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Clandestine radio receiver
The type 31/1, also known as Sweetheart, is one of the most beautiful
clandestine receivers of WWII.
It was developed for use by the SOE
in 1943 by the Norwegian Willy Simonsen.
It consists of a small body-wearable receiver, a separate battery unit
and a tobacco tin that contains the earphones. Receiver and battery case
are painted in blue/grey wrinkle paint.
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In 1942, the Norwegian graduate electronics engineer Willy Simonsen
escaped to England where he started working for the Inter Services
Research Bureau (ISRB).
He used his knownledge and experience with Norwegian resistance
work, to design a small pocket-size receiver with low power consumption,
that could be driven for a long time by standard domestic batteries.
For the design of the Sweetheart, he was not allowed to use military-grade
components. As a result, the receiver had to be built from standard
domestical (unpreferential) components.
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Furthermore, the receiver could not be built by the highly skilled
craftsmen that assembled the other spy radio sets, so they had to revert
to a 'standard' manufacturer.
As it was built under supervision of the SOE, the receiver was given the designation
Type 31/1. It was nicknamed Sweetheart, probably because of a nice young lady who worked on the
project with Simonsen.
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Approximately 50,000 Sweethearts were built
by Hale Electric Co. Ltd., at a price of just 8 GBP each.
About 5000 units went to the Norwegian
government in exile, and were subsequently dropped over occupied
Norwegian territory, intended for use by the resistance.
The image on the right shows three young men in a hole in a forest
near Hvarnes, using the Sweetheart receiver.
The image was taken in the summer of 1944.
From left to right: Josef Haraldsen (district leader),
Erling Slorvik (radio telegrapher) and Hans Lien (chief arms officer).
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Together they were the leaders of MILORG D-15 (Vestfold).
Hidden safely in the wide forest near Hvarneskollen, south-east of Oslo,
they exchanged hundreds of messages with London [3]. More images of the
D-15 Vestfold group below (copyright unknown).
MILORG (short for: Military Organisation) was the organised Norwegian
Resistance that served directly under the Norwegian Armed Forces High
Command (FOR) at Kingston House in London, where also the King
and the Norwegian Government in exile were located [4].
During WWII, MILORG executed a range of resistance and sabotage
activities.
By spring 1945, MILORG had about 40,000 men under arms in Norway.
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One way of keeping power consumption as low as possible was to use
crystal earphones instead of normal headphones and transformers.
Such crystal earphones were not a comodity in the UK in 1943, so Simonsen
had to order them from the USA. They were manufactured by Brush (USA)
for use in hearing aids, another type of devices that require low
power consumption.
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These earphones used Rochelle salt for the piezo-crystal element
and could not withstand the low pressure in an airplane.
To allow the Sweetheart units to be dropped over occupied territory,
they were therefore packed in hermetically sealed tobacco tins.
A warning in the user instructions says that the receiver should not
be flown over 5000 meters, unless the headphones were hermetically
sealed (please check the user instructions in the download section below).
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The Type 31/1 Sweetheart receiver is extremely small for its time, and was designed
for low power consumption. It was powered by two
batteries: one standard 4.5 Volt torch battery for the filament
and a small 30 Volt HT battery, commonly used in hearing aids, for the
anode voltage.
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The Sweetheart would still work when the HT voltage
dropped to 20 Volt, allowing the HT battery to last for 150 to 200 hours,
whilst the LT battery (4.5 Volt) had to be replaced approx. every 50 hours.
The image on the right show the interior of the Sweetheart after removing
its cover.
It contains only three valves of the type 1T4 that, like the earphones,
were manufactured in the USA.
The three miniature valves are mounted in a small subframe with a range of
colourful passive compontents soldered to their sockets.
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The receiver covers a frequency range from 6 to 12 MHz in a single band,
with the small dial calibrated in metres.
As they were ideal for the reception of BBC newscasts in Western Europe,
Sweetheart receivers were also dropped by the SOE over other parts of
Europe during WWII. They are therefore also known as the Propaganda Set.
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© Copyright 2009-2013, Paul Reuvers & Marc Simons. Last changed: Wednesday, 30 April 2014 - 21:37 CET
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