Radio Wave Propagation Paths
Question:
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?
Answer:
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.