High frequency direction finder, HF/DF or Huff Duff as it was known was a device for determining the direction of short wave radio signals. It enabled their source to be located by taking bearings from two or more sources.
Huff Duff was part of the armoury of radio transmission analysis known as ELINT. ELectronic signals INTelligence covered intelligence gathering by use of electronic sensors. The system focused on non-communications intelligence. It was not necessary to know the content of the transmission. The signal itself could be used to locate the source of the transmission and thereby enable it to be used to locate ships, submarines and downed aircraft.
Along with ASDIC [sonar], Ultra code breaking SIGINT and radar, Huff Duff was a valuable part of the Allies armoury for detecting German U-Boats and commerce raiders during the Battle of the Atlantic.
Whenever a U-Boat transmitted a message Huff Duff could get bearings on the approximate position of the boat. Because it worked on the electronic emission and not the content it did not matter that the message was encrypted. The key feature and main advantage of Huff Duff was the use of an oscilloscope and fixed aerial which could instantaneously reveal the direction of the transmission without the time taken in conventional direction finding to rotate the aerial. U-Boat transmissions were deliberately kept short and it was wrongly assumed this would avoid detection. One quarter of all U-Boat losses in WW2 can statistically be associated with HF/DF operations [Foundation for German Communications and Related Technologies].
The site was ½ to ¾ of a mile NE of the aerodrome and built on boggy ground at Tresmarrow. A boggy site was desirable as it helped to enhance the signal. It had a rest room (MR 195) with sleeping accommodation and stove for heating and was constructed of concrete block. The main building (MR 194) was the HF/DF block and it's associated aerial. The building housed radio transmission and receiving equipment. Because of the height of the aerial it was secured by wire stays. There was an M and E plinth (MR 196) which brought in the power supply.
The site crew operated under the Station Signals Officer. When on duty they were able to sleep on site. WAAF Bobby Prime was one of the wireless operators who was called on to work the equipment.
The site is no longer extant. Everything was removed after the war.