INVITED ARTICLE
Jiahao Zhang, Sen Yan, Guy A.E. Vandenbosch
IMAGES LICENSED BY INGRAM PUBLISHING
With the development of modern wireless communication systems, the demand for high-performance, multifunctional, low-cost, and compact antennas has been increasing substantially. The greatest challenge when designing this type of antennas is the tradeoff between complexity and performance. More design freedom is therefore highly desired. As an important subtype of metamaterials, composite right/left-handed transmission line (CRLH-TL) metamaterials have paved the way to achieve this compromise. In this review article, first, the possible CRLH-TL topologies that can be used in antenna design are discussed, involving structures based on not only Cartesian but also polar coordinates and 2D CRLH-TLs. The reconfigurability of the CRLH-TLs in terms of operating mechanisms, reconfigurability methods, and control techniques is then reviewed. By applying these structures to electrically small antenna design, considerable extra design freedom can be obtained. In the literature, a whole range of novel antennas with remarkable performances have been reported, holding great potential for various applications, such as wireless body area networks (WBANs), multiple-input, multiple-output (MIMO), radio-frequency identification devices (RFIDs), wireless power harvesting, and so on. All of these applications are briefly discussed.
With the development of modern communication systems, a great variety of applications and technologies proliferate rapidly, which triggers the demand for highly efficient multifunctional antennas for both smart devices and processing centers. The possible scenarios involving indoor and wearable applications for modern small antennas are shown in Figure 1 and illustrated in [1], [2], and [3]. To meet complex and various requirements, more degrees of design freedom are necessary in designs. Small antennas that can support as many services as possible while maintaining good performance, with a low fabrication cost and occupying a miniaturized space, have attracted more and more research attention. Mature small antenna topologies include traditional dipole antennas, loop antennas, microstrip antennas, slot antennas, and so on. Developed from these basic antennas, novel designs are proposed to meet increasing demands, including planar inverted-F antennas, folded patch antennas, differentially fed antennas, Huygens dipoles, and so on [4], [5]. Excellent performance in a specific area is achieved in these designs. However, the greatest challenge is the tradeoff between complexity and the multiple functions since the degree of freedom is limited.
Figure 1. (a) A multistandard base station (BS) antenna for indoor applications [1]. (b) A frequency-tunable access point antenna for indoor scenarios [2]. (c) A pattern-reconfigurable antenna for on-body applications [3]. ISM: Industry, Science, Medicine; WiMAX: Worldwide Interoperability for Microwave Access.
As a kind of planar metamaterial, the CRLH-TL is widely used in antenna and circuit design [1]. By introducing LH structures, new characteristics are obtained compared with traditional right-handed (RH) TLs, and thus, more design freedom can be achieved. In recent years, many new concepts have been introduced into CRLH-TL-based antennas: novel topology-based CRLH-TL antennas [2], [7], [8], [9], reconfigurable CRLH-TL-based antennas [3], [10], [11], [12], and CRLH-TL antennas realized in new materials [11], [13], [14], [15]. By employing these concepts, multifunctional antennas for modern systems are realized.
The performances and characteristics of metamaterials highly depend on their topologies. CRLH-TLs are usually realized in a linear (1D) topology [16], [17]. When the unit cells are replicated along two orthogonal Cartesian directions, the 2D CRLH-TL is realized [19], [20], [21]. The characteristics of the 3D CRLH-TL are also studied in [22]. The CRLH-TL topology based on polar coordinates has been described rather recently, including the annular CRLH-TL topology and the radial CRLH-TL topology [2], [8], [23]. Inspired by the unique characteristics of annular and radial propagation, many novel small antennas have been proposed. Similar to 2D CRLH-TLs in Cartesian coordinates, 2D CRLH-TLs in polar coordinates have also been studied [1].
Reconfigurable metamaterials are drawing a lot of attention now since they give the designer an extra set of degrees of freedom for the synthesis of innovative active systems. The unique electromagnetic field manipulation properties enabled by reconfigurable metamaterials make them an attractive solution in a wide set of applicative scenarios [24], including antenna design [25]. Reconfigurability is mostly done in an electric way by using RF microelectromechanical systems (MEMS) [26] or, mainly, p-i-n diodes [3] and varactor diodes [2], because of the low cost, simple control strategy, and ease of integration. By combining reconfigurability with CRLH-TL topologies, novel designs with various advantages have been achieved thanks to the extra design freedom. This advantage is crucial for multifunctional and high-performance electronically small resonant antennas.
In this review article, three main aspects of CRLH-TL-inspired small antennas are reviewed: topology, reconfigurability, and applications. Traditional CRLH-TL topologies and novel CRLH-TL topologies in polar coordinates are reported in the “Topologies” section. The latest progress in reconfigurable CRLH-TL-inspired antennas is discussed in the “Reconfigurability” section, including the operating mechanism and the reconfigurability and control methods. Based on the novel topologies and various reconfigurability approaches, tremendous applications can be reached based on CRLH-TL metamaterial-based structures. This is discussed in the “Operational Features” section. Possible future research is sketched in the “Dedicated Applications” section. Note that although CRLH-TLs are also widely implemented in leaky wave antennas and arrays, this topic is beyond the scope of this review article. Supplementary material accompanying this article is available at https://www.doi.org/10.1109/MAP.2022.3201194.
The conventional CRLH-TLs are normally constructed in linear 1D topologies [see Figure 2(a)]. The mushroom structure-based CRLH-TL unit cell is shown [6], where the yellow indicates metal and the cylinder indicates a shorted pin. By introducing the LH characteristics, the zeroth mode (0 mode) and the negative modes can be achieved. These artificial modes can be utilized to design omnidirectional antennas, compact antennas, and other high-performance antennas [11], [16], [17], [18].
Figure 2. (a) A linear CRLH-TL. (b) An RH-TL + CRLH-TL structure. (c) A 2D CRLH-TL.
By combining an RH-TL part with a CRLH-TL part, the RH-TL + CRLH-TL topology is achieved [see Figure 2(b)]. More degrees of freedom are obtained by introducing the RH-TL part when compared with structures that contain only CRLH-TLs. Diverse antenna designs are realized by the hybrid TL structures [3], [19]. In [19], the hybrid structure is realized by a partially filled conventional patch antenna with LH-TLs.
A linear CRLH-TL can be extended to a 2D or 3D CRLH-TL [1], [20], [21], [22] by periodically implementing the unit cell along two or three orthogonal directions [see Figure 2(c)]. The 2D or 3D CRLH TL supports wave propagation in any bidimensional or tridimensional direction within the structure. Examples are shown in [20] and [21], where a compact antenna and a gain-enhanced antenna are achieved. A 3D CRLH-TL is analyzed in [22] in terms of its dispersion relation, Bloch impedance, principal axes propagation, and mode excitation.
Note that unlike electrically large metamaterial-based antennas, such as leaky wave antennas and metasurface-based antennas, CRLH-TL-inspired small antennas normally have a compact size and contain a limited number of unit cells. Despite this difference, the analysis method of traditional CRLH-TL metamaterials can be applied to CRLH-TL-inspired small antennas, for example, using equivalent circuit models. It is a convenient and useful way to describe a certain resonant structure by using the CRLH-TL concept since, by introducing LH structures, more degrees of freedom are obtained, and the antenna performance can be easily evaluated.
An annular CRLH-TL is shown in Figure 3(a). The unit cells of the annular CRLH-TL are repeated along the annular direction (i direction). Similar to the “mushroom-type” linear CRLH-TL [1], [3], the annular TL provides the series inductance (LR) and the shunt capacitance (CR). The gap between the annular TLs provides the series capacitance (CL). The shorted pin connected to the ground provides the shunt inductance (LL). Some i-direction CRLH-TL-based antennas are presented in [7], [9], and [23].
Figure 3. (a) An annular CRLH-TL. (b) A radial CRLH-TL. (c) A 2D CRLH-TL in polar coordinates.
A radial CRLH-TL is shown in Figure 3(b). The periodicity occurs along the radial direction (r direction). The slots between the rings provide the series capacitances, and the metallic pins provide LL. The other parameters, including LR, CR, and CL, are obtained similarly to the linear “mushroom-type” topologies. Note that to obtain a quasi-homogeneous metamaterial topology, the density of the metallic pins has to be quasi-constant over the area of the rings. In a radial CRLH-TL structure, this means that the number of pins has to grow as the radius of the cell increases. Very few researchers focus on the radial CRLH-TL topology. Two related works were reported in [2] and [27]. The antenna topology and the equivalent model can be found in [27].
Note that the propagation direction of an annular CRLH-TL is along the i direction in polar coordinates, and the propagation direction of a radial CRLH-TL is along the r direction. By combining an annular CRLH-TL and a radial CRLH-TL, a 2D CRLH-TL in polar coordinates can be achieved [see Figure 3(c)]. A multifunctional antenna inspired by this topology was first proposed in [1]. Similar to the CRLH-TL in Cartesian coordinates, the CRLH-TL in polar coordinates can be analyzed by using equivalent circuits, which provide not only physical understanding but also an initial design with rough structure dimensions (see [2] and [3]). Some representative antenna designs based on CRLH-TLs in polar coordinates are shown in Figure 4.
Figure 4. (a) An annular CRLH-TL-inspired antenna [8]. (b) A radial CRLH-TL-inspired antenna [2]. (c) A 2D CRLH-TL in polar coordinates-inspired antenna [1].
Normally, a CRLH-TL-inspired structure has six parameters that can be independently controlled: four CRLH-TL parameters, CR, CL, LR, and LL, and two conventional RH-TL parameters, CRH and LRH. By tuning these parameters, the dispersion properties of the CRLH-TL can be reconfigured. When the CRLH-TL is applied to an antenna, the working mode of the antenna is related to the dispersion properties of the CRLH-TL. Therefore, by reconfiguring the CRLH-TL parameters, the resonant mode of a CRLH-TL-based antenna can be changed in a discrete way or in a continuous way.
One way to reconfigure the CRLH-TL is to tune CL by loading diodes. Figure 5(a) gives an example where switch diodes/varactor diodes are loaded. When the switch diode is in the ON state, the gap is shorted, which means that CL is eliminated. When the switch diode is in the OFF state, CL is determined by the gap and the OFF state capacitance of the switch diode. When a varactor diode is loaded, the capacitance of the varactor diode and the capacitance introduced by the gap together determine CL. Tuning CL by loading switch diodes/varactor diodes was introduced in [3], [12], and [28]. Another way to tune CL is to use a fluid, as in [29]. By changing the permittivity of the injected fluid, CL can be continuously tuned. Although this method is implemented there for a phase shifter, it can easily also be utilized in antenna design.
Figure 5. The reconfigurability methods for (a) CL and (b) LL. Note that the dc bias control network is omitted for simplicity. (c) The reconfigurability method for CR and CRH.
Another way to reconfigure a CRLH-TL is to change LL. Figure 5(b) gives an example of how LL can be tuned. A varactor diode is loaded between the shorted pin and the TL. When the capacitance of the varactor diode is changed, consequently, LL changes. The varactor diode can be replaced by a switch diode. When the switch diode is in the OFF state, LL is eliminated. In this case, the CRLH-TL contains only CL, CR, and LR. When the switch diode is in the ON state, LL is introduced. Examples can be found in [11] and [30]. In [11], the resonant mode can be changed from the 0 mode to the 1 mode, validating the used concept.
In addition, CR and CRH can be tuned. Normally, in a TL-based structure, CR and CRH are determined by the parallel capacitance between the trace and the ground, which is affected by the permittivity of the substrate, the width of the trace, the height of the substrate, and so on. Figure 5(c) indicates a way to tune CR and CRH. By arranging a dielectric fluid channel between the metallization and the ground, the permittivity of the substrate can be tuned, thus changing CR and CRH. The CR can also be changed by using diodes in a coplanar CRLH-TL topology [10]. A similar concept can be also found in [31], where a liquid crystal is used.
Here, LR and LRH are mainly related to the length and the width of the metallization of the TL. To the best knowledge of the authors, no real-time control methods to tune these two parameters have been proposed yet in the open literature.
Besides the aforementioned methods, reconfigurability can also be realized in other ways. For example, two or more parameters of CRLH-TLs can be tuned simultaneously [32]. Also, reconfigurability can be achieved by tuning the termination load [33] and by switching the termination states (open ended or short ended) [34] of the CRLH TL.
It has to be emphasized that reconfigurability can be realized by numerous methods, e.g., p-i-n switch diodes, varactor diodes, RF MEMS, dielectric fluid channels, microfluidics, mechanical reconfiguration, liquid crystals, non-Foster materials, electrically/magnetically biased materials, and so on. Among these approaches, p-i-n switch diodes and varactor diodes are the ones mainly used because of the low cost, simple control strategy, and ease of integration [25].
Frequency reconfigurability is desired to increase the operating bandwidth to cover as many commercial bands as possible. This can be successfully achieved by using CRLH-TL structures [2], [9], [10], [12], [23], [28], [29]. The mechanism is to reconfigure the excited operating mode of the CRLH-TL by tuning the dispersion properties. Different from pattern/polarization reconfigurability, the same operating mode is excited during the frequency reconfiguration. In other words, the dispersion curves are shifted, not really reconstructed. This can normally be realized by using varactor diodes. More complicated frequency reconfigurability can also be achieved, such as dual-band frequency reconfigurability and one tunable band + one stable band reconfigurability, as introduced in [2].
Multiband antennas are also a possibility when various standards are integrated in a single system and operate in different frequency bands. CRLH-TLs have been successfully used in the design of multiband antennas [1], [2], [3], [12], [14], [35], [36]. In [2] and [12], dual-band operation is achieved by exciting the 0 mode and 1 mode. In [35], triband operation is achieved by exciting the ±1 mode and the 0 mode. Multiple bands can not only be achieved by a CRLH-TL with a single propagation direction but they can also be achieved by CRLH-TLs that can support different propagation directions. For example, in [1], multibands were separately obtained by using the two orthogonal propagation directions in a 2D CRLH-TL.
Pattern/polarization-reconfigurable antennas can be used to avoid in-band interference, filter the detrimental fading loss caused by the multipath effect, and boost the system capacity. The reconfigurability between circular and linear polarization based on the CRLH-TL structure is reported in [37] and achieved by tuning the LH inductance. It is important to realize that the ±1 mode results in broadside radiation patterns, while the 0 mode results in omnidirectional radiation patterns. These features can be exploited to develop pattern/polarization-reconfigurable antennas. This is achieved by reconfiguring the operating mode at a certain frequency from the ±1 mode to the 0 mode. As a result, the antenna can switch its radiation pattern from a broadside pattern to an omnidirectional pattern [3], [11]. To completely switch the operating mode at a certain frequency, the dispersion properties of the CRLH-TL have to be significantly reconfigured (see the “Reconfigurability” section). Note that in these kinds of applications, the design of the feeding network is quite challenging since the two completely different operating modes have to be well excited with the same feeding network. One way to achieve this is to introduce an impedance matching network [3].
Antenna diversity is a key benefit of MIMO systems. A patch-like radiation pattern (broadside) and dipole-like radiation pattern (omnidirectional) are often combined when implementing pattern diversity since they have “orthogonal” radiation directions and may have orthogonal polarizations. The CRLH-TL can be designed to work in both the ±1 mode and the 0 mode simultaneously. The use of the CRLH-TL can thus be quite beneficial to achieve pattern diversity since it has the ability to achieve both broadside and omnidirectional patterns by working with the different modes. Pattern diversity antennas based on the CRLH-TL and 2D CRLH-TL can be found in [1], [7], and [9].
Omnidirectional antennas are very advantageous, for example, in access points to provide wide communication coverage. Omnidirectional radiation can be obtained with the 0 mode of a CRLH-TL [3], [11]. However, it has to be emphasized that it is not straightforward to achieve omnidirectional patterns in multiple bands simultaneously by using linear CRLH-TLs since the ±1 mode of a linear CRLH-TL results in a patch-like broadside pattern. Topologically, a linear topology indeed does not have circular symmetry (see [3]). This problem can be overcome by using a CRLH-TL in polar coordinates. An r-direction CRLH-TL-based antenna is proposed in [2]. Omnidirectional patterns are obtained for both modes, due to the circular symmetry linked with the r direction in the CRLH-TL.
Miniaturized and low-profile antennas hold a great benefit in a variety of wireless communication systems. They have been achieved by using CRLH-TLs in [1], [7], [9], [10], [38], and [39]. As mentioned before, the CRLH-TL structure can be excited in the 0 mode and the negative modes. Miniaturization is realized by using the fact that when the antenna works in the 0 mode or a negative mode, the resonant frequency is lower than for the corresponding conventional RH-TL-based antenna.
For example, in [1], such a miniaturized multiband antenna is designed based on a 2D CRLH-TL. The operation mechanism is the following. Split-ring resonator (SRR) structures are loaded around shorting pins. By adding these SRR structures, the surface current via the shorting pins is meandered. Thus, the LH inductance introduced by the shorting pins is enhanced. Therefore, the resonances of the CRLH-TL move to lower frequencies [1]. In other words, the electrical size of the 2D CRLH-TL-inspired antenna is miniaturized by loading the SRR around the shorting pins. A low-profile design can be easily achieved with a planar CRLH-TL structure. Some recent low-profile designs are presented in [7], [9], and [10].
Antennas that can support as many services as possible while maintaining good performance, with a low fabrication cost and occupying a miniaturized space, have attracted more and more research attention. The application of CRLH-TLs has facilitated the design of a wide variety of antennas for different standards, including but not limited to the previously mentioned antennas. Starting from the conventional CRLH-TLs, novel CRLH-TL topologies and reconfigurability provide even more design freedom. Multistandard CRLH-TL-based antennas can be realized by combining the use of the different characteristics of CRLH-TLs.
A frequency-reconfigurable antenna with pattern diversity is realized for the first time in [9]. Frequency reconfigurability is achieved as discussed before, and pattern diversity is obtained based on a novel i-direction CRLH-TL topology. A multistandard CRLH-TL antenna with a triband, pattern diversity, and miniaturization is realized in [1]. More multistandard CRLH-TL antennas can be found in [2], [3], [12], [35], and [40].
The flexible operational features of CRLH-TLs enable designing antennas within the context of a wide range of different application scenarios. In the information transfer realm, CRLH-TL-inspired wearable antennas for WBANs are presented in [11], [13], [14], and [41]. A CRLH-TL-based miniaturized antenna for RFID applications is proposed in [38]. For 5G applications, a circularly polarized CRLH-TL antenna is realized in [42], and a multifunctional CRLH-TL antenna is proposed in [43]. In the power transfer realm, a CRLH-TL-based antenna for energy harvesting is studied in [44]. CRLH-TL antennas have been designed for applications ranging from millimeter-wave to terahertz bands [45], [46]. In summary, the CRLH-TL can be used in various antennas for application domains ranging from WBANs to 5G. These domains are vital subareas within the modern wireless society.
In most previous research, only one equivalent parameter of the CRLH-TL is utilized in the reconfigurability. Antenna performance can be further boosted by lifting this limitation and leveraging several parameters simultaneously in more complex antenna designs.
Programmable CRLH-TL metamaterials are a kind of inhomogeneous metamaterial (with nonperiodic structures). They provide considerably more freedom to control electromagnetic field behavior, which is a huge advantage in antenna design. Programmable metamaterials have great potential in phased arrays, beamforming, wireless power transfer, and so on.
CRLH-TLs can be realized in printed circuit board, substrate-integrated waveguide, and other waveguide technologies, especially focusing on high-power applications. Although diodes are widely used for an easy realization of reconfigurability, most of the designs neglect their nonlinear effects. The maximum input power of diode-loaded antennas is mainly limited by the power handling capacity of the diodes. In addition, the nonlinear characteristics of the diodes may cause problems, such as harmonic distortion and gain compression. These factors all still have to be investigated.
Most of the reconfigurable CRLH-TLs have been realized by using diodes or MEMS. Other control approaches can be investigated to meet different requirements for various applications, from microwaves to optics, such as the use of microfluidics and dielectric fluids, liquid crystals, thermally controlled methods, optically controlled methods, and so on.
In this article, the CRLH-TL-inspired small antennas published in the literature were overviewed in terms of topology, reconfigurability, operational features, and various applications. By introducing new topologies and new reconfigurability techniques, more freedom and flexibility can be obtained compared to traditional design approaches. This extra freedom is the key benefit to achieve multifunctional antennas for increasingly complex wireless applications, including information exchange and power transfer. Although CRLH-TL-inspired antennas already have shown great promise, it is the opinion of the authors that further great improvements can be expected in the near future.
This work was supported, in part, by the National Natural Science Foundation of China (Grants 62001496 and 52177012), Young Elite Scientists Sponsorship Program by CAST, and the Foundation of National Key Laboratory of Science and Technology on Vessel Integrated Power System (Grant 6142217210501). Jiahao Zhang is the corresponding author of this article. This article has supplementary downloadable material available at https://www.doi.org/10.1109/MAP.2022.3201194, provided by the authors.
Jiahao Zhang (jiahao.z@hotmail.com) is with the National Key Laboratory of Science and Technology on Vessel Integrated Power System Technology, Naval University of Engineering, Wuhan 430033, China.
Sen Yan (sen.yan@xjtu.edu.cn) is with the School of Information and Communications Engineering, Xi’an Jiaotong University, Xi’an, 710049 China. He is a Member of IEEE.
Guy A.E. Vandenbosch (guy.vandenbosch@esat.kuleuven.be) is with the ESAT-WAVECORE Research Division, KU Leuven, 3001 Leuven, Belgium. He is a Fellow of IEEE.
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Digital Object Identifier 10.1109/MAP.2022.3201194