Chapter | 7 Radio/Wireless 157 absorbed and attenuated by the Earth, trees, buildings, and other obstructions. Depending on the frequency of operation, transmitter power, and the nature of the terrain, distances are limited to several hundred miles. Distances can be extended with greater power or if the signal travels over water. High Frequencies In the HF or shortwave frequency range, the ground waves are extremely weak and travel only short distances, usually several miles. Instead, propagation is by way of refraction of the ionosphere. The ionosphere is an electrified area above the Earth located from approximately 30 miles to 250 miles above the Earth. The Sun ionizes the gases in the air, thereby giving them electrical char- acteristics. The closer the ionosphere to the Sun, the greater the degree the ion- ization. The ionosphere is actually several different layers or ionization levels as shown in Figure 7.5 . The upper layers have greater ionization than the lower layers. Obviously, the degree of ionization is determined by whether it is day or night. During the daylight hours, the ionosphere is highly charged electri- cally. At night, the ion layers essentially disappear. A radio wave in the 3- to 30-MHz range is transmitted up to the ionosphere where it is refracted or bent by ionization. The degree of bending depends on the