So many different types of antennas have been used for EW, SIGINT, and radar it might be hard to believe that others remain unexplored. But just as the challenges faced by these systems increase, antenna technology must meet them, and there remains no shortage of new problems to solve. What’s different today is that DoD is not mostly alone in facing them, as the commercial wireless industry has issues that, if not entirely the same, are equally daunting.
They’re due in large part to the enormous promises the industry has set forth for itself. So vast in fact that it encompasses virtually every commercial application and some that have not even yet emerged. In addition to the wireless industry, the booming market for small satellites is creating the need for more advanced antennas. Both DoD and commercial markets have equal need to ensure the security of GPS. All these markets are fertile ground for antenna R&D, and advances are taking place in every area from materials through systems.
There’s no better place to start this discussion than EW because of its inherent need to receive and transmit over very wide bandwidths and increasing threats faced at higher frequencies. So, antennas that can achieve both requirements (and others) while reducing size and cost, and increasing overall performance are extremely appealing. With the most boots on the ground, the Army has sometimes relied on antennas that hindered movement, exposed positions, and burdened warfighters with everything from HF radios to satcom terminals. These challenges make for a great “new problem” to be solved by designers.
The AESA architecture (Figure 1), which is on most people’s list of the greatest advances for radar and EW, is also undergoing regular improvement. Although most of the media attention focuses on RF power generation (GaN), receiver performance, signal processing, computational prowess, and higher efficiency, the array itself is a subject of extensive R&D. For example, MIMO and digital beamforming, now a staple of devices from Wi-Fi access points to base stations, and even smartphones, is also a candidate for radar (and EW) systems. There is debate over its advantages when compared to traditional active phased arrays that have the same capabilities and already employ digital beamforming along with some features of MIMO.
Some of the most advanced concepts are coming from the commercial market, where the fifth generation of the wireless industry is faced with the self-inflicted wound of operating at very high frequencies that only a decade ago would have been considered fanciful, at best. This is great news for DoD, as it is accelerating the development of the AESA architecture, the use of new types of materials, advanced MIMO for radar and EW, and others as well. This work is being conducted in the U.S., Europe, and Asia, and although the initial applications are typically for enabling 5G, the benefits in most cases are also well suited for defense applications.
The development of millimeter-wave radar for defense systems, which has been used for years in missile seekers, has also benefited from the need for better radars used in most vehicles, even at the lower end of the market. The systems are beginning to incorporate more sophisticated capabilities, such as providing high angular resolution in azimuth and elevation, which provides a far better view of the environment. In addition, they will eventually employ phased-array antennas along with digital rather than analog beamforming, which although complex, reduces the number of RF components, a key consideration within the confines of a vehicle.
Another application for which millimeter-wave antenna design is critical is the passive millimeter-wave camera that, along with lidar and electro-optical sensors, is one of the fundamental sensors in enhanced vision systems (EVS) that enhance situational awareness for pilots by showing everything present in the scan, including terrain, buildings, highways or obstacles. Their predecessor, the synthetic vision system (SVS) is a standard component in the cockpits of commercial airliners and increasingly in military aircraft and UAS.
Millimeter-wave technology generates interest due to its ability to penetrate clouds, fog, rain or sandstorms. In fact, attenuation is about a million times smaller than visible light. Passive Millimeter Wave (PMMW) cameras (Figure 2) do not emit radiation. They collect radiation naturally emitted by objects or coming from other sources that reflect in objects. A Passive Millimeter Wave camera obtains a multidimensional map from the radiation of the scene, reflected or emitted by objects.
There are four types of PMMW imaging systems: mechanical scanning, phased-array, synthetic aperture and focal-plane array (FPA); among which the single channel mechanical scanning imaging system is currently more favorable due to its simplicity and low cost.
One of the most promising technologies for antenna construction involves the use of metamaterials, which although having been explored in the early 1900s, are only recently beginning to reach their true potential. The use of metamaterials in antenna design not only dramatically reduces antenna size but can also improve other antenna parameters such as enhancing bandwidth, increasing gain, or generating multiband frequencies of operation.
Although extensive research has been conducted by industry, DoD, and academia, materials that have a negative index of refraction are of the most interest. They’re called, appropriately, negative-index metamaterials (NIMs), as they have a negative refractive index (smaller size) than the wavelength of the frequency range applied to them.
Metamaterials are artificial materials that possess characteristics not commonly found in natural materials. They’re made from composites such as metal or plastic that contain elements (unit cells) that are arranged in repeating patterns to form an array. Each unit cell is tuned to respond in a specific way based on the desired characteristic of the array. The geometry, size, and orientation of cells in the array allows them to block, absorb, enhance, or bend electromagnetic waves.
This allows control over material parameters known as permittivity and magnetic permeability that together determine the propagation of electromagnetic waves, whether in the region of radio wavelengths or light. The result is a structure that can achieve remarkable capabilities that are beyond the means of conventional materials. When arranged in an array on an electrically-thin surface, the resulting antenna can achieve the same performance as conventional three-dimensional antennas. In addition, their smaller size reduces loss over the array, and their fabrication cost is also low because they can be printed using standard lithographic processes.
A discussion of metamaterials in an antenna context requires ignoring conventional antenna theory. That is, they are inherently “electrically shortened,” which in traditional antenna theory translates into reduced efficiency—the shorter the antenna compared to the operating frequency of the system, the more inefficient it will be. However, while a metamaterial-based antenna is very electrically short at a given frequency (about 1/10th of a wavelength or less), its properties are not the same as conventional types and cannot be considered in the same discussion.
In natural materials, properties such as magnetic permeability and electric permittivity are determined by the response of the material’s atoms and molecules to the electromagnetic wave passing through it. In metamaterials, these properties are determined by the arrangement of the unit cells, and these small structures can be designed to interact with electromagnetic waves to create finely tuned resonances and other properties.
One of the leaders in making these antennas commercially available is Kymeta, whose antennas use a diffractive rather than a refractive surface to define an antenna beam holographically. This method of forming an electromagnetic beam is known as holographic diffraction, and Kymeta’s commercial approach to fabrication is called metamaterial-surface antenna technology (Figure 3).
This holographic approach to beamforming uses the resonant frequency of each unit cell to create a dynamically reconfigurable diffraction grating. An antenna with hundreds of thousands of elements is placed next to a broad wall of a rectangular waveguide feed structure that couples all the elements to an electromagnetic wave generated by a single RF power source.
The elements are spaced so that their radiated waves are in phase (i.e., coherent) at the desired scan angle of the beam to scatter strongly, while elements out of phase are detuned and do not radiate. The scan angle is defined as the angle between the beam and an axis that is perpendicular to the plane of the antenna surface. The Kymeta metasurface uses the controllable permittivity of liquid crystals to tune each radiating element. The design is compatible with liquid crystal display fabrication processes, positioning this technology for high-volume production by leveraging the established manufacturing infrastructure of the LCD industry.
The antenna technologies described above are just a few of the many advances being made to accommodate both 5G, IoT, and defense systems. As the wireless industry moves toward 6G at the end of the decade and defense systems operate at higher wavelengths, everything from materials science to antennas architectures and software will converge to solve the problems inherent when operating at frequencies of 100 GHz and perhaps higher.