The history.

The Parasets were built in Whaddon England by a division of the Special Operations Executive abbreviated SOE a subsection of the well known MI6. It was founded to drop secret agents, with or without including with a transceiver, behind enemy lines in France, Belgium, the Netherlands and Norway and possibly other countries.

The first PARASETs were manufactured in an antique travel case as shown in the picture. In the case was also the adapter to run the set on the mains as well as on a 6 Volt battery. The suitcase also contained a few spare radio tubes, headphones and a number of crystals to operate on different frequencies. However, German occupiers soon noticed that someone walking on the street with a suitcase transported a transceiver, with capture and the further known serious consequences. Later they started to manufacture the PARASET in a separate box, first out of wood as in the replica and later entirely of metal. The occupying forces eventually recognized not only the secret agents by the suitcases but also by their English clothing cut. So the SOE also had to do something about that too. The SOE also manufactured other transceiver models that were slightly larger and looked slightly different. But the PARASET, with it's small size and charming appearance, has become so popular to radio amateurs that many amateurs all over the world have built a working replica of the PARASET. Or have found an authentic PARASET from the war in an abandoned attic or cellar and are fascinated to make radio connections all over the world with Morse telegraphy and thus contribute to the plans to declare Morse telegraphy a cultural heritage (plans 1917).

Original version of 1941

How does a PARASET work and how is it operated?

The PARASET consists of only three radio tubes (lamps) in metal, 1 x 6V6 for the transmitter and 2 x 6SK7 for the receiver.The transmitter supplies approximately 5 watts to the antenna. That is enough to make connections throughout Europe and beyond if atmospheric conditions are favorable. The transmitter is constructed in such a way that a wide variety of antennas can be used because the secret agents usually had to work with very temporary antennas that were quick to set up and also quickly break down. Sometimes the washing line was used as an antenna or a piece of wire on the ceiling or under the carpet. Two simple bicycle taillight lights showed whether the transmitter was properly adjusted to give its maximum energy. The transmitter has a built-in signal key to transmit the Morse characters.

The receiver is of the regenerative type, also known as a feedback detector. This type of receiver was invented in 1912 by Edwin Armstrong who worked at the University of Columbia America. A regenerative receiver is very simple in design, but it may be said, at the same time also quite difficult in it's operation. That is why the secret agents are all the more honored that they could work successfully with this. Nevertheless, regenerative receivers have been very popular in the early days of the radio and also later. One of the reasons for this was that radio tubes were still very expensive at the time and only one radio tube was needed for a regenerative receiver. Despite it's simplicity, a regenerative receiver is very sensitive and selective. That is the ability to separate closely adjacent transmitters. Where does a regenerative receiver get these properties from? In a regenerative receiver, the signal that comes out is partly returned to the input, also referred to in English as "feedback or reaction". Maybe a bit similar to the turbo in a car. The level of the returned signal is adjusted so that, simply said, the receiver is almost going crazy and is about to start operating as a transmitter because it will oscillate itself. Because the receiver then tends to amplify the output signal again and again, the sensitivity and the selectivity are increased many times. So far everything positive, but what makes the operation so difficult? An American professor of electronics called it a "Two hands needed receiver" which means that it takes two hands at a time to tune the receiver. Actually even three, but that is difficult.

The image shows a very simply principle of a regenerative receiver. To give the receiver the right amount of feedback for optimal operation, two control knobs are required, hence the "Two hands". In the image on the left, these buttons can be recognized as R2 and C2. R2 controls the level of the feedback and C2 controls the frequency at which we want to receive. The disadvantage of a regenerative receiver, however, is that both schemes are very interactive. This means that they strongly influence each other. This creates that when the knob is turned to adjust the feedback the frequency also changes slightly and vice versa. Also C1 which is adjustable and has to ensure that the antenna good works, works interactively, hence the "three hands"

Except to these disavantages, there is another disavantage that in the ordinary use of a regenerative receiver is not so great, but posed a great danger to the secret agents. This is because the receiver also worked as a transmitter if it was set up to receive Morse telegraphy signals, which forced it to oscillate to make the signals audible. This meant that the a secret agent was traceable even when he was not transmitting himself. I do not know whether the designers of the PARASET were aware of this. In addition to the enemy trying to find out the secret agent, they also cut off the mains streetwise. When the sending suddenly stopped, they already knew in which street the secret agent was on. At least when he was not working on the battery but on the mains.

The schematic:

The transceiver consists of only three tubes, one tube 6V6 for the transmitter and two tubes 6SK7 for the receiver. The first 6SK7 is connected as a feedback detector and the second as a low-frequency amplifier. The transmitter is crystal controlled for stability.

The reception range is from 3.0 MHz to 7.6 MHz and thus includes the 40 and 80 meter band. The transmission range is divided in two with a switchable range from 3.3 MHz to 4.5 MHz and a range from 4.5 MHz to 7.6 MHz.

The tuning (aligning) of the transmitter must be done by means of the Arialtuning and Tanktuning buttons to let the indicator lights give maximum light. Preferably, the Arial indicator light should provide the most light.

Tuning the receiver must be done with the Receiver tuning and the Reaction buttons. To make Morse (CW) signals audible, the feedback detector must be adjusted with the Reaction knob just over the point with the most sensitive reception so that it will oscillate weakly. This point is audible when the noise level suddenly increases.

However, due to the necessary oscillation, a weak signal is also transmitted via the antenna, even when the transceiver is on reception. Through which the secret agent can be located with the danger to his own life if he is arrested by the enemy.

Tuning and aligning requires some experience because the settings of both buttons influence each other. In other words adjusting the Reaction not only changes the feedback but also the reception frequency in a greater or lesser degree and vice versa. The antenna length and arrangement can also affect both settings.

The transceiver does not have a so-called sidetone, which should make it possible to hear the Morse characters that you are sending your selvesit to make it easier to signal with the Morsel key. However, a well-trained operator can also transmit Morse code without sidetone. For security purposes, the secret agents had to signal mistakes that had been spelled out, such as, for example, every sixteenth letter, the so-called "security check". So that the London headquarters knew that they would not signal in captivity under pressure from the enemy.

The frequency reading for the receiver is done with a graph on which the frequency can be read on the basis of the 0 to 100 scale on the large receiver button, which is not always exact due to the interaction of the buttons mentioned. Which can make it extra difficult for the operator to go on the correct frequency of the counter station quickly enough.

This makes it clear that a secret agent operator should preferably be well trained to stop working with the transceiver as necessary so as not to fall into the hands of the enemy unintentionally.