The electromagnetic field: a brief review 15 in Software

Printer DataMatrix in Software The electromagnetic field: a brief review 15

The electromagnetic field: a brief review 15
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2-7 Radio propagation in the ionosphere is affected by a number of different physical factors: cosmic rays, atomic particles, solar radiation
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16 Radio-wave propagation daylight hours, but the ionization begins to fall off rapidly after sunset, altering the radio propagation characteristics after dark The ionization does not occur at lower altitudes because the air density is such that the positive and negative ions are numerous and close together, so recombination occurs rapidly
EM wave propagation phenomena
Because EM waves are waves, they behave in a wave-like manner Figure 2-8 illustrates some of the wave behavior phenomena associated with light and radio waves: reflection, refraction, and diffraction All three play roles in radio propagation Reflection and refraction are shown in Fig 2-8A Reflection occurs when a wave strikes a denser reflective medium, as when a light wave strikes a glass mirror The incident wave (shown as a single ray) strikes the interface between less dense and more dense mediums at a certain angle of incidence (ai ), and is reflected at exactly the same angle (now called the angle of reflection (ar ) Because these angles are equal, a reflected radio signal can often be traced back to its origin Refraction occurs when the incident wave enters a region of different density, and thereby undergoes both a velocity change and a directional change The amount and direction of the change are determined by the ratio of the densities between the two media If Zone B is much different from Zone A, then bending is great In radio
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EM wave propagation phenomena 17 systems, the two media might be different layers of air with different densities It is possible for both reflection and refraction to occur in the same system Indeed, more than one form of refraction might be present These topics will be covered in greater depth shortly Diffraction is shown in Fig 2-8B In this case, an advancing wavefront encounters an opaque object (eg, a steel building) The shadow zone behind the building is not simply perpendicular to the wave, but takes on a cone shape as waves bend around the object The umbra region (or diffraction zone) between the shadow zone ( cone of silence ) and the direct propagation zone is a region of weak (but not zero) signal strength In practical situations, signal strength in the cone of silence rarely reaches zero A certain amount of reflected signals scattered from other sources will fill in the shadow a little bit The degree of diffraction effect seen in any given case is a function of the wavelength of the signal, the size of the object, and its electromagnetic properties
Propagation paths
There are four major propagation paths: surface wave, space wave, tropospheric, and ionospheric The ionospheric path is important to medium-wave (MW) and HF propagation, but is not important to VHF, UHF, or microwave propagation The space wave and surface wave are both ground waves, but they behave differently enough to warrant separate consideration The surface wave travels in direct contact with the earth s surface and it suffers a severe frequency-dependent attenuation caused by absorption into the ground The space wave is also a ground-wave phenomenon, but it is radiated from an antenna many wavelengths above the surface No part of the space wave normally travels in contact with the surface; VHF, UHF, and microwave signals are usually space waves There are, however, two components of the space wave in many cases: direct and reflected (see Fig 2-9) The tropospheric wave is lumped with the direct space wave in some texts, but it has properties that actually make it different in practical situations The troposphere
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