To understand sky wave propagation, you need to consider the effects of the ionosphere and solar activity on HF radio propagation. You must also be familiar with the techniques used to predict propagation and select the best frequencies for a partic-ular link at a given time. Let’s start with some definitions.

The Ionosphere, Nature’s Satellite

The ionosphere is a region of electrically charged particles or gases in the earth’s atmosphere, extending from approximately 50 to 600 km (30 to 375 miles) above the earth’s surface. Ionization, the process in which electrons are stripped from atoms and produces 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? Well, this blanket of gases is like nature’s satellite, actually making most BLOS radio communications possible. When radio waves strike these ionized layers, depending on frequency, some are completely absorbed, others are refracted so that they return to the earth, and still others pass through the ionosphere into outer space. Absorption tends to be greater at lower frequencies, and increases as the degree of ionization increases. The angle at which sky waves enter the ionosphere is known
as the incident angle (Figure 2-1). This is determined by wave-length
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 it hits it. Thus, the incident angle is an important factor in determining communications range. If you need to reach a station that is relatively far from you, you would want the incident angle to be relatively large. To communicate with a nearby station, the incident angle should be relatively small. The incident angle of a radio wave is critical, because if it is too
nearly vertical, it will 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. So, incident angle must be sufficient for bringing the radio wave back to earth yet not so great that it will lead to absorption.

Layers of the Ionosphere

Within the ionosphere, there are four layers of varying ioniza-tion (Figure 2-2). Since ionization is caused by solar radiation, the higher layers of the ionosphere tend to be more dense, while the lower layers, protected by the outer layers, experience less ionization. Of these layers, the first, discovered in the early 1920s by Appleton, was designated E for electric waves. Later, D and F were discovered and noted by these letters. Additional iono-spheric phenomena were discovered through the 1930s and
1940s, such as sporadic E and aurora. A, B, and C are still avail-able for further discoveries. In the ionosphere, 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 occa-sionally 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 there-fore 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.

Factors Affecting Atmospheric Ionization

The intensity of solar radiation and therefore ionization varies periodically. Hence, we can predict solar radiation intensity
based on time of day and the season, for example, and make adjustments in equipment to limit or optimize ionization effects. Ionization is higher during spring and summer because the hours of daylight are longer. Sky waves are absorbed or weakened as they pass through the highly charged D and E layers, reducing, in effect, the communication range of most HF bands. Because there are fewer hours of daylight during autumn and
winter, less radiation reaches the D and E layers. Lower frequen-cies
pass easily through these weakly ionized layers. Therefore, signals arriving at the F layer are stronger and are reflected over greater distances. Another longer term periodic variation results from the 11-year
sunspot cycle (Figure 2-3). Sunspots generate bursts of radiation that cause higher levels of ionization. The more sunspots, the greater the ionization. During periods of low sunspot activity, frequencies above 20 MHz tend to be unusable because the E and F layers are too weakly ionized to reflect signals back to earth. At the peak of the sunspot cycle, however, it is not unusual to have worldwide propagation on frequencies above 30 MHz. In addition to these regular variations, there is a class of unpre-dictable phenomena known as sudden ionospheric disturbances
(SID), which can affect HF communications as well. SIDs, random events due to solar flares, can disrupt sky wave communication for hours or days at a time. Solar flares produce intense ionization of the D layer, causing it to absorb most HF signals on the side of the earth facing the sun. Magnetic storms often follow the eruption of solar flares within 20 to 40 hours. Charged particles from the storms have a scat-tering effect on the F layer, temporarily neutralizing its reflective properties.

Frequency and Path Optimization

Because ionospheric conditions affect radio wave propagation, communicators must determine the best way to optimize radio frequencies at a particular time. The highest possible frequency that can be used to transmit over a particular path under given ionospheric conditions is called the Maximum Usable Frequency (MUF). Frequencies higher than the MUF penetrate the ionosphere and continue into space. Frequencies lower than the MUF tend to
refract back to earth. 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, therefore, lies between the MUF and LUF. TheFrequency of Optimum Transmission (FOT) is nominally 85 percent of the MUF. Generally, the FOT is lower at night and higher during the day. These frequencies are illustrated in Figure 2-4. In addition to frequency, the route the radio signal travels must also be considered in optimizing communications. A received signal may be comprised of components arriving via several
routes, including one or more sky wave paths and a ground wave path. The arrival times of these components differ because of differences in path length; the range of time differences is the multipath spread. The effects of multipath spread can be mini-mized by selecting a frequency as close as possible to the MUF.

Propagation Prediction Techniques

Since many of the variables affecting propagation follow repet-itive cycles and can be predicted, techniques for effectively determining FOT have been developed. A number of propagation prediction computer programs are
available. One widely used and effective program is Ionospheric Communications Analysis and Prediction (IONCAP), which predicts system performance at given times of day as a function of frequency for a given HF path and a specified complement of equipment. Of course, since computerized prediction methods are based on historic data, they cannot account for present conditions affecting communications, like ionospheric changes caused by random phenomena such as interference and noise (more about these later). A more immediate automated prediction method involves ionospheric sounding. One system, the Chirpsounder TM, uses remote stations to transmit test signals (chirps) that sweep through all frequencies from 2 to 30 MHz. The receiver tracks the signal, analyzes its reception on assigned operating frequen-cies, and displays frequency ranges for optimum propagation. In addition, modern HF communications systems are increas-ingly making use of Link Quality Analysis (LQA) techniques. In these systems, transmitting and receiving stations cooperate to assess automatically the quality of the channels available to them. When the need to communicate arises, the LQA data is used to select the best frequency. We’ll take a closer look at this technique in Chapter 6.

SUMMARY

• The ionosphere is a region of electrically charged particles or gases in the earth’s atmosphere, extending from 50 to 600 km (approximately 30 to 375 miles) above the earth’s surface.

• There are layers of varying electron density in the ionosphere that absorb, pass, or reflect radio waves, depending on the density of the layer, the angle with which the radio waves strike it, and the frequency of the signal.

• Ionization, caused by solar radiation, strips electrons from atoms, producing electrically charged particles.

• The density of ionospheric layers varies with the intensity of solar radiation, which changes according to time of day, season, and sunspot cycle.

• The angle of radiation is determined by the wavelength of a signal and the type of antenna used.

• Radio waves are absorbed as they pass through the ionosphere. The absorption rate increases as frequency decreases.

• Communications is best at the frequency of optimum trans-mission (FOT), nominally 85 percent of the maximum usable frequency (MUF).

• Sunspots increase and decrease in 11-year cycles. Higher sunspot numbers increase ionization, lower sunspot numbers cause less ionization.

• Solar flares cause sudden ionospheric disturbances (SIDs), which can disrupt HF communications.

• Propagation prediction techniques, such as IONCAP, deter-mine the MUF, LUF, and FOT for a given transmission path and time of day. Other methods include ionospheric sounding and Link Quality Analysis (LQA).