Axum

A Non-Directional Beacon (NDB) is one of the oldest and simplest radio navaids. It transmits a steady signal in the low/medium-frequency band (about 190–535 kHz) that carries no directional information itself — the work is done in the aircraft. An Automatic Direction Finder (ADF) receiver senses the bearing to the station and swings a needle that points straight at it, so the pilot flies "to the needle" to track toward the beacon.

Because LF/MF signals follow the curvature of the earth, an NDB can be received far beyond line of sight, which made it valuable over water and remote terrain. The trade-off is accuracy: the signal bends near coastlines (shoreline effect), scatters after dark (night effect), and the needle is drawn toward thunderstorms, so readings must be treated with caution. NDBs support NDB instrument approaches, holding and en-route tracking, but they are being steadily decommissioned as GPS takes over.