Automatic Directional Finder (ADF)

• ADF Function

  It determines the A/C route by following the ADF GND stations using the radio signals to indicate the relative bearing to any station.
  
  
  
  
   
  
  
  
  
  -Transmitter & Receiver Location:
    Transmitter is the NDB GND station, receiver is on A/C.
  -Antenna Location:
    On the top surface of the A/C
  -No. of ADF Systems on the A/C:
    2 ADF systems on the A/C
  
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  • ADF Principle of  Operation

    •The radio wave is an electromagnetic field.
    •The E is parallel to the radiating element while the H is perpendicular to the radiating element.
  
  
  Loop Antenna
  
  
  •It is a directional antenna.
  •It indicates the direction of the station, but cannot determine if the station is in front/back or to the right/left of the aircraft.
  • It is in the shape of rectangular or circular loop.
  •It uses magnetic field component of the radio wave.
  
  
  
  
   
  Sense Antenna
  •Its function is to indicate if the station is in front / back or to 

      the left/right of the aircraft.

  
  

  •It is a capacitive antenna.

  
  

  • It couples only with the electrical component of the radio 

     wave.

  
  

  • It is an omni-directional antenna, its signal strength is the

      same all around.

   
  Homing  
  
  
  ØThe A/C tries to follow the wave from the ADF GND station by directing the loop antenna to face the ADF GND station. When the A/C arrives the ADF station, the power sign is reversed to indicate that an old station is passed and a new station is ready for tunning.
  
  
  
  ADF  operation 
  
  
  •The loop antenna receives the H components of the wave then shifted by 90 0 to be in phase or out of phase with the E
         component.
  •The balanced modulator achieves AM by 47 Hz.
  •The mixer mixes the H and E to be ready for detecting.
  •The audio detector is used to obtain the audio frequency
   signal with its phase.
  • this signal is compared with the original 47 Hz to get the (+) 
  or (-) sign. This sign is fed to a motor, synchro and modulated 
  by a 400 Hz AC power to determine the rotation direction of 
  the loop antenna. 
  
  

  
   
  

  •It is not strictly required to rotate the loop antenna to find 

  the null position. A fixed non rotating loop antenna is used

   to be trouble free due to fewer moving parts.

  
  

  •A fixed loop antenna consists of 2 perpendicular antennas 

  and connected to 2 coils of a resolver stator.

  
  

  •The fixed loop antenna has low probability of failure and 

  easy for maintenance.

   •Correction is done for sense antenna and loop antenna. 

  •A quadrature error correction (QEC) is a group of variable

   capacitors and coils that are adjusted to correct the loop 

  antenna reading. A coupler is used to match the sense

  capacitive antenna to the receiver.