Practical_Antenna_Handbook_0071639586

154 p a r t I I : F u n d a m e n t a l s
Outer circle = 1.6 dB more
than for a single dipole.
Figure 5.2B Overall pattern for two-element broadside
array in (A).
2
I 1
I 1
= I 2
I 2
Array axis
r
r
Figure 5.2C Two-element broadside array of dipoles
spaced l/2 at right angles to the array axis.
r
A
two dipoles were far enough apart to have
negligible interaction.
But suppose we place one of the dipoles
l/2 above the other, as depicted in
Fig. 5.2C. Each dipole “sees” the other dipole
differently, and their mutual impedances
are different from those of Fig. 5.2A
as a result. As reported in Fig. 5.2D, the
overall gain of this two-element broadside
array is 3.8 dB more than that of a single
dipole. Clearly, if one is going to use two
driven dipoles to maximize forward gain
in a broadside array, the configuration of
Fig. 5.2C is preferable to that of Fig. 5.2A.
Note that in either case, however, the dipoles
are oriented so that the direction(s) of
maximum innate radiation for the basic dipole
element coincides with the desired direction
of maximum radiation for the array
as a whole when operated in its broadside
mode—that is, with in-phase currents at
the feedpoints of the dipoles. Along the
axis of the array (i.e., the extension of the
line drawn between the two antennas of
the array), then, not only is the array pattern
trying to bring the radiated field
strength to zero, so too is the inherent pattern
of the dipoles themselves. The overall
pattern of Fig. 5.2D will be of particular interest
to us later when we discuss the effect
of ground on a dipole’s pattern—especially
when we vary the height of the dipole.
Another popular form of array operation
is the end-fire mode. Here the objective
is to maximize radiation along the axis connecting the elements. One popular way to do
this is by spacing the elements l/2 apart and feeding them 180 degrees out of phase. By
the time the out-of-phase field from element A travels along the axis to element B it will
have shifted another 180 degrees and be back in phase with the field from B, thus effectively
doubling the radiated field along the axis, relative to a single element. For two
identical radiators spaced l/2 apart and fed with out-of-phase excitations of equal amplitude,
the shape of the array pattern is given by
⎛ π ⎞
AP = sin⎜
cosθ⎟ (5.6)
⎝ 2 ⎠
From our earlier discussion about attempting to align the inherent pattern of the elements
of the array with the main lobe of the array pattern, we conclude that horizontal