RESEARCHERS AT the Georgia Institute of Technology have designed and built a prototype Rotman Lens that operates at millimeter wave frequencies. Because it has no moving parts, no phase shifters and can be implemented in plastic, the electronically scanned device offers an inexpensive, rugged, reliable and compact alternative to current millimeter wave antenna technologies.

PHOTO BY STANLEY LEARY Dr. Otto Rausch adjusts the Rotman Lens. Variations of it could be used for many military and civilian applications.

The Georgia Tech prototype is believed to be the first Rotman Lens to operate at a frequency as high as 37 GHz. Variations of the antenna could be used in a range of military and civilian applications, including tank radars, all-weather aircraft landing systems, communications equipment, missile seekers and automobile collision avoidance systems.

"We have taken a first step toward the goal of a really inexpensive millimeter wave antenna that would be useful in a growing number of applications," says Dr. Ekkehart (Otto) Rausch, senior research scientist at the Georgia Tech Research Institute. "Radars and communications devices in the millimeter wave region are becoming more widely used. There are many applications where you would like to have an antenna that is very low-cost, simple, rugged and reliable."

The research was supported by the U.S. Army Research Laboratory.

Through a Glass Brightly

The device got its name because of its ability to focus microwave or millimeter wave energy coming from a particular direction by passing the electromagnetic energy through a pair of parallel plates that are shaped like a lens. Beam-forming or focal ports are located on one side of the plates, fed by a switch array. The array ports are on the opposite side, each connected to an antenna element. Energy fed into a specific focal port will emerge from the antenna elements and produce a beam along a particular direction.

Switching the input from focal port to focal port steers the beam electronically in one direction across a 45- degree arc. The switching could be done with pin diode switches, which are also simple, reliable and inexpensive.

Previous Rotman lens antennas have been developed at frequencies of 18 GHz or below, Rausch says. Most have been produced in microstrip. Microstrip, however, is very lossy at high frequencies, and therefore is not suitable in the millimeter wave region. Instead waveguides and an air dielectric must be used between the parallel plates to reduce the losses to an acceptable value between 1 and 2 dB.

With modeling assistance from Dr. Andrew F. Peterson in Georgia Tech's School of Electrical and Computer Engineering, Rausch designed and fabricated an antenna milled out of a solid block of aluminum. Production of the aluminum antenna demanded tolerances of 0.0005 inches. Rausch talked with more than 20 fabrication facilities before locating a New Jersey company capable of using electrical discharge techniques to carve out the necessary shapes at those tolerances.

"Everything about this lens, from the width of the waveguides to the shape of the absorber foam, matters a great deal," he says. "The surface roughness and even the placement of the screws all have been designed according to strict design principles."

The lens was designed with the assistance of Jay Sexton and fabricated with the technical expertise of Mitch Cole, who laid out the design in AutoCAD. Greg Hampton made the accurate waveguide extensions required for the measurements. Measurements were made by Mitch Kappa, and Kevin Murphy assisted with analysis.

Production antennas could be hot-pressed in plastic, which would then be coated with a conductor like gold. The antenna feed horns and switch array could be made the same way, allowing the antennas to be very low in cost.

Besides the low cost, compact size and ruggedness, the Rotman lens antenna also offers very low throughput loss and sidelobe emissions. In the prototype developed by Peterson and Rausch, sidelobe power can be suppressed by a factor of one-thousand below the energy of the main beam. The power loss through the lens itself is less than 2 dB.

Most antennas operating at millimeter wave frequencies use mechanical scanning or phase shifters, both of which have disadvantages. Mechanically steered antennas are slow in response and suffer reliability problems due to shock and vibration. Phase shifters are costly to fabricate and introduce considerable RF losses. By avoiding those drawbacks, the Rotman lens antenna could open new applications for millimeter wave radar.

Potential Applications

To be successful in some applications that Rausch envisions, the antenna's operating frequency must be expanded, and the capability to scan in two dimensions added. Potential applications include:

• Autonomous aircraft landing systems: Poor visibility caused by heavy fog can keep pilots from seeing enough of the runway to allow a safe landing. A synthetic vision system based on millimeter wave radar could produce images through the fog, allowing aircraft to land even when runways are obscured. Such a system would require a reliable, compact and inexpensive antenna system to be affordable.

• Synthetic vision for ground vehicles: Operators of ground vehicles such as tanks also need to see through fog and smoke, but the vibration and harsh operating conditions limit use of conventional antennas. A Rotman lens antenna could be integrated into the tank's structure, eliminating the need for an external dish and providing necessary reliability and ruggedness, Rausch says.

• Automobile collision avoidance systems: Collision avoidance systems built into automobiles could provide drivers with warnings of approaching vehicles. If implemented in plastic, the new antenna could lower the cost of such systems enough to make them practical.

• Commercial communications: Rotman lens antennas could be used in short-range, building-to-building wireless communications. Implementation in plastic could help lower the capital costs for such systems.

• Missile seekers: Its low cost, reliability and compactness could find application in airborne systems such as missile seekers.

A paper on the Rotman lens was submitted to the 1996 Antenna Applications Symposium. Another abstract on the Rotman lens was submitted to the 1997 National Radar Conference.

Further information is available from Dr. Otto Rausch, Georgia Tech Research Institute, Sensors and Electromagnetic Applications Laboratory, Georgia Institute of Technology, Atlanta, GA 30332-0857. (Telephone: 770/ 528-7777) (E-mail: ekkehart.rausch@gtri.gatech.edu)

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Last updated: 27 Nov. 1996