High-frequency direction finding, usually known by its abbreviation HF/DF or nickname huff-duff, is a type of radio direction finder (RDF) introduced in World War II. High frequency (HF) refers to a radio band that can effectively communicate over long distances; for example, between U-boats and their land-based headquarters. HF/DF was primarily used to catch enemy radios while they transmitted, although it was also used to locate friendly aircraft as a navigation aid. The basic technique remains in use as one of the fundamental disciplines of signals intelligence, although typically incorporated into a larger suite of radio systems and radars instead of being a stand-alone system. In earlier RDF systems, the operator mechanically rotated an antenna or solenoid and listened for peaks or nulls in the signal to determine the bearing to the transmitter. This took considerable time, on the order of a minute or more. In HF/DF systems, a set of antennas received the signal in slightly different locations or angles, and then used the slight differences in the signal to display the bearing on an oscilloscope display essentially instantaneously, allowing it to catch fleeting signals, such as from the U-boat fleet. The system was initially developed by Robert Watson-Watt starting in 1926, as a system for locating lightning. Its role in intelligence was not developed until the late 1930s. In the early war period, HF/DF units were in very high demand, and there was considerable inter-service rivalry involved in their distribution. An early use was by the RAF Fighter Command as part of the Dowding system of interception control, while ground-based units were also widely used to collect information for the Admiralty to locate U-boats. Between 1942 and 1944, smaller units became widely available and were common fixtures on Royal Navy ships. It is estimated HF/DF contributed to 24% of all U-boats sunk during the war. The basic concept is also known by several alternate names, including Cathode-Ray Direction Finding (CRDF), Twin Path DF, and for its inventor, Watson-Watt DF or Adcock/Watson-Watt when the antenna is considered. History Before HF/DF Radio direction finding was a widely used technique even before World War I, used for both naval and aerial navigation. The basic concept used a loop antenna, in its most basic form simply a circular loop of wire with a circumference decided by the frequency range of the signals to be detected. When the loop is aligned at right