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EM wave propagation phenomena 33
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2-21A Terrain masking of VHF and higher-frequency signals
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pact points on plane [Bl-B2 ] form radii Rn called Fresnel zones The lengths of the radii are a function of the frequency and the ratio of the distances D1 and D2 (see Fig 2-21B) The general expression is N FGHz D1D2 D1 + D2
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Rn = M where
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[228]
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Rm is the radius of the nth Fresnel zone FGHz is the frequency in GHz D1 is the distance from the source to plane B1-B2 D2 is the distance from destination to plane B1-B2 N is an integer (1, 2, 3, ) M a constant of proportionality equal to 173 if Rm is in meters and D1, D2 are in kilometers and 721 if Rm is in feet and D1, D2 are in statute miles If you first calculate the radius of the critical first Fresnel zone (R1), then you can calculate the nth Fresnel zone from Rn = R1 n [229]
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Example 2-2 Calculate the radius of the first Fresnel zone for a 25-GHz signal at a point that is 12 km from the source and 18 km from the destination
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34 Radio-wave propagation
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a b a
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a b a b
B2 D1 D2
D1 D
A-S S-C
(Rotated view)
2-21B Fresnel zone geometry
Solution: R1 = M N FGHz D1D2 D1 + D2 1 25 12 18 12 + 18
R1 = (173)
EM wave propagation phenomena 35 Rt = (173) (04) (72)
R1 = (173) (288) = 294 m For most terrestrial microwave systems an obstacle clearance of 06R1 is required to prevent diffraction attenuation under most normal conditions However, there are conditions in which the clearance zone should be more than one Fresnel zone Fading mechanisms Fading is defined as a reduction in amplitude caused by reduced received signal power, changes in phase or polarization, wave cancellation, or other related problems that are not caused by a change in the output power level or other parameters associated with either the transmitter or its antenna system You would not ordinarily think that line-of-sight radio relay links would experience fading, but that is not true Fading does, in fact, occur, and it can reach levels of 30 dB in some cases (20 dB is relatively common) In addition, fading phenomena in the VHF-and-up range can last several hours, with some periods being reported of several days in duration (although very rare) There are several mechanisms of fading, and these will be dealt with in this section HF fading caused by ionospheric mechanisms will be covered later Any or all of the mechanisms shown in Fig 2-22 can occur in a given system In all cases, two or more signals arrive at the receiver antenna (Rx) Ray A represents the direct path signal that is, ideally, the only signal to reach the destination But it is also possible that a signal, in an elevated layer or other atmospheric anomaly, will cause refraction or subrefraction of the wave creating a second component, B If this second signal arrives out of phase with A, then fading will occur (signal reinforcement
Higher level layer
A TX RX
D C C
2-22 Multiple paths for signal to take between transmitter and receiver
36 Radio-wave propagation in phase can also occur) It is also possible to see subrefraction fading, as in D The classical multipath situation represented by ray C and its reflected component C' is also a source of fading These mechanisms are frequency-sensitive, so a possible countermeasure is to use frequency diversity Hopping over a 5 percent frequency change will help eliminate fading in many cases In cases where either system constraints, or local spectrum usage prevents a 5 percent delta (change), then try for at least 2 or 3 percent Over ocean areas or other large bodies of water there is a possibility of encountering fair weather surface ducting as a cause of fading These ducts form in the mid latitudes, starting about 2 to 3 km from shore, up to heights of 10 to 20 mi; wind velocities are found in the 10 to 60 km/h range The cause of the problem is a combination of power fading, due to the presence of the duct, and surface reflections (see Fig 2-23) Power fading alone can occur when there is a superrefractive duct elevated above the surface The duct has a tendency to act as a waveguide and focus the signal (Fig 2-24) Although the duct shown is superrefractive, it is also possible to have a subrefractive duct Attenuation in weather Microwave communications above about 10 GHz suffer an increasingly severe attenuation because of water vapor and oxygen in the atmosphere Figure 2-25 shows the standard attenuation in dB/km for microwave frequencies Note that there are several strong peaks in an ever-increasing curve Setting a system frequency in these regions will cause poor communications or will require a combination of more transmit power, better receiver sensitivity, and better antennas on receiving and transmitting locations The curves shown in Fig 2-25 assume certain standardizing conditions Rain and other weather conditions can severely increase the attenuation of signals In addition to attenuation, radar exhibits severe clutter problems when signals backscatter from rain cells
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