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inversion region, it increases) The inversion layer forms a duct that acts like a waveguide In Fig 2-19, the distance Dl is the normal radio horizon distance, and D2 is the distance over which duct communications can occur Ducting allows long-distance communications from lower VHF through microwave frequencies, with 50 MHz being a lower practical limit and 10 GHz being an ill-defined upper limit Airborne operators of radio, radar, and other electronic equipment can sometimes note ducting at even higher microwave frequencies Antenna placement is critical for ducting propagation Both the receiving and transmitting antennas must be either (1) inside the duct physically (as in airborne cases) or (2) able to propagate at an angle such that the signal gets trapped inside the duct The latter is a function of antenna radiation angle Distances up to 2500 mi
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High temp Low temp
Refracted path
Direct path
Hot desert Cold sea
2-18 Superrefraction phenomena
Tropospheric wave
Temper atu inversio re n zone
Spac
e wa
D1 D2
th's
surf
2-19 Ducting phenomenon
or so are possible through ducting Certain paths where frequent ducting occurs have been identified: the Great Lakes to the Atlantic seaboard; Newfoundland to the Canary Islands; across the Gulf of Mexico from Florida to Texas; Newfoundland to the Carolinas; California to Hawaii; and Ascension Island to Brazil Another condition is noted in the polar regions, where colder air from the land mass flows out over warmer seas (Fig 2-20) Called subrefraction, this phenomena bends EM waves away from the earth s surface thereby reducing the radio horizon by about 30 to 40 percent
32 Radio-wave propagation
Low temp Higher temp
Direct path
R e fr a cte d p a
Cold land mass
Warmer sea
2-20 Subrefraction phenomena
All tropospheric propagation that depends upon air-mass temperatures and humidity, shows diurnal (ie, over the course of the day) variation caused by the local rising and setting of the sun Distant signals may vary 20 dB in strength over a 24hour period These tropospheric phenomena explain how TV, FM broadcast, and other VHF signals can propagate great distances, especially along seacoast paths, at some times while being weak or nonexistent at others Diffraction phenomena Electromagnetic waves diffract when they encounter a radio-opaque object The degree of diffraction, and the harm it causes, is frequencyrelated Above 3 GHz, wavelengths are so small (approximately 10 cm) compared to object sizes that large attenuation of the signal occurs In addition, beamwidths (a function of antenna size compared with wavelength) tend to be small enough above 3 GHz that blockage of propagation by obstacles is much greater Earlier in this chapter, large-scale diffraction around structures (such as buildings) was discussed The view presented was from above, so it represented the horizontal plane But there is also a diffraction phenomenon in the vertical plane Terrain, or man-made objects, intervening in the path between UHF microwave stations (Fig 2-21A) cause diffraction, and some signal attenuation There is a minimum clearance required to prevent severe attenuation (up to 20 to 30 dB) from diffraction Calculation of the required clearance comes from Huygens-Fresnel wave theory Consider Fig 2-21B A wave source A, which might be a transmitter antenna, transmits a wavefront to a destination C (receiver antenna) At any point along path A-C, you can look at the wavefront as a partial spherical surface (Bl-B2 ) on which all wave rays have the same phase This plane can be called an isophase plane You can assume that the dn/dh refraction gradient over the height extent of the wavefront is small enough to be considered negligible Using ray tracing we see rays ra incoming to plane [Bl-B2 ], and rays rb outgoing from plane [Bl-B2 ] The signal seen at C is the algebraic sum of all rays rb The signal pattern will have the form of an optical interference pattern with wave cancellation occurring between rb waves that are a half-wavelength apart on [Bl-B2 ] The ray im-
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