Radio Wave Propagation
Propagation modes
Ground wave follows the Earth's surface and supports MF and lower HF over limited distances. Skywave relies on refraction by the ionosphere: the D layer absorbs HF by day, the E layer supports short skip, and the F layers (F1 and F2) support long-distance communication, especially around the solar maximum. The critical frequency is the highest frequency returned vertically; the MUF is the highest usable frequency for a given path, and the LUF is limited by absorption and noise. Above the MUF, signals pass through the ionosphere into space.
VHF and above
VHF and UHF propagation is mainly line of sight, with a radio horizon slightly beyond the optical horizon. Tropospheric ducting, temperature inversions, sporadic E and rain scatter can extend range. Fading results from multipath and polarisation changes; obstruction of the Fresnel zone degrades paths even when the antennas are visible.
Traps and rule of thumb
Questions often confuse MUF with LUF, or assume HF skywave is reliable at all times. Remember that the D layer is an absorber, not a reflector, and that higher frequencies generally need a higher MUF.
Worked example: the radio horizon for an antenna at 9 m is about 3.57 × √9 ≈ 10.7 km; for two such antennas, add the two horizons for roughly 21 km.