Low Frequency Radio
Radio Wave Propagation – Low Frequency Radio
Low Frequency (LF) radio in this section is shown as 200–400 kHz, and in aviation this is the band associated with NDB navigation.
The main reason it is useful is propagation: LF energy tends to travel as a ground wave, which means it can bend/diffract and continue following the Earth’s surface rather than only travelling in a straight line like line-of-sight VHF.
In the diagram, the beacon is transmitting energy in many directions, but the important part for navigation is the energy that stays close to the surface and continues over terrain/sea. That is why reception can still be available well beyond what you would expect from simple visual horizon limits. As distance increases, signal strength reduces, but the wave can still propagate over long ranges compared with higher-frequency line-of-sight systems.
Operationally, this is why LF has historically been used for long-distance communication, and in aviation why it underpins Non-Directional Beacons (NDBs).
Key Points
LF band shown: 200–400 kHz.
LF uses long wavelengths compared with VHF systems.
Long wavelength behaviour helps the signal follow Earth’s curvature (ground-wave effect).
The beacon radiates in many directions, but navigation usefulness comes from surface-following propagation.
Reception is possible over long distances, although signal strength decreases with range.
In aviation, LF in this context is used for NDB operations.