Рабочий лист для специальности радиотехник

Text 1.2: IONOSPHERE AND HF RADIO PROPAGATION

Grammar: PARTICIPLE II

Task 11. Look through the table and study the functions of Participle II.

Task 12. Find Participle II in the English sentences. Pay attention how it is expressed in the Russian equivalent.

Task 13. Skim the following text and try to understand the subject-matter of the text.

TEXT 1.2:

IONOSPHERE AND HF RADIO PROPAGATION

1. 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 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.

2. 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.

3. The angle at which sky waves enter the ionosphere is known as the incident angle. (Figure 3).

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 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.

4. 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, the incident angle must be sufficient for bringing the radio wave back to the earth yet not so great that it will lead to absorption.

5. Within the ionosphere, there are four layers of varying ionization. (Figure 4).

Since the ionization is caused by the 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 ionospheric phenomena were discovered through the 1930s and 1940s, such as sporadic E and aurora. A, B, and C are still available for further discoveries.

6. 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.

7. 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.

8. 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.

9. 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.

10. 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.

Task 14. Match up the words with the definitions.

Task 15. Comprehension check. Answer the following questions:

1. What is the ionosphere?

2. What happens when radio waves strike?

3. What does figure 3 illustrate?

4. What is the ionization caused by within the ionosphere?

5. Can you name the layers of the ionosphere that you know?

6. What is the lowest region affecting HF radio waves in the ionosphere?

7. Do the irregular cloud-like formations of ionized gases occasionally occur in the D layer or in the E layer?

8. Is the F layer the most heavily ionized region of the ionosphere?

9. What layer is the most important for long haul communications?

10. What are the most effective frequencies for long-haul nighttime communications?