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A metamaterial antenna lens can have the shape of a wing leading-edge while focusing EM waves like a parabolic lens. (BAE Systems)

Hideaway antennas for aircraft

The modern airplane has become a flying antenna farm. Aerials and antennae have sprouted up all along airframes during the past few decades, some resembling wind-sculpted trees or truncated stumps, others flat and embedded in the skin. Meanwhile, large dishes and phased-array panels, especially on passenger planes and military aircraft, often hide behind protective nose radomes. They are all there, of course, to feed the assorted radios, radars, data links, beacons, monitors, and altimeters that enable today’s pilots and aircrews to function.

And increasingly, to serve the passengers’ needs as well. Long-term bans on in-flight use of personal electronics devices are falling away, and if the trends toward ubiquitous wireless connectivity and sensors continue, even more antennas will soon find their way onto aircraft.

But just as aerospace engineers face a proliferation of aviation antennas, they are simultaneously gaining a solution. New high-performance antennas made from “designer” substances called metamaterials are not only more compact and lightweight than current counterparts, they also offer promising new ways to direct the behavior of electromagnetic waves. Some indication of the novel capabilities of these developmental devices are hinted at by recent news reports of metamaterial “invisibility cloaks” that can bend light around themselves and “superlenses” that can resolve sub-wavelength light beyond the diffraction limit.

Small, sensitive antennas

Researchers create metamaterials by carefully designing and fabricating novel structures to exhibit patterns of electromagnetic properties—specifically, dielectric permittivity and/or magnetic permeability—at the micro- or nano-scale. This special spatial arrangement of elements ensures that the volumetric arrays interact with electromagnetic fields in desirable ways.

The resulting materials (or devices) can, for example, efficiently absorb or emit EM radiation across a wide bandwidth, or do so along specified directions, making them perfect for sensitive, highly directional antennas or beam-steering arrays. Such high-function, low-form-factor metamaterial antennas could be fully embedded in the skin and even conform to airframe contours without compromising aerodynamic performance.

It is little wonder then that a variety of companies are working in this field, according to a recent report by technology consultants Lux Research. Lead author Anthony Vicari and his Lux colleagues cited vigorous R&D effort on metamaterial antennas at startups such as Kymeta, Fractal Antenna Systems, and Metamaterial Tech; industrial firms including Harris Corp., Kyocera Wireless, and EMW; large aerospace and defense contractors like Lockheed Martin, Boeing, and Raytheon; and even consumer electronics giant, Samsung. Among the leading academic institutions in this area, they said, are Duke University and Queen Mary’s University of London.

Another big player in metamaterial antenna technology is BAE Systems, the British multinational aerospace and defense firm. Earlier this year, the company announced that company researchers had built a prototype metamaterial antenna lens that was significantly flatter and more compact than the conventional analogue on which it was based.

Flat Luneburg lens

A team at BAE Systems’ Advanced Technology Center fabricated a compact version of a Luneburg lens, which is a spherical gradient-index lens that focuses incoming plane waves to a point or, conversely, converts waves that are emitted from a point source into plane waves. The metamaterial version of the Luneburg lens, a flattened, thin cylinder, demonstrates proof-of-concept, said Sajad Haq, group leader for functional materials.

Working with researchers at Queen Mary’s, who supplied the basic electromagnetic design methodology, BAE System engineers manufactured a composite structure—“essentially a millimeter-scale particulate filler dispersed within a polymer matrix”—that displays the specific pattern of dielectric permittivities that enabled the flattened device to emulate the wave-manipulating functions of the original spherical lens, Haq explained.

To make a flat panel redirect electromagnetic energy like a curved lens, a three-dimensional “map” is created, which lays out the physical properties that determine how the EM energy is to be refracted. This map is then used to deposit precise layers of a metamaterial in such a way that the performance of the curved lens is emulated by the flat panel.

“The mathematical technique transforms the properties of a given space or volume to another one, essentially discretizing the structure so that it can undertake the fabrication of the new device,” he said. The approach also allows the use of a degree of simplification in the pattern that nonetheless closely approximates the targeted performance, making manufacturing easier.

Transformation optics

Experts at Queen Mary’s developed mapping methodology using a mathematical technique known as transformation optics, whereby they analyzed the original lens’ ability to bend electromagnetic waves and calculated how that same functionality could be transferred to a different, more compact shape. For the Luneburg lens, he said, university researchers did this by modifying the volumetric distribution of the particulate filler so that the resulting dielectric permeability of the new device would mimic that of the original.

In some ways the approach is analogous to the creation of an optical Fresnel lens, in which curved sections of glass of a standard concave lens are “reassembled” in a flatter space, but would diffract light in much the same fashion as the original lens. The mathematics involved with the transformation optics procedure performed on antennas are considerably more complex, however, and the resulting metamaterial devices operate on the microscale, since they are to manipulate radio waves and microwaves rather than visible light.

“This is not a ‘conventional’ metamaterial antenna as has been developed previously,” Haq warned. First-generation metamaterial antennas are based on arrays or superlattices of active electronic elements laminated onto something like printed circuit boards, whereas the BAE System devices use only the dielectric or magnetic properties of substances distributed within bulk composite materials. Those earlier antennas, he stated, have limited bandwidth, whereas “ours can have a wide frequency range. The test item, for example, has a pretty wide bandwidth, from 1-2 GHz to 18-20 GHz.”

“The nice thing about this composites technology is its flexibility,” he said. “You can cast it or mold it to shape using standard composite methods, and you can change its electromagnetic properties as needed just by varying the polymer matrix or the filler, or the latter’s dispersion, orientation, grading, particle shape, and so forth.”

In practice, conductive particles, rods, and traces are used to tune the electric and magnetic parameters of bulk polymers or ceramics. It’s the feature-sizes of the fillers that determine the wavelength with which they most strongly interact. High-resolution 3D printing processes should help produce metamaterials with the precision structures that are needed to tailor the antennas’ electromagnetic properties.

“Right now, we’re doing a more detailed assessment of the benefits and trade-offs,” Haq noted. “We’re looking at a set of different options regarding systems issues, how to mature the technology and integrate it onto a platform.”

Beyond smaller antennas, he said, in the future metamaterial antennas may also perform their aerial magic by shrinking radar cross-sections, and masking reflective airplane features and signatures for stealth purposes.

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