ADF & NDB

062 Radio NavigationChapter 3

Relative bearing and QDM

Formula — QDM from relative bearing

QDM = MH + RB

Key fact

The ADF measures the relative bearing (RB) of an NDB from the nose of the aircraft. Maintaining RB = maintaining constant track (not homing). RB is the angle between aircraft heading and where the ADF needle points.

ADF displays

Display	QDM shown?
Fixed RBI	No — mental maths required
RBI with moving compass card	Yes, when current MH is set
RMI	Always — linked to compass

Key fact

Arrow at the tip of the RMI needle indicates QDM. True bearing from a VDF station is always given as QTE (from the station).

NDB characteristics

Key fact

Omnidirectional, vertically polarised, LF/MF 190–1750 kHz, surface wave propagation (sky waves may interfere at night). Range 10–500 nm (most common 200–500 nm). Power 25 W–10 kW. Ident: 2/3 Morse code letters. NON A1A — unmodulated carrier; NON A2A — AM modulated (lower range as power used for modulation).

Formula — NDB range from power

Land range (nm) = 2 × √Power (watts)
Sea range (nm)  = 3 × √Power (watts)
Power new ÷ Power old = (Range new ÷ Range old)²

Key fact

To double range, power must increase × 4. Increase TX power = increase range; increase frequency = decrease range. NDB range determined by power and surface (land/water), not aircraft height.

Loop and sense aerial

Key fact

Different loop orientations give different voltage differentials — maximum when parallel, minimum when perpendicular to the signal. A sense aerial resolves the 180° ambiguity (two positions give the same differential). Loop (figure-8) + sense (circular) = cardioid. Direction of zero signal strength points toward the NDB. Due to 0 V, ident must be performed regularly to confirm the NDB is active.

Key fact

Fixed loop theory: rotating loop replaced by a pair of fixed loops 90° apart with an electromagnetic field. ANT switch (REC/OMNI/SENSE): sense aerial only — needle should point to 90°; when deselected, needle should point to beacon.

BFO selector

Key fact

Used to ident NON A1A (unmodulated) beacons. NON A2A are AM modulated — no BFO required for ident. NDB frequencies are outside audio range — an appropriate sideband must be created (e.g. 300 kHz NDB mixed with 299 kHz BFO → 1 kHz beat frequency). BFO also used for manual tuning of ADF. Even for NON A2A, BFO produces higher-quality signal as loop aerial is removed — useful to check interference.

ADF selection procedure

1. Check aircraft within NDB stated range
2. Increase gain
3. Select frequency
4. Select ANT to test
5. Select BFO as required
6. Check ident
7. Select ADF

NDB errors

Error	Cause / mitigation
Static (precipitation)	Dust/water droplets on aerial — must make physical contact
Static (thunderstorms)	ADF points toward lightning — vicinity sufficient
Mountain / multipath	Reflection/refraction in mountainous areas
Coastal refraction	RBI points toward coast — fly higher or move NDB closer to coast
Night effect	Sky waves from other NDBs — listen on BFO and ident
Mutual interference	Same/similar frequency NDBs — cannot use in overlapping area; stick to published range (day only)
Quadrantal	Signal at 45° to fuselage bent by metal — normally fixed internally
Dip error	In bank, PD distorted — needle dips toward lower wing (~10° light aircraft)
Lack of failure warning	0 V also present when NDB off or in cone of silence above NDB

Key fact

NDB accuracy: ± 5° (day). ADF accuracy: ± 5° generally. NDB locator — low-powered NDB supplementing ILS at outer/middle marker sites.

Formula — Distance from NDB

Distance = (Elapsed Time × GS) ÷ Change in Radial

Exam trap

Homing vs tracking: homing flies toward the station (QDM changes); tracking maintains constant track (RB constant). Do not bank when using VDF. Variation at aircraft for ADF/NDB; variation at station for VOR. EN-route NDBs: long range, LF A1A.