Metamaterial antenna
Metamaterial antennas are a class of antennas which use metamaterials to increase performance of miniaturized 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 or double negative metamaterial 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 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 with DNG slabs, or epsilon-negative slabs with mu-negative slabs are employed in the subsystems. Antenna subsystems that are currently being researched include cavity resonators, waveguides, scatters and antennas. 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
et al. were able to show that a three-dimensional array of intersecting, thin wires could be used to create negative values of permittivity, and that a periodic array of copper split ring resonators could produce an effective negative magnetic permeability.In May 2000, a group of researchers, Smith et al. were the first to successfully combine the split-ring resonator, 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 metamaterials was introduced that employs periodic reactive loading of a 2-D transmission line as the host medium. This configuration used positive index material with negative index material. 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.
Phase compensation due to negative refraction
DNG can provide phase compensation due to their negative index of refraction. This is accomplished by combining a slab of conventional lossless DPS material with a slab of lossless DNG metamaterial.DPS has a conventional positive index of refraction, while the DNG has a negative refractive index. Both slabs are impedance-matched to the outside region. The desired monochromatic plane wave is radiated on this configuration. As this wave propagates through the first slab of material a phase difference emerges between the exit and entrance faces. As the wave propagates through the second slab the phase difference is significantly decreased and even compensated for. Therefore, as the wave exits the second slab the total phase difference is equal to zero.
With this system a phase-compensated, waveguiding system could be produced. By stacking slabs of this configuration, the phase compensation would occur throughout the entire system. Furthermore, by changing the index of any of the DPS-DNG pairs, the speed at which the beam enters the front face, and exits the back face of the entire stack-system changes. In this manner, a volumetric, low loss, time delay transmission line could be realized for a given system.
Furthermore, this phase compensation can lead to a set of applications, which are miniaturized, subwavelength, cavity resonators, and waveguides with applications below diffraction limits.
Transmission line dispersion compensation
Because of DNG's dispersive nature as a transmission medium, it could be useful as a dispersion compensation device for time-domain applications. The dispersion produces a variance of the group speed of the signals' wave components, as they propagate in the DNG medium. Hence, stacked DNG metamaterials could be useful for modifying signal propagation along a microstrip transmission line. At the same time, dispersion leads to distortion. However, if the dispersion could be compensated for along the microstrip line, RF or microwave signals propagating along them would significantly decrease distortion. Therefore, components for attenuating distortion become less critical, and could lead to simplification of many systems. Metamaterials can eliminate dispersion along the microstrip by correcting for the frequency dependence of the effective permittivity.The strategy is to design a length of metamaterial-loaded transmission line that can be introduced with the original length of microstrip line to make the paired system dispersionless creating a dispersion-compensating segment of transmission line. This could be accomplished by introducing a metamaterial with a specific localized permittivity and a specific localized magnetic permeability, which then affects the relative permittivity and permeability of the overall microstrip line. It is introduced so that the wave impedance in the metamaterial remains unchanged. The index of refraction in the medium compensates for the dispersion effects associated with the microstrip geometry itself; making the effective refractive index of the pair that of free space.
Part of the design strategy is that the effective permittivity and permeability of such a metamaterial should be negative – requiring a DNG material.
Innovation
Combining left-handed segments with a conventional transmission line results in advantages over conventional designs. Left-handed transmission lines are essentially a high-pass filter with phase advance. Conversely, right-handed transmission lines are a low-pass filter with phase lag. This configuration is designated composite right/left-handed metamaterial.The conventional Leaky Wave antenna has had limited commercial success because it lacks complete backfire-to-endfire frequency scanning capability. The CRLH allowed complete backfire-to-endfire frequency scanning, including broadside.