Metamaterial Enhances Microstrip Antenna Gain

July 29, 2016
Single and multiple layers of metamaterials formed as lenses can improve the impedance matching, gain, and fractional bandwidth of compact microstrip antennas.
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High antenna gain is generally desirable for communications systems, whether terrestrial or based on satellites. The use of metamaterials in single or multiple layers has been shown to contribute a great deal to the design of a high-gain microstrip antenna at 10.5 GHz. With a double-negative electromagnetic (EM) constant, these metamaterials exhibit properties not mormally found in circuit materials, such as negative permittivity and negative permeability.

Structures formed of the metamaterials act like a lens for a microstrip antenna, boosting gain and enhancing the radiation pattern for greater coverage. In addition to providing more than 6-dB gain compared to a conventional microstrip antenna, these metamaterial-equipped antenna designs also feature increased bandwidth (from 3.64% to 4.68%) when compared to a conventional microstrip design.

1. These are the dimensions of the microstrip antenna.

Patch antennas are often desirable for wireless communications systems for their low profile, compact size, ease of implementation, and low implementation cost. Unfortunately, patch antennas typically exhibit low gain and narrow bandwidths. Several approaches have been presented to overcome these disadvantages.1-3 For example, arrays of several patch antennas have been used to achieve increased gain. However, this approach must overcome losses associated with the feed network and coupling between antenna elements in the array.

Metamaterials can provide EM properties not found in nature that can help enhance antenna gain. Left-handed materials (LHMs) were theorized in 1967 as EM plane wave propagation in a lossless medium with simultaneous negative real permittivity and permeability at a given frequency.4 LHM is characterized by antiparallel phase and group velocities as well as nonlinear phase characteristics.5-7 These properties have enabled the development of compact microwave components, such as power dividers, filters, phase shifter, resonators, and antennas.8-11

2. The plots compare the simulated and measured return loss of the microstrip antenna, designed for 10.5 GHz.

Metamaterials have several advantages, such as achieving miniaturization and increasing the gain performance. The recent revival of interest in double-negative media began with Smith, Schultz, and Shelby, as inspired by the work of Pendry.12, 13 Smith demonstrated a new metamaterial that simultaneously achieved negative permittivity and permeability.14 Different shapes of metamaterial cells have also been used to achieve double-negative characteristics.15-21

3. The radiation patterns show simulated and measured responses at 10.5 GHz in the (a) E-plane and (b) H-plane.

To better understand the benefits of using metamaterial structures with microstrip antennas, an antenna was designed with a unit-cell metamaterial lens structure; the performance of the antenna was evaluated with the Microwave Studio full-wave EM simulation software from CST. Figure 1 shows a two-dimensional (2D) layout of the microstrip antenna. It was designed on RT/duroid 5880 circuit material from Rogers Corp. with relative dielectric constant (εr) of 2.2 in the z-axis (thickness) at 10 GHz, dielectric loss tangent of 0.0009, and thickness of 0.787 mm.

4. The dimensions of the metamaterial unit cell or lens are shown here.

The optimization of the antenna is carried out using CST Microwave Studio commercial software. The antenna measures 50 × 60 mm. 50 mm × 60 mm. The patch has length, Lp, of  8.8 mm, width, Wp, of 13 mm, and thickness of 35 μm. The antenna is impedance-matched by means of a quarter-wavelength (λ/4) transformer with length, Lt, of 5 mm and width, Wt, of 0.9 mm. The antenna is fed by means of 50-Ω microstrip line with width, Wf, of 2 mm and length, Lf, of 20 mm.

5. The plots show the real and imaginary values for the metamaterial unit cell (a) permittivity and (b) permeability.

Figure 2 compares measured and simulated values for return loss, which are in good agreement. From the simulations, it is apparent that the antenna has an impedance bandwidth from 10.30 to 10.75 GHz. However, from the measurements, the fabricated antenna exhibits an impedance bandwidth from 10.20 to 10.58 GHz, with a center frequency, f0, of 10.43 GHz. Figure 3 shows the antenna’s simulated and measured radiation patterns, in the E and H planes at 10.5 GHz, which also agree closely. The measured gain is 5.8 dB.

6. This is a simple layout of the microstrip antenna showing the distance to a single-layer metamaterial unit cell.

Figure 4 presents (a) two-dimensional (2D) and (b) three-dimensional (3D) layouts of a single metamaterial unit cell for an omega structure. In this omega structure, two perfect electric conductors (PECs) with thickness t = 0.035 mm are integrated on RO4003 circuit material from Rogers Corp. with relative dielectric constant (εr) of 3.55, dissipation factor of 0.0027, and thickness of 0.813 mm. This structure is a complex design whereby the rod and ring are coupled, resulting in the permittivity and permeability being coupled.22

7. The plots show the simulated return loss of the microstrip antenna with a single-layer metamaterial lens at different values of distance d1.

The effective permittivity and the effective permeability of the omega structure can be calculated by an approach based on extraction from the transmission and reflection characteristics of the metamaterial omega structure.22,23 This extraction technique consists of several steps. First, the complex normalized wave impedance (z) and refractive index (n) are retrieved from the S-parameters. Second, the effective permittivity (εeff) and permeability (μeff) are computed from the n and z values. The data are calculated with the aid of the MATLAB mathematics-based simulation software from MathWorks. Figures 5(a) and (b) show the effective permittivity and permeability of the omega structure. It can be seen that both parameters exhibit a real part from 10 through 15 GHz, which leads to a negative refractive index through these frequency bands.

8. These plots show simulated gain for the microstrip antenna with a single-layer metamaterial lens at different values of distance d1.

To enhance microstrip antenna performance, it was necessary to understand the impact of the size of the omega structure serving as a lens on the radiation patterns and antenna performance. To ensure the effect of the periodic structure size and to obtain the optimum return loss and radiation parameters, a parametric study on the antenna loaded with metamaterial lens at specific separation and different dimensions for the periodic structure are carried out.24 A 6 × 5 periodic structure was considered to yield optimum return loss and optimum radiation properties.

9. These photographs show different views of the fabricated antenna with a single-layer metmaterial lens.

Figure 6 shows the antenna layout, with a single-layer metamaterial lens loaded at a distance, d1, from the patch antenna. The effect of d1 on performance was evaluated, with simulated return loss and gain for different values of d1 shown in Figs. 7 and 8, respectively.

From these evaluations, it is clear that d1 influences the impedance bandwidth and the gain. When d1 equals 8 mm, at around 0.25λ0, the impedance bandwidth and gain are affected compared to the antenna without metamaterial lens, where the bandwidth and gain are less. When d1 equals 16 mm, at around 0.50λ0, the impedance bandwidth and gain are increased, with gain growing from 5.8 to 9.8 dB.

10. The return loss of the microstrip antenna was measured with and without a single-layer metamaterial lens.

When d1 is increased to 32 mm, at around 1.0λ0, the impedance bandwidth is reduced and the operating frequency is shifted toward 10.7 GHz. The gain is reduced to 8 dB, but it is still more than a conventional antenna without metamaterial lens. For optimum performance with a single-layer metamaterial lens, d1 should be equal to 16 mm at this frequency, or around 0.50λ0 for a frequency of interest.

11. The gain of the microstrip antenna was at 10.5 GHz with and without metamaterials in (a) the E-plane and in (b) the H-plane.

To better understand the effects of the metamaterial structure, the proposed antenna was fabricated and characterized. Foam material with relative dielectric constant approximately equal to that of air (1) and thickness (d1) was added as a spacer between the metamaterial and the patch antenna (Fig. 9).

Figure 10 shows measured return loss with and without the metamaterial. As can be seen, the metamaterial lens enhances the matching characteristics of the antenna. The fractional bandwidth of the antenna is increased from 3.64% to 4.68% as well. Figure 11 shows the measured E- and H-plane radiation patterns of the antenna at 10.5 GHz with and without the metamaterial lens. The metamaterial lens enhances the gain by 4 dB (from 5.8 to 9.8 dB).

12. This layout shows the concept of the microstrip antenna with two metamaterial lenses on the left and the spacing of the lenses on the right.

If one metamaterial layer can help microstrip antenna performance, perhaps two layers might provide greater benefits. To explore this possibility, the effects of adding another layer of metamaterial were investigated, at a distance, d2, from the first layer above the antenna. An optimum value of d1 = 16 mm was used for the first layer. Figure 12 shows the proposed antenna with two metamaterial layers, one above the other.

13. The antenna return loss was simulated for different values of distance d2.

The second layer was placed at different distances of 0.25λ0, 0.5λ0, and λ0 above the first layer, with simulated return loss and gain plotted in Figs. 13 and 14, respectively. As can be seen from the plots, d2 also impacts impedance bandwidth and gain, although the impedance bandwidth was not affected when the second layer was at a distance of 0.5λsub>0 from the antenna with a first metamaterial layer.

14. These plots compare the simulated gain of the microstrip antenna without a metamaterial lens and with different values of distance d2.

With the added metamaterial layer, the gain of the antenna increased from 5.8 to 12 dB at 10.5 GHz. It also increased from 11.8 to 12.4 dB at the lower-frequency edge of the band at 10.3 GHz and at the upper-frequency edge of the band at 10.7 GHz at a distance of d2 = 16 mm for high gain throughout the antenna bandwidth. When d2 = 32 mm, the gain of the antenna increased from 5.8 to 11.4 dB at 10.5 GHz. The optimum distances d1 and d2 for good antenna performance in the 10-GHz band were d1 = 16 mm and d2 = 16 mm, which also contributed to reducing the size of the antenna by having both layers at the same distance.

Adel B. Abdel-Rahman, Professor

Department of Electronics and Communications Engineering, E-Just-Alexandria, Egypt

Faculty of Engineering, South Valley University-Qena, Egypt

Ahmed A. Ibrahim, Professor

Faculty of Engineering, Minia University, Egypt

Institute of Micro and Sensor Systems, University of Magdeburg, 39106, Magdeburg, Germany


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