Radio Wave Propagation Paths


This is about the space wave & ground wave again. In the previous chapter, it is said that if the frequency is higher, it has greater penetration power and thus space wave can penetrate through the ionosphere. But now, in a new chapter, it is said that 'range of travel of wave is limited by the extent to which energy is absorbed from it by the ground, the higher the frequency, the greater the absorption of wave'. The two statements seem to be contradicting. Should higher frequency waves have more penetration power or will be more easily absorbed? Or do they have any special meaning?


You might think of the wave emanating from an antenna as radiating in all directions. The components radiating downward into the ground is so quickly absorbed that we may discard them immediately. That leaves us with waves radiating along the surface of the Earth and up into the sky. Those waves traveling along the surface of the Earth are ground waves. Their interaction with the Earth's surface has two effects. Their path tends to be bent to follow the curvature of the Earth, similar to diffraction of light, and they are absorbed as some of their energy is transferred to the Earth's surface. The bending tends to be greater at greater wavelengths, where the wavelength covers enough of the Earth's surface that the curvature becomes noticeable. The absorption tends to increase with increasing frequency, where the excitation of the material on the surface of the Earth is higher. These phenomena combine to make low frequency, long wavelength waves travel better along the ground than do high frequency short wavelength waves.

Those wave components which radiated upward into the sky do not interact with the surface of the Earth. Their fate is determined by the interaction with the atmosphere. The atmosphere, including the ionosphere tends to absorb or reflect low frequency waves more than high frequency, giving the high frequency waves more penetration power.