Electro-Optics - Applied Optics Fundamentals and Device Applications - page 10

are determined by the relative current phases of the radiating elements, the
array controller has complete and nearly instantaneous control over the
beam direction and pattern. For the same reasons, the phased array system
is an ideal candidate for computer control due to the ability to control the
array beam characteristics via the phase shifters located in the T/R modules.
This ability is enhanced since the phase shifters most commonly chosen for
modern T/R modules are digital; therefore, the operator could run any num-
ber of phase sequencing algorithms from a computer controller to maneuver
the radar beam.
The physical layout of the phased array provides some additional advan-
tages over the nonarray radar. First, since there are typically thousands of
radiating elements in a phased array radar, the system may exhibit non-
catastrophic degradation. If a small percentage of the radiating elements
are nonoperational, the radar will function properly but generally with an
increase in peak sidelobe level [10]. Also, since phased arrays are typically
built in a planar arrangement, they are more resistant to blasts, making the
phased array attractive for use in hostile environments.
One of the major design decisions for a phased array radar is the choice
of an array feed formation. Several options are available including the one
source, one feed design implied in Figure 2.4. Figure 2.6 depicts a few other
alternatives. One common implementation is the multiple-source corporate
fee. An example of this is the COBRA DANE system located on Shemya
Island, Alaska. This inbound and outbound missile tracking radar uses 96
traveling wave tubes to illuminate its 15,360 active elements [11]. The serpen-
tine feed structure is being incorporated in the S-band AR320 3-D Radar [12].

Corporate feed Multiple source corporate feed

T/R

T/R

Load

Secondary
transmit
array

Serpentine feed

T/R

Primary
transmit
antenna

T/R

Space feed

FIGURE 2.6
Array antenna feed configurations.