Technology Today Volumn 3 Issue 1 - Raytheon

PHASED ARRAY SYSTEMS
Continued from page 21
to distribute the energy from the highpower
sources to the aperture. The corporate
feed, in turn, interconnected with the
phase shifters and corresponding radiating
elements.
Common phase shifters for both corporateand
space-fed arrays included ferrite (the
most popular type), and PIN diode. The ferrite
phase shifters used the principle of a
variable magnetic field which altered the
wave propagation characteristics to set the
desired phase. PIN diodes were used as RF
switches for combining varying lengths of
transmission lines or as a termination in a
transmission line to alter the phase. Ferrite
phase shifters were most popular at S-band
frequencies and above, since the waveguide
that housed the ferrite was reasonably
sized, provided lower transmission loss,
and was capable of supporting higher
RF power.
Transformation from
Passive to Active, Solid
State Comes of Age
Passive phased arrays provided new capabilities
for radar systems, agile beams and
improved reliability. However, passive
phased array architectures had their problems.
They were heavy, due to the need for
a low loss feed structure like a metallic
waveguide, and/or bulky because of the
depth required of the space feed approach.
Furthermore, their reliability was typically at
the mercy of the high power RF transmitter.
The high power transmitter was a singlepoint
failure risk. An attempt to improve
the reliability was introduced by using a distributed
configuration of lower power
tubes, combined with solid state driver
amplifiers known as Microwave Power
Modules (MPMs). While the MPM
approaches improved reliability, they didn’t
achieve the ultimate goal: an amplifier at
every element of the phased array. Nor did
they afford thinner and lighter-weight
approaches that would have revolutionized
the application of phased arrays to an
unprecedented number of airborne, space
22
and ground platforms. The evolution
of solid-state microwave
devices lagged the digital revolution
that produced personal computers,
and the solid-state transistor
radio, primarily due to the
industry’s inability to produce
devices in volume with features
(e.g., circuit line widths and material
characteristics) that would provide
acceptable performance at
microwave frequencies. Silicon was
the material of choice, as it is today,
for PCs and most consumer electronics.
As previously mentioned, it wasn’t until the
1980s that the U.S. government provided
industry and academia with the funding
needed to develop and mature a technology
that revolutionized phased arrays (and
many other commercial telecom applications.)
It wasn’t, however, until the early
1990s that visionaries at Raytheon and two
key customers — Strategic Missile Defense
Command (SMDC), now known as the
Missile Defense Agency (MDA) and a commercial
venture of Motorola sought ways to
produce (in volume) active, electronically
scanned arrays. SMDC for many years had
been visualizing radar systems in support
of missile defense. In 1992, Raytheon competed
for, and won the Ground Based
Radar Program (now know as the Terminal
High Altitude Area Defense, THAAD Radar).
During the next three years, Raytheon
developed the largest and most powerful
solid-state, phased array radar, consisting
of more than 25,000 T/R modules.
AESAs — particularly at L-band and above
— were enabled by GaAs technology. With
this development each radiating element of
the array had its own power and low-noise,
amplifiers, digital phase and attenuation
controls. This new generation of phased
arrays were highly reliable now that the RF,
Power and Control subsystems were all distributed,
i.e., eliminating single point failures.
That is, performance would degrade
gracefully as the element level electronics
began to fail. AESAs allowed unprecedented
capabilities in beam pointing, sidelobe
control, polarization versatility, multiple
The GBR/THAAD AESA.
beams, instantaneous bandwidth and
packaging, just to name a few. Today,
platforms — especially in the air and in
space — could benefit from the features
that phased arrays afforded.
Array Packaging:
Brick vs. Tile
The first evolution of AESAs used what is
commonly referred to as a “brick” style
packaging. Brick packaging arranges the
active electronics (and some of the beamforming)
in the plane orthogonal to the
aperture surface (see Figure 3).
Figure 3. Brick and Tile Packaging Architectures
Examples of brick style packaging include
THAAD, SPY-3, GBR-P, F-15, etc. For example,
most of the ground/shipboard and earlier
versions of the airborne-style radar