FH4 "Huff-duff" equipment on the museum ship HMS Belfast
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 to this day 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.
HF/DF used a set of antennas to receive the same signal in slightly different locations or angles, and then used those slight differences in the signal to display the bearing to the transmitter on an oscilloscope display. Earlier systems used a mechanically rotated antenna ( or solenoid ) and an operator listening for peaks or nulls in the signal, which took considerable time to determine, often on the order of a minute or more. HF/DF's display made the same measurement essentially instantaneously, which allowed it to catch fleeting signals, such as those 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. 
Early DF systems used a loop antenna that could be mechanically rotated. The operator would tune in a known radio station and then rotate the antenna until the signal disappeared. This meant that the antenna was now at right angles to the broadcaster, although it could be on either side…