December 1971 Popular Electronics
Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
published October 1954 - April 1985. All copyrights are hereby acknowledged.
Steerable phased array antenna systems used
to be the exclusive domain of military and aerospace radar and electronics warfare
systems. The expense involved in both the hardware (structure, antenna elements,
electric power, cooling) and the electronics required for controlling the beam was
expensive and complicated. Larger phased array antennas for lower frequency (longer
wavelength) bands are still relatively expensive. However, small cell wireless phone
and WiFi applications in the 2.4 GHz and higher bands are seeing the development and
deployment of phased arrays that will search for and track individual users in order
to allocate antenna gain and signal power where it is needed, rather than using an
omnidirectional radiation pattern. Physically steered directional antennas are not
capable of the speeds needed to do the job. In the last couple years, MMIC (microwave
monolithic integrated circuit)
phased antenna arrays have begun appearing in the news for
millimeter-wave systems. Construction of
(Precision Acquisition Vehicle Entry Phased Array Warning System, AN/FPS-115)
location #3 was just getting started at Robins Air Force Base, Georgia, when I left
there in 1982. It operated in the 420-450 MHz UHF band.
Giant Billboard Antennas for Space-Age Radars
Stationary "Phased Arrays" Move the Beams
By Edward A. Lacy
Conventional radar installations, with their spiderwork parabolic antennas and block-house-like
equipment structures, are easy to spot and identify. Even the "golf ball" domes that
are used to protect radar antennas from the elements are of no help in hiding the identity
of a radar installation. However, today there is a new crop of radar that is so unobtrusive
and adaptable to almost any type of architecture that recognizing a radar installation
can be rendered almost impossible.
The story dates back more than half a decade when the Air Force was faced with a mounting
problem: The number of satellites and amount of junk orbiting the Earth over the United
States could not be reliably tracked with conventional radars. What was needed was a
system that could track all of these objects and any additional ones that would be put
into orbit. So, the engineers put their heads together and devised a whole new radar
system to meet the Air Force's needs.
Thus emerged the phased array (or electronically steerable array) radar. The phased
array radar is unique in that it contains no moving parts. Instead of a rotating parabolic
dish antenna to steer the radar beam, the beam itself is positioned and repositioned
through a system of electronics only.
The new phased array radars have been functioning for quite a while now. The biggest
one, the AN/FPS-85, (below) located at Eglin Air Force Base, was designed to meet the
need for space tracking. Built by Bendix Corp., the radar is housed in a wedge-shaped
building the size of a football field. Seen from either end, the building has the shape
of a right triangle. Its sloping roof contains the individual transmitting and receiving
modules. Head-on, the building looks more like a giant billboard instead of a radar installation.
The Army's use of phased array radar includes an installation at the White Sands Missile
Range known as HAPDAR (Hard Point Demonstration Array Radar) which was designed and installed
by Sperry Corp. for the Army Missile Command. A more recent phased array radar installation
under the auspices of the Army is located at Kwajalein Island, in the West Pacific.
Hughes Aircraft Co. ADAR phased array radar has antenna with space-age
Nor has the Navy been dragging its feet, although so far, the carrier Enterprise is
the only place in which the Navy is employing phased array radar.
At present, and after some six years of operational tests, phased array radars are
still classified as experimental. But it won't be long before these radars become regular
inventory items by the various services.
In a typical phased array radar system, one set of antennas is used for transmitting,
another for receiving. Each system consists of hundreds, even thousands, of separate
antennas. The AN/FPS-85, for example, contains 5776 transmitting and 19,500 receiving
The antenna sets can be subdivided into subsets, with each subset connected to a receiver
or transmitter. In some cases, each antenna is coupled to its own transmitter or receiver.
All of the transmitters in a phased array system are on the air simultaneously when
a pulse is being transmitted. Since the output power of each transmitter is additive,
theoretically an unlimited amount of output power can be generated and radiated.
To move the radar beam from one point to another, it is only necessary to change the
phase of the signal delivered to each antenna. The antenna itself remains stationary.
The result of all-electronic beam steering is inertialess tracking. Hence, the beam can
be steered from point to point in microseconds.
To provide the proper phase shift to each antenna at the high speeds required, computers
are used. In the case of the AN/FPS-85, three IBM computers are employed, while HAPDAR
employs a Univac computer.
Radar users are continually demanding more power, larger antennas, greater receiver
sensitivity, and the ability of the radar to keep track of the multitude of high-velocity
flying and orbiting objects. Such demands are mostly beyond the capabilities of conventional
radar techniques-but not beyond the capabilities of the phased array.
Parabolic radar antennas must conform to hard-to-meet mechanical tolerances and are
so heavy that enormous, impractical drive systems are required to overcome the inertia
involved in moving and re-positioning them fast enough to search for incoming targets
and simultaneously keep track of those targets already spotted.
Conventional radars (Western Electric's DEW Line system shown) are
easy to identify.
Also, the waveguide that feeds the parabolic antenna limits the peak power that can
be transmitted by the system. Exceeding the power-handling capacity of the waveguide
results in inevitable breakdown. But even before this happens, the generation and control
of high voltages required to operate the super-power klystrons in radar systems become
The phased array radar overcomes another of the disadvantages of conventional radars.
Its thousands of active transmitters preclude system failure even if dozens of transmitters
are out of operation. (It has been estimated that in a given day, as many as 100 transmitter
modules fail in the AN/FPS-85 alone.) If a transmitter fails, it can simply be unplugged
and replaced by an identical unit.
Another important advantage of the phased array system is that virtually any surface
will accommodate it. In aircraft, consequently, better aerodynamic stability and less
crowding in the nose can be achieved when the radar is flush-mounted in the fuselage.
In space satellites, balance stability is obtained from the phased array due to its lack
of moving parts.
The basic principle of the phased array is not new. In fact it has been known for
many years that a directional beam can be formed with an array of antennas. Only the
technique of electronically varying the phase of the signal at each antenna to obtain
inertialess beam steering is new.
In operation, the signals from each antenna in a phased array form a wavefront close
to the array. Farther out, the wavefront forms a directional beam. Beam shaping is determined
by the number of antennas and their spacing within the array. For the narrow beam essential
to satellite tracking, a large number (the larger the better) of antennas must be used.
Conversely, for a broad-beam surveillance radar, a few antennas are sufficient.
When the antenna elements are excited in phase with each other, the direction of the
beam is broadside to the face of the array. By introducing a different phase displacement
in the current delivered to each antenna, however, the beam can be moved almost instantaneously
from one position to another.
Phase shifter consists of a ferrite rod in waveguide. (Courtesy of
The phase shifter consists of a ferrite rod that is placed inside the waveguide as
shown in the drawing. This rod forms the center of a solenoid which is wrapped around
the waveguide. By varying the current through the solenoid coil, the permeability of
the ferrite rod is changed, thereby changing the velocity of propagation through the
shifter system. The result is that the delay in propagation causes a phase shift in the
With a high-speed computer in control of the changes in current through the solenoid
coil, it is possible to create an almost unlimited variety of beam scanning patterns.
For example, the computer can be programmed to produce a beam that will "skip" a nearby
mountain but scan on both sides of the mountain for targets. Furthermore, the high speed
of the computer and low inertia of the radar beams make possible tracking any number
of objects in several different directions while at the same time scanning for new targets
coming into range.
Obviously, ponderous conventional radars with their enormous inertia cannot do the
job effectively. They are limited to the number of objects they can track simply because
they have to "pause" between pulse transmissions to wait for the return echo before moving
on. In the interim, they must remain idle.
A phased array radar, on the other hand, can transmit a pulse, move on to other positions
to transmit more pulses, and return to "listen" for the echoes from each target spotted.
Naturally, the closest targets are spotted first. During the receiving phase, the appropriate
amount of phase shift is applied to the signals being received so that the signals add
coherently. In effect, the received beam is "steered" in a manner not unlike the steering
of the transmitted beam.
Inertialess beam steering is not obtained without difficulty. Mutual coupling between
the radiating elements of the antennas in the array is a major problem. Then, too, as
the beam is steered or scanned away from boresight, spurious multiple beams - commonly
known as grating lobes - appear, giving the array a tendency toward tunnel vision instead
of the 180° scan angle it has in theory.
To scan a full 360°, it is necessary to use three or four phased array radar systems-a
factor that can multiply over-all systems cost well beyond so-called practical cost/use
limits. Phased array radars, after all, are not inexpensive. The total reported cost
of the AN/FPS-85 is $62,000,000. But this figure also includes the cost of rebuilding
the original system which was destroyed by fire in 1965 shortly before it was to undergo
The new techniques and devices (especially the use of IC's) just might cut the cost
of phased array radars dramatically. For example, RCA's Missile and Surface Radar Division
has already developed a solid-state phased array antenna design technique which is said
to yield power densities 100 times those of conventional radars. Texas Instruments Inc.
has developed a microradar so small that a complete transmitter, receiver, and antenna
can fit into the palm of a hand. And other companies - Sperry Rand, Raytheon, RCA, Hughes
Aircraft - are working feverishly to develop compact, reliable, and low-cost phased array
No matter what the cost of the phased array radar, it does a job that no other type
of radar can approach. For satellite tracking, defense surveillance, and insuring air
traffic safety, the phased array radar is a must. Bear in mind, however, that no plans
are on the drawing board to substitute the phased array where a conventional radar will
suffice. The conventional radars will be with us for a long time to come, but they will
eventually have to give way to the Space Age Radar - the phased array - which has already
Posted December 11, 2018