Wireless electronic systems have been relying on dish antennas to send and receive signals. These systems have been widely used where directivity is important and many of those systems work well at a relatively low cost after years of optimization. These dish antennas having a mechanical arm to rotate the direction of radiation does have some drawbacks, which include being slow to steer, physically large, having poorer long-term reliability, and having only one desired radiation pattern or data stream.
The demands on Wireless communications and radar systems have been continuingly increasing as the need for accuracy, efficiency, and more advanced metrics become increasingly important. Many new applications will only be possible with antennas that consume less power in a lower profile than traditional mechanically steered dish antennas. For example, the ongoing New Space revolution which has planned up to 50,000 active satellites to be in orbit over the next 10 years. All these satellites have complex and variegated sets of orbits and waveforms that satellite communication (SATCOM) networks need to support. This drives the need for SATCOM operators to create flexible and adaptable networks capable of operating on a myriad of different waveforms, orbits, and constellations—while simultaneously maintaining service quality and profitability.
Researchers are always looking for new materials with novel properties. A metamaterial is a kind of artificial synthetic composite material with a specific structure, which exhibits properties not found in natural materials. They are formed by assembling multiple elements made from composite materials such as plastics or metals. Metamaterials have received increasing attention due to their unique electromagnetic properties. One of the most important applications of metamaterials is antenna design. Due to the unusual properties of metamaterials, we can achieve antennas with novel characteristics which cannot be realized with traditional materials. Recently, the use of metamaterials in antenna systems has resulted in performance enhancement to systems in many applications, including wireless communication, space communications, GPS, satellites, space vehicle navigation, airplanes, and military systems.
For broadband satellite communications applications where the platform is mobile, where the satellite is non-geostationary or both, a scanning antenna is required. The satellite communications industry, however, is dominated by dish antennas mounted on motorized gimbals for these applications. These solutions are too large, heavy, and power-consuming to offer solutions for consumer mobile applications such as the connected car or a personal satellite terminal.
Electronically steerable antennas (ESAs), often referred to as flat panels, are the critical link for next-generation constellations. Compared with their bulkier mechanical cousins, flat-panel antennas offer greater efficiency and performance while being modular and dynamically steerable—all of which are needed for the future ground segment.
While ESAs’ flat and conformal characteristics can have aesthetic benefits, the real benefit comes from their performance. LEO and MEO satellites require the ability to track and communicate with two or more satellites in view at the same time, and this can only be done with multiple mechanical antennas. With no moving parts, ESAs are more reliable and efficient as they can connect to multiple satellites at the same time. This gives a single ESA the ability to interoperate with multiple orbits—not just GEOs.
However, Phased array technology, is typically available only to government and military customers because of its expense and power consumption.
Metamaterials are a basis for further miniaturization of microwave antennas, with efficient power and acceptable bandwidth. Using metamaterials in metasurface antennas reduces the size of the antenna and increases the power it radiates, while also allowing it to absorb light waves. Conventional antennas that are very small compared to the wavelength reflect most of the signal back to the source. A metamaterial antenna behaves as if it were much larger than its actual size, because its novel structure it stores and re-radiates energy.
Metamaterials permit smaller antenna elements that cover a wider frequency range, thus making better use of available space for space-constrained cases. In these instances, miniature antennas with high gain are significantly relevant because the radiating elements are combined into large antenna arrays. Furthermore, metamaterials’ negative refractive index focuses electromagnetic radiation by a flat lens versus being dispersed. Established lithography techniques can be used to print metamaterial elements on a PC board.
The usage of metamaterial antennas have been increasing over the past few years. A company named Kymeta Corporation stated that its sales of Ku-band metamaterial antenna increased in 2018, as compared to previous years. The demand for such antennas is due to their use in vessels, radar, and special smart phones.
China’s new stealth fighter, the J-20, utilize metamaterials which could make it an electromagnetic force, according to state media. The metamaterials on the J-20 are likely to be used for as antennas and absorbers, given that the facility making them specializes in electromagnetic tech. Metamaterial antennas can increase radiated power, resulting in longer-range and more precise radar, as well as powerful jammers and datalinks. In turn, by fine-tuning their structures, metamaterial absorbers can be engineered to absorb specific wavelength ranges, such as those from the radars of enemy fighters and missiles. Such absorbers would likely be put on areas likely to reflect radar waves, such as the edges of canards, weapon bay doors, and engine nozzles.
Various types of metamaterial have been proposed with different characteristics, e.g. e.g. negative permittivity or permeability, zero refractive index, and huge chirality, etc. These unusual properties play an important role in modern antenna design, which can provide better performance, more functions, and more flexibility. Antennas employing metamaterials offer the possibility of overcoming restrictive efficiency-bandwidth limitations for conventionally constructed, miniature antennas. These novel antennas aid applications such as portable interaction with satellites, wide angle beam steering, emergency communications devices, micro-sensors and portable ground-penetrating radars to search for geophysical features.
Researchers have proposed two classification of metamaterial based antennas . The first category is the concept of a transmission line composed of a periodic repetition of a unit cell comprising a series capacitance and a shunt inductance. This category is a direct application of the leaky-wave metamaterial antennas, which consists of a cascaded series of unit cells lying on a matched microstrip line. This type is preferred for beam scanning applications.
In the second category are the resonant antennas, which, in opposition to the first category, are obtained by terminating the structure to the free space by a short or open circuit. The metamaterial based resonant types of antenna structures allow dual-band, multiband behaviours and can be miniaturized but do not increase the bandwidth of the antenna.
Some of the advantages of Metamaterial Antennas are:
- High gain, electrically configurable beam forming maximizes channel efficiency
- Ultra-fast reconfiguration allows SDAs to realign on a frame-by-frame basis
- Self-alignment eliminates the need for expensive technician installations or mechanical steering gimbals, as well as self-recovery from displacement
- Active dynamic null generation allows mitigation of interfering signals when used in cluttered spectrum
- Lightweight, compact and capable of being ruggedized for size-sensitive applications in harsh environments
- Conformal form factor enables geometry-flexible antennas to be placed where conventional antennas could not be located
- Support for a wide spectrum of frequencies across the RF, microwave, and millimeter wave spectrums
- It is difficult to manufacture metamaterial based antennas in large quantities.
- It works for limited range of wavelengths.
- The shape of the antenna can not be changed during operation.
- They are lossy.
RF Energy Harvesting Platform
RF Energy Harvesting PlatformAn RF energy harvesting platform that converts Wi-Fi and other RF bands to electricity, to power IoT sensors. It consists of a metamaterial-inspired antenna and a custom rectifying circuit. There are two classes of prototypes that we have demonstrated: hybrid (printed antenna with integrated silicon chips) and all-printed devices. The performance and bandwidth of the RF energy harvesters exceed by at least an order of magnitude that of the state of the art.
A Broadband Left-Handed Metamaterial Microstrip Antenna with Double-Fractal Layers
Antennas are essential for wireless communication systems. The size of a conventional antenna is dictated mainly by its operating frequency. With the advent of ultra-wideband systems (UWB), the size of antennas has become a critical issue in the design of portable wireless devices. Consequently, research and development of suitably small and highly compact antennas are challenging and have become an area of great interest among researchers and radio frequency (RF) design engineers
In commercial wireless communication systems, the antenna remains a key element of the communication chain. The efficiency of a radio broadcasting system is directly related to the characteristics of its antennas. In addition, future communication systems using cognitive radio or flexible radio will need smaller wideband antennas.
On one of the common antenna designs is microstrip patch antenna. This design has many advantages; it can be easily fabricated using a lithographic technique, it has a low profile, it has a low production cost, and its structure is fairly simple. However, these advantages are offset by the narrow bandwidth of the antenna. To date, several approaches have been proposed to address this deficiency. In most cases, the proposed solution was to increase the thickness and decrease the dielectric constant of the substrate at the same time. However, these attempts did not produce significant bandwidth enhancements in redesigned antennas.
With the development of new materials called left-handed materials (LHM), or left-handed metamaterials, it is possible to achieve a significantly wider frequency range. As a result, many antennas with LHMstructures with better performance than conventional microstrip patch antennas were proposed.
Planar left-handed metamaterial structures were proposed a few years ago. The discussed structures consist of 2D periodic arrays of unit cells. This concept was applied to LHMantennas, resulting in broadband and high gain designs. The periodic patterns which showed left-handed characteristics were applied to rectangular conventional microstrip patch antennas. These configurations allowed obtaining a frequency range several times wider than the same patch antenna without the metamaterial pattern.
Researchers Roman Kubacki and others from Military University of Technology,Warsaw, Poland have proposed a microstrip patch antenna based on the left-handed metamaterial concept, using planar periodic geometry, which results in improved characteristics. This periodic geometry is derived from fractal shapes, which have been widely used in antenna engineering. The metamaterial property was obtained as a result of the double-fractal structure on both the upper and the bottom sides of the antenna. The upper side of the antenna follows the shape of crossbar fractals, with Minkowski fractals on the lower layer. The proposed self similarity and ease of repetitiveness of the geometry make these designs attractive for creating a periodic structure.
The final structure has been optimized to enhance bandwidth, gain, and radiation characteristics of the microstrip antenna. This combination significantly improved antenna performance; our design could support an ultrawide bandwidth ranging from 4.1 to 19.4GHz, demonstrating higher gain with an average value of 6 dBi over the frequency range and a peak of 10.9 dBi and a radiation capability directed in the horizontal plane of the antenna.
Duke-Northrop Grumman Partnership Seeks to Unflatten Metamaterial Antennas
Researchers from Duke University are teaming up with colleagues at the University of Washington and industry experts at Northrop Grumman to develop metamaterial antennas tailored to the contours and angles of modern military aircraft. The collaboration is the result of a new pilot program from the Department of Defense called the Defense Enterprise Science Initiative (DESI). Backing five university-industry teams pursuing independent projects, DESI is focused on accelerating the impact of basic research on defense capabilities.
“Optimizing a metamaterial antenna’s shape for both aerodynamics and electromagnetic performance simultaneously is a challenging task,” said David Smith, the James B. Duke Professor of Electrical and Computer Engineering at Duke and one of the original co-inventors of the technology. “But with the help of Northrop Grumman’s expertise and capabilities in the realm of aeronautics, we’re confident we can produce aerodynamic metamaterial antennas with extraordinary mission flexibility.”
Metamaterials are artificial materials that manipulate waves like light and sound through properties of their structure rather than their chemistry. Most metamaterials are composed of a grid of repeating cells, each of which can be individually tuned to steer electromagnetic waves along a gradient—a concept called transformation optics.
Computational tools have been built to design metamaterials for reconfigurable antennas that can focus in any direction without moving. Several commercial companies have been founded on the technology, including Kymeta, which builds flat-panel satellite antennas, and Echodyne, which makes compact phased-array radar technology.
One feature both of these technologies share is that their devices are flat. Introducing curves and angles to the device itself wreaks havoc on the computations that allow the antennas to dynamically refocus. The new DESI grant seeks to change that by developing the tools needed to design and operate metamaterial antennas that can include curves and angles.
With these modeling tools in hand, the researchers will then build prototypes that can aerodynamically conform to the geometric needs of military aircraft and form predictable sets of radiation patterns. This flexibility will allow the same metamaterial antenna to rapidly reconfigure to switch between communications and radar taskings, for example. “Programs like DESI are vital to foster collaboration in the research ecosystem and accelerate the transition of ground-breaking basic science to transformative capabilities,” said Dr. Bindu Nair, Deputy Director for Basic Research at the Department of Defense. “I look forward to seeing how these teams can help us address our unique and challenging defense problem sets.”
Fractal Firm Confirms Breakthrough Metamaterial Antenna Technology
Fractal Antenna Systems has confirmed that it has developed a new proprietary antenna technology with broad applications, particularly in point to point access with directional antennas. The new technology is enabled by the firm’s fractal metamaterial discoveries and inventions. Fractal metamaterial devices are populated by closely packed ‘self similar’ shaped electromagnetic structures. Developed by the firm, the use of fractal metamaterials has already resulted in a broad range of critical attributes. Now magnification ability publicly joins the list of essential practical advantages.
The new antenna technology, referred to as “FM/R”, has the advantages of smaller size, wider bandwidths, and high efficiency, at high magnification, or “gain”. In addition, it has a unique characteristic of being nearly agnostic to its form factor shape. This means that most conventional, prescribed ‘fishbone’, ‘arrowhead’, and ‘bubble’ shapes for directional antennas are obsolete, or severely limited in comparisons of their footprint, supporting electronics, and cost. In addition, the FM/R antennas may replace several directional antennas at once, diminishing coveted tower and building real estate needs for antennas.
Notes CEO and inventor Nathan Cohen: “Others have oversold the case of metamaterials for lens-like applications, and ended up with ‘me-too’ technology of limited practical value. We’ve delivered on the promise.” Cohen attributes previous impediments to a failure to recognize the potential afforded by: “Greater sampling of the nearfield, through fractals. The physics was sound, but the assumptions about how to apply it were stuck in an age-old rut.”
Metamaterials surface antenna technology
For broadband satellite communications applications where the platform is mobile, where the satellite is non geostationary, or both, a scanning antenna is required. Metamaterials surface antenna technology (M-SAT) is an invention that uses metamaterials to direct and maintain a consistent broadband radio frequency beam locked on to a satellite whether the platform is in motion or stationary. Gimbals and motors are replaced by arrays of metamaterials in a planar configuration.
For broadband satellite communications applications where the platform is mobile, where the satellite is non geostationary, or both, a scanning antenna is required. The satellite communications industry, however, is dominated by dish antennas mounted on motorized gimbals for these applications. These solutions are too large, heavy, and power-consuming to offer solutions for consumer mobile applications such as the connected car or a personal satellite terminal. Another alternative is phased array technology, but this technology is typically available only to government and military customers because of its expense and power consumption.
Kymeta has addressed these obstacles by developing an electronically-scanned antenna technology, based on a diffractive metamaterials concept, called Metamaterial Surface Antenna Technology (MSAT). Electronic scanning is achieved through the use of high-birefringence liquid crystals. The use of liquid crystals (LC) as a tunable dielectric at microwave frequencies permits large-angle (> 60°) beam scanning with power consumption of < 10 Watts and antenna thickness ~ 5.0 cm, with no moving parts.
Kymeta’s engineering approach, through the use of LC and optimization of the materials and design for compatibility with liquid crystal display (LCD) manufacturing processes, positions the technology for mass production by leveraging the capital infrastructure of the LCD
Conventional three-dimensional metamaterials rely on bulky structures where resonant phenomenon are used to achieve the desired effective medium properties, e.g., negative refractive index. This resonant behavior dramatically limits their bandwidth, efficiency, and ultimate utility for point-to-point communications links. In addition the tolerances required to maintain narrow resonances over physically large structures (such as the aperture sizes required for Ku- and Ka-band satellite communications) prohibits the manufacturing of such materials at consumer electronics scale and cost.
Kymeta is leveraging a metasurface concept, in conjunction with holographic beamforming principles to commercialize MSAT. Metasurfaces have a number of advantages, namely that they take up less physical space and have the potential for less-lossy structures. Metasurfaces are characterized by both the periodicity of scatterers and thickness of the surface being small relative to the wavelength of interest.
In the MSAT approach, the metasurface is treated as a diffractive element, rather than a refractive one. The metasurface is introduced along one of the guiding surfaces of a guided-wave feed structure, such that the scattering elements are weakly coupled to the feed wave. In this particular example, the metasurface is placed across the broad wall of a rectangular waveguide.
The scattering elements in the metasurface radiate with a periodicity that corresponds to the
scan angle of the beam. The scattering strength is controlled by tuning the resonance frequency of the individual elements. Near their resonant frequency, the scattering elements couple energy from the feed wave and radiate this energy from the surface of the antenna. Detuned elements do not scatter energy from the feed wave at the frequency of interest.
Kymeta’s alpha prototypes have demonstrated combined transmit and receive functions from a single aperture, dynamically controllable polarization, ~ 30 dB of cross-polarization discrimination, full 360 degrees of azimuth scanning, and elevation scanning > 60 degrees with
negligible impact to return loss. The size, weight, power, performance and manufacturing of MSAT make it wellpositioned to address high-volume, low-cost mobile satellite opportunities.
Metamaterials Electronically Scanned Array (MESA)
A low-cost, high-performance RF beam steering module that can be adapted for a broad range of applications, including: collision avoidance system for self-driving cars or drones, broadband satellite internet/radio, hypothermia treatment, wireless communications, etc. The key performance feature of PARC’s MESA is its capability to maintain a high signal-to-noise ratio and high-resolution, simultaneously.
Metamaterial-Based Radar Lets Drones Fly beyond Visual Line Of Sight
This system uses metamaterials—engineered, artificial materials with properties not found in nature—to build a new architecture for an all-electronic scanning radar system. The use of metamaterials means MESA has significantly lower cost, size, weight and power-usage than other radar systems. The compact, lightweight MESA radar units have the potential for multiple applications in a variety of border security scenarios. CBP procured a pilot quantity of MESA radar units and intends to test their efficacy in two programs and evaluate them for the ability to improve border situational awareness.
“Radar is an ideal sensor technology for all sorts of scanning and imaging applications, especially when environmental conditions are less than ideal,” explained Thomas Driscoll, Chief Technology Officer for Echodyne. “Our radar thrives over other sensors in unpredictable weather conditions, can rapidly scan a broad field of view, can track Cessna-sized targets at distances greater than two kilometers, and dramatically increases situational awareness for UAS operators.”
Echodyne Corporation, a developer of metamaterials-based radar systems, says it has completed testing on its airborne Metamaterial Electronically Scanning Array (MESA)-Detect and Avoid (DAA) radar on a small Unmanned Aerial Vehicle (sUAV). “Echodyne’s airborne detect-and-avoid radar is made especially for small to medium UAS and enables safe beyond visual-line-of-site operations – in all environments and conditions,” said Jerry Hendrix, Executive Director for the LSUASC. “Before the MESA-DAA became commercially available, there were no options for long-range radar on small to medium commercial drones.”
In addition to this testing, Echodyne’s solution is currently being used as the primary detection and cueing component on autonomous surveillance towers currently deployed in the San Diego Sector . These towers are being piloted with the potential of incorporation into border surveillance programs.
Echodyne’s radar array is made of multiple layers of carefully patterned copper wiring, and beam control results from heating specific areas of the wiring, according to IEEE Spectrum. The smaller design is less powerful and has shorter range, but it also is more affordable to build, and is no less than effective for most commercial applications.
METamaterials for Active ELEctronically Scanned Arrays (METALESA)
An AESA, as core component of modern military radar systems, is a type of phased array, whose transmitter and receiver functions are composed of numerous small transmit/receive modules.
The objective of the METALESA project was to employ MetaMaterials in order to increase the efficiency and reliability of operating radar systems Electromagnetic MTMs are artificial materials with unusual macroscopic propagation properties of electromagnetic waves, which are normally generated by microscopic periodic metallo-dielectric structures.
Four main topics were identified as critical in the AESA design process, where MTM concepts could be applied, and have been analysed, prototyped and tested methodically in the project:
- Expensive RF feeding networks, with considerable space requirements.
- The coupling between radiating elements of the antenna array is the principal source of the scanning angle limitations, causing the undesired angular blind spots.
- The parasitic back-lobe and side-lobe radiation caused by the antenna´s finite dimensions can cause unwanted disturbances of other systems or in the system itself.
- A MTM based radome is proposed to reject the out of band interference and simultaneously, the in band back-lobe and side-lobe radiation
Smart Metamaterial Antennas
Highly reconfigurable metamaterial antennas are a natural evolution of the MESA architecture. They are tailored for 4G LTE/5G bay stations and for satellite communications.
US DOD’s Requirement of Antenna Electronic Beam Steering for Missile Seekers
US DOD has issued SBIR to investigate low-cost alternatives to steerable antennas for the munitions application. The performance enhancements afforded by electronically steerable antennas is of high interest to the radar seeker community. Traditionally phase array antennas require beam forming networks with distributed phase shifters or time delay mechanisms and additional control circuits, to perform beam steering that lead to expensive and complicated circuitry not economically feasible for use in small missile radar seekers.
Recent breakthroughs in engineered electronic and electromagnetic materials and continuous transverse stub arrays have made agile, reconfigurable apertures possible where the beam-forming function is integrated in to the aperture. These technologies are opening avenues to provide new levels of real-time control of the aperture and performance as well as affordability, says SBIR.
Army Aviation requirement
Army aviation has the need of a broadband antenna with reduced size and weight while obtaining enhanced power efficiency and ultrawide bandwidth (UWB) frequency capability. Such a capability would contribute to the Army’s superiority in multiple abilities, to include conducting joint air-ground operations, maintaining manned-unmanned teams, and operating in contested airspace. In addition, a physically low profile is needed for airborne applications to maintain aircraft aerodynamics and minimize surface clutter and interaction with other pre-existing equipment. Airborne platforms have limited surface area for antenna mounting, and it is often non-electrically conductive. These constraints require the antenna(s) to be “electrically small,” with a total height less than one quarter of a wavelength at its center frequency. It is envisioned that emerging developments and technology using metamaterials could provide a solution for such an antenna.
The focus of this SBIR is to conduct research culminating in the development of a metamaterial-based antenna that will provide the desired performance for aircraft applications. Metamaterials are artificial engineered materials that possess a negative index of refraction for an isotropic medium. Their properties can be tailored to provide anomalous interactions with the electromagnetic field. These electromagnetic interactions include the effect of causing the electromagnetic waves phase and group velocity to have a reversed direction of propagation with the respect to the direction of energy flow, as well as, preventing reflected and scattered radiation return on active antenna surfaces. Utilization of left-handed materials (LHM), such as double negative materials (DNG) and epsilon near-zero permittivity (ENZ) materials enable the antenna size to be reduced while increasing the antenna efficiency and bandwidth. Legacy techniques, such as, utilization of split ring resonators (SRR) and complimentary SRRs (CSRR) in the substrate and superstrate of the PCB based antennas are known to increase the radio frequency (RF) bandwidth and directivity by manipulation of permittivity and permeability in specific frequency bands.
Much metamaterial research and work has been competed during the last fifteen years. The challenge presented in this SBIR will be to combine multiple efforts and create a product that can perform in the aviation adverse environment and endure that grueling environment without failure in high operational situations. This also implies that the antenna must survive in extreme environmental conditions, to include heavy rainfall, high heat desert type temperatures, very cold environments covered with ice and snow, and continual abrading due to small particles such as sand. The antenna is expected to interface with a 50 Ohm coaxial cable with a suitable high frequency capable connector. If power is required, it will be derived from 28 VDC aircraft power.
Metamaterials Antenna Market
The global aerospace and defense industry is estimated to dominate the metamaterial market during the forecast period 2018-2023. Reliable aerospace, defense, and military communications depend on advanced antenna technologies. The success of network centric operations on the digital battlefield requires reliable, efficient, and real-time communications to present accurate information to the right person at the appropriate time. The antenna is an integral, if not often overlooked, component of military, aerospace, and defense systems. Additionally, military antennas are being used in radars to provide missile guidance for accurate target tracking.
The rise in border infiltration, terrorism activities, inter country conflicts, and attack prone borders are expected to drive the demand for military antennas. High defense budgets of various countries and increasing demand for economic and security issues have increased the need for RADAR systems. Additionally, technological advancements and increasing demand for ranging and detection of objects are fuelling the demand for RADAR systems. As the defense spending and production of aircrafts and satellites continue to grow at a steady rate, the demand for antennas is also set to witness a healthy growth rate.
Major Players: Echodyne Corp., Evolv Technologies Inc., Fractal Antenna Systems Inc., NKT Photonics A/S, JEM Engineering LLC , Kymeta Corporation, Metamaterials Technologies Inc., MetaShield LLC, TeraView Ltd, and Multiwave Technologies AG, among others.
Metamaterials in Antenna Design
One of the most important applications of metamaterials is antenna design. Due to the unusual properties of metamaterials, we can achieve antennas with novel characteristics which cannot be realized with traditional materials.
Electrically small antennas based on zeroth resonant mode
In mobile communication systems, electrically small antennas (ESA) are desired. Modern integrated circuit technology has the ability to miniature circuits to a very small size. However, in a traditional design, the performance of the antenna is related with its size. The antenna usually has dimensions in the order of the operating wavelength. This sets boundaries for the size of the whole system.
A ZIM medium, whose refractive index is near zero, shows an operating wavelength that is infinite at an arbitrary designed frequency. This phenomenon is named zeroth resonant mode. Since the wave number in this antenna is zero, in theory, the physical size of the antenna can be made independent of its working frequency. Because the operating wavelength is infinite, the field distribution and the radiation pattern are different from the normal ones.
Dual-band and multi-band antennas
Normal dual-band antennas are realized with different resonant structures, or different resonant modes in one structure. The main disadvantage of this technique is that the field distributions in these structures can hardly be the same in both bands. This means that the radiation patterns in the operating bands are different. Since metamaterials can support a negative refractive index, the resonant modes can be selected as a symmetric pair, i.e. so-called negative and positive modes. The field distributions of these two modes can be very similar, and thus also the radiation patterns.
Negative and positive modes can be designed together with a zeroth-order mode. This yields a multi-band antenna with a specific pattern for each mode. An extra advantage of a metamaterial-loaded multi-band antenna is the fact that its size is usually smaller than in a traditional design, where the size is decided by the lowest operating frequency.
Low Profile planar reflectors
In an electric dipole antenna positioned parallel on top of a PEC plane, the distance between the dipole antenna and the reflector should be approximately a quarter wavelength. Indeed, since the reflective phase at the PEC plane is 180°, the radiation of the image of the electric dipole will start to cancel the radiation of the dipole itself if it is located closer to the reflector.
However, if the reflector is a PMC plane, the reflective phase is zero, and the image of the electric dipole will enhance the radiation when the dipole is located near the PMC plane. This technique allows designing low profile reflectors for electric dipole antennas.
Conversely, magnetic dipoles, in practice realized by slots or apertures in a ground plate, are also not suitable for placement near any PEC plane because of the generation of parallel plate modes between the two metal planes, which considerably distorts the characteristics. An AMC plane can help to suppress any parallel plate modes. Also in this case, low profile structures become feasible.
Antenna lenses and polarizers
Dielectric lenses can be used to improve the directivity and gain of an antenna. However, the cost to fabricate a 3D lens is large. Further, the location of the lens should be carefully chosen in relation with the phase centre of the antenna. A metamaterial lens can be formed by a flat 2D structure. Their manufacturing cost is much lower. They can even be integrated with the planar antenna structure to reduce the profile and size of the antenna system.
A polarizer can be based on a chiral medium which has the capability to transform a linearly polarized wave into a circularly polarized wave. This opens a way to design circularly polarized antennas based on existing linearly polarized antennas.