NDBs are non-directional beacons that transmit a simple carrier wave with a modulation to overlay the morse code identifier. They are a ground station consisting of just one aerial for transmission.
The ADF (Automatic Direction Finder) is the equipment in the cockpit which measures the direction from which the NDB carrier wave signal came, and points a needle directly towards the NDB in question on one of the pilot's instruments.
NDBs operate between 190 and 1750 kHz, which spans both the LF (Low Frequency, 30 - 300 kHz) and MF (Medium Frequency, 300 - 3000 kHz) frequency bands.
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