There are four major propagation paths: surface wave, space wave, tropospheric, and ionospheric. The ionospheric path is important to medium-wave and HF propagation, but is not important to VHF, UHF, or microwave propagation.

The space wave and surface wave are both ground waves, but behave differently. The surface wave travels in direct contact with the Earth’s surface, and it suffers a severe frequency dependent attenuation due to absorption into the ground.

The space wave is also a ground wave phenomenon, but is radiated from an antenna many wavelengths above the surface. No part of the space wave normally travels in contact with the surface; VHF, UHF, and microwave signals are usually space waves.

There are, however, two components of the space wave in many cases: direct and reflected (Figure 1.2). The ionosphere is the region of the Earth’s atmosphere that is between the stratosphere and outer space.

The peculiar feature of the ionosphere is that molecules of atmospheric gases (O2 and N2) can be ionized by stripping away electrons under the influence of solar radiation and certain other sources of energy (see Figure 1.1).
In the ionosphere the air density is so low that positive ions can travel relatively long distances before recombining with electrons to form electrically neutral atoms. As a result, the ionosphere remains ionized for long periods of the day – even after sunset.

At lower altitudes, however, air density is greater, and recombination thus occurs rapidly. At those altitudes, solar ionization diminishes to nearly zero immediately after sunset or never achieves any significant levels even at local noon.

Ionization and recombination phenomena in the ionosphere add to the noise level experienced at VHF, UHF, and microwave frequencies. The properties of the ionosphere are therefore important at these frequencies because of the noise contribution.

In addition, in satellite communications there are some transionospheric effects.

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