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Long-distance propagation in the HF region depends upon the ionospheric phenomena called skip (See Chap 2 for a more extensive explanation) In this type of propagation, the signal leaves the transmitting antenna at some angle a, called the angle of radiation, and enters the ionosphere where it is refracted back to earth at a distance D from the transmitting station The signal in the zone between the outer edge of the antenna s ground-wave region and the distant skip point is weak or nonexistent The distance covered by the signal on each skip is a function of the angle of radiation Figure 7-2 shows a plot of the angle of radiation of the antenna, and the distance to the first skip zone The angle referred to along the vertical axis is the angle of radiation away from the antenna relative to the horizon For example, an angle of
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Distance to first reflection (miles) 7-2 Effect of radiation angle on skip communications
10 is elevated 10 above the horizon Shorter distances are found when the angle of radiation is increased At an angle of about 30 , for example, the distance per skip is only a few hundred miles Although you might expect on first blush to see a single line on the graph, there is actually a zone shown (shaded) This phenomenon exists because the ionosphere is found at different altitudes at different times of the day and different seasons of the year Generally, however, in the absence of special event phenomena in the ionosphere, you can expect from 1500 to 2500 mi per bounce in the HF bands for low angles of radiation Note, for example, that for a signal that is only a degree or two above the horizon the skip distance is maximum
Gain in vertical antennas 179 At distances greater than those shown in Fig 7-2, the signal will make multiple hops Given a situation where the skip distance is 2500 mi, covering a distance of 7500 mi requires three hops Unfortunately, there is a signal strength loss on each hop of 3 to 6 dB, so you can expect the distant signal to be attenuated from making multiple hops between the earth s surface and the ionosphere For maximizing distance, therefore, the angle of radiation needs to be minimized So what is the ideal angle of radiation It is standard but actually erroneous wisdom among amateur radio operators (and even commercial operators, it turns out) that the lower the angle of radiation, the better the antenna That statement is only true if long distance is wanted, so it reflects a strong bias toward the DX community The correct answer to the question is: It depends on where you want the signal to go For example, I live in Virginia If I want to communicate with stations in the Carolinas or New England, then it would behoove me to select a higher angle of radiation for radio conditions represented in Fig 7-2 so that the signal will land in those regions But if I wanted to work stations in Europe or Africa or South America, then a low angle of radiation is required Because of the difference between performance of high and low angles of radiation, some stations have two antennas for each band: one each for high and low angles of radiation Figure 7-3 shows a signal from a hypothetical antenna located at point 0, in order to show what angle is meant by angle of radiation The beam from the antenna is elevated above the horizon (represented by the horizontal tangent to horizon line) The angle of radiation a is the angle between the tangent line and the center of the beam This angle is not to be confused with the beamwidth, which is also an angle In the case of beamwidth, we are talking about the thickness of the main lobe of the signal between points where the field strength is 3 dB down from the maximum signal (which occurs at point P); these points are represented by points X and Y in Fig 7-3 Thus, angle b is the beamwidth, and angle a is the angle of radiation
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