Radio Wave Propagation
Ground wave and space wave
At MF and below, the ground wave follows the earth's curvature and carries 2182 kHz traffic for hundreds of miles. Above roughly 30 MHz, propagation is mostly line of sight plus slight diffraction: the radio horizon extends beyond the geometric one, roughly d(nm) = 1.23 x square root of h(ft) for one antenna. Two antennas combine their horizons.
Sky wave
The ionosphere has D, E, F1, and F2 layers. The D layer absorbs MF and HF by day and largely vanishes at night; F2 is the main long-haul refracting layer, highest in the afternoon and during high sunspot years. Critical frequency is the highest returned straight up; the MUF is the highest usable on a given path and time; the LUF is set by absorption and noise. Between them lies the usable window. The skip zone is the gap between the end of ground-wave coverage and the first returning sky wave.
Anomalies
Sporadic E clouds reflect unexpectedly high frequencies over short distances, mostly in summer. Tropospheric ducting bends VHF and UHF along temperature inversions, extending range. Auroral and polar paths are noisy and fluttery. Solar flares cause sudden ionospheric disturbances that black out HF for minutes to hours.
Traps
Higher frequency, higher MUF, longer skip; but a frequency above the MUF passes through the layer instead of returning. At night the skip distance on a given frequency increases. VHF and UHF are not strictly line of sight: ducting and scatter matter.
Rule of thumb: for an antenna 100 ft high the radio horizon is about 12 nautical miles per antenna.