Metamaterial-Inspired Engineering of Antennas

INV ITED P A P E R Metamaterial-Inspired Engineering of Antennas Some applications of metamaterials in efficient, electrically small antennas are reviewed in this paper; multiband and mutifrequency antennas are also discussed. By Richard W. Ziolkowski, Fellow IEEE, Peng Jin, Member IEEE, and Chia-Ching Lin, Student Member IEEE ABSTRACT | A variety of antennas have been engineered with metamaterials (MTMs) and metamaterial-inspired constructs to improve their performance characteristics. Examples include electrically small, near-field resonant parasitic (NFRP) anten- nas that require no matching network and have high radiation efficiencies. Experimental verification of their predicted beha- viors has been obtained. Recent developments with this NFRP electrically small paradigm will be reviewed. They include considerations of increased bandwidths, as well as multiband and multifunctional extensions. KEYWORDS | Antenna efficiency; antenna theory; electrically small antennas; metamaterials (MTMs); parasitics; quality factors I . INTRODUCTION While double negative (DNG)metamaterials (MTMs) were proposed over 40 years ago [1], they have been experimen- tally demonstrated only in the last decade [2]–[7]. The adaptation of a variety of epsilon-negative (ENG), mu- negative (MNG), and DNG MTMs or simply MTM unit cells to achieve enhanced performance characteristics of antenna systems has since received considerable research attention. This includes studies, for instance, of small antennas [9]–[34]; multifunctional antennas [35]–[43]; infinite wavelength antennas [44]–[46]; patch antennas [47]–[50]; leaky-wave antenna arrays [6], [7], [51], [52]; higher directivity antennas [53]–[57]; low profile antennas [58]–[60] achieved with a variety of modified ground planes, e.g., artificial magnetic conductors; and dispersion engineering of time domain antennas [52], [61]. The proliferation of wireless devices for communication and sensor applications has restimulated interest in many different types of antennas. The often conflicting require- ments, for instance, of efficiency, bandwidth, directivity, weight, and cost have made the design tasks onerous for antenna engineers with traditional schemes. The MTM-inspired engineering of antennas and their perfor- mance characteristics has provided an alternative approach to addressing these pressing issues. In this paper, the MTM-inspired engineering of electrically small antennas will be emphasized. II . MTM SHELL-BASED ANTENNAS The idea of using a resonant MTM object in the near-field of an electrically small radiator to significantly enhance its performance was introduced in [9], [10], [14], and [22]. The theoretical models began with enclosing a radiating dipole with DNG or single negative, spherical MTM shells. For instance, nearly complete matching to a 50 � source was achieved for a coax-fed dipole (loop) antenna within an ENG (MNG) or DNG shell without any external match- ing circuit and high radiation efficiencies were realized giving overall (realized) efficiencies near 100%. One of the original designs is shown in Fig. 1. The bandwidth was commensurate with the electrical size of the antenna. It was also demonstrated [15] that with an active ENG shell, the bandwidth could be increased considerably beyond the well-known Chu [62] and Thal [63] limits. Manuscript received June 7, 2010; revised August 29, 2010; accepted November 2, 2010. Date of publication December 23, 2010; date of current version September 21, 2011. This work was supported in part by the Defense Advanced Research Projects Agency (DARPA) under Contract HR0011-05-C-0068 and by the Office of Naval Research (ONR) under Contract H940030920902. R. W. Ziolkowski and C.-C. Lin are with the Department of Electrical and Computer Engineering, University of Arizona, Tucson, AZ 85721-0104 USA (e-mail: ziolkowski@ece.arizona.edu). P. Jin was with the Department of Electrical and Computer Engineering, University of Arizona, Tucson, AZ 85721-0104 USA. He is now with Broadcom Corporation, Irvine, CA 92617 USA (e-mail: pengjin@broadcom.com). Digital Object Identifier: 10.1109/JPROC.2010.2091610 1720 Proceedings of the IEEE | Vol. 99, No. 10, October 2011 0018-9219/$26.00 �2010 IEEE It was then realized that the ENG shell is an electrically small resonator, i.e., its core is an electrically small region excited by the electric field of the driven dipole and, hence, it acts as a capacitive element. Similarly, its shell is also excited by that electric field and has a capacitive response but is filled with a negative permittivity and, hence, acts as an inductive element. The combination of the lossy capacitive and inductive elements, i.e., the juxtaposition of the positive and negative material regions, yields a lossy (RLC) resonator. The driven element, the electrically small dipole antenna, has a large negative reactance, i.e., it too is a capacitive element. Because the lossy resonator is in the extreme near-field of the driven element, the fields involved and the subsequent responses are large. It was found that the reactance of this near-field resonant parasitic (NFRP) element, the ENG shell, can be conjugate matched to the dipole reactance by adjusting their sizes and material properties to achieve an antenna resonance at fres ¼ 1 2� 1ffiffiffiffiffiffiffiffiffiffiffiffiffi LeffCeff p (1) where Leff and Ceff are, respectively, the effective inductance and capacitance of the system, in order to have the total reactance equal to zero. (It is noted that in the dual case, a loop antenna and an MNG shell, the first antenna resonance is generally an antiresonance.) Moreover, by tuning the effective capacitances and inductances of both the driven and parasitic elements, the entire antenna can be nearly completely matched to the source, i.e., the NFRP element also acts as an impedance transformer. By arranging the NFRP element so that the currents on it dominate the radiation process, a high radiation efficiency and, conse- quently, a very high overall efficiency, i.e., the ratio of the total radiated power to the total input power, can be realized. This basic physics of the NFRP element-based electrically small antenna is depicted in Fig. 2. For example, the 300-MHz version of the dipole-ENG shell antenna shown in Fig. 1 was predicted to have an overall efficiency greater than 97% when ka ¼ 0:12 [14]. We note, however, that because the antenna is electrically small, one would expect its directivity to be near that of a small dipole element, i.e., 1.76 dB. Furthermore, the bandwidth will be small. In particular, if a is the radius of the smallest sphere enclosing the entire antenna, k ¼ 2�=�res ¼ 2�fres=c is the free space wave number at the resonance frequency, andRE is the radiation efficiency of the antenna, then the antenna is electrically small if ka � 0:5 ð1:0Þ if a (no) ground plane is involved. The Chu lower bound on the quality factor of an electrically small antenna is [64] Qlb ¼ RE� 1ðkaÞ3 þ 1 ka " # (2) giving FBWub � 2=Qlb as the upper bound of the half- power voltage standing wave ratio (VSWR) fractional bandwidth. An important practical difficulty with the MTM-shell concept is the need to have extremely small unit cell sizes. For instance, if a ka ¼ 0:10 antenna is desired, the thickness of the unit cell would be on the order of �res=100, thus requiring unit cells at least �res=300 in size to have three unit cells across the shell thickness and, consequently, something like a bulk MTM. Some of the smallest unit cells fabricated to date are �=75 at 400 MHz [4]. Furthermore, when there are losses associated with each unit cell, having many of them can lead to a large cumulative loss value. This behavior has been verified [27] with a dual version of the system shown in Fig. 1. An antenna consisting of an electrically small driven magnetic loop and a mu-negative sphere was designed and fabricated. The measured results demonstrated that the MNG sphere did provide matching of the antenna to the Fig. 1. Original MTM-based, efficient electrically small antenna consisting of a center-fed dipole antenna surrounded by an ENG shell [14]. Fig. 2. The basic physics governing the behavior of an electrically small NFRP antenna. Ziolkowski et al. : Metamaterial-Inspired Engineering of Antennas Vol. 99, No. 10, October 2011 | Proceedings of the IEEE 1721 source. However, because of the losses associated with each unit cell and because there were many cells involved in the overall design, it also did not achieve the expected high radiation efficiency. III . NEAR-FIELD RESONANT PARASITIC ANTENNAS In contrast, it was found [22] that an NFRP element constructed from a single MTM unit cell is sufficient to achieve the desired matching and high radiation efficiency properties. The resulting radiating systems were termed MTM-inspired antennas rather than MTM-based antennas because only a single MTM unit cell was used and not a bulk medium. One does not need a large resonator around the entire radiating element, but rather only a single unit cellVan electrically small resonatorVin the near-field of the driven radiator to achieve nearly complete matching to the source without any matching circuit and nearly 100% overall efficiency. The initial designs made use of the analytical results that matched the type of driven element with the appropriate type of MTM. Both electric and magnetic coupling mechanisms between the driven and NFRP elements have since been explored. These MTM-inspired NFRP elements have led to a variety of interesting electrically small antenna systems. Several of these MTM-inspired NFRP designs have been fabricated and tested; the measured results are in good agreement with their simulated values. A. Electric NFRP, Electric Coupling The first NFRP antenna that was tested for its overall efficiency performance was the electric 2-D EZ antenna shown in Fig. 3. The term EZ was chosen to reflect the fact that these original NFRP antennas were Beasy[ to design and fabricate. It was a Rogers Duroid 5880 design. A monopole was printed on one side of the Duroid sheet and was coaxially fed through a copper ground plane. The NFRP element was a small meander line connected to the ground plane on the other side of the Duroid sheet. It has been demonstrated [65], [66] that this is a unit cell of an ENG MTM. It was demonstrated experimentally [22] that a 2-D electric EZ antenna with fres ¼ 1.37 GHz and ka � 0:49 was nearly completely matched to the 50 � source and had an overall efficiency �94%, with a 4.1% fractional bandwidth. The NFRP element is electrically coupled to the driven mono- pole. The fields generated by the currents on the horizontal lines are nearly canceled out by their images through the ground plane. The vertical elements radiate coherently and provide the high radiation efficiency. With an interest to achieve an adjustable and poten- tially tunable version of this NFRP antenna, the Z antenna, whose 570-MHz, 31-mil, 2-oz Duroid 5880 realization is shown in Fig. 4(a) and (b), was designed, fabricated by Boeing Research and Technology, Seattle, WA, and tested in the reverberation chambers at the National Institute ofFig. 3. ENG-based NFRP element, 2-D electric EZ antenna. Fig. 4. Fabricated 570-MHz Z antenna on its small circular copper insert. (a) Z element side and (b) monopole element side [32]. Ziolkowski et al. : Metamaterial-Inspired Engineering of Antennas 1722 Proceedings of the IEEE | Vol. 99, No. 10, October 2011 Science and Technology (NIST), Boulder, CO [32]. The meanderline was reduced to two simple J-elements connected with a lumped element inductor. The bottom J-element is connected to the ground plane. The monopole is coaxially-fed through the ground plane. It was a 30 mm� 30 mm design incorporating a CoilCraft 47-nH inductor. Measurements were taken with both the small ground plane version (120.6-mm diameter copper disc) shown in Fig. 4 and a larger ground plane version (the small ground plane version was inserted into an 18 in � 18 in ¼ 457.2 mm � 457.2 mm copper ground plane). A physical comparison of the Z antenna and the reference ETS LINGREN 3106 double-ridged waveguide horn, which is about 94% efficient in its 200 MHz–2 GHz frequency band, is shown in Fig. 5. An overall efficiency equal to 80% was measured at the resonance frequency fres ¼ 566.2 MHz ðka ¼ 0:398Þ with a half-power fractional bandwidth FBW ¼ 3.0%, giving Q ¼ 4:03 Qlb. There was little difference between the small ground plane and larger ground plane results. The predicted gain patterns in the small and large ground plane configura- tions are shown in Fig. 6, confirming that the Z antenna acts like a small vertical monopole with a finite ground plane. A second Z antenna, a 40 mm� 40 mm design incorporating a CoilCraft 169-nHmaxi inductor, had a 46%measured overall efficiency at fres ¼ 294.06 MHz ðka ¼ 0:276Þ. Both sets of experimental results demonstrated, as predicted, the ability to obtain a lower resonance frequency with a simple redesign using a larger lumped element value. These experiments not only confirmed the predicted controllability of the resonance frequency, but they also provided information on how to treat the lumped element inductor in the electromagnetic simulations. Based on these results, an updated Z antenna was designed with an overall efficiency of OE ¼ 82.3% (jS11j ¼ �25.44 dB) at fres ¼ 285.6MHz ðka ¼ 0:428Þ [32]. As it was recognized that the NFRP elements were the key to these electrically small designs, several variations have been designed to allow for other functionalities, which will be described in the following. For instance, the NFRP element associated with the Z antenna can be simplified considerably [25], [38], [40]. The version shown in Fig. 7(a) consists of a split vertical segment connected by a lumped element inductor. The size of the upper horizontal metal rectangle is adjustable to tune the input reactance. ANSOFT high frequency structure simulator (HFSS) simulations of the currents at the resonance frequency confirm that they are mainly on the vertical segments, which means this antenna radiates as a monopole over the ground plane and that the overall efficiency is above 90% [40]. The second version in Fig. 7(b) utilizes a distributed NFRP element. The vertical strip couples directly to the driven monopole; the metal arc provides the inductance; and the horizontal strips provide additional capacitance. In resonance, HFSS simulations predict that the overall efficiency is above 90% and confirm that the currents, as shown in Fig. 7(b), are mainly on the vertical segment of the NFRP element, which again indicates why this antenna radiates as a monopole over the ground plane [67]. B. Magnetic NFRP, Magnetic Coupling In a similar fashion, the NFRP element can be driven with the magnetic field of the driven element. In conjunction with the analytical solutions, we first consid- ered a driven magnetic semiloop antenna coaxially fed through a finite ground plane. This led to the magnetic 2-D and 3-D EZ antennas [19], [20], [29], [34]. This feed- coupling scheme is graphically illustrated in Fig. 8, which illustrates a variation of the 2-D magnetic EZ antenna. The parasitic element is a capacitively loaded loop (CLL), Fig. 5. Physical comparison of the 570-MHz small ground plane Z antenna and the dual-ridged reference horn in the NIST-Boulder reverberation chamber [32]. Fig. 6. HFSS-predicted patterns for the Z antenna at fres have the expected electric monopole with a finite ground plane shapes [32]. Ziolkowski et al. : Metamaterial-Inspired Engineering of Antennas Vol. 99, No. 10, October 2011 | Proceedings of the IEEE 1723 which was originally used successfully to achieve an artificial magnetic conductor MTM (without any ground plane) [59]. Both distributed and lumped element versions have been fabricated and tested successfully [22]. The 3-D magnetic EZ antenna is composed of an electrically small loop antenna that is coaxially fed through a finite ground plane and that is integrated with an extruded CLL element. This 3-D CLL structure is designed to be the NFRP element. The measured results [29] for the 3-D magnetic EZ antenna demonstrated that for an electrical size, ka � 0:43, at 300.96 MHz, nearly complete matching to a 50 � source, and a high overall efficiency (> 94%) was achieved. These results and those in [58] established that the CLL-based elements can work in many frequency bands, e.g., from the ultrahigh frequency (UHF) band to the X band. While negative permeability cannot be ascribed to either the 2-D or 3-D CLL elements themselves, an MTM constructed with them as its unit cell inclusion would exhibit MNG properties. Nonetheless, as with the electric source-electric coupling cases, it is the electrically small, magnetic-based NFRP CLL structure that provides the ability to match the electrically small loop antenna to the source. This can be visualized with the configuration dual to that shown in Fig. 2. The NFRP element again enhances the radiation process to achieve high radiation efficiencies. In particular, the CLL element can be engineered to control the strong magnetic flux generated by the small driven loop antenna and convert it into the appropriate currents flowing on the CLL element. Furthermore, this magnetic coupling process between the driven loop and the NFRP CLL element can be adjusted to tune the resonance of the entire antenna system according to (1) [22], [29]. These MTM-engineered NFRP antennas again help overcome the loss issues associated with an actual MTM-based antenna design [27]. We note that wire versions of the CLL NFRP antennas have also been designed and show similar performance characteristics [39]. A similar low profile (height � �res=25) 3-D magnetic EZ antenna was designed for operation at 100 MHz. The fabricated antenna is shown in Fig. 9. The measured and simulated results were in very good agreement. This ka ¼ 0:46 antenna was measured to haveOE � 95% and a half-power VSWR fractional bandwidth of 1.52% ðQ ¼ 11:06 QlbÞ at fres ¼ 105.2 MHz. This design utilized a quartz spacer ð"r ¼ 3:78Þ to help lower the resonance frequency and to provide mechanical stability during Fig. 7. Electrically small antenna designs incorporating generalized NFRP elements and coax-fed monopoles. (a) Rectangular NFRP with lumpedelement inductor. (b)Egyptianaxeantennawith its distributed NFRP element. The current densities represent their basic behavior at the first resonance of the antenna. Fig. 8. Themagnetic flux of theprinted semiloopantenna coaxially fed through a finite ground plane drives the CLL NFRP element in this variation of the 2-D magnetic EZ antenna. Fig. 9. The fabricated 100-MHz 3-D magnetic EZ antenna in its small ground plane configuration [34]. Ziolkowski et al. : Metamaterial-Inspired Engineering of Antennas 1724 Proceedings of the IEEE | Vol. 99, No. 10, October 2011 shipping and operation. Similar antennas have been designed for operation at 20 MHz using simply a "r ¼ 100 spacer [34]. The corresponding HFSS-predicted gain patterns are shown in Fig. 10; the maximum gain value is 5.94 dB. Because the gain patterns are symmetric, it can be im- mediately inferred that the surface currents induced on the electrically small CLL element by the flux of the driven semiloop antenna are uniform and symmetric. This behavior is verified with the HFSS-predicted vector surface current distributions. This current distribution also demonstrates that this electrically small antenna system is radiating as a magnetic dipole over a finite ground plane [34]. C. Magnetic NFRP, Electric Coupling Several important features distinguish