Practical_Antenna_Handbook_0071639586

272 P a r t I V : D i r e c t i o n a l H i g h - F r e q u e n c y A n t e n n a A r r a y s
360-Degree Directional Array
The phased vertical antenna concept can be used to provide round-the-compass steering
of the antenna pattern. Figure 11.5 shows how three l/4 verticals (arranged in a
triangle that is a half-wavelength on each side) can be used to provide unidirectional
and bidirectional patterns through combinations of in-phase, out-of-phase, and
grounded elements. For a specific pattern, any given element (A, B, or C) is grounded
(“passive”), fed at 0-degree phase, or fed with 180-degree phase shift.
Table 11.1 lists for each compass direction the drive to each of the three elements,
the maximum forward gain of that configuration and the elevation angle at which it
occurs, and a rough front-to-back (F/B) number. The figures in Table 11.1 are specifically
for a three-element 80-m array with identical 66-ft verticals spaced 136 ft (l/2) apart.
The design frequency is 3.6 MHz. Each element has fifteen 65-ft radials at its base. Any
time an element is labeled “Passive” in the table, it is unfed and directly grounded to
the radial field beneath it.
NW2
N
NE1
NW1
C
NE2
W
A
B
E
SW2
SE1
SE2
S
SW1
Figure 11.5 Three-element phased array.