EL2 or EL6 valve, but due to wartime shortages, it was also built with EL3, 6L6 or PE06/40. Although this circuit is much simpler than the original design, and produces less power output, it was very successful and was much more stable [1].

PE06/40 valve
PE06/40 is penthode, developed by Philips for use in RF and AF amplifiers. In the OD Transmitter, two such valves were used in a balanced oscillator circuit, that delivered an output of approx. 100 Watts. As the circuit was rather unstable, some users modified it into a one-valve oscillator and an one-valve power amplifier (PA), which produced less power (35W) but was much more stable. Below is the pinout of the P-version of the PE06/40, as seen from the bottom of the valve.

 PE06/40 datasheet

EL2 valve
In the alternative design of the OD Transmitter, made by Piet Neve (PA0PN), a single EL2 valve is used, in a simple oscillator/doubler configuration. If the EL2 was not available, the EL6 or PE06/40 was used as an alternative. Below is the pinout of the EL2, as seen from the bottom.

 EL2 datasheet

EL6 valve
In the alternative design of the OD Transmitter, the EL6 penthode was used as an alternative to the EL2, when the latter wasn't available. Below is the pinout of the EL6, as seen from the bottom.

 EL6 datasheet

6L6 valve
In the alternative design of the OD Transmitter, the American 6L6 beam tetrode was sometimes used as an alternative to the EL2 or EL6 valves, when the latter weren't available due to wartime shortages. Below is the pinout of the 6L6, as seen from the bottom.

 6L6 datasheet

  • Design
    ir. J.P. Heyboer (Philips), Jan Thijssen
  • Manufacturer
    Philips (Eindhoven), Jan Hendrik (Hein) Op den Velde (Zaandam)
  • User
    Binnenlandse Radiodienst (BR) of the Ordedienst (OD)
  • Circuit
    Self-excited push-pull
  • Frequency
    2.85 - 3.15 MHz
  • Modulation
  • Output
    100 W
  • Valves
    2 x PE06/40
  • Power
    Mains PSU or rotary DC converter
  • Dimensions
    ~ 180 x 160 x 160 mm
  • Weight
Alternative design
  • Design
    Piet Neve (PA0PN)
  • Manufacturer
    Piet Neve (PA0PN)
  • User
    Binnenlandse Radiodienst (BR) of the Ordedienst (OD)
  • Circuit
  • Frequency
    3.5 - 4 MHz
  • Output
    15 to 30W, depending on valve type
  • Valves
    1 x EL2, EL6, EL3, 6L6 or PE06/40
  1. Louis Meulstee, Wireless for the Warrior, volume 4
    ISBN 0952063-36-0, September 2004

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

  3. Wikipedia, Battle of the Scheldt
    Retrieved December 2020.  Dutch version

  4. Wikipedia, De Slag om de Schelde (film)
    Retrieved December 2020 (in Dutch language).

  5. Wikipedia, Tri-tet oscillator
    Retrieved December 2020.

  6. 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.

  7. Wikipedia (Netherlands), Henk op den Velde
    Retrieved December 2020.

  8. Medica Amsterdam, Luxor - Bestralingsapparaat
    1938. In Dutch language. CM301583/3/5.

  9. D.W. (Dick) Rollema (PA0SE), Radioverbindingen van het Verzet in Zeeland
    VERON Electron, May 1987

  10. D.W. (Dick) Rollema (PA0SE), Station G11 van de Binnenlandse Radiodienst
    VERON Electron, May 1988
Further information
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Crypto Museum. Last changed: Tuesday, 19 January 2021 - 20:53 CET.
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