Friday, January 17, 2014

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


    • 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.

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