Practical_Antenna_Handbook_0071639586

230 P a r t I I I : H i g h - F r e q u e n c y B u i l d i n g - B l o c k A n t e n n a s
Vertical Dipole
To develop our understanding of vertical antennas, we start with a half-wave dipole,
oriented with one end directly “above” the other relative to earth, as in Fig. 9.1. If we
place this antenna many, many wavelengths
above the earth’s surface, we can guess that its
Feedline
Earth
h
Figure 9.1 l/2 vertical dipole far from earth’s
surface.
pattern will be very close to its free-space pattern,
yet we have preserved a sense of what is
“vertical” and what is “horizontal” to make
our discussion of this antenna easier to follow.
The vertical dipole is constructed in exactly
the same manner as the horizontal dipole
but is mounted or supported in the vertical
plane. In general practice, the radiating element
that is closer to the ground is connected
to the shield or outer conductor of any unbalanced
feedline, such as coaxial cable or CATV
hard line, but there is no factual basis for doing
so in a properly engineered installation.
Like the horizontal dipole, the approximate length of the vertical dipole is calculated
from
where L = total length of dipole, in feet
F = operating frequency, in megahertz
468
L (feet) =
(9.1)
F (MHz)
Example 9.1 Calculate the length of a half-wavelength vertical dipole for operation on
a frequency of 14.250 MHz in the 20-m amateur radio band.
Solution
L = 468
F
L = 468 = 32.8 ft
14.250
Note: The 0.8-ft part of this calculated length can be converted to inches by multiplying
by 12:
0.8 × 12 = 9.6 in
Each leg of the vertical dipole in this example is one half of the calculated length, or
32.8
2
= 16.4 ft