Practical_Antenna_Handbook_0071639586

152 p a r t I I : F u n d a m e n t a l s
Figure 5.1C Array pattern for the two-element
array of (B) in broadside (fed in phase) mode.
Outer circle = 3.8 dB
(relative to a single
radiator)
Array axis
radiated power in other directions—specifically,
those broadside to the elements
of the array.
If we move our receiving antenna
around the compass at a constant distance
from the center of the array, we find
that the received signal varies from zero
on the array axis to a maximum broadside
to the array, as shown in Fig. 5.1C.
Any time the two elements of an
array are odd multiples of l/2 apart and
the currents at the two elements are in
phase, radiation along the axis of the
array will be zero and radiation broadside
to the line connecting the two elements
will be a maximum. In our first
example we calculated a gain of 3 dB over
a single element, but that is only a rough approximation because we ignored possible
interactions between the two elements. In truth, if the two elements are l/2 apart, onaxis
radiation is everywhere zero, but interaction between the two elements alters their
feedpoint impedances and the resulting gain in the desired (broadside) direction is not
exactly 3 dB greater than that of a single radiating element. In fact, for l/4 verticals
spaced l/2 apart, it’s a bit higher than 3 dB; as shown in Fig. 5.1C, it’s 3.8 dB. For other
spacings, such as 3l/2, 5l/2, etc., and for other
antenna orientations (dipoles end to end, for instance)
the mutual impedances of the elements
can cause the array gain to be either greater than
or less than 3 dB for a fixed input power to the
array. (Also, for other than l/2 spacing, additional
side lobes form.) As we shall see in Chap. 12,
it is these same interactions between closely
spaced conductors that allow us to obtain array
gains from multielement antennas having only
one element directly connected to a feedline!
If now the RF signal being applied to one of
the two radiating verticals in Fig. 5.1B is shifted
180 degrees so that the two antennas are being
driven out of phase, the pattern of Fig. 5.1D results.
Note that the pattern is not just rotated 90
degrees but is somewhat “squashed” compared
Outer circle = 2.3 dB
(relative to a single
radiator)
Figure 5.1D Array pattern for the two-element array of
(B) in end-fire (fed 180 degrees out of phase) mode.