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.
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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vrijdag 04 oktober 2019