Metamaterial Extends Microstrip Antenna

Metamaterial Extends Microstrip Antenna

The use of metamaterials can dramatically improve the performance and reduce the size of a patch antenna.

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Left-handed metamaterials (LHMs), although predominantly artificial in nature, have been shown to offer some advantages in high-frequency designs. For example, these electromagnetic-bandgap (EBG) materials can be used to increase the gain and bandwidth of a microstrip patch antenna while at the same time reducing its volume. To demonstrate the capabilities of these materials, based on an ordinary patch antenna, a microstrip patch antenna was designed with isolated complementary double-ring and crossed stripline gaps etched on the metal patch and ground plane.

Simulated and measured results for the antenna design agree closely and indicate that the bandwidth can be greatly increased through the use of the metamaterials, reaching a bandwidth of 10.8 GHz with high gain. The antenna, which achieves maximum gain of 8.89 dB while exhibiting a voltage standing wave ratio (VSWR) that remains below 2.0:1, has dimensions of 30.6 x 35.3 x 0.8 mm3. Wave propagation along the patch induces the strongest radiation in horizontal direction, rather than in the vertical direction of a conventional patch antenna.

Demand for wireless communications and portable, compact wireless-communications devices continues to grow, and designers everywhere are grappling with ways to make these devices smaller. One of the most important components in any wireless communications system is its radiating element, which should be compact but also provide high directivity, efficiency, generous gain, and broadband operation. Numerous broadband techniques have been investigated to overcome the trade-off between antenna size and minimum quality factor (Q), as dictated by Chu formulations.1 These techniques are mainly involved with increasing the thickness of the substrate, using different shaped slots or radiating patches;2-4 stacking different radiating elements or loading of the antenna laterally or vertically;5-7 using magnetodielectric substrates;8 and engineering the ground plane, as in the case of EBG metamaterials.9

As alluded to above, LHMs are artificially structured materials that provide electromagnetic (EM) properties not encountered in nature. The electrodynamics of hypothetical materials having simultaneously negative permittivity and permeability was first theoretically predicted by Veselago in 1968.10 Over the past decade, studies of the EM properties of left-handed materials and their applications have drawn wide attention, with wide use in the design of different microwave components and applications.11-13

For antenna applications, split-ring resonators (SRRs) and some other planar structures have been applied in some antenna fabrications to enhance the radiation and minimize the size.14,15 In some other designs, artificial magnetic materials with stacks of SRRs under patch antennas have been proposed. It was found that the resonant frequency of the original patch antenna can be significantly decreased through the use of these artificial materials.

There are, however, still fundamental issues at microwave frequencies, including narrow bandwidths and high losses (due to ohmic losses and radiation losses of the circuit elements). These limitations become especially serious when SRRs and other types of metamaterials are used as substrates for patch antennas.16,17 For the current study, a different planar left-handed material was used to enhance the bandwidth and gain of a conventional patch antenna. This was accomplished by applying the planar complementary double-ring directly on the upper patch and bottom ground plane of the dielectric substrate, enabling the patch antenna to achieve excellent performance.

A conventional microstrip patch antenna is usually mounted on a substrate, which is backed by a conducting ground plane. A different approach was used in the current report: A planar LHM pattern on a rectangular patch antenna mounted on the substrate was used to enhance the antenna’s horizontal radiation pattern and — via coupling with the conducting ground plane backing the substrate and LHM pattern—to increase the antenna’s effective bandwidth.

Metamaterial Extends Microstrip Antenna, Fig. 1
1. These 2D views of the proposed antenna show (a) a top view and (b) a bottom view, with the middle pattern representing an LHM unit cell.

Metamaterial Extends Microstrip Antenna, TableOn the upper patch [Fig. 1(a)], there are 12 complementary double-ring resonators (CDRRs), arranged in a 3 x 4 array. The left-handed characteristics of these patterns were demonstrated in Ref. 18 and will not be discussed further here. On the bottom ground plane [Fig. 1(b)], periodically distributed cross stripline gaps are included. An off-centered microstrip line feeds the patch antenna. Its geometrical parameters are presented in the table and a prototype is shown in Fig. 2. The width of the feeding line is 3 mm, the gap between the two etched rings is 0.2 mm, and all design parameters have been optimized for best performance. The substrate is a nonmagnetic circuit material from Rogers Corp. with a relative permittivity of 2.3 and loss tangent of 0.0004; the thickness of the substrate is 0.8 mm.

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LHM Prototypes

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Prototypes of the LHM patch antenna were numerically simulated, fabricated, measured, and characterized. Computer simulations were based on the standard finite-difference-time-domain (FDTD) method with commercial simulation software from Computer Simulation Technology (CST). The return loss of the antenna was measured with a Model N5244A PNA-X vector-network analyzer (VNA) from Agilent Technologies, with measured and simulated results depicted in Fig. 3.

Metamaterial Extends Microstrip Antenna, Fig. 2
2. These photographs detail the top (left) and bottom (right) of the proposed antenna.

Metamaterial Extends Microstrip Antenna, Fig. 3
3. These plots display the measured and simulated return loss for the antenna design, from 2 to 15 GHz.

As that figure shows, the measured and simulated results are in good agreement, and the impedance bandwidth of 124% extends from approximately 3.2 to 14 GHz with a center frequency of 8.7 GHz. The bandwidth is enhanced by the fact that different sections of the LHM cells are excited at different frequencies; it is a reason why the structure achieves broadband-frequency operation.

Metamaterial Extends Microstrip Antenna, Fig. 4
4. This is the simulated gain for the proposed antenna, from 3 to 15 GHz.

The gain of the novel LHM patch antenna was simulated within the frequency band of interest, as shown in Fig. 4. Antenna gain was generally greater than 6 dB, with peak gain of 8.9 dB. Compared with a conventional microstrip patch antenna, the gain of the proposed antenna has been improved.19 As Fig. 5 shows, the measured VSWR remains well below 2.0:1 for the full operating frequency band.

Metamaterial Extends Microstrip Antenna, Fig. 5
5. This is the measured VSWR for the microstrip antenna design, from 3 to 15 GHz.

Because of the transmission characteristics of left-handed materials, the wave propagation along the patch induces the strongest radiation in the horizontal direction rather than the vertical direction of a conventional patch antenna.16 The measured radiation patterns of the antenna in the X-Y and Y-Z planes at 6.2 and 11.0 GHz — both within the working bandwidth — are shown in Fig. 6. In the X-Y plane, the radiated energy is mainly focused in the Y-direction in the case of the copolarization.

Metamaterial Extends Microstrip Antenna, Fig. 6
6. These measured radiation patterns show copolarized (red traces) and cross-polarized (black traces) performance at 6.2 GHz (left-hand side) and 11.0 GHz (right-hand side).

In the Y-Z plane, in the case of the copolarization, the radiation level is well suppressed except in the Y-direction. This indicates that the strongest radiation is in horizontal direction and the cross-polarized radiation pattern is always orthogonal with the copolarized radiation pattern. Across the full frequency range, the radiation pattern of the proposed antenna maintains good performance. The antenna achieves a broad impedance bandwidth of 124% from 3.2 to 14.0 GHz with maximum gain of 8.89 dB.

Due to the transmission characteristics of LHM materials, the overall size of this patch antenna is smaller than a conventional patch antenna on standard substrates. The proposed antenna is simple in its design and fabrication. It demonstrates that LHM materials offer many potential opportunities, and that better antenna performance can be obtained by loading left-handed materials. This new antenna design may find use in a variety of applications, including in satellite-communications (satcom) and mobile-communications systems.

Jiangpeng Liu, Professor

Yongzhi Cheng, Engineer

Yan Nie, Engineer

Rongzhou Gong, Engineer

Huazhong University of Science and Technology, Department of Electronic Science & Technology, 430074, Luoyu Road No.1037, Wuhan, Hubei, People’s Republic of China; +86 13720361628, FAX: 027-87547337.

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1. L.J. Chu, “Physical limitations on omnidirectional antennas,” Journal of Applied Physics, Vol. 19, 1948, p.1163. 

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15. Z.K. Zhu, C.R. Luo, and X.P. Zhao, “A novel microstrip antenna with dendritic-structured negative permeability metamaterials,” Acta Physics Sin Vol. 58, 2009, pp. 6152-6156.

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18. W.B. Lu, Z.F. Ji, Z.G. Dong, X.W. Ping, and T.J. Cui, “Left-handed transmission properties of planar metamaterials based on complementary double-ring resonators,” Journal of Applied Physics, Vol. 108, 2010, p. 3717.

19. M.J. Lee, S. Pyo, W.S. Yoon, I.C. Shin, and Y.S. Kim, “A size reduced crlh resonant antenna based on interdigital capacitors with defected ground structure,” Microwave and Optical Technology Letters, Vol. 52, 2009, p. 2142.

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