Definition
Two adjacent bands of the radio spectrum used by certain aviation navigation aids. Low frequency (LF) covers 30 to 300 kilohertz (kHz), and medium frequency (MF) covers 300 to 3,000 kHz. In aviation, signals in these bands are used by non-directional beacons (NDBs) and the automatic direction finder (ADF) equipment that receives them.
Plain English
Two ranges of radio waves, sitting next to each other on the radio dial, that some older aviation navigation beacons broadcast on.
Context Anchor
Seen in instrument flying when studying NDB navigation and the aircraft receiver used to point toward an NDB signal.
Derivation
The names come straight from where these bands sit on the radio spectrum. They were labeled 'low' and 'medium' historically, when these were among the first frequencies put to practical use. Higher bands (VHF, UHF) were developed later and named in relation to them.
Why Pilots Care
LF/MF signals travel long distances and follow the curvature of the earth, giving pilots reliable bearings for navigation when GPS or VHF aids are unavailable.
Analogy
Think of frequency like the pace of a flashing light. A low frequency flashes slowly, a medium frequency flashes faster, and a high frequency flashes much faster.
Grounding Statement
LF/MF describes how fast the radio signal repeats, not how strong or reliable the signal is.
Intuition Check
Low or medium does not mean the signal is weak or only moderately useful. Here, low and medium describe the radio wave’s frequency range.
Example Sentence 1
The NDB the instructor tuned was broadcasting in the LF/MF range, so the ADF needle was sensitive to nearby thunderstorm activity.
Example Sentence 2
Because LF/MF signals propagate well at night, the NDB remained usable for navigation over water after sunset.