Why is the ionosphere important?
The ionosphere is a region of electrically charged particles or gases in the earth’s atmosphere, extending from approximately 30 to 375 miles above the earth’s surface.
Ionization, the process in which electrons are stripped from atoms and produce electrically charged particles, results from solar radiation. When the ionosphere becomes heavily ionized, the gases may even glow and be visible. This phenomenon is known as Northern and Southern Lights. Why is the ionosphere important in HF radio? This blanket of gases is like nature’s satellite and makes most Beyond Line Of Sight (BLOS) radio communications possible. When radio waves strike these ionized layers, some are completely absorbed. Depending on frequency, others are refracted so that they return to the earth while still others pass through the ionosphere into outer space. Absorption tends to be greater at lower frequencies and increases proportionally with the degree of ionization.
The angle at which sky waves enter the ionosphere is known as the incident angle. This is determined by wavelength and the type of transmitting antenna. Like a billiard ball bouncing off a rail, a radio wave reflects from the ionosphere at the same angle at which it hits. Incident angle is an important factor in determining communications range; to reach a station that is relatively far away, the incident angle has to be relatively large. In order to communicate with a nearby station the incident angle should be relatively small. If the incident angle is nearly vertical, it may pass through the ionosphere without being refracted back to earth. If the angle is too great, the waves will be absorbed by the lower layers before reaching the more densely ionized upper layers. Therefore, the incident angle must be sufficient to bring the radio wave back to earth, yet not so great that it will lead to absorption.
Within the ionosphere, there are four layers of varying ionization. Since ionization is caused by solar radiation, the higher layers of the ionosphere tend to have a greater density, while the lower layers experience less ionization. Of these layers, the first, discovered in the early 1920s, was designated E for electric waves. Layers D and F were discovered later.
Additional theories related to the ionosphere were explored through the 1930s and 1940s, such as sporadic E and aurora. The D layer is the lowest region affecting HF radio waves. Ionized only during the day, the D layer reaches maximum ionization when the sun is at its zenith and dissipates quickly toward sunset.
The E layer reaches maximum ionization at noon. It begins dissipating toward sunset and reaches minimum activity at midnight. Irregular cloud-like formations of ionized gases occasionally occur in the E layer. These regions, known as sporadic E, can support propagation of sky waves at the upper end of the HF band and beyond.
The most heavily ionized region of the ionosphere, and therefore the most important for long-haul communications, is the F layer. At this altitude, the air is thin enough that the ions and electrons recombine very slowly, so the layer retains its ionized properties even after sunset. In the daytime, the F layer consists of two distinct layers, F1 and F2. The F1 layer, which exists only in the daytime and is negligible in winter, is not important to HF communications. The F2 layer reaches maximum ionization at noon and remains charged at night, gradually decreasing to a minimum just before sunrise.
During the day, sky wave reflection from the F2 layer requires wavelengths short enough to penetrate the ionized D and E layers, but not so short as to pass through the F layer. Generally, frequencies from 10 to 20 MHz will accomplish this, but the same frequencies used at night would penetrate the F layer and pass into outer space. The most effective frequencies for long-haul nighttime communications are normally between 3 and 8 MHz.
As frequency is reduced, the amount of absorption of the signal by the D layer increases. Eventually, the signal is completely absorbed by the ionosphere. The frequency at which this occurs is called the Lowest Usable Frequency (LUF). The “window” of usable frequencies lies between Maximum Usable Frequency (MUF) and LUF.
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