A Dipole-Fed Multi-Band Spiral Antenna Mayumi Matsunaga Dept. of Electrical and Electronic Engineering Ehime University Matsuyama, Ehime, Japan
[email protected] Abstract— A novel structure of multi-polarization spiral antennas is presented. The spiral antenna radiates not only linearly polarized waves but also circularly polarized waves simultaneously without phase shifters. In this paper, the way to tune the antenna into three bands, GPS and two mobile phone bands, is delineated. The return-loss characteristics as regards various lengths of the appropriate elements that can control utilized frequencies of the antenna are discussed to show that the antenna can be tuned into multi-bands. The return-loss and radiation patterns are delineated by comparing simulation results and measurement results. These results show that the antenna radiates good circularly polarized waves as well as good linearly polarized waves at a GPS and mobile phone bands, respectively.
I.
waves. By making these gaps, electromagnetic waves distribute on the spiral element in the desired way to radiate circularly polarized waves. Therefore, thanks to the gaps, the CSA is able to be fed by a dipole element without interference of electromagnetic waves on the spiral and dipole antennas.
INTRODUCTION
The demand for a simple high performance antenna is increasing to pursue smaller size and higher specs of a smartphone. A multi-polarization antenna could be one of these antennas that can solve the problem to reduce the space for the antenna inside mobile communication devices as keeping each antenna's performance. The multi-polarization antenna presented in this paper has been developed from the cross-shaped spiral Antenna (CSA), which is invented by the author as a good design that makes spiral/loop antennas radiating circularly-polarized waves without phase shifters [1]. By making a gap on the spiral element, CSA has developed into a multi-polarization antenna [2]. To improve the polarization characteristics, we have employed a dipole antenna as a feed element of the multipolarization CSA [3]. In this paper, the new CSA that is simplified and improved its performance is presented. The structure of the new CSA, which radiates circularly and linearly polarization waves at three bands simultaneously, is explained first. Then next, the return-loss characteristics and radiation patterns obtained by testing the prototype antenna are delineated. Finally, the return loss characteristics are shown as changing the length of the element that can tune each resonant frequency into an exact band. II.
THE DESIGN OF THE MULTI-POLARIZATION CSA
Figure 1 shows the structure of the presented new CSA. Detailed measurements of the drawing are shown in table 1. Cross-shaped spiral element is fed by a dipole element located in the center of the CSA. The spiral element is a parasitic one, having gaps at point A and B for controlling electromagnetic
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Figure 1. Drawing of the CSA basic design. TABLE I. DIMENSIONS OF THE CSA. (IN CASE THE DIELECTRIC SUBSTRATE WHOSE εR IS 4.4, TANδ IS 0.016, AND THICKNESS IS 1.6 mm IS USED.). w 1.0
L1 52.2
L2 60.8
L3 59.8
L3 5.0
s1 0.5
s2 3.0
s3 0.5 Unit is mm.
III.
CHARACTERISTICS OF THE CSA
Figure 2 shows a return loss characteristic of the CSA. There are three major resonance frequencies. For convenience, these frequencies are named f1, f2 and f3 from the highest ones. There is good agreement between the result obtained by testing the prototype antenna actually and the result obtained by computer simulations. In figure 3, the beam patterns at y-z plane for the circularly polarized waves of 1.57 GHz and the linearly polarized wave of 1.8 GHz are shown. Simulation results also agree with measurement results very well. According to the radiation patterns, a linearly polarized, circularly polarized and linearly polarized wave radiate at f1, f2 and f3, respectively.
AP-S 2013
Figure 2. Return loss characteristic of the CSA.
Figure 4. Return loss characteristic for various lengths of L1.
Figure 3. y-z cut radiation patterns of circularly polarized waves at 1.57 GHz and 1.8GHz.
Figure 5 Return loss characteristic for various lengths of L4.
At the f2, the phase difference between the EM waves on the upper and lower wings and those on the right and left wings is almost π/4. On the other hand, at the f3, there is almost no phase difference between the EM waves on these wing parts of the spiral element. For these reasons, circularly polarized waves are radiated at f2 and linearly polarized waves are radiated at f3. IV.
THE WAY TO TUNE THE CSA
First, let us examine an element contributed to f1. It can be easily understood that a main element controlling the resonant frequency f1 is a dipole antenna. The return losses simulated with varying length of L1 is shown in figure 4. Figure 4 shows that f1 can be tuned by the length of L1. Next, let us study the resonant frequencies f2 and f3 at the same time. Spiral elements mainly work at these frequencies. Figure 5 and 6 show the return loss characteristics when the length of L4 and s2 vary, respectively. From these results, L4 controls f2, and s2 controls the frequency span between f2 and f3.
Figure 6. Return loss characteristic for various lengths of s2. REFERENCES [1]
[2]
ACKNOWLEDGEMENT The authors would like to thank the Ministry of Education, Culture, Sports, Science and Technology of Japan who funded part of this research with a Grant-in-Aid for Young Scientists (A) (21686035).
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[3]
M. Matsunaga, T. Matsuoka and T. Matsunaga, “A Suggested Shape of Spirals for Expanding the Half-Power Beamwidths of UHF Band RFID's Planar Spiral Antennae,” Proc. of International Symposium on Antennas and Propagation, pp. 1422–1425, Oct. 2008. M. Matsunaga, K. Kakemizu and T. Matsunaga, “A cross shaped spiral antenna radiating omnidirectional circularly and linearly polarized waves”, IEICE Electron. Express, Vol. 9, No. 4, pp.256-262, 2012. M. Matsunaga and T. Matsunaga, “A Dual-Polarization Single-Layered Antenna for GPS and ISM Bands,” Proceedings of the 2012 IEEE International Symposium on AP-S/USNC-URSI, July, 2012.
