Long wave or shortwave converter
Device, generating short wave (or long wave) signals by changing the frequency determining circuits in a radio receiver. The short wave unit (Unit 1C) consists of two short wave coils that can be connected in parallel with the existing tuning coils in the radio by using a built-in switch.
When switching these external coils in parallel with the coils in the tuned circuits in the radio, the self-inductance of the combination becomes lower and the resonance frequency higher. The receiving frequency of the radio then shifts from medium wave to short wave.
The unit can be mounted on the back of a radio; a number of wires connect the device to the coils inside the radio.
Developed in World War II by David Berreklouw, owner of the Amsterdam-based company Sonotron, located at the 1e van der Helststraat 21-25.

In the Netherlands, listening to foreign radio stations was prohibited by the Germans from 1 July 1940. Broadcasts directed to the occupied Netherlands were jammed by the Germans since that date; on short wave this was a little more difficult.
By purchasing the Sonotron, which was suitable for a large number of medium wave radios used during that time, it became possible to listen to the Dutch "Radio Oranje" BBC broadcasts on short wave.
The sale of the Sonotron Units took place in all openness, although it was clear that the the ban on listening to foreign radio stations was ignored.
The original pice was f 18,-.
Data
Serial number: none
Sonotron Technische Mededeelingen 1-4, 7-9
Dimensions: 7.5 x 2.2 cm
Made in: 1941-1943
Purchased in: 2017
 

Listen to the first 4 minutes of a Radio Oranje transmission from 1941
Connections at the rear
Side view of the unit
 
Advertisement in the newspaper Nieuwsblad van het Noorden of November 5, 1941, in which representatives for Sonotron are sought for the province of Groningen.
Below, an advertisement in the Twentsche Courant of February 7, 1942, with an offer for installing a Sonotron Unit.
  Advertisement in the Twentsche Courant. March 14, 1942. Especially the suggestive text: "Are you missing something ??" is noticeable.
Installing a Sonotron Unit at a radio trader cost f 2.50; The unit itself could be purchased for 18, -.
There were units for 800-2000 meters (Unit A), 55-200 meters (Unit B) and 18-55 meters (Unit C).
Circuit, showing the Sonotron connections for a Philps 203U. Image from the patent application of David Berreklouw from 1941. The patent was not granted because the patent office was not convinced of its originality.
Front and back of the warranty label
A Sonotron Unit, mounted on the back of a Philips 203U radio.

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