1. Non-Directional Beacons (NDB) & Automatic Direction Finding (ADF)
General Principles

2. NDB Radiation Pattern
Omni-directional polar diagram

3. Types of Modulation NØNA1A = Unmodulated identification
NØNA2A = Modulated identification

4. NØNA1A – Carrier Wave only

5. NØNA2A – Modulated Carrier Wave

6. Max. Range = 3 Power (Watts)
Range of an NDB
Max. Range = 3 Power (Watts)
e.g. A 100W transmitter will have an approximate range of 30NM

7. An NDB ground station

8. An NDB ground station

9. An NDB ground station

10. The “cone of silence” above the NDB station in which
the signal strength may
be too low to be used.
Radio Navigation – Chapter 3

11. ADF Principle of Operation - Antenna System
2 aerials are required:
Loop antenna (to detect null directions)
Sense antenna (to resolve ambiguity of null directions)
Fixed Loop system

13. Loop antenna polar diagram
Null
Sense antenna
(Omni-directional)
Null

14. The resultant cardioid polar diagram of the ADF aerial.
Radio Navigation – Chapter 3

15. Typical ADF control panel.
Frequency
range
Typical ADF control panel.
Radio Navigation – Chapter 3

16. 190 - 1750 kHz (ICAO) Aeronautical Range MF 315 - 405 kHz
NDB frequency range
kHz (ICAO)
Aeronautical Range
MF kHz
LF kHz
Marine kHz
Broadcast
LF kHz
MF kHz

17. A rotatable card type of indicator is exactly like a fixed card indicator, except that the card can be rotated to reflect the aircraft’s heading.
Radio Navigation – Chapter 3

18. The most common type of presentation is the RMI - Radio Magnetic Indicator
Radio Navigation – Chapter 3

19. Homing will point the nose of the aircraft directly to the station as long as the ADF needle is kept pointing to the top of the indicator.
Radio Navigation – Chapter 3

20. Relative Bearing Indicator
HDG
= 075° RB
= 345°
(= –15°)
QDM = HDG + RB = = 420°
420 – 360 = 060°;
or 075 – 015 = 060°

21. Relative Bearing Indicator
Required QDM = 035°
HDG = 075°
Required RB to intercept radial
= QDM – HDG =
= – 040; or
= 320°

22. What relative bearing do we require to intercept radial 085?
Intercept of an outbound track 085° from a NDB.
HDG
125ºM RB
100º
QDM = 225°M
QDR = 045°M
RB = ( ) = 140°
What relative bearing do we require to intercept radial 085?
Radio Navigation – Chapter 3

24. NDB & ADF Limitations & Accuracy
Radio wave propagation
LF/MF
Ground wave - Day/Night
Sky wave - Night
Direct wave - line of sight only

25. This gap is called the skip zone.
The ground wave coverage might extend out to 300 nm, while the first skywave returns at 1000 nm.
This gap is called the skip zone.
Radio Navigation – Chapter 3

26. Quadrantal Error No error
The quadrantal error is maximum on relative bearings 045°, 135°, 225° and 315°
No error
Radio Navigation – Chapter 3

27. Coastal Refraction
The effect of coastal refraction is least when bearings are 90° to the coastline
Radio Navigation – Chapter 3

28. Coastal Refraction
Note: The further inland the NDB’s are, the worse is the error
NDB1
NDB2
Apparent position of a/c

29. Other ADF errors Dip (Bank) Error Mountain effect - multipath signals
Noise:
Static Interference (worse at LF)
Electric storms
(increasing in summer & towards the equator)

30. Electric Storms (Thunderstorms)
A lightning discharge near the aircraft may give erroneous readings
Can result in a complete reversal of the ADF needle
Effect will fade with increasing distance from cloud

31. Other ADF errors Noise: S/N ratio 3:1
Man-made, e.g. from power lines, also:
electronic equipment, especially cellphones, laptop computers, electronic games, etc.
Station Interference due to sky waves at night time

32. Skywave problems - Night Effect

33. Skywave problems - Night Effect
Symptoms - hunting of the indicator needle and signals fading or fluctuating in strength
Condition is most noticeable around dawn or dusk (the “Twilight Zone” or Transition Period)

34. (ICAO) ± 5° — Daytime only
Accuracy of an NDB
(ICAO) ± 5° — Daytime only

35. Questions What is the basic information given by ADF?
The relative bearing from the aircraft to the NDB
On a manually rotateable card type indicator, what should be indicated at the top?
The aircraft heading

36. More Questions
Whilst correctly tuned to an NDB transmitting a N0NA2A signal, with the BFO off, what should you hear?
The station identification only
What is the ICAO frequency allocation for ADF receivers?
190 to 1750 kHz

37. More Questions
An aircraft is heading 330°M, the relative bearing to an NDB is 190°, what is the QDM?
= 520 (-360) = 160°M
What is the usual effect of coastal refraction?
It puts the apparent position of the aircraft closer to the coast than the real position
When will bearings be most affected by coastal refraction?
When the NDB is further inland

38. More Questions What aerials are used with ADF? Loop and sense aerials
At what time of day are bearings likely to be least accurate?
Around dawn or dusk
What symptoms does night effect produce?
Hunting of the needle and fading of the signal