Why is the ADF ground Tx known as a non-directional beacon (NDB)?
It transmits omnidirectionally
What is 500 kHz used for?
International distress frequency
Describe the transmissions from a NDB?
- Vertically polarised
- Modulated at 1020 Hz
- Keyed CW or keyed MCW
How does ADF determine whether ADF beacon is in front or behind antenna?
Signal at loop RF either leads or lags the sense RF by 90°
Incoming loop RF is phase shifted by 90°, which either causes it to be in phase or 180° out of phase with sense RF signal
How are relative bearings measured?
Clockwise from the forward axis of the aircraft
What is the top surface of an ADF antenna covered with?
Conductive antistatic paint for the purpose of providing precipitation static protection
What should the bonding resistance be between the indicated measuring point of an ADF antenna and the aircraft skin?
Less than the maximum allowable value of 0.0015Ω
Describe a fixed loop ADF antenna?
- ‘Fore and aft’ coil having maximum response to stations in front of and behind aircraft
- ‘Transverse’ coil having maximum response to stations located at each side of the aircraft’s direction of flight
- Sensitivity of one coil may be reduced in manufacture to provide correction for Quadrantal Error
- Each coil has a centre tap which is connected to ground via the loop feeder cable
What does the centre tap of the coils in a ADF antenna provide?
Loop is balanced in relation to the Electrical component of the electromagnetic field acting upon it
What is the QE corrector loop equaliser for?
Compensating C and L for shorter than specified lengths of loop cable
What length of loop cable are ARINC specification equipment ADF receivers designed to work with?
30 ft
What total capacitance of sense aerial circuit are ARINC specification equipment ADF receivers designed to work with?
3000 pf
What is the maximum length of sense cable to be used in ARINC installations?
60 ft
What may be used in the sense aerial circuit where shorter lengths of cable than 60 feet are used in ARINC installations?
- Sense aerial matching unit
- Sense aerial cable equaliser
- Susceptiformer
What are the ARINC specifications that relate to separate sense and loop antennae in ADF installations?
ARINC 550
What should be the relative positions of the ADF sense and loop antennae?
Sense - at the electrical centre of the aircraft
Loop - within 0.25° of the aircraft longitudinal line
Why is the position of the sense aerial important?
Affects the over-station performance
What is caused by inverting the loop antenna?
Goniometer field will rotate in the wrong direction and be 180° out of phase
What connections must be changed when the sense antenna is moved from the top to the bottom in a synchro driven ARINC installation?
Synchro rotor C and H connections
What connections must be changed when the loop antenna is moved from the top to the bottom in a synchro driven ARINC installation?
Synchro rotor C (R1) and H (R2) connections, as well as position synchros X (S1) and Y (S3)
What are the three types of error in an ADF installation?
- Quadrantal error
- Loop alignment error
- Field alignment error
What gives the quadrantal error its name?
Maximums occur approximately at the centre of each of the quadrants formed by the horizontal axis of the aircraft
What causes the field alignment error?
The loop aerial is installed offset from the aircraft centre line
At what intervals are initial loop swings and check swings of the ADF carried out?
Initial loop swings - 15°
Check swing - 45°
