Sky waves

Sky Waves

For frequencies from roughly a few MHz up to about 30-40 MHz, long-distance communication becomes possible because radio waves can be reflected back down to earth by the ionosphere - this is sky-wave propagation, the mechanism used by short-wave broadcast services (see Figure 15.4, which shows rays launched from a ground tower being bent back to earth by the D, E, F1 and F2 layers, with waves reflected off higher layers landing farther away).

The ionosphere gets its name from the large density of ions (charged particles) it contains, and it extends from about 65 km to about 400 km above the earth's surface. This ionisation occurs because air molecules absorb ultraviolet and other high-energy radiation from the sun. Ionisation density is not uniform with height: close to the ground, molecular concentration is high but the ionising radiation has already been absorbed higher up, so ionisation stays low; at very great heights, radiation is intense but molecules are too sparse to ionise much, so ionisation is again low; the maximum ionisation occurs at some intermediate height (as tabulated in Table 15.3). …