RNAV: ADF / NDB

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Last updated 10:09 AM on 9/18/26
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8 Terms

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What frequency does NDB / ADF use & what range

  • 190 to 1750khz LF / MF

  • 300-500nm


Locator 10-25nm ←2 or 3 letter ident.

Enroute - 25-50nm

Long range - 200-500nm


For range to double, power needs to x 4


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How does NDB Work

  • NDB outputs a omni direction surface wave, (not line of sight)

  • The receiver is a loop antenna, when the wave crosses both sides of the loop at the same phase (so the antenna is parallel to the wave)
    you know the NDB is in front or behind,

  • a second vertical aerial can detect whether its front or behind.


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NDB Errors:

Night Effect:

  • NDB uses surface waves, at night when the ionosphere is less dense, the lower levels disappear (D/E)

  • now the LF/MF wave can get to the F layer before being absorbed, so it can now return

  • The range is extended due to the return, but there is multi path interference.

  • The return wave is of different polarisation so can’t be used


Thunderstorms:

  • The needle will point to storms due to static


Coastal Effect:

  • wave speed up over sea as less absorption.

  • The wave refracts towards the coast

  • The minimise, fly higher, use the NDB closer to the coast, use signals right angle to coast

Mountain Effect: Diffraction



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NDB Homing vs Tracking

Homing - you follow TO only, doesn’t take into account wind, curved arc to NDB.


Tracking - account for wind to fly direct to NDB, to or from.

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NDB Question Example:
Compass heading: 330
ADF: 240
Dev +3
VAR 6E

What is True From NDB to Aircraft?

You are on compass heading, 330, and the RB on the ADF is showing 240.

Convert Compass heading to true heading
330 + 3 + 6 =339

Add the RB to get True TO
339 + 240 =579 - 360 = 219
we want FROM so 219 - 180 = 39°.

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NDB Question Example:
If QDR is 075, Magnetic HDG is 295, what is RB?

So the QDR (magnetic from NDB to aircraft) is 075 , + 180 = 255 QDM

QDM = MHDG + RB =
255 = 295 + ?
? = -40

To get RB 360 - 40 =320 °

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VHF Direction Finding?

Is VHF Radio direction uses VHF radio only

2 stations required for position


VDF Accuracy:

A = +-2°

B = +-5°

C = +-10°

D = more than 10°

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