Radio Direction Finder

Radio Direction Finder

1. Automatic Direction Finder

   • Automatically & continuously display bearing to radio staion
   • Tune to NDB in low wave 190 - 535 kHz
   • ADF tunes to correct frequency
     • Verify identity using morse code
   • Moves RMI to show direction of beacon
   • Will always point to the station regardless of aircraft heading
   • Problem
     • Dip error
       • Dip in direction of the turn
       • Loop antenna banking in direction of turn
         • Loop antenna being different angle to beacon
       • Loop antenna is shielded by direction sensing antenna during turn
     • Quadrantal error
       • Radio waves bounced off or reradiated by airframe
       • Does not affect cardinal points (NSEW)
       • Affect quadrantal points (45, 135, 225, 315)
   • Station Passage
     • Flying over the top of any navigation aid
     • Approaching station
       • ADF increased sensitivity
       • Small derivation, large deflections
       • Provides position fix for navigator
       • First indications of station proximity to positive station passage varies with altitude
         • Few moments at low levels to several minutes at high altitude
   • Homing
     • Flying straight ahead, pointing to 0 degrees
     • Tune to station, identity morse code, turn aircraft to 0 degrees
     • Poor piloting technique
       • Affected by cross wind and off course
       • Fly further & longer than direct track
   • Tracking
     • Tracked desired course
     • Allow wind aloft, wind blowing aircraft off course

2. Radio Magnetic Indicator (RMI)

   • Incorporate a compass card
     • Actuated by aircraft's compass system
   • Reading magnetic bearing to & from transmitting station instead of relative bearing
   • Most have 2 direction needle
     • Thicker double barred needle connected to ADF
     • Slimmer single barred connected to VOR
   • Navigator able to triangulate & fix position w/o station passage
   • Able to replace magnetic compass
     • But not heading indicator
   • Being replaced by Horizontal Situation Indicator (HSI) with VOR

3. Operation

   • Comparing signal strength of directional antenna pointing at different directions
   • Tune to correct frequency of station
     • Manually turn loop to listen high pitch tone decrease to none or watch s-meter at the highest peak
       • Listen to station's morse code identifier