WordNet (r) 3.0 (2006):

direction finder
    n 1: radio; determines the direction of incoming radio waves

Moby Thesaurus II by Grady Ward, 1.0:

87 Moby Thesaurus words for "direction finder":
   Bourdon tube, Gyropilot, Gyrosin compass, HFDF, Mach meter, RDF,
   absolute altimeter, accelerometer, aerial reconnaissance camera,
   aerometer, aeroscope, air log, altigraph, altimeter, ammeter,
   anemograph, anemometer, anemoscope, aneroid, automatic pilot,
   autosyn, barometer, bombing locator, bombsight, calorimeter,
   card compass, chronometer, compass, control rod, evaporimeter,
   flight recorder, galvanometer, gosport, gosport tube, gyrocompass,
   gyroscopic compass, gyrostatic compass, horn, hub dynamometer,
   hygrograph, hygrometer, hypsometer, inclinometer,
   induction compass, inertial navigation system, joy stick, loran,
   magnetic compass, magnetic needle, meteorograph, micrometer,
   needle, octant, optical altimeter, ozonometer, photometer,
   pitot-static tube, pluviometer, potentiometer, pyrometer, quadrant,
   radar, radio, radio compass, radio direction finder,
   radiogoniometer, rate-of-climb indicator, recording hygrometer,
   robot pilot, sextant, shoran, spirit level, static tube, sting,
   strike radar scanner, sun compass, tachometer, thermograph,
   thermometer, thermostat, throttle, transit instrument, variometer,
   venturi tube, viscosimeter, wireless compass, yoke