Which of the following is/are correct ? 1. Ground wave is also known as a direct wave. 2. Skywave is a radio wave propagated upward from earth, whether reflected by the ionosphere or not.
Ground wave: The wave when propagates through the Earth’s atmosphere is known as the ground wave. Ground wave propagation of the wave follows the contour of the earth . Such a wave is called a direct wave . Hence, Statement 1 is correct. A wave is termed a reflected wave. The direct wave and reflected wave together contribute to the signal at the receiver station. The signal is filtered to avoid distortion and amplified for clear output. Skywave: Skywave is a radio wave propagated upward from earth , whether reflected by the ionosphere or not . Hence, Statement 2 is correct. In the frequency range from a few MHz up to 30 to 40 MHz, long-distance communication can be achieved by the ionospheric reflection of radio waves back towards the earth . This mode of propagation is called skywave propagation and is used by short wave broadcast services. The ionosphere is so-called because of the presence of a large number of ions or charged particles. It extends from a height of ~ 65 Km to about 400 km above the earth’s surface. Ionization occurs due to the absorption of the ultraviolet and other high-energy radiation coming from the sun by air molecules. The degree of ionization varies with height. The density of the atmosphere decreases with height. At great heights the solar radiation is intense but there are few molecules to be ionized. Close to the earth, even though the molecular concentration is very high, the radiation intensity is low so that the ionization is again low. However, at some intermediate heights, there occurs a peak of ionization density. The ionospheric layer acts as a reflector for a certain range of frequencies (3 to 30 MHz). Electromagnetic waves of frequencies higher than 30 MHz penetrate the ionosphere and escape.