Radio direction finding (RDF) was developed as a navigational aid in the 1920s and was widely used in the decades that followed. It works by using a rotatable aerial to determine the direction in which the signal from a known radio transmitter comes through most strongly. It was particular useful when sailing fairly close to the shore, up to about 25 miles (40 km).

This RDF receiver has a circular aerial loop that has a pointer to indicate its direction. The receiver can be set for three wavelength ranges: long wave (LW), medium wave (MW) and trawler band (TB).