TC 9-64 _________________________________________________________________________
2-48. The ionosphere extends upward from about 31.1 miles (50 km) to a
height of about 250 miles (402 km). It contains four cloud-like layers of
electrically charged ions, which enable radio waves to be propagated to great
distances around the earth. This is the most important region of the
atmosphere for long distance point-to-point communications. This region is
discussed in detail later in this chapter.
2-49. There are two principal ways in which electromagnetic (radio) energy
travels from a transmitting antenna to a receiving antenna. One way is by
ground waves and the other is by sky waves. Ground waves are radio waves
that travel near the surface of the earth (surface and space waves). Sky
waves are radio waves that are reflected back to Earth from the ionosphere
(see figure 2-11).
Figure 2-11. Ground Waves and Sky Waves
2-50. The ground wave is actually composed of two separate component
waves. These are known as the surface wave and the space wave (figure
2-11). The determining factor in whether a ground wave component is
classified as a space wave or a surface wave is simple. A surface wave travels
along the surface of the earth. A space wave travels over the surface.
2-51. Surface wave. The surface wave reaches the receiving site by
traveling along the surface of the ground as shown in figure 2-12. A surface
wave can follow the contours of the earth because of the process of diffraction.
When a surface wave meets an object and the dimensions of the object do not
exceed its wavelength, the wave tends to curve or bend around the object.
The smaller the object, the more pronounced the diffractive action will be.