ut in desert High-Frequency Sky Wave Propagation 2 MHz Seawater 2 MHz Land-Sea-Land Fig. 8.6 Ground wave field strength curve for a hypothetical 100 km 200 km 300 km nonhomogeneous path. Distance from Transmitter conditions this might be limited to tens of kilometers unless very high powers and direc- tional antennas are used. A limitation that often occurs in medium-frequency broadcasting is interference between ground wave and sky wave from the same transmitter during dark- ness because of the absence of the absorbing D layer, causing deep and rapid fading in received signals. The only practical solution is limiting power radiated at night, when the D layer is no longer present, and designing the antenna to minimize high angle radiation. At LF and VLF, very-long-distance and global communications can be achieved, but this necessitates the use of high transmitter powers and very large antenna systems. High-Frequency Sky Wave Propagation Propagation in the HF band from 3 to 30 MHz is probably the most variable and least pre- dictable of all transmission modes because it depends on the height and intensity of the ionized layers in the ionosphere. A wealth of data has been collected and incorporated in comprehensive computer programs to enable predictions to be made with a reasonable degree of statistical accuracy, and modern ionospheric sounders can evaluate conditions on a real-time basis. However, unpredictable events, such as solar flares, mean that HF propa- gation always contains some degree of uncertainty (as does most radio wave propagation), and predictions can only be made on a statistical probability basis. The ionosphere, which is the primary influence on HF propagation, extends from approximately 80 to 300 km above the Earth's surface, and it divides into a number of dis- tinct ionized layers. Variations in the height and intensity of the layers occurs on a diurnal and seasonal basis-the result of the rotation and position of the Earth in relation to the Sun-and on the longer-term 11-year sunspot cycle