High-frequency direction finding
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.
FH4 "Huff-duff" equipment on the museum ship HMS Belfast
"Super Duff" equipment on the museum ship HMS Belfast. The circular indicator provides a direct reading of the relative bearing from-which signals are received - red numerals for to port of the ship, green for to starboard
Direction finding (DF), or radio direction finding (RDF), is the use of radio waves to determine the direction to a radio source. The source may be a cooperating radio transmitter or may be an inadvertant source, a naturally-occurring radio source, or an illicit or enemy system. Radio direction finding differs from radar in that only the direction is determined by any one receiver; a radar system usually also gives a distance to the object of interest, as well as direction. By triangulation, the location of a radio source can be determined by measuring its direction from two or more locations. Radio direction finding is used in radio navigation for ships and aircraft, to locate emergency transmitters for search and rescue, for tracking wildlife, and to locate illegal or interfering transmitters. During the Second World War, radio direction finding was used by both sides to locate and direct aircraft, surface ships, and submarines.
Radiotriangulation scheme using two direction-finding antennas (A and B)
Direction finding antenna near the city of Lucerne, Switzerland
W.G. Wade of the National Bureau of Standards uses a large multi-loop antenna to perform RDF in this 1919 photo. This is a fairly small unit for the era.
This Royal Navy model is typical of B–T goniometers. The two sets of "field coils" and the rotating "sense coil" are visible.