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2-9 Space wave propagation
is the region of earth s atmosphere between the surface and the stratosphere, or about 4 to 7 mi above the surface Thus, most forms of ground wave propagate in the troposphere But because certain propagation phenomena (caused mostly by weather conditions) only occur at higher altitudes, tropospheric propagation should be differentiated from other forms of ground wave The ionosphere is the region of earth s atmosphere that is above the stratosphere The peculiar feature of the ionosphere is that molecules of air gas (O2 and N) can be ionized by stripping away electrons under the influence of solar radiation and certain other sources of energy In the ionosphere, the air density is so low that ions can travel relatively long distances before recombining with oppositely charged ions to form electrically neutral atoms As a result, the ionosphere remains ionized for long periods of the day even after sunset At lower altitudes, however, air density is greater and recombination thus occurs rapidly At those altitudes, solar ionization diminishes to nearly zero immediately after sunset, or never achieves any significant levels even at local noon Ionization and recombination phenomena in the ionosphere add to the noise level experienced at VHF, UHF, and microwave frequencies The properties of the ionosphere are important to microwave technology because of the noise contribution In satellite communications, there are some additional transionospheric effects
Ground-wave propagation
The ground wave, naturally enough, travels along the ground, or at least in close proximity to it There are three basic forms of ground wave: space wave, surface
EM wave propagation phenomena 19 wave, and tropospheric wave The space wave does not actually touch the ground As a result, space wave attenuation with distance in clear weather is about the same as in free space (except above about 10 GHz, where H2O and O2 absorption increases dramatically) Of course, above the VHF region, weather conditions add attenuation not found in outer space The surface wave is subject to the same attenuation factors as the space wave, but in addition it also suffers ground losses These losses are caused by ohmic resistive losses in the conductive earth In other words, the signal heats up the ground Surface wave attenuation is a function of frequency, and it increases rapidly as frequency increases Both of these forms of ground-wave communications are affected by the following factors: wavelength, height of both the receiving and transmitting antennas, distance between the antennas, and the terrain and weather along the transmission path Figure 2-10 is a nomograph that can be used to calculate the line of sight distances in miles from a knowledge of the receiver and transmitter antenna heights Similarly, Figs 2-11A and 2-11B show power attenuation with frequency and distance (Fig 2-11A) and power attenuation in terms of field intensity (Fig 2-11B) Ground-wave communications also suffer another difficulty, especially at VHF, UHF, and microwave frequencies The space wave is like a surface wave, but it is radiated many wavelengths above the surface It is made up of two components (see Fig 2-9 again): the direct and reflected waves If both of these components arrive at the receiving antenna, they will add algebraically to either increase or decrease signal strength There is always a phase shift between the two components because the two signal paths have different lengths (ie, D1 is less than D2 + D3) In addition, there may possibly be a 180 ( radians) phase reversal at the point of reflection (especially if the incident signal is horizontally polarized), as in Fig 2-12 The following general rules apply in these situations: A phase shift of an odd number of half wavelengths causes the components to add, increasing signal strength (constructive interference) A phase shift of an even number of half wavelengths causes the components to subtract (Fig 2-12), thus reducing signal strength (destructive interference) Phase shifts other than half wavelength add or subtract according to relative polarity and amplitude You can characterize the loss of signal over path Dl with a parametric term n that is defined as follows: Sr n= [211] Sf where n is the signal loss coefficient Sr is the signal level at the receiver in the presence of a ground reflection component Sf is the free-space signal strength over path D1 if no reflection took place
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