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Propagation by Reflection

Author: Edmund A. Laport

High-frequency propagation takes place between the surface of the earth and the one or more shells of ionized atmosphere that exhibit sufficient conductivity to reflect the waves back to earth. One reflection from the ionosphere is called one "hop", and the number of hops made by a wave before arriving at the desired receiving point is the number of successive reflections from the ionosphere. We shall use the word "ionosphere" in its most general sense, sometimes involving three or more distinct layers at different heights, each with its own relative reflectivity as determined by its state of ionization.

Transmission over long distances takes place by successive reflections from the ionosphere to earth and back into upper space. There is some attenuation to a wave traversing the lower atmosphere during the daytime, when solar bombardment induces a condition of ionization of the molecules of the air. This solar ionization causes reduction in field strength. During prolonged darkness, after equilibrium of the atmosphere has been reestablished, the neutral atmosphere acts very much as free space from an absorption standpoint.

There is always some loss of wave energy at reflection from any boundary between mediums which are imperfect conductors, or dielectrics. The amount of loss is a function of the angle of incidence of the wave, the frequency of the wave, and the directions of the electric and magnetic vectors with respect to the plane of the boundary. The ionosphere is therefore an imperfect reflector, and there is a loss of energy at reflection which depends upon the state of ionization where the reflection occurs.

The earth's surface is also an imperfect reflector for downcoming waves.

When the point of earth reflection is on a large body of water, such as the ocean, the reflection loss is small. When reflection occurs at a point where the terrain is very rugged, such as a region of mountains, the loss by poor reflectivity and wave scattering can be very large.

In high-frequency communication engineering, consideration is given to the condition of the ionosphere and the characteristics of the earth at all points along the wave path. The total attenuation over a path is the sum of attenuations due to solar absorption in the atmosphere, all of the losses due to ionosphere reflections, and all of the losses due to earth reflections, together with the inverse-distance attenuation due to normal expansion of the wavefront with distance. It is evident that a space circuit having the fewest losses at reflections and the lowest solar absorption will yield the highest field strength at the receiving point.

In multihop circuits it will be recognized that successive ionosphere reflection points are a considerable distance apart. Therefore it is unlikely that the ionosphere will have the same characteristics at any two such points. If one point happens to be in a daylight region and another in a night region, the conditions for reflection of the same frequency can be vastly different. The optimum working frequency for one reflection point may completely penetrate at another. The maximum frequency that can be used for the circuit is therefore the one that will be low enough to reflect from the point having the lowest maximum usable frequency at that instant along the entire circuit.

Thus a multihop optimum working frequency may be a frequency that gives high losses at some reflection points; but if reflection can occur, the signal will be transmitted to the destination, however much reduced in intensity. Under such a condition, relatively high power may be needed for acceptable communication.

The maximum usable frequency for any given path involving ionospheric reflections varies continuously. However certain days are considered typical within certain seasons, and operating frequencies are based on them. It is naturally impractical to have a continuous change of operating frequency on a circuit that follows exactly the variations of the maximum usable frequency, and so some compromise frequency is used for certain hours during which the maximum-usable-frequency changes are within certain limits. Then at the times when the maximum usable frequency changes greatly, some new compromise frequency is chosen for a certain other number of hours.

Last Update: 2011-03-19