Definition
A range of radio frequencies used for aviation navigation and communication, where low frequency (LF) covers 30 to 300 kHz and medium frequency (MF) covers 300 to 3,000 kHz. In aviation, L/MF most commonly refers to the band used by Non-Directional Beacons (NDBs) and the Automatic Direction Finder (ADF) receiver in the aircraft.
Plain English
A specific slice of the radio spectrum that older navigation beacons broadcast on. The aircraft's ADF receiver tunes into these signals and points toward the beacon.
Context Anchor
Seen in instrument flying discussions of radio navigation aids, especially older beacon-based navigation systems.
Derivation
Low' and 'medium' here describe where these signals sit on the radio spectrum compared to other bands like VHF (very high frequency) used for VOR navigation and voice radio. Lower frequencies travel farther and follow the curve of the Earth, which is why they were chosen for long-range beacons before higher-frequency systems became common.
Why Pilots Care
L/MF signals behave differently than the VHF signals most pilots use day to day. They can be affected by terrain, thunderstorms, shoreline effects, and night-time atmospheric changes, so bearings from L/MF beacons require careful interpretation.
Intuition Check
Low or medium frequency does not mean a weak or average-quality signal. It names the part of the radio-frequency range where the signal is transmitted.
Example Sentence 1
The NDB on the approach plate transmits in the L/MF band, so the pilot tuned the ADF receiver to its published frequency.
Example Sentence 2
Older charts still show L/MF facilities that remain useful as backups when GPS is unavailable.