Practical_Antenna_Handbook_0071639586
C h a p t e r 5 : a n t e n n a A r r a y s a n d A r r a y G a i n 167 Figure 5.8 Image antenna for ground-mounted vertical monopole. Lossy Ground The discussion of ground reflection effects thus far assumes perfectly conducting ground; for average or poor grounds, the effect is to partially fill in some of the nulls in the elevation pattern (so they seldom get deeper than perhaps 15 dB down) and to reduce the maximum amplitude of the array factor in its favored directions—especially as the frequency increases. Lobes above the lowest angle lobe are reduced even further. Note Especially at the lower elevation angles, the ground characteristics that are important are not the ones for the ground directly beneath the antenna; rather, it is the ground many wavelengths from the antenna that establishes the efficiency of the reflection that can add as much as 6 dB to the signal compared to the same antenna in free space. For instance, for a horizontal dipole or Yagi at height l, a 10-degree takeoff angle is reinforced by the ground reflection 5.7 l away; on 14 MHz that’s about 400 ft! At 1 degree (the “sweet spot” in the minds of many DXers), the first bounce is nearly a mile away! Also, in real life, with real earth beneath an antenna, the effective ground may well lie some distance beneath the physical surface. How far beneath will depend on the operating frequency and the actual characteristics of the earth at that frequency, but distances of a few inches to a few feet are common. Figures 5.9 and 5.10 at the end of this chapter display the ground reflection factors for horizontally polarized and vertically polarized antennas, respectively, at selected heights above ground. (The charts of Fig. 5.9 are for an azimuth broadside to the axis of the dipole.) Each individual chart shows, for a given height, both the theoretical elevation pattern over perfectly conducting ground (broken line) and the equivalent pattern (solid line) over average ground (s = 0.005 S/m and e r = 13). In all of these charts, the outer ring of the plot corresponds to 7.0 dBd—i.e., to a maximum signal strength that is 7.0 dB greater than would be obtained from an identical dipole in free space. Clearly, the ability to determine the optimal height(s) for maximizing an antenna’s utility over the intended range of communications distances is a very powerful tool. Especially for individuals planning to erect MF or HF antennas, Figs. 5.9 and 5.10 are potentially among the most useful charts of the entire book. Note The charts of Figs. 5.9 and 5.10 assume flat ground under and near the antenna. The effect of ground that gradually slopes down as one gets farther and farther from the antenna is to enhance the very low angle signal strength in the same manner that hoisting the antenna higher can. Conversely, ground that slopes up tends to mimic lower antenna heights. As discussed in Chap. 2, terrain analysis programs can help identify or predict the effect of a specific terrain contour on the performance of a specific antenna installation.
168 p a r t I I : F u n d a m e n t a l s Example 5.2 An amateur radio operator’s favorite band for casual operating is 20 m, but she also spends time DXing on 15 and 10 m when those bands are open. She is planning to purchase a triband Yagi antenna but is trying to decide whether to put up a 35-ft house-bracketed tower or a guyed 70-ft tower to support the antenna. From what we know about the characteristics of those bands throughout most years of a typical sunspot cycle, we don’t expect them to be open for short skip very often, so she should try to select a height that favors elevation angles below 50 degrees on 20 m, somewhat lower angles on 15, and even lower angles on 10. A height of l/2 on any band provides a very broad lobe centered on 25 degrees—right in the middle of her desired range of angles. On 20 m, l/2 corresponds to 35 ft, which is just about the practical minimum if the antenna is to be above nearby obstructions. On 10 m, however, a triband beam at 35 ft is a full l high and the peak amplitude of the main lobe has moved lower in takeoff angle even as a second lobe, slightly weaker than the main lobe, appears in the elevation pattern, as seen in Fig. 5.9H. Of course, average ground will act more like a dielectric and less like a perfect mirror at 28 MHz, so the nulls will partially fill in. Except at times of extremely high sunspot activity or sporadic E-skip, it’s very unlikely that the ionosphere will support high-angle F-layer paths at 28 MHz, so the most useful elevation angles on 10 m are almost always lower than those of 15 or 20. If our operator is interested primarily in working very long haul DX on 20 m, she might want to consider heights closer to l (70 ft) at the risk of inserting some nulls at useful angles in her 15- and 10-m elevation patterns. By raising her triband beam from 35 to 70 ft over average ground, she gains not only another decibel of forward gain at the peak of the main lobe, but the elevation angle of that peak drops from 25 degrees to 14 degrees. At the low elevation angles that are so useful for long-haul DXing, the resultant overall improvement from doubling the height of her beam from l/2 to l is about 5 dB—equivalent to tripling her transmitter output power! In return, a null appears at 30 degrees (reducing her signal for some short-haul elevation angles on 20 m) and the elevation patterns on 15 and 10 break up into multiple lobes, albeit with lower peak elevation angles. In short, a height of 70 ft or as close to it as she can get will put most of her output power in the right range of takeoff angles on the band where she will need it the most—20 m—at a cost of some higher-angle nulls on all three bands.
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C h a p t e r 5 : a n t e n n a A r r a y s a n d A r r a y G a i n 167<br />
Figure 5.8 Image antenna for ground-mounted<br />
vertical monopole.<br />
Lossy Ground<br />
The discussion of ground reflection effects thus far assumes perfectly conducting<br />
ground; for average or poor grounds, the effect is to partially fill in some of the nulls in<br />
the elevation pattern (so they seldom get deeper than perhaps 15 dB down) and to reduce<br />
the maximum amplitude of the array factor in its favored directions—especially<br />
as the frequency increases. Lobes above the lowest angle lobe are reduced even further.<br />
Note Especially at the lower elevation angles, the ground characteristics that are important<br />
are not the ones for the ground directly beneath the antenna; rather, it is the ground many<br />
wavelengths from the antenna that establishes the efficiency of the reflection that can add as<br />
much as 6 dB to the signal compared to the same antenna in free space. For instance, for a<br />
horizontal dipole or Yagi at height l, a 10-degree takeoff angle is reinforced by the ground<br />
reflection 5.7 l away; on 14 MHz that’s about 400 ft! At 1 degree (the “sweet spot” in the<br />
minds of many DXers), the first bounce is nearly a mile away!<br />
Also, in real life, with real earth beneath an antenna, the effective ground may well lie<br />
some distance beneath the physical surface. How far beneath will depend on the operating<br />
frequency and the actual characteristics of the earth at that frequency, but distances<br />
of a few inches to a few feet are common.<br />
Figures 5.9 and 5.10 at the end of this chapter display the ground reflection factors<br />
for horizontally polarized and vertically polarized antennas, respectively, at selected<br />
heights above ground. (The charts of Fig. 5.9 are for an azimuth broadside to the axis of<br />
the dipole.) Each individual chart shows, for a given height, both the theoretical elevation<br />
pattern over perfectly conducting ground (broken line) and the equivalent pattern<br />
(solid line) over average ground (s = 0.005 S/m and e r = 13). In all of these charts, the<br />
outer ring of the plot corresponds to 7.0 dBd—i.e., to a maximum signal strength that is<br />
7.0 dB greater than would be obtained from an identical dipole in free space.<br />
Clearly, the ability to determine the optimal height(s) for maximizing an antenna’s<br />
utility over the intended range of communications distances is a very powerful tool.<br />
Especially for individuals planning to erect MF or HF antennas, Figs. 5.9 and 5.10 are<br />
potentially among the most useful charts of the entire book.<br />
Note The charts of Figs. 5.9 and 5.10 assume flat ground under and near the antenna. The<br />
effect of ground that gradually slopes down as one gets farther and farther from the antenna<br />
is to enhance the very low angle signal strength in the same manner that hoisting the<br />
antenna higher can. Conversely, ground that slopes up tends to mimic lower antenna<br />
heights. As discussed in Chap. 2, terrain analysis programs can help identify or predict the<br />
effect of a specific terrain contour on the performance of a specific antenna installation.