Antenna Designing Using Metamaterial
Recent advances in metamaterials research have highlighted the possibility to create novel devices with unique electromagnetic (EM) functionality. Indeed, the power of metamaterials lies in the fact that it is possible to construct materials with a user-designed EM response at a precisely controlled target frequency. This is especially important for the technologically relevant terahertz (THz) frequency regime with a view toward creating new component technologies to manipulate radiation in this hard to access wavelength range. Considerable progress has been made in the design, fabrication, and characterization of metamaterials with HFSS at THz frequencies. The several antennas based on metamaterials are designed, including dual-band patch antennas based on the linear composite right/left-handed transmission line, radial patch antennas operating at zeroth-order mode and negative modes, and low-profile antennas loaded with an artificial magnetic conductor plane. All the antennas are designed using simulations and verified through experiments. Their performances improve compared with conventional antennas.
Recently, artificially structured electromagnetic (EM) materials have become an extremely active research area because of the possibility of creating materials which exhibit novel EM responses not available in natural materials. Artificially structured materials with precise shape, geometry, size, orientation and arrangement of metamaterials give smart properties to electromagnetic waves e.g blocking, absorbing, enhancing, or bending waves.
This includes negative refractive index 1, super resolution imaging 2,invisibility cloaking 3, and more generally, coordinating transformation materials. This has generated tremendous worldwide interdisciplinary efforts including physicists, material scientists and engineers. For the most part, these composites, often called metamaterials, are subwavelength composites, where the EM response originates from oscillating electrons in highly conducting metals such as gold or copper allowing for a designed specific resonant response of the electrical permittivity (?=?1+i?2) or magnetic permeability (?=?1+i?2).
Continuous media with negative parameters, namely, with negative ?r or ?r have been known in EM theory for a long time. The Drude–Lorentz model, applicable to many materials in nature, predicts that above resonance there exists a region, where ?1 or ?1 is negative. If the losses are sufficiently low, it becomes possible to take advantage of this negative response. Media with negative ?1 over a broad frequency range are found in nature 4. The best-known examples are metals or doped semiconductors, where ?1 is negative below the plasma frequency 5. However, media with negative ?1 are less common in nature, which is partly due to the weak magnetic interactions in most of the naturally existing materials. At terahertz (THz) frequencies, it has long been known that antiferromagnets such as FeF2 or MnF2 exhibit a resonant magnetic response. In artificial materials, negative ?1 was first realized with split-ring resonators (SRRs). SRRs are composed of metallic rings with gaps as theoretically introduced by Pendry et al. 6 and experimentally verified by Smith et al. in 2000 7. As shown in Fig. 1, an appropriately designed combination of two sets of resonators, namely, metallic wires and SRRs, can yield negative ?1 and ?1 in the same frequency band resulting in a negative refractive index. Furthermore, these metamaterials structures are scalable to operate over most of EM spectrum spanning from microwaves to optical frequencies 8.The THz regime of the EM spectrum extends from 100 GHz to 10 THz (1 THz = 1012 Hz, where 1 THz corresponds to a wavelength of 300 ?m and photon energy of 4.1 meV) 9. This region, alternatively called the far-IR, lies below the visible and IR frequencies and above the microwave frequencies, as shown in Fig. 1. The response of natural materials, arising from interactions of the EM field with the electron, forms the basis for the construction of most modern devices. However, the nature of the EM response of materials changes as a function of frequency. At frequencies of a few hundred gigahertz and lower, the motion of free electrons forms the basis of most EM devices. On the other hand, at IR through optical/UV wavelengths, photon-based
Figure 1:THz regime of the EM spectrum extends from 100 GHz to 10 THz, which lies below visible and infrared (IR) wavelengths and above microwave wavelengths.
devices are dominant. In between these two regions, there exists the so-called “THz gap,” where the efficiency of electronic and photonics responses tends to taper off. As such, the THz regime is arguably the least developed and least understood portion of the EM spectrum scientifically and technologically. Recently, there have been important advances using electronic and optical techniques to generate and detect THz radiation. During the past two decades, significant progress has been achieved in THz science and technology. As examples, the emergence of THz timedomain spectroscopy (THz-TDS) 10 and THz quantum cascade lasers 11 have been spectacular in advancing the state of the art. Since the EM response of metamaterials can be designed over a large portion of the EM spectrum by, to first order, simply scaling the dimensions of the structures, metamaterials have played an increasingly important role particularly in the construction of functional THz devices. For THz metamaterials, the unit cell is few tens of microns with critical feature sizes of a few microns. For these length scales, conventional microfabrication techniques offer considerable flexibility to experimentally realize novel metamaterials structures and devices. This paper focuses on metamaterials that are designed to operate in the THz regime with the emphasis on the recent progress on developing THz metamaterials devices, which may have real-world applicability.
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.
1. 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. 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 12.
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. 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 13.
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 14.
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 center of the antenna 15. 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.
5. Other antennas and structures involving metamaterials
There are a lot of other types of antennas and structures involving metamaterials, e.g. leaky wave antennas, magnetodielectric microstrip antennas, ultra-wideband (UWB) antennas with notched bands, metamaterial based isolators, series power divider, dual-band splitters and delay lines. All of these designs have a relatively better performance than the corresponding conventional designs.
Electrically small antennas continue to be a critical enabling technology for many wireless applications. The design, fabrication, and testing of a variety of metamaterial-inspired near-field resonant parasitic antenna systems have demonstrated that they can meet many of the performance demands for those applications, including high efficiency, broad bandwidth, and multifunctionality.
Electromagnetics, classical optics, microwave and antenna designing ,optoelectronics, materialsciences, nanoscience and semiconductor engineering are the basic applications of metamaterials. If metamaterial used it as a substrate in antenna designing the gain, return loss and bandwidth improves significantly. Metamaterials antenna used to improve radiated power which give high efficiency and broad bandwidth. It is due to negative permeability it has ideal properties e.g small antenna size, high directivity and tunable frequency.There will be certain additional fundamental advances during the next decade coupled with the implementation of metamaterials into real-world THz applications.
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