Definition
A radio frequency band designation covering low frequency (LF, 30–300 kHz) and medium frequency (MF, 300–3,000 kHz). In aviation, LF/MF is used by older long-range navigation aids and beacons, most commonly non-directional beacons (NDBs), as well as some long-range communication systems.
Plain English
A range of relatively low radio frequencies used by certain navigation beacons and long-range radios. It sits well below the VHF band that most aircraft radios use for everyday communication.
Context Anchor
Seen in FAA acronym lists, radio navigation discussions, equipment descriptions, and some chart or procedure references involving older ground-based navigation signals.
Derivation
“Low frequency” and “medium frequency” are international labels assigned by the radio spectrum allocation system. Each band is defined by the wavelength range of its signals, with lower frequencies producing longer waves that travel farther and bend around the Earth's curve.
Why Pilots Care
LF/MF signals travel long distances and follow the Earth's surface, which is why they are still used for some en route beacons and oceanic communication. However, they are more vulnerable to atmospheric noise, thunderstorms, and night-time interference than VHF, so pilots using them need to be aware of reliability limits.
Grounding Statement
If an aircraft receiver is using an LM/MS beacon, it is listening to a lower-frequency radio signal sent from the ground.
Intuition Check
Low/medium does not mean the signal is weak or only moderately useful. It means the signal is in specific lower radio-frequency ranges.
Example Sentence 1
The chart legend showed the beacon transmitting in the LF/MF band, identifying it as an NDB.
Example Sentence 2
Many remote airports still publish approaches that rely on LM/MS transmissions for guidance.