.net barcode reader library Radio-wave propagation in Software

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50 Radio-wave propagation
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Ionosphere
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Transmitter 2-34 Effects of radiation angle on distance
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Receiver
Nets on 21290 MHz (15 m) daily from about 1700Z to 2100Z (ending time dependent upon traffic) Multihop skip is responsible for the reception of the signal from transmitter T at site R3 The signal reflects (not refracts) from the surface at R1, and is retransmitted into the ionosphere, where it is again refracted back to earth Figure 2-35 shows a situation where skip signals are received at different distances depending upon the angle of radiation of the transmitting antenna A high angle of radiation causes a shorter skip zone, but a lower angle of radiation results in a longer skip zone Communication between any particular locations on any given frequency requires adjustment of the antenna radiation angle Some international shortwave stations have multiple antennas with different radiation angles to ensure that the correct skip distances are available Great circle paths A qreat circle is a line between two points on the surface of a sphere, such that it lays on a plane through the earth s center and includes the two points When translated to radiospeak, a great circle is the shortest path on the surface of the earth between two points Navigators and radio operators use the great circle for similar, but different, reasons The navigator s reason is in order to get from here to there, and the radio operator s is to get a transmission path from here to there The heading of a directional antenna is normally aimed at the receiving station along its great circle path Unfortunately, many people do not understand the concept well enough, for they typically aim the antenna in the wrong direction For example, I live near Washington, DC, which is on approximately the same latitude as Lisbon, Portugal If I catch a lift on Superman s back, and he flies due East, we ll have dinner in Lisbon, right Wrong If you head due east from Washington, DC, across the Atlantic, the first landfall would be west Africa, somewhere near Zaire or Angola Why Because the great circle bearing 90 takes us far south The geometry of spheres, not flat planes, governs the case
EM wave propagation phenomena 51
Ionosphere
A1 > A2, so D1 < D2
Earth
D1 D2
2-35 Skip propagation
Figure 2-36 shows a great circle map centered on the Washington, DC area These maps, or computer tabulations of the same data, can often be purchased for your own location by supplying your latitude and longitude to the service company that does the job By drawing a line from your location at the center of the chart to the area you want to hear, and then extending it to the edge of the chart, you will obtain the beam heading required Long path versus short path The earth is a sphere (or more precisely, an oblate spheroid), so from any given point to any other point there are two great circle paths: the long path (major arc) and the short path (minor arc) In general, the best reception occurs along the short path In addition, short-path propagation is more nearly textbook, compared with long path reception However, there are times when long path is better, or is the only path that will deliver a signal to a specific location from the geographic location in question Gray line propagation The gray line is the twilight zone between the nighttime and daytime halves of the earth This zone is also called the planetary terminator (Fig 2-37) It varies up to +23 either side of the north-south longitudinal lines, depending on the season of the year (it runs directly north-south only at the vernal and autumnal equinoxes) The D layer of the ionosphere absorbs signals in the HF region This layer disappears almost completely at night, but it builds up during the day Along the gray line, the D layer is rapidly decaying west of the line, and has not quite built up east of the line Brief periods of abnormal propagation occur along the gray line Stations on either side of the line can be heard from regions, and at distances, that would otherwise be impossible on any given frequency For this reason, radio operators often prefer to listen at dawn and dusk for this effect
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