Metamaterial antennas are a class of antennas which use metamaterials to increase performance of miniaturized (electrically small) antenna systems. Their purpose, as with any electromagnetic antenna, is to launch energy into free space. However, this class of antenna incorporates metamaterials, which are materials engineered with novel, often microscopic, structures to produce unusual physical properties. Antenna designs incorporating metamaterials can step-up the antenna's radiated power. 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 stores and re-radiates energy. Established lithography techniques can be used to print metamaterial elements on a printed circuit board. 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. Some applications for metamaterial antennas are wireless communication, space communications, GPS, satellites, space vehicle navigation and airplanes.
Antenna designs Antenna designs incorporating metamaterials can improve the radiated power of an antenna. The newest metamaterial antennas radiate as much as 95 percent of an input radio signal. Standard antennas need to be at least half the size of the signal wavelength to operate efficiently. At 300 MHz, for instance, an antenna would need to be half a meter long. In contrast, experimental metamaterial antennas are as small as one-fiftieth of a wavelength, and could have further decreases in size. Metamaterials are a basis for further miniaturization of microwave antennas, with efficient power and acceptable bandwidth. Antennas employing metamaterials offer the possibility of overcoming restrictive efficiency-bandwidth limitations for conventionally constructed, miniature antennas. 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.
The DNG shell The earliest research in metamaterial antennas was an analytical study of a miniature dipole antenna surrounded with a metamaterial. This material is known variously as a negative index metamaterial (NIM) or double negative metamaterial (DNG) among other names. This configuration analytically and numerically appears to produce an order of magnitude increase in power. At the same time, the reactance appears to offer a corresponding decrease. Furthermore, the DNG shell becomes a natural impedance matching network for this system.
Ground plane applications Metamaterials employed in the ground planes surrounding antennas offer improved isolation between radio frequency, or microwave channels of (multiple-input multiple-output) (MIMO) antenna arrays. Metamaterial, high-impedance groundplanes can also improve radiation efficiency and axial ratio performance of low-profile antennas located close to the ground plane surface. Metamaterials have also been used to increase beam scanning range by using both the forward and backward waves in leaky wave antennas. Various metamaterial antenna systems can be employed to support surveillance sensors, communication links, navigation systems and command and control systems.
Novel configurations Besides antenna miniaturization, the novel configurations have potential applications ranging from radio frequency devices to optical devices. Other combinations, for other devices in metamaterial antenna subsystems are being researched. Either double negative metamaterial slabs are used exclusively or combinations of double positive (DPS) with DNG slabs, or epsilon-negative (ENG) slabs with mu-negative (MNG) slabs are employed in the subsystems. Antenna subsystems that are currently being researched include cavity resonators, waveguides, scatters and antennas (radiators). By integrating dynamic elements, such as liquid crystals, into their structure, metamaterial antennas can be made tunable within different frequency ranges, from optical to RF. Metamaterial antennas were commercially available by 2009.
History Pendry et al. were able to show that a three-dimensional array of intersecting, thin wires could be used to create negative values of permittivity ("ε"), and that a periodic array of copper split ring resonators could produce an effective negative magnetic permeability ( "μ"). In May 2000, a group of researchers, Smith et al. were the first to successfully combine the split-ring resonator (SRR), with thin wire conducting posts and produce a left-handed material that had negative values of ε, μ and refractive index for frequencies in the gigahertz or microwave range. In 2002, a different class of negative refractive index (NRI) metamaterials was introduced that employs periodic reactive loading of a 2-D transmission line as the host medium. This configuration used positive index (DPS) material with negative index material (DNG). It employed a small, planar, negative-refractive-lens interfaced with a positive index, parallel-plate waveguide. This was experimentally verified soon after. Although some SRR inefficiencies were identified, they continued to be employed as of 2009 for research. SRRs have been involved in wide-ranging metamaterial research, including research on metamaterial antennas. A more recent view is that by using SRRs as building blocks, the electromagnetic response and associated flexibility is practical and desirable.
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