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WWII
OD
  
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OD Wavemeter
Frequency standard used by resistance

The handheld device shown below, is an absorption wavemeter, made during World War II (WWII) by the Ordedienst (OD) – one of three important resistance organisations in the Netherlands – to adjust the frequency of free-running OD Transmitters, that were used as part of a Dutch national clandestine wireless communications network. It uses a neon lamp as the absorption indicator.

The device is housed in an aluminium enclosure that measures 114 x 114 x 55 mm. A 110 mm long wooden grip at the bottom, allows it to be held in the hand without touching the metal.

At the top is a removable coil that is installed into a two-pin socket of which the pins are 19 mm apart. The coil is 40 mm wide and extends from the top of the device by approx. 50 mm.

At the front panel is a large circular bakelite knurled knob, of which one half (i.e. 180°) carries a linear scale, marked from 0 to 100.
  
xFrequency meter for OD transmitter

In the upper left corner is a neon lamp that protrudes the front panel. It acts as a power indicator, and lights up brighter when more power is absorbed by the instrument. With the wooden grip, the bakelite knob and the coil, the device measures 274 x 114 x 87 mm and weights 460 grams.

Frequency meter for OD transmitter
xFrequency meter for OD transmitter
Plug-in coil
Tuning lamp
Rear side
OD Wavemeter and plug-in coil
 Plug-in coil separated from the wavemeter
Letter 'W' imprinted in the front panel
A
×
A
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Frequency meter for OD transmitter
A
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xFrequency meter for OD transmitter
A
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Plug-in coil
A
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Tuning lamp
A
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Rear side
A
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OD Wavemeter and plug-in coil
A
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 Plug-in coil separated from the wavemeter
A
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Letter 'W' imprinted in the front panel

Features
The image below gives an overview of the features of the OD Wavemeter. It should be held by the wooden grip – which is similar to the grip of, say, a screwdriver of the era – so that the aluminium enclosure is left untouched. Next, the black frequency dial is set to the desired frequency, using a separately supplied calibration chart (missing here). This is done by looking up the frequency and finding the corresponding linear number. The dial is then set with this number at the index line.

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Next, transmitter is turned on, and the coil of the wavemeter is brought close to the antenna or to the antenna coil of the transmitter. The frequency adjustment of the transmitter is now turned until the indicator (neon lamp) on the wave meter lights up as bright as possible. If the light is too bright, move the wavemeter away from the transmitter and try again. When the maximum is found, the frequency of the transmitter matches that of the tuned circuit inside the wavemeter.

Related items
Complete OD radio set
OD transmitter
TX
OD receiver
RX
Morse key (made by NSF for use in airplanes)
Key
Poem Code used by the Ordedienst (OD)
History
The wavemeter shown here, was used by the OD radio post of Region 13 (The Hague), that was alternately located (clandestinely) at the office of the Radio Monitoring Service (RCD), the house of RCD employee Anton van Schendel, the house of radio technician Dick Reijns at Torenstraat, and the attic of the CJMV building at the corner of Prinsegracht and Boekhorststraat in The Hague [1].

The CJMV building – shown in the 1943 image on the right [5] – was selected because of its strategic position. The radio station was located in the attic, whilst the windows at the top floor offered a good view of three surrounding streets: Prinsegracht, the corner with Boekhorststraat and, opposite the front door, Jan Hendrikstraat.

The building was also chosen because of the presence of electric overhead lines of the tram. It was assumed that these lines would cause inter­ference when the Germans tried to locate the transmitter by means of radio direction finding.

The transmitter was usually operated by wireless operators Adri van Mansum (of Delft) and Joop Ketting (of The Hague), and occasionally also by Dick Reijns, who had been appointed by wireless coordinator Anton van Schendel as the radio officer of OD Region 13 [3]. Reijns was a radio amateur (PA0RS) and had a radio repair shop at Jan Hendrikstraat, close to the CJMV building. He was responsible for the contact with London, as well as for the contact with other OD regions, via the secret national OD communications network.
  
The CJMV building at Prinsengracht 4 in The Hague in 1943 - one of the locations of the transmitter of OD Region 13

Close to the end of the war – on 18 February 1945 – during communication with the OD post in Eindhoven (Region 18) 1 Reijns and van Mansum were arrested by the German Sicherheitsdienst (SD) [3]. They were executed less then three weeks later, on 8 March 1945 at Waalsdorpervlakte [6]. Joop Ketting and Anton van Schendel both survived the war. The wavemeter featured on this page and a morse key made by NSF, are the only remains of the OD radio post in The Hague [1].

 Further history of the OD equipment

  1. Eindhoven was already liberated at this point.

Circuit diagram
Below is the circuit diagram of the OD Wavemeter [7]. It is basically a simple tuned circuit, that consists of a coil (L) and a variable capacitor (C) connected in parallel to a neon lamp (La). The resonance frequency of the tuned circuit (f0) is calculated with the formula at the right. The coil (L) is mounted outside the enclosure, so that it can absorb some energy from the transmitter.


When the resonance frequency of the tuned circuit is the same as the frequency emitted by the transmitter, the tuned circuit absorbs the maximum amount of energy, which is then fed to the (high-impedance) neon lamp. The device is fully passive, which means that it does not require an external power source, such as a battery, for its operation. It is driven by the absorbed energy.



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Interior
The interior of the OD Wavemeter can be accessed by removing the eight screws along the edges of the rear panel, after which the rear panel can be taken off, as shown in the image above. Inside the enclosure are just to components: (1) a large variable capacitor (the black disc at the centre) and a regular neon lamp mounted in the upper corner. A socket for the coil is in the other corner.

Rear side
Rear panel removed
Interior
Wiring of the coil socket
Neon lamp
ID stamps
B
×
B
1 / 6
Rear side
B
2 / 6
Rear panel removed
B
3 / 6
Interior
B
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Wiring of the coil socket
B
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Neon lamp
B
6 / 6
ID stamps

References
  1. Cor Moerman, OD Wavemeter - THANKS !
    Received November 2020.

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

  3. Verslag A.S.M. van Schendel
    Post-war debriefing.

  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. Photographer Ojen, Image of CJMV Building at Prinsegracht 4 in The Hague in 1943
    Collectie Haags Gemeentearchief. ID 0.58587.

  6. Oorlogsgravenstichting, Gerardus Bernardus Reijns (1907-1945)
    Retrieved December 2020.

  7. Wikipedia, Absorption wavemeter
    Retrieved December 2020.
Further information
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Crypto Museum. Created: Tuesday 29 December 2020. Last changed: Thursday, 01 April 2021 - 10:15 CET.
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