I thought this article also deserved a page of its own as this predates radar! Got your ears on Good buddy? :o) Yes these where actualy used to listen out for aircraft!

Article kindly donated by - MIKE G8KMP 28/10/2003

During a conversation at work one day, the subject of Dishes at Greatstone came up. This took me back to 1980, while on holiday there.

From what I remember, some unusual looking parabolic dishes not too far away from the holiday site could be seen, so I went to investigate. I managed to walk around the site at that time.

Here is a brief history of these Dishes actually called Mirrors.

Around October 1918, acoustic methods of detecting aircraft were carried out experimentally, with mirrors erected at Joss Gap on the Thanet coast and also at Fan Hole, Dover.

During 1920 and 1936 more experiments were carried out. Several sites around the coast were considered, with all sites being connected by telephone. This was to form an early warning system to detect aircraft. Only a few of the sites were ever completed.

There were three types. A 20 foot concave mirror of concrete. The listener stood on a small platform and moved a sound collector on an arm to pick up the maximum sound reflected from the mirror. Bearings were then read off graduated scales on the wall.

A 30 foot hemispherical bowl, with the listener sitting in a chamber beneath the mirror.

Each 20 foot mirror would cost about £650 to construct in May 1927.

A strip mirror some 200 foot long, by 26 foot high. The 20 foot and 30 foot mirrors collected sounds from the surface of the dish by acoustic methods using stethoscopes. The 200 foot mirror had several microphones placed in front of the wall.

The original plan was to have a 200 foot mirror every 25 miles around the coast, with two 20 or 30 foot mirrors between each of them.

Denge Site

Denge (Near Greatstone) was chosen as the nearest approach to silence that we can expect.

The 20 foot mirror at Denge was completed in 1928, and the 30 foot mirror was completed early 1930. This later dish was inclined at an angle upwards towards the sky.

In 1927 it was planned to build a 200 foot mirror at Denge and work started in 1929, while at the same time the Romney Hythe & Dymchurch Railway was also being extended to Dungeness. This ran in front of the mirrors and was used to carry materials for the building of the 200 foot mirror. In 1929, 891 tons of materials were carried on this railway.

The 200 foot mirror was completed in June 1930. Only one microphone was used at any time, so bearings were accurate to 5 degrees, and could hear aircraft about 20 miles away. With the unaided ear this was 6.5 miles. Later with improved microphones, tests gave an accuracy of 1.5 degrees. A wooden hut behind the mirror was used to monitor the sound from the microphones, but in 1933 it was desired to construct a concrete listening room

1933 seems to be when a lot of testing was carried out with the 200 foot mirror, as the RAF were being trained to use the system. The exercises carried on until about 1936.

1937 saw the mirrors only used occasionally due to the developing of houses nearby.

In May 1939, a decision was made to abandon the mirrors, as radar development proved to be far more effective than the acoustic mirrors. Biggin hill now became the home of the Air Defence.

The abandoned mirrors can still be seen in various conditions along the Kent coast.

Maybe these Remnants of our acoustic air defence system could be preserved for years to come. The Denge three are on PrivateLand owned by a gravel company at Greatstone.

More information can be seen by using the web and searching for Sound Mirrors. There are several sites with photos of the mirrors on some of them.

A book Mirrors by the Sea and written by Richard N Scarth was available in 1995 and published by the Hythe Civic Society It has 40 pages with many photos taken at the time of use of these mirrors.

I believe this book may now be called Echoes from the Sky

Author: MIKE G8KMP 28/10/2003