Definition
A grouping of two adjacent radio frequency bands used by certain ground-based aviation navigation aids. Low Frequency (LF) covers 30 to 300 kHz, and Medium Frequency (MF) covers 300 to 3,000 kHz. In aviation, the LF/MF range is most commonly associated with Non-Directional Beacons (NDBs) and the receivers (Automatic Direction Finders, or ADFs) that work with them.
Plain English
LF/MF is a low end of the radio spectrum where some older navigation beacons transmit. When a chart or handbook says a navigation aid is 'LF/MF,' it means the signal is on a long-wavelength frequency that an ADF receiver in the cockpit can pick up.
Context Anchor
Seen in instrument flying discussions of navigation aids, especially when learning about nondirectional beacons and ADF equipment.
Derivation
Low Frequency' and 'Medium Frequency' are formal labels assigned by international radio authorities to specific slices of the radio spectrum. 'Frequency' here means how many wave cycles pass a point each second, measured in kilohertz (kHz). Lower frequencies have longer wavelengths, which is why LF/MF signals can travel long distances and follow the curve of the earth — a useful property for early navigation beacons.
Why Pilots Care
Knowing a NAVAID is in the LF/MF range tells you what receiver you need (an ADF, not a VOR receiver) and what to expect from the signal. LF/MF signals are more vulnerable to interference from thunderstorms, terrain, and shoreline effects than higher-frequency NAVAIDs, so pilots have to interpret the indications with that in mind.
Grounding Statement
An LF/MF station is a ground transmitter sending a low-range radio signal that an aircraft receiver can pick up and use for direction information.
Intuition Check
LF/MF is not the name of one specific navigation aid. It names the radio frequency ranges used by certain navigation aids.
Example Sentence 1
The NDB at the field transmits in the LF/MF band, so the pilot tuned the ADF rather than the VOR receiver.
Example Sentence 2
LF/MF signals remain usable at low altitudes where higher-frequency aids may be blocked.