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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

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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