High-frequency direction finding

High-frequency direction finding, usually known by its abbreviation HF/DF or nickname huff-duff, is the common name for a type of radio direction finder (RDF) introduced in World War II. High frequency (HF) refers to a radio band that can efficiently 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.