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Jones, S-27 Ham Radio & Knickebein Beams

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German Knickebein Navigation Beams


Physicist, Dr. R.V. Jones

Photo of Physicist, Dr. R.V. Jones, on display at the National Electronics Museum.

The British declared war on Germany September 3, 1939, as a result of the German invasion of Poland and the German threat to the British homeland was in the form of bombing raids and the possibility of a land invasion. A young British physicist named R. V. Jones had been called upon to join the Intelligence section of the Air Ministry to aid in the defense of Britain from a German air attack.


During the summer of 1940, a recovered German message included the term Knickebein radio beacon1 and Jones began his investigation. As more evidence mounted, Jones believed that the Germans were broadcasting radio waves from two stations off the coast of Europe, directed so as to intersect over a certain target in England. The German bombers would then orient off the beams of the first transmitter, which emitted the sound of dots on one side and dashes on the other.



Diagram of German Knickebein radio beams.

German Knickebein radio beams. Diagram from National Electronics Museum. I have added the aircraft silhouette in the overlap area of the beams. Second transmitter beam (for target location) is not shown.

When located in an area of overlap of these two beams, the bomber would hear a continuous sound, which alerted it as to the correct course, and maneuvered right or left to avoid the dots or dashes which acted as sort of an electronic guard rail. On reaching the target, where the beam of the second transmitter intersected the beams of the first, the bomber's receiver would pick up the signal to drop the bombs.


The Hallicrafter's S-27 Ham Radio Finds the Beams


Avro Anson with S-27 ham radio

According to the National Electronics Museum, the S-27 ham radio was installed in an Anson twin-engine reconnaissance aircraft for what became the first airborne ELINT mission. Photo on display at the National Electronics Museum.

Jones sent up an Anson twin-engine reconnaissance aircraft with a radio set and instructions to listen for the dots and dashes. The set was a U.S. built Hallicrafter's S-27 ham radio (right out of the box), because it could detect signals near 30 MHz (which Jones suspected as the frequency of the Knickebein beams).


After several attempts, the report came back that the dots and dashes had been found. The beams intersected over Derby where the Rolls Royce factory was producing engines for the RAF.


Interference Beams to Confuse the Bombers

Photo of a Hallicrafter's S-27 ham radio receiver at the National Electronics Museum in Linthicum, Maryland.

Hallicrafter's S-27 ham radio receiver, on display at the National Electronics Museum in Linthicum, Maryland.

Now that the beams had been confirmed, countermeasures were introduced. The first efforts used a medical diathermy set which had components within that could be modified to transmit at the required frequency. This was replaced later with a more effective jammer.


The transmitted interference caused the German bombers to be unable to hear the continuous sound from the overlapping beams which indicated when they were on the correct path. The effect was a constant changing of direction, in search of the continuous sound. Interestingly, the beams were code named headaches, and the countermeasure, Aspirins.

The Germans introduced other beam systems which were increasingly more difficult to counter but by February 1941, the Battle of the Beams was as good as won. All three major German systems, Knickebein, X and Y, were defeated.2


Photos taken by WW2HQ staff with permission of the National Electronics Museum.


Notes
1Prior to WWII the Germans had developed the Lorentz blind landing radar, the development of which send them to the forefront in navigational radar. The Knickebein was based on the Lorentz system but used receivers with much greater sensitivity.

2R. V. Jones. Most Secret War. London: Hamilton, 1978. p179.

Additional Resources
1. Information from the signs at the National Electronics Museum.
2. Louis Brown. A Radar History of World War II – Technical and Military Imperatives. Bristol and Philadelphia: Institute of Physics Publishing, 1999.



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British improvisation & the
Battle of the Beams

England was dependent upon imports for many items including electronic equipment. The Germans had placed mines around England, and were bombing and torpedoing Allied ships. The situation was complicated by the fact that most of the young men had been drafted into military service leaving a shortage of dock workers.

To compensate, the British needed to improvise, and civilian equipment was recruited for military service. Also, civilian equipment was recruited to implement plans for military items that had not yet been built but were urgently needed. One of the items was the U.S. built Hallicrafter's S-27 ham radio, and another was the medical diathermy set. See main article.

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Jones' battle for the beams

Proving the existence of the German Knickebein beams turned out to be a battle for 28 year old Jones. It became necessary for him to convince his superiors, since Eckersely, an expert on radio waves, believed that a navigational beam was not possible due to the earth's curvature at that distance.

British thought at the time also was against Jones in that it was believed that night bombings could be accurate without additional navigational aids, and that the Knickebein beam was possibly a hoax.

But Jones was so convinced, that when the Principle Deputy of Signals tried to cancel one of his flights in search of the beams, Jones told him that the Prime Minister would be informed of the countermand. After this confrontation the flight was approved, and turned out to be the very mission which discovered the existence of the beams.

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What it means to bend a beam

To visualize radio beams (waves), think of dropping a pebble in a pond and picture the waves it creates. If another pebble is dropped near the location of the first, its waves can affect the waves which were set in motion by the first pebble. This illustrates in part, what would have happened had the British jamming beam actually bent the waves of the German Knickebein.

Radio waves are, however, a bit different from the waves caused by dropping a pebble into water in that they are not composed of the medium in which they travel. So, unless two radio waves are synchronized, one would just pass through the other, and not interfere with its movement. However, in the case of jamming the Knickebein beam, the interference wave was not synchronized and so did not bend it as was rumored on both sides. But the wave could be heard over the radio receiver and confused the German bombers as to their location.