NDB and ADF
Core concepts
An NDB transmits in the LF/MF band, typically 190–1750 kHz, and the ADF receiver derives a relative bearing to the station using a directional antenna (loop or sense antenna combination). The magnetic bearing to the station equals the aircraft magnetic heading plus the relative bearing. Tracking to or from an NDB requires applying wind drift; the fixed-card ADF simply shows relative bearing, while a radio magnetic indicator (RMI) shows magnetic bearing directly. NDB signals follow the terrain well, giving useful range at low altitude, but they are vulnerable to errors.
Traps in questions
Expect questions on: night effect (sky-wave interference causing bearing errors, worst at dusk and dawn); coastal refraction (the wavefront bends as it crosses a coastline at an oblique angle, so the bearing bends toward the normal); mountain effect (terrain re-radiation); thunderstorm static; and station interference on the same frequency. Another trap is the difference between relative bearing and magnetic bearing and the sign convention when heading exceeds 360°. Also remember that an ADF gives no distance information on its own.
Worked example
Heading 310°M, relative bearing 060°. Magnetic bearing to station = 310 + 60 = 370, so 010°M. To fly directly to the station, turn right to 010°M, then correct for drift once established.
Rule of thumb
MB = MH + RB, subtracting 360 if the result exceeds 360. Treat any ADF bearing at night or near a coastline as suspect and cross-check with another aid.