Fractal and Multiband Communication Antennas

In this paper we present two novel multiband antenna designs for multiple frequency applications. The first of these, which is based on fractal concepts, is a Sierpinski gasket type of fractal configuration, printed on a dielectric substrate backed by a ground plane. We propose a novel approach to enhancing its multiband performance, in terms of impedance matching characteristics as well as radiation patterns, at three operating frequencies. The second design combines a rectangular microstrip patch with a cylindrical dielectric resonator, and operates at multiple frequencies. Specifically, we describe a stacked configuration of a dielectric resonator antenna on a microstrip patch for three frequency bands. Circular polarization is obtained by utilizing four coax feeds in phase quadrature. This configuration is found to exhibit good cross-pol rejection characteristics. Sierpinski grid antenna and modification The basic configuration, which is the starting point of the design, is a gridded version of the second-order Sierpinski gasket patch antenna, shown in Fig. 1. The height of the patch antenna is 44mm, and all of the triangular sub-grids are identical except for the truncated top ends. The patch is placed on a substrate whose thickness is 3.175mm and the dielectric constant is 2.2. Three shorting pins are inserted into the structure, with one of them mounted at the top center and the other placed at the two sides of the Sierpinski grid (indicated by arrows in Fig. 1). It is found that the above modifications, viz., the grid truncation and inclusion of the shorting pins, help improve the matching characteristics at the low frequency bands. To model the antenna design we simulate it using an MoM code. The patch is fed by a coaxial probe near the apex, and assumed to be located above an infinite ground plane. The S11 characteristic of the antenna is plotted in Fig. 2, which shows that the shorting pins do improve the matching performance at the lowest frequency band. The radiation patterns of the antenna operating in the first two bands (3 and 6 GHz) are desirable types; however, the pattern at 12 GHz, which corresponds to the resonant frequency of the bottom-most triangular grid, develops multiple lobes because the current flow is not confined to the bottom region alone. One solution to this problem is to decouple the bottom grid from the rest of the structure in the high frequency band by introducing inductor elements shown in Fig. 3, with a view to providing low-resistance conducting path at the low frequency bands, but sufficiently high impedance at the higher frequencies. This helps suppress the currents in the middle and top regions of the patch that cause the pattern to have multiple lobes at higher end of the band. The three shorting pins remain in their previous locations, but ten chokes are now inserted in the mid-region of the patch. With this modification, the resonance frequencies shift to 3, 5.2 and 10.2 GHz  which is desirable. We noted the radiation patterns for the above three resonance frequencies are very smooth in nature, and achieving this behavior is usually an important design goal. Multiband antenna configuration using DRA and microstrip patch The DRA/microstrip patch antenna configuration investigated is shown in Fig. 4. A rectangular microstrip patch lies upon a dielectric substrate, whose dielectric constant is 2.2, the thickness is 3.175mm, and the length is 40mm. The length of the rectangular patch is 30mm, and it 0-7803-8197-1/03/$17.00 (c)2003 IEEE is fed by two 50Ω coaxes (at ports 1 and 3) in phase quadrature, to obtain circular polarization. This feed configuration requires the isolation of the splitters for the phase-quadrature feeds, so as to suppress the multiple reflections between the feeds, which lead to increased level of cross-polar radiation. The rectangular patch antenna is designed to cover the two lower bands (2 and 3.2GHz), and the operation at the third frequency (5.1GHz)is achieved by stacking the cylindrical dielectric resonator atop the circular patch. The first band is linearly polarized for which either port-1 or port-3 can be excited, while the unexcited port is terminated by 50Ω, which provides the matching load. The dimensions of the DRA are: radius r=7mm; height h=10.8mm; the dielectric constant εrDRA=10. The DRA is also fed by two coaxes (ports 2 and 4) in phase quadrature. The antenna has been simulated by using the ANSOFT-HFSS code. The simulated directivity patterns for three operating bands showed good cross-pol rejection characteristics. ConclusionsIn this paper, we have introduced two novel antenna designs for multiband applications. Forthe Sierpinski grid antenna, desirable input impedance and radiation pattern characteristics havebeen realized by using a combination of shorting pins and chokes that control the current flows onthe radiating structure. For the second design, have shown how a stacked configuration of theDRA and a rectangular patch antenna can be combined to provide a multi-frequency operation,while retaining the ability to scale their resonant frequencies independently. References[1] D.H. Werner and R. Mittra, Frontiers in Electromagnetics, IEEE Press, 2000.[2] C. Puente, J. Romeu, R. Bartolome, and R. Pous, “On the Behavior of the SierpinskiMultiband Antenna,” IEEE Transactions on Antennas and Propagation, Vol. 46, No.4,pp. 517-524, April 1998.[3] Stuart A. Long, Mark W. McAllister, and Liang C. Shen, “The Resonant CylindricalDielectric Cavity Antenna,” IEEE Transactions on Antennas and Propagation, Vol. 31,No.3, pp. 406-412, May 1983. 051 01 5-3 0-2 5-2 0-1 5-1 050S 1 1 C h a ra c te r is t i c F re q u e n c y (G H z )S11(dB) S im u la t io n W / O s h o r t in g p in sS im u la t io n W i t h s h o r t in g p in sE x p e r im e n t Figure 1. Sierpinski grid antenna Figure 2. S11 characteristic of the Sierpinski grid antenna