Giant Billboard Antennas for Space-Age Radars
December 1971 Popular Electronics

December 1971 Popular Electronics Table of Contents Wax nostalgic about and learn from the history of early electronics. See articles from Popular Electronics, 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 PAVE PAWS (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.

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 antennas.

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.

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 serious problems.

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.

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 transmitter's signal.

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 operational tests.

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 radars.

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 proved itself.

Posted December 11, 2018