NDB & ADF Flashcards
How does an NDB work
A simple radio transmitter, radiating a vertically polarised signal in all directions simultaneously.
signal is a carrier wave transmitted at the published frequency of the NDB and amplitude modulated by a tone.
NDB provides an omnidirectional signal source from a known location. Aircraft use ADF to determine bearing to/from the NDB
Describe the basic features of a non directional beacon (NDB), including:
(a) the range of frequencies usually employed;
NDBs operate in the Low Frequency (LF) and Medium Frequency (MF) bands: 190kHz - 1750kHz.
Describe the basic features of a non directional beacon (NDB), including: the factors affecting operational range; (topography, transmitter power, NDB frequency, Time of day, type of surface, efficiency of receiving antenna)
Topography – Line of sight. Hills, large buildings and other obstacles for the signal will cause interference.
Transmitter Power – Most beacons are between 100 – 500 watts. Generally the higher the power, the greater the range before signal is lost through attenuation.
NDB Frequency – Lower frequency = greater range (generally). Taumarunui is higher frequency to minimize effects of terrain.
Time of Day – rated coverage may be reduced at night due to “sky effect” (discussed later).
Type of Surface – range is greater over water than over land (attenuation).
Efficiency of Receiving Antenna – e.g. if the receiver is experiencing precipitation static, range is reduced.
Describe the basic features of a non directional beacon (NDB), including: typical name, frequency, identification, and location details
Name, frequency and location of each beacon is shown on the enroute charts and in volumes 1/2/3.
- Ident code (morse) and identifier are also listed on the charts in universal order.
- You must correctly identify the selected beacon before using it for navigation
Describe the basic components of an aircraft automatic direction finder (ADF). (5)
ADF radio receiver
- Loop antenna (aerial)
- Sense antenna
- Control panel
- Instrument to display indication to the ADF station. (RBI or RMI)
Explain the basic principles of loop direction finding, including the:
(a) generation of maximum and null signals in different loop positions;
Loops rotated around vertical axis and the voltage decreases as the difference in phase experienced by each side of the loop decreases.
Voltage decreases until its returned to the plane of radio wave propagation and here the same phase is experienced by each side of the loop meaning theres no voltage.
Consider that the loop can be rotated 360°. This results in 2 positions that will produce maximum current and 2 that will produce zero current (null).
Explain the basic principles of loop direction finding, including the: use of polar diagrams
Imagine two circles OO
When the two antennas are in line with the beacon, maximum current in induced in the loop (notice the distance the green line passes through the circles O-O)
When the beacon is at an angle to the loop, the current is reduced (distance the line passes through the circles is reduced O/O).
When the beacon is a right angles to the loop, the line does not pass through either position and no current is produced. This is the null point. (OIO)
What is the null position? And where is it?
The null position is sharp, with a small rotation of aerial resulting in greater current change than it would from the maximum current position.
For this reason the null position is used to indicate position of NDB rather than the maximum signal. However now we have 180° ambiguity.
Bearing to the beacon can be found by rotating the loop until the null position is found. All that is required to obtain an accurate bearing to the beacon is to remove the ambiguity of the 180° null position.
(90-270º)
Explain the basic principles of loop direction finding, including the: what a sense aerial is and the application of a sense aerial to resolve ambiguity
An antenna with the same sensitivity as loop aerial in its maximum position so it generates a continuous maximum current.
Produces a cardioid (heart circle) by adding the steady signal from the sense aerial to the alternating signal from the loop as it rotates, theres only one position as the loop rotates where theres null.
This acts as a phase position reference point and can be used as the alignment with the fore/aft axis of the aircraft.
Explain the basic principles of loop direction finding, including the: automatic seeking of the null position (ADF)
The null produced is not accurate enough to satisfy the ICAO requirement of +/-5°.
In order to meet this requirement, the polarity of the sense antenna is reversed at a rate of approximately 120Hz. This produces both a left and right cardioid with a more precise null, increasing the accuracy of the ADF.
The resultant signal from both aerials is amplified and phase detected and used to drive motor attached to loop.
The loop is always driven towards the null. The loop position is relayed by a synchroniser to ADF needle, which then remains pointing at the beacon at all times.
Describe the basic principle of operation of a fixed loop
fixed loop antenna is two fixed loops, arranged so that one is aligned with the aircrafts longitudinal axis, and the other is facing at 90° to it. The signal from the loops is fed to stator coils of a receiving resolver which establishes a magnetic field across the resolver which is parallel to the direction of the received NDB signal.
The fixed loop then operates in much the same way as the rotatable loop ADF. A sense aerial is used to remove the 180° ambiguity from the output signal of the resolver motor.
Advantages of fixed loop antenna
Simplicity (less moving parts)
More aerodynamic shape.
Less susceptible to icing.
Describe the function of the controls on a typical ADF control panel, including:
ADF
Direction information is given, and the instrument can be used for navigation.
The selected station can also be identified but a large amount of static may be present.
Describe the function of the controls on a typical ADF control panel, including:
(a) ANT (or REC or VOICE) mode;
Selecting this position switches off the loop aerial, allowing a clear strong signal for station identification.
To indicate the loss of direction information the needle will deflect to 90° relative. Ensure you return it back to ADF mode!
Describe the function of the controls on a typical ADF control panel, including: TEST mode
Used to check system operation, placing the ADF in this mode should deflect the needle 90° relative. Newer models don’t have this function.
