ULTRAWIDEBAND (UWB) communications systems work across broad bandwidths and can provide an effective means of transferring large amounts of data. But these systems also place great demands on the operating bandwidths of their components, including the antennas, forcing designers to develop structures that are not only small in size but capable of extremely wide bandwidths. Fortunately, fractal microstrip antenna structures show great promise for handling wide bandwidths at microwave frequencies with moderate gain and excellent return-loss performance. Simulations performed with a commercial circuit simulator show that this example design is capable of operating from 5 to 50 GHz with quite reasonable performance. In addition, the fractal antenna geometry is fairly easy to fabricate, for low manufacturing costs.

Traditional antennas are based on quarter-wavelength elements, which dictate the size of a printed-circuit antenna for a given frequency. But fractal antennas are based on self-similar elements iterated in various directions and can provide broadband frequency operation without the size restrictions of quarter-wave elements. Their total forms do not change with increasing iterations because their small sections are reduced-size copies of the whole form. The multiband and broadband properties of fractal antennas draw from the self-similarity of the fractal elements.^{1-4}

The space-filling property of fractals leads to curves which have long electrical length but fit into a compact physical area.^{5-10} Compared to conventional square microstrip patch antennas, for example, the space-filling and self-similarity properties of fractal-shaped patches, such as Koch patches, can be as much as 40 percent smaller than a conventional Euclideantype microstrip patch antenna, without sacrificing radiation pattern performance.

A number of different UWB antenna configurations based on fractal geometries were investigated, including Koch, Sierpinski, Minkowski, Hilbert, Cantor, and fractal tree antennas, to compare performance as well as manufacturability. The antennas were modeled with method-of-moments (MoM) electromagnetic (EM) simulation software and were also fabricated as prototype circuits and measured to evaluate actual performance. The new fractal antenna reported on here is yet another antenna configuration which makes use of this circuit approach, with considerably larger bandwidth than these other conventional fractal antenna types.

The Microwave Studio suite of software design tools from Computer Simulation Technology (CST) was used to design and simulate the new fractal microstrip antenna configuration. The results of the simulation show a usable bandwidth of 5 to 50 GHz. Radiation patterns were studied at different frequencies, and the gain was found to be reasonable across the entire 45-GHz operating bandwidth.

The new fractal shape starts with the base shape shown in Fig. 1. The base shape is formed by two 45-deg. crossed squares with length of 2 cm for each side. This shape is iterated twice.

The second iteration of Fig. 1 was taken as the new fractal antenna geometry. The fractal antenna is based on the use of alumina circuit substrate with relative dielectric constant (e_{r}) of 9.4 and 1.6-mm thickness. The dimension of the ground plane was chosen to be 3 x 3 cm^{2}. The location of the coaxial feed was placed on the patches 13 mm from the center at the corner of the structure (Fig. 2).

The performance of the microstrip fractal antenna was simulated with Microwave Studio. Simulations were performed assuming ideal ground plane and conductors. Simulations were performed from 5 to 50 GHz. Figure 3 shows the simulated reflection coefficient as a function of frequency from 5 to 50 GHz.

According to the simulated reflection coefficients, this fractal microstrip antenna is usable across a total frequency range of 5 to 50 GHz frequency range. To study the radiation pattern, Fig. 4 shows the maximum gain versus frequency and Fig. 5 shows the radiation patterns (E-Field) for 10, 20, 30, and 40 GHz for the x-y, x-z, and y-z planes.

Abolfazl Azari is a Member of Young Researchers Club, Islamic Azad University, Gonabad Branch, Iran; e-mail: azari@ieee.org.

References

1. A. Azari, and J. Rowhani, "Ultra Wideband Fractal Microstrip Antenna Design," Progress In Electromagnetics Research C, Vol. 2, 2008, pp. 7-12.

2. A. Azari, "A New Fractal Antenna for Super Wideband Applications," Progress in Electromagnetic Symp., Cambridge, MA, 2010.

3. J. P. Gianvittorio and Yahya Rahmat Samii, "Fractal Antennas: A Novel antenna Miniaturization Technique and Applications," IEEE Antenna and Propagation Magazine, Vol. 44, No. 1, Feb. 2002.

4. A. Azari, "A New Ultra Wideband Fractal Antenna," paper presented at the Electromagnetic Theory Symposium, Berlin, Germany, 2010.

5. D.H. Werner, R.L. Haupt, and P.L. Werner, "Fractal antenna engineering: The theory and design of fractal antenna arrays," IEEE Antennas and Propagation Magazine, Vol. 41, No. 5, 1999, pp. 37-59.

6. J. Gouyet, Physics and Fractal Structures, Springer, New York, 1996.

7. D. H. Werner and Raj Mittra, Frontiers in Electromagnetics, IEEE Press, New York, 2000.

8. A. Azari, "Super Wideband Fractal Antenna Design," IEEE MAPE, Beijing, China, 2009.

9. A. Azari, "A new fractal monopole antenna for super wideband applications," IEEE MICC, Kuala Lumpur, Malaysia, 2009.

10. A. Azari, and J. Rowhani, "Ultra Wideband Fractal Antenna Design," IASTED ARP, Baltimore, MD, 2008.