While VHF (including both communication radio and VOR navigation) is quasi-optical and limited by altitude, NDB (Non-Directional Beacon) stations operate on lower frequencies (typically 190–415 kHz). These lower frequencies propagate quite differently.

NDB signals can propagate both via ground wave (following the Earth's surface over considerable distances) and via sky wave (bouncing off the ionosphere, especially at night). This means NDB range is significantly greater than VHF range, often exceeding 100 nautical miles even from low altitude.

However, NDB propagation is less predictable than VHF, especially over long distances and at night. Factors such as time of day, seasonal variations, terrain conductivity (whether you are over land, sea, or mixed terrain), and atmospheric disturbances affect NDB signal quality. Additionally, NDB signals are susceptible to interference from electrical storms and other sources.

As a VFR pilot using an ADF receiver, you may rely on NDB stations for navigation and position fixing. You do not transmit on NDB frequencies; you only receive and take bearings. The greater range of NDB compared to VHF is an operational advantage, but you must be aware of the lower reliability over long distances.

The key distinction for this lection: VHF communication range is limited by line-of-sight altitude factors, while NDB navigation range can extend much farther due to the nature of low-frequency propagation. Understanding this difference helps you plan your communication and navigation strategy, especially on long-distance VFR flights.