Practical_Antenna_Handbook_0071639586

C h a p t e r 9 : V e r t i c a l l y P o l a r i z e d A n t e n n a s 237
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Figure 9.5B Accounting for ground losses over real earth.
network components, and the connections themselves—just ask any serious user of HF
mobile equipment!
Ground Systems for the Grounded Monopole
Earlier in this chapter, we discussed how the vertical antenna radiates equally well in
all (azimuthal) directions. Over the years, some wags have said the vertical radiates
equally poorly in all directions. Which of these opinions is correct?
In fact, either can be true. The secret of success is this: The grounded vertical monopole
antenna works well only when placed over a good ground system. (This comment
does not apply to a vertical dipole in free space, many wavelengths away from the nearest
conductors.) As we discussed earlier, in a ground-mounted vertical the ground system
provides an electrical “return” for antenna currents; it is the replacement for the
“other” half of the dipole. To the extent that the ground return is “lossy” or incomplete
in its coverage of the radiation field of the vertical, a significant ground loss resistance
is added to the total resistance seen by the RF energy delivered to the antenna terminals
by the transmitter and feedline. If drawn schematically, ground losses and antenna radiation
resistance are in series and, hence, the transmitter output power is divided between
the radiation resistance of the antenna and the loss resistance of the ground
return system. With radiation resistances in the 2- to 37-Ω range, it does not take much
loss resistance to steal half or more of your output power!
Ground return loss is one of the primary reasons mobile installations generally do
not get out as well as home station antennas. Because mobile whips (usually operated
as grounded verticals) are so much shorter than a quarter-wavelength, their input impedance
is very low—usually just a few ohms, at best.
The usual way to provide a good ground for a ground-mounted vertical is to use a
system of radials such as those shown in Fig. 9.8. (We will take up elevated ground-