History of the Kyynel radio – the most advanced HF transceiver of its time
The Kyynel (”Tear”) was a lightweight portable long-range patrol radio. It was designed and developed prior to the Winter War by the Finnish Armed Forces radio intelligence unit – one of the best in the world - based on the needs of Finnish long-range reconnaissance patrols. Probably the best patrol radio available during the WW2 anywhere in the world, the Kyynel was a light, small and efficient shortwave radio that was used by Finnish special forces soldiers operating behind enemy lines to contact their headquarters. The genesis of the device came about in 1937.
Design and development by the Finnish radio intelligence unit for such a radio began in the spring of 1937. Colonel Reino Hallamaa assigned Captain Osmo Töyrylä to lead a team on this project. The development of this radio was seen so important that a large group was formed, with the lead development engineer being a reserve officer, Captain Holger Jalander. The team was largely made up of radio amateurs as they had considerable experience designing, building and operating their own short-wave transmitters and receivers and were accustomed to working with weak radio signals in a variety of conditions.
Development and testing was made at a rapid pace and under conditions of high security. Transmission experiments were limited to minimum as Colonel Hallamaa feared the enemy might be listening to the transmissions. One of the most demanding development tasks was making the unit watertight, because the operator had to have access to the switches and tuning knobs inside the cover. Captain Holger Jalander had the idea of using foundry-cast aluminium boxes as the casing for the radios. Using special threaded lids with seals, the casing was found to be almost completely watertight. The tubes and other electronic components were mounted side-by-side – no circuit boards were available at that time. This kind of assembly proved to be advantageous because of short interconnections and, thus, minimal stray capacitance and series inductance. Moreover, the design led to the whole unit being very rugged and capable of sustaining a drop from several meters – also with a parachute. Before a mission, the operating frequency was set to the correct base and screw -secured to stay in place. At the same time, the radio was provided with an antenna length specific to the selected operating frequency.
The M-10 Kyynel used in 1942 was a complete transceiver in one die cast aluminium cover , weighting only 1.5kg. The transceiver was operating between approximately 3.3 MHz to 4.8 MHz and a half-wave dipole in V-form was used as the antenna. Because the Kyynel used HF frequencies, the communication range was a function of the frequency being used and the time of the day. Reliable communication could be maintained at ranges from 60 km up to 600km (and sometimes even longer if propagation was good). The device had one lockable tuning knob for the transmitter and two knobs for the receiver – one for RX-tuning and the other for sensitivity control. The receiver was very sensitive, stable and easy to tune due to the successful feedback control arrangement.
- The components of the Kyynel that was issued as standard to all Finnish units tasked with operating behind enemy lines from mid-1939 onwards. Because their secrecy was to be maintained at any price, these radios were fitted with a detonator and a 200 grams of TNT to destroy the set in case of capture. Unauthorised opening of one of the threaded covers would also set off the explosive charge.
It was more or less at this time, in mid-1939 as the radio began to be issued, that the patrol-radio became known as the “Kyynel.” The name of the transmitter has two explanations: A typical operating position of the unit was somewhere in the deep forest under big trees -which, in peace-time Finland, would have been an ideal place for an illegal booze factory (in Finnish: “korpikuusen kyynel”). Another, perhaps more technical translation was that when the radio was turned on, it made a chirpy noise that sounded as if the radio was weeping – which was highly annoying to the chief designer, engineer Ragnar Lautkari.
The Kyynel would remain in service with the Finnish military well into the 1950’s. The Finnish Radio Intelligence Headquarters and the operational units using this radio succeeded in keeping the device secret for the duration of WW2.