Computer simulations of |S11|, aperture efficiency, directivity and the radiation patterns in the E- and H-planes are shown in Figure 3. At 5.8 GHz, the gain is 12.95 dBi and the aperture efficiency is 86.75 percent. The impedance bandwidth is 340 MHz, i.e., |S11| < ‐10 dB from 5.73 to 6.07 GHz. Directivity is between 12.77 and 13.07 dBi, and aperture efficiency is between 83.34 and 86.75 percent.

Figure 3

Figure 3 Computer simulation of conformal antenna |S11| (a), directivity and aperture efficiency (b) vs. frequency and E-plane (c) and H-plane (d) patterns at 5.8 GHz.

Figure 4

Figure 4 Influence of the cylindrical radius on |S11| (a), E-plane (b) and H-plane (c) antenna patterns.

Sensitivity Analysis

To evaluate the robustness of the design, the influence of factors affecting the stability of antenna performance was explored: the radius of the cylinder, the dielectric constant and the spacing of layers. Figure 4 shows the variation of |S11| and the antenna radiation patterns with the radius of cylinder Sr1 at 58, 63 and 68 mm. With decreasing radius, the center frequency increases, accompanied by a sharp decline in impedance bandwidth. With increasing radius, the impedance bandwidth is affected to a lesser degree. When the radius is 58 mm, its impact on the pattern, especially in the E-plane, is large; the side lobe and back lobe grow significantly. For a radius of 68 mm, the impact on the pattern is relatively small.

Figure 5

Figure 5 conformal antenna.

Figure 6

Figure 6 Measured vs. simulated |S11| .

The dielectric constant of the teflon dielectric plate changes with frequency and temperature. The influence of dielectric constant on antenna performance was evaluated for εr values of 2.05, 2.55 and 3.05. The corresponding antenna gains are 13.6, 13.7 and 13.4 dBi, indicating the dielectric constant has little influence on the pattern. For a double-layer antenna, the spacing between the parasitic layer and radiation layer has a strong influence on impedance bandwidth but little influence on operating frequency. Radiation patterns with different layer spacing show that with hz spacings of 2.05, 2.35 and 2.65 mm, antenna gains are 12.3, 12.95 and 12.46 dBi, respectively, indicating that variation in the spacing of layers over a narrow range has little influence on gain.

Experimental results

The prototype antenna is shown in Figure 5. Its reflection coefficient |S11| was measured with a vector network analyzer, and a comparison of the measured and simulated results is shown in Figure 6. They are in good agreement, demonstrating an impedance bandwidth of 320 MHz (|S11| < ‐10 dB from 5.73 to 6.05 GHz). Antenna patterns were measured at 5.7, 5.8 and 5.9 GHz (see Figure 7). They agree well with the simulation, especially at the center frequency of 5.8 GHz.

Figure 7

Figure 7 Measured vs. simulated antenna patterns at 5.7 (a), 5.8 (b) and 5.9 (c) GHz.

Conclusion

This article describes a novel, two-layer, stacked, microstrip, cylindrical conformal antenna using cross snowflake fractal patches. Based on the planar cross snowflake fractal patches, the structure of the conformal antenna was optimized by FEM to achieve high aperture efficiency and wideband properties at a center frequency of 5.8 GHz. The measured impedance bandwidth extended from 5.23 to 6.54 GHz (19.75 percent). The gain was
12.1 dBi, and the corresponding aperture efficiency was between
87.5 and 89.75 percent.

Acknowledgment

This work was supported by the National Natural Science Foundation of China (Grant Number: 61372043) National Basic Research Program of China (Grant Number: 2013CB328905) and the Sichuan Science Fund for Distinguished Young Scholars (Grant Number: 2015JQ0034).

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