Old Technology Solves New Problems
Versatile antenna goes with the flow of communications.
Engineers have updated and improved a 60-year-old lens antenna technology to create a low-cost, high-gain steerable microwave antenna for satellite tracking applications. Conceived in 1944, the Luneberg lens currently is being employed to maintain two-way satellite contact when a satellite, a receiver or both are moving.
Previously, Luneberg lenses have been used primarily as reflectors or radar beacons because when combined with ground planes they offer a large return over a wide range of angles. Their use as an antenna lens has been limited because of the difficulty in manufacturing, which resulted in high cost and low performance. With new material technologies and design capabilities, high-performance, low-cost lenses can now be constructed.
The Luneberg lens is a special class of lens that focuses microwaves at two conjugate focal points. It converges incoming microwaves to a point that is at or near the surface of the lens. This is done using materials having a dielectric constant ranging from 2 at the center of the sphere to 1 at the surface. Dielectric constant is a measurement of a material’s capacity to modify electromagnetic waves. In practice, engineers approximate the material gradient by using discrete dielectric layers where the dielectric constant of the layer is the average of what it should be at its inner and outer diameters. Scanning the antenna merely requires moving the relatively small feed around the lens.
Dielectric lens antennas have been used for many years in various applications. Like an eyeglass lens, the purpose is to focus incoming plane waves to a point. Constant-K lenses, the most common and simplest to construct, consist of a single material with a homogenous dielectric constant. The shape of the material determines lens focusing. From a material standpoint, they are easy to make; however, the cost of machining to shape could be prohibitive. In addition, changing the beam direction of the antenna would entail turning the entire lens. R.K. Luneberg found a solution to this issue in the 1940s.
Receiving a satellite signal from a geostationary satellite at a fixed groundstation requires a high-gain antenna that is fixed in position and beam direction. However, if either the satellite or the receiver is moving, the antenna must be constantly adjusted to maintain signal.
With the geostationary satellite band filling up, many organizations are turning to low-earth-orbit (LEO) satellites requiring the groundstation to track the satellite. Often a mobile groundstation is desired. In either case, the satellite must be tracked to maintain the signal. Reorienting a dish antenna requires physically moving the entire structure. To guarantee that there will be no signal interruption, the next satellite would need to be acquired before the first one drops below the horizon. This process would necessitate using either two dishes or a very smooth and quick mechanical movement.
A Luneberg lens antenna solves these problems. Because of its spherical symmetry, scanning requires moving the relatively small feed around the lens while the lens itself remains stationary. Acquiring another satellite signal before losing the first can be accomplished by employing dual feeds because the lens can support simultaneous beams.
A second type of application involves the receive station moving relative to either a geostationary or LEO satellite. For example, a mobile ground vehicle or an aircraft needs to maintain constant satellite contact. A Luneberg lens antenna can be more easily steered than a dish antenna.
Although phased array antennas are one possible solution to the problems with moving stations, performance deteriorates at low scanning angles, and high cost limits their use. Performance deterioration becomes relevant, for example, in an aircraft on a transpolar route when it is attempting to communicate with a geostationary satellite above the equator. In addition, phased array antennas tend to be narrowband, which limits their effectiveness in broadband applications.
The Luneberg lens is broadband because the dielectric constant shows little variation with frequency in the microwave band. Its bandwidth is determined by the bandwidth of the feed. Because the feed is generally a low-gain antenna, it is easy to construct a broadband unit.
In practice, Luneberg lenses have been used in airborne applications as hemispheres mounted on ground planes that essentially duplicate the performance of a full lens at half the height thus reducing weight and drag. Datron Systems Incorporated has constructed a system using four Luneberg hemispheres that are phase combined to make a high-gain, easily steerable antenna system for satellite communications.
Designing a Luneberg lens requires approximating the dielectric gradient with discrete layers of a given dielectric constant. A larger number of layers for a given radius lens will better approximate the pure Luneberg lens but at a greater manufacturing cost. Diminishing returns are seen as more layers are added. In addition, a smaller number of layers will result in a higher dielectric constant for the outermost layer, which reduces lens efficiency.
A simple ray-tracing program can be used to design a Luneberg lens; however, this yields no information regarding gain or side-lobe level. New prediction and design software that uses spherical wave functions and boundary-value matching are now employed for accurate prediction of lens performance. A genetic algorithm or similar method can then be used to find the optimum design.
Lens radius, feed location and desired gain are key parameters in the design. Gain performance close to the theoretical maximum for the aperture size can be achieved.
The manufacture of high-performance Luneberg lenses requires the development of low-loss materials with dielectric constants between 1 and 2. Tight control of the dielectric constants of each layer must be maintained to ensure lens performance. The ability to accurately measure material dielectric constant within ±0.005 is crucial. Dielectric constant errors will increase the side-lobe level, and air gaps between the layers will adversely affect both side-lobe level and gain.
Traditional Luneberg lenses were made from expanded polystyrene (EPS). Each layer was created in a hemispherical shell and then assembled. Using this process, it is very difficult to avoid air gaps between the layers, and the dielectric loss in EPS affects performance and limits its use in high-power applications.
New generation lenses are manufactured using low-loss composites with tightly controlled dielectric constants in a manufacturing process that eliminates the air gaps. This advancement in Luneberg lens technology is made possible by the development of new composites that possess three critical characteristics. First, they feature low dielectric constants and loss tangents that are within the range Luneberg lenses need. Second, the dielectric constant and specific gravity do not change during the curing/hardening process. These properties simplify the manufacturing process, which improves the homogeneity within the lens and eliminates air gaps. Third, the dielectric constant of a specific composite is directly proportional to its density and can be calculated using a single equation. The result is that Luneberg lenses can be manufactured with consistent performance both from all angles of view and from lens to lens.
Paul Dixon is the senior microwave engineer at Emerson & Cuming Microwave Products, Randolph, Massachusetts.