Practical_Antenna_Handbook_0071639586

234 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
Voltage
/ 4 Radiator
Current
Coax to
XMTR
Max
Min
Figure 9.4B Current and voltage distribution along
l/4 vertical.
Figure 9.4C Simple coaxial-fed vertical antenna.
/ 4 Radiator
/ 4 Radials
Transmission
line
The radials of a ground-plane vertical antenna
are typically installed at some vertical
angle between horizontal (0 degrees) and, say,
45 degrees below horizontal. Figure 9.4D is an
example of the latter case, showing a groundplane
antenna made with “drooping” radials.
Similarly, Fig. 9.4E is an example of a groundplane
vertical having perfectly horizontal radials.
Sometimes the feedpoint VSWR of the
antenna can be improved by adjusting the degree
of “droop” in the vertical’s radials.
Unlike ground-mounted verticals, the
“ground plane” formed by the radials of Fig. 9.4E
does not preclude the existence of a far E-field
below the horizontal plane of the radials, as suggested
by the (solid line) pattern in the figure.
Figure 9.4D Mast-mounted vertical uses radials as a
counterpoise ground.