Practical_Antenna_Handbook_0071639586

C h a p t e r 5 : a n t e n n a A r r a y s a n d A r r a y G a i n 155
dipoles oriented parallel to the array axis
are not the best choice for elements in this
end-fire array. For instance, if we feed
one dipole in the array of Fig. 5.3A with a
drive current that is out of phase with the
current in the other element, we obtain
the azimuthal pattern of Fig. 5.3B—not
totally useless, but probably not what we
expected, either. Along the array axis, the
dipole patterns radiate very little horizontally
polarized energy to the far field,
and at right angles to the array axis the
element spacing and drive current phase
Outer circle = 3.8 dB
relationships conspire to create a similar
more than for a single
null. But the radiated energy has to go
dipole.
somewhere, and it is “easiest” for it to go
off at 45 degrees to either axis, looking
like a perfect four-leaf clover!
Figure 5.2D Overall pattern for the two-element
Instead, the horizontal dipoles
broadside array in (C).
should be rotated on the page 90 degrees,
as shown in Fig. 5.3C. The resulting
overall pattern of Fig. 5.3D is much more in line with our intentions and our
expectations. (As noted before, the overall gain relative to a single antenna is a bit less
than 3 dB because of the effect of the mutual impedances between the elements for this
spacing and relative element orientation.)
If the user wishes to switch a single
fixed physical array between
broadside and end-fire patterns, the
ideal element may well be a vertical
dipole or monopole, since those antenna
types enjoy omnidirectional azimuthal
radiation patterns and can be
equally at home in either broadside or
end-fire applications without having
to be physically rotated or relocated
2
1
r
A
I 1
= I 2
180˚
r
I 2
r
Outer circle = 0.5 dB
more than for a single
Array axis
dipole.
Figure 5.3A Two-element end-fire array of l/2
dipoles spaced l/2 apart on the array axis.
FIGURE 5.3B Overall pattern for the two-element
end-fire array in (A).