Practical_Antenna_Handbook_0071639586

166 p a r t I I : F u n d a m e n t a l s
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radiated field strength begins to drop. However, no matter how high the antenna is
raised, as long as it is above a ground plane, the horizontally polarized radiation at 0
degrees elevation angle (i.e., the horizon when flat land is involved) will always be zero
because the image antenna and the real antenna are always exactly out of phase on
these equidistant paths.
Now consider a vertically polarized dipole not in contact with the earth, as shown
in Fig. 5.7. Further consistent with the mirror analogy, boundary conditions at the
ground plane require the image antenna’s vertically polarized (with respect to the
ground plane) E-field to be in phase with the corresponding component of the original
antenna’s radiated field. Thus, the ground beneath a vertical l/2 dipole, for instance, is
replaced with an identical dipole having the same vertical orientation and a standing
current that is exactly in phase with the drive current of the aboveground dipole. Figure
5.10 at the end of the chapter consists of a series of graphs depicting the ground reflection
factor for vertically polarized antennas as a function of their height above ground.
For tilted antennas, or antennas otherwise having a mix of both horizontal and vertical
polarization, visualizing the two cases separately may be the easiest way to understand
the overall effect of nearby ground on the radiated signal. Of course, if antenna
modeling software is available, all the hard work is done for you by the computer!
For the specific case of a ground-mounted monopole, the image antenna is best visualized
as an identical but upside-down vertical with its upper end just “touching” the
base of the real antenna. If the current flow at some instant is downward in the real
monopole, it is also downward in the image monopole. Because the vertical component
of the two currents is in phase, the current distribution in the two verticals appears
continuous across the ground plane; consequently, the effect of the ground-plane
boundary conditions is to cause the image antenna to act like the other half of a vertical
dipole. Like any other dipole, the standing current is maximum at the center of the resulting
antenna structure and in phase across the entire combined length of the two
pieces, as suggested by Fig. 5.8.
Because the radiated E-fields from the real and image vertical antennas are in phase
and at right angles to the ground plane, there is no automatic cancellation of the radiated
field at 0 degrees elevation angle. In the presence of a perfectly conducting ground
plane, a vertical enjoys the same 6-dB “boost” in received signal strength at certain
wave angles as do horizontal dipoles but—because its E-field is at right angles to the
ground plane—this boost occurs at 0 degrees. The ground-mounted vertical is, therefore,
unsurpassed in its ability to produce a strong signal at the horizon and very low
elevation angles. As discussed in Chap. 2 (“Radio Wave Propagation”), however, the
losses inherent in real earth cause the distant radiated field from any vertical to be
zero near the horizon. Despite that, numerous
DXpeditions have demonstrated that groundmounted
monopoles at the edge of ocean saltwater
have a distinct advantage over virtually
any other antenna for intercontinental communications
on the MF and HF bands.
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Figure 5.7 Image antenna for vertical dipole not in
contact with the ground.