High-frequency Propagation

Author: Edmund A. Laport

The antenna engineer must have a comprehensive understanding of wave propagation to adapt his antennas to the most favorable conditions of radiation. High-frequency propagation is a very complex field of study, and new facts of basic importance are appearing from time to time. It is a statistical subject. The extent of this treatment must of necessity be limited to the barest essentials for the guidance of the antenna designer.

The ionosphere is composed of layers of free electrons of heights and thicknesses that vary over wide limits. During the hours of darkness when the upper atmosphere is not in sunlight, there is a relatively permanent layer known as the "F layer." During daylight hours, starting as soon as sunlight bombards the upper atmosphere, this layer usually breaks into two strata, known as "F₁" and "F₂", the latter being the higher in altitude. At sunrise there is formed a lower layer of ionization called the "E layer", which is quite stable and constant. At sunset it disappears. There are transient regions of ionization called "E sporadic" (E_s) which may exist during the day and night hours more or less at random.

The virtual heights of the ionosphere layers vary with latitude, with the hours of the day and the seasons, and with sunspot activity. Over the earth on any particular day the variations of height are of the order of 2 to 1. Monthly average heights at any one location usually follow a fairly constant pattern. In older texts typical heights were assigned to these layers, but such rough information can be misleading. For specific information the reader should consult refs. 1 and 2 at the end of this section.

In terms of the earth's radius (6,378.4 kilometers) the various ionosphere layers form a thin shell around the earth. The height of the F layer averages around 5 percent of the earth's radius but in some instances reaches nearly 10 percent. The E-layer height is generally of the order of 1.8 percent.

When the electromagnetic wave enters one of these ionized regions, it will do one of three things, depending on the frequency of the wave. If the frequency is above a certain critical value, the wave will pass through the ionized region and out into space. If the frequency is equal to or less than the critical frequency, the wave will be bent or reflected back toward earth. If the frequency is much less than the critical frequency, much of the energy will be absorbed in the ionized layer. Thus the critical frequency is the maximum usable frequency (MUF) that will be reflected by the ionized layer back to the earth at a given point.

Since the maximum usable frequency is rather critical, stable operation in practice requires the use of a frequency near to maximum usable frequency but somewhat below it. Experience has established the optimum working frequency (OWF) at about 85 percent of maximum usable frequency.