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Data reception
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ONCE A SIGNAL HAS LEFT A TRANSMITTER, THE IMPULSES TRAVEL, OR propagate, in a cable, optical fiber, or space. In cables, the signals are ac (usually) or dc (sometimes). In fiberoptic systems, the signals are infrared or visible light. The signals are confined in cables and fibers; the only important variable is the attenuation per kilometer. This depends on the ac frequency, the thickness of wire, or the clarity of optical fiber material. In communication via electromagnetic waves, the propagation is affected by several factors.
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Radio wave propagation
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Here is a summary of the main things that affect EM wave communications.
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Polarization
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The orientation of the E flux is the polarization of an EM wave. If the E flux lines are parallel to the earth s surface, you have horizontal polarization. If the E flux lines are perpendicular to the surface, you have vertical polarization. Polarization can be slanted at any angle between horizontal and vertical. The orientation of the E flux lines sometimes rotates as the wave travels through space. This is circular polarization if the E-field intensity remains constant. If the E-field intensity is more intense in some planes than in others, the polarization is said to be elliptical. Rotating polarization can be either clockwise or counterclockwise, viewed as the wavefronts approach you. This is the sense of polarization.
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The line-of-sight wave
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Electromagnetic waves follow straight lines unless something makes them bend. Line-of-sight propagation can take place even when the receiving antenna can t be seen 499
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500 Data reception from the transmitting antenna. To some extent, radio waves penetrate nonconducting objects such as trees and frame houses. The line-of-sight wave consists of two components: the direct wave and the reflected wave. The direct wave The longest wavelengths are least affected by obstructions. At very low, low and medium frequencies, direct waves can diffract around things. As the frequency rises, especially above about 3 MHz, obstructions have a greater and greater blocking effect. The reflected wave Electromagnetic waves reflect from the earth s surface and from conducting objects like wires and steel beams. The reflected wave always travels farther than the direct wave (Fig. 27-1). The two waves are usually not in phase at the receiving antenna. If they re equally strong but 180 degrees out of phase, a dead spot occurs. This is most common at the highest frequencies.
27-1 The reflected wave travels farther than the direct wave.
At VHF and UHF, an improvement in reception can result from moving the transmitting or receiving antenna just a few inches. In mobile VHF/UHF operation, when the transmitter and/or receiver are moving, dead spots produce holes or picket fencing in the received signal.
The surface wave
At frequencies below about 10 MHz, the earth s surface conducts ac quite well. Because of this, vertically polarized EM waves follow the surface for hundreds or even thousands of miles, with the earth helping to conduct the E flux. The lower the frequency, the lower the ground loss and the farther the waves travel by surface-wave propagation. Horizontally polarized waves do not travel well in this mode, because horizontal E flux is shorted out by the earth. Above about 10 MHz, the earth becomes lossy, and surface-wave propagation is not useful for more than a few miles. Significant surface-wave communications are done mainly at very low, low, and medium frequencies (up to 3 MHz).
Sky-wave EM propagation
The earth s ionosphere, at altitudes from about 35 miles to 250 miles, has a great effect on EM waves at frequencies below about 100 MHz.
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Radio wave propagation 501 The ionosphere consists of three layers, called the D layer, E layer, and F layer. The layers form at different altitudes, with the D layer lowest and the F layer highest (Fig. 27-2). The D layer absorbs radio waves at frequencies below about 7 MHz. The E and F layers return radio waves to the earth by a process called ionospheric refraction.
27-2
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