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POLARIZATION - 14182_70
Refraction - 14182_72

Neets Module 10-Introduction to Wave Propagation, Transmission Lines, and Antennas
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2-11 If you know the directions of the E and H components, you can use the "right-hand rule" (see figure 2-6) to determine the direction of wave propagation. This rule states that if the thumb, forefinger, and middle finger of the right hand are extended so they are mutually perpendicular, the middle finger will point in the direction of wave propagation if the thumb points in the direction of the E field and the forefinger points in the direction of the H field. Since both the E and H fields reverse directions simultaneously, propagation of a particular wavefront is always in the same direction (away from the antenna). Figure 2-6.—Right-hand rule for propagation. Q8.   If a transmitting antenna is placed close to the ground, how should the antenna be polarized to give the greatest signal strength? Q9.   In the right-hand rule for propagation, the thumb points in the direction of the E field and the forefinger points in the direction of the H field. In what direction does the middle finger point? ATMOSPHERIC PROPAGATION Within the atmosphere, radio waves can be reflected, refracted, and diffracted like light and heat waves. Reflection Radio waves may be reflected from various substances or objects they meet during travel between the transmitting and receiving sites. The amount of reflection depends on the reflecting material. Smooth metal surfaces of good electrical conductivity are efficient reflectors of radio waves. The surface of the Earth itself is a fairly good reflector. The radio wave is not reflected from a single point on the reflector but rather from an area on its surface. The size of the area required for reflection to take place depends on the wavelength of the radio wave and the angle at which the wave strikes the reflecting substance. When radio waves are reflected from flat surfaces, a phase shift in the alternations of the wave occurs. Figure 2-7 shows two radio waves being reflected from the Earth's surface. Notice that the positive and negative alternations of radio waves (A) and (B) are in phase with each other in their paths toward the Earth's surface. After reflection takes place, however, the waves are approximately 180 degrees out of phase from their initial relationship. The amount of phase shift that occurs is not constant.






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