HF/DF

…these devices were known as HF/DF, or Huff Duff. The use of HF/DF is given much credit, along with microwave radar and Ultra (a project for decoding encrypted German military messages), for the eventual defeat of the very serious German submarine threat.

…contribution to the development of high-frequency direction finders (HF/DF, or “Huff Duff”) permitted the U.S. Navy during World War II to detect enemy transmissions.