Definition
Medium Frequency (MF) is the radio frequency band from 300 kHz to 3,000 kHz (3 MHz). In aviation, MF is the band used by Non-Directional Beacons (NDBs) and by Automatic Direction Finder (ADF) receivers, which typically operate in the lower portion of this band (roughly 190–535 kHz, including the adjacent low frequency range).
Plain English
MF is a range of radio frequencies that older navigation aids use. Ground beacons transmit on these frequencies, and a cockpit receiver points a needle toward the beacon so the pilot knows which direction it is.
Context Anchor
Seen in instrument procedure references, avionics discussions, and descriptions of radio navigation equipment.
Derivation
Called 'medium' because it sits between low frequency (LF) and high frequency (HF) on the radio spectrum. The labels low, medium, and high were assigned historically as the radio spectrum was divided into bands.
Why Pilots Care
MF supports reception of Non-Directional Beacon signals for en route and approach navigation.
Intuition Check
MF does not mean a generally average radio setting. It means the specific 300 to 3,000 kilohertz radio-frequency band.
Example Sentence 1
The NDB used for that approach transmits in the medium frequency band, so the ADF receiver must be tuned to its assigned MF frequency.
Example Sentence 2
MF signals remain usable at greater ranges than higher-frequency bands in many conditions.