A Spiral-Dipole Antenna for MIMO Systems - Microwave & Antenna ...

IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 7, 2008

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A Spiral-Dipole Antenna for MIMO Systems Yun-Taek Im, Jee-Hoon Lee, Rashid Ahmad Bhatti, and Seong-Ook Park, Member, IEEE

Abstract—A new circular polarization spiral-dipole antenna has been proposed for multilple-input–multiple-output (MIMO) systems. A dipole antenna is loaded with spirals at both of its ends to generate omnidirectional left-hand or right-hand circular polarization. The sense of polarization [left-hand circular polarization (LHCP) or right-hand circular polarization (RHCP)] depends on the orientation of the spirals. The measured bandwidth of the antenna is 8% at the center frequency of 5.2 GHz with 3.6-dBic gain. The isolation ( 21 ) between collocated LHCP and RHCP antennas is better than 30 dB that makes them suitable for MIMO. Index Terms—Dipole, multiple-input–multiple-output (MIMO), pattern diversity, polarization diversity, spatial diversity, spiral.

I. INTRODUCTION

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ECENT research on multiple-input–multiple-output (MIMO) systems has been carried out by many system designers. MIMO wireless communication system promise high channel capacity and improved overall performance [1]. The capacity of an MIMO system depends on the correlation between received signals. The concept of diversity is used in order to decorrelate the received signals. Spatial, pattern, and polarization diversities are commonly used to achieve better MIMO performances. The polarization diversity technique is promising when collocated antennas are required with a high degree of isolation. High isolation between antenna elements also ensures low mutual coupling among them. Strong mutual coupling adversely affects the antenna efficiency which results in low signal-to-noise-ratio (SNR) leading to degraded MIMO system capacity [1], [2]. Ideally, only two cases can be considered for perfect polarization mismatch. The first case is that of vertical polarization (V-Pol) versus horizontal polarization (H-Pol). The second case can be that of right-hand circular polarization (RHCP) and left-hand circular polarization (LHCP). Both cases require H-Pol that can be realized from small antennas of which the total length should be less than one-tenth of a wavelength. However, a small-loop antenna is quite reactive and difficult to match, so it is hardly used to produce H-Pol. In this regard, a simple combination of a dipole and a loop antenna is impractical [3], [4]. In this letter, an omnidirectional circular polarization (CP) Manuscript received April 24, 2008; revised June 07, 2008. First published June 27, 2008; current version published January 23, 2009. The authors are with the School of Engineering, Information and Communications University, Daejeon 305-732, Korea (e-mail: [email protected]; [email protected]).This work was supported by the Korea Science and Engineering Foundation (KOSEF) through Acceleration Research Program funded by the Ministry of Science and Technology (No. R17–2007-023–01001-0). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LAWP.2008.2001395

antenna operating at 5.2 GHz has been reported. The proposed antenna consists of a dipole and spiral, which are for the V-Pol and H-Pol, respectively. The proposed antenna is a much simpler and more practical configuration than that of the previous studies [2], [5]–[8]. All simulations are verified through computer simulation technology (CST) microwave studio (MWS). II. PROPOSED ANTENNA DESIGN A simple half-wavelength dipole antenna is shown in Fig. 1(a), and the current distribution is the same on the upper and lower parts of the dipole. If we increase the total dipole length beyond one wavelength, the current distribution is different from the half-wavelength dipole antenna, which means that the opposite current occurs at the upper and lower parts of the dipole. Generally, this case looks undesirable due to the opposite currents. However, the bent dipole antenna in Fig. 1(c) will have similar radiation performances as that of the simple half-wavelength dipole antenna, because the radiations due to the opposite current distributions at the end parts are cancelled. Then, the antenna can maintain the omnidirectional V-Pol pattern which is the same as that of the half-wavelength dipole. In Fig. 1(d), the spiral-shaped wires are added at the end of the dipole. Fig. 1(e) shows the top view of current distribution of Fig. 1(d). From the points A, B, C, and D in Fig. 1(e), we can find that each point has the same magnitude and the phase that looks like the formation of the virtual small-loop current. Since points A and C of the upper spiral are of the same magnitude and phase, points B and D also have the same magnitude and phase. So we can draw another virtually uniform loop current by connecting each point. Fig. 1(e) shows only four branches, but the addition of more spiral branches will result in a much smoother horizontal radiation pattern. Fig. 2 shows the whole geometry of the spiral-dipole antenna. The upper spiral is shown in Fig. 2(a), and the lower spiral is exactly the opposite shape of the upper one as shown in Fig. 2(b) and (c). The combined antenna, consisting of a dipole and spirals shown in Fig. 2(c), is fabricated on the substrate with the dielectric constant 2.2 and thickness of 0.508 mm. The dipole antenna consists of two layers—the top and the bottom layers—connected through the vias of radius 0.25 mm. The tapered balun and the input impedance transformer are used for easy fabrication. The optimized dimensions are listed in Table I. Fig. 3 shows the simulated and measured results of return loss and radiation patterns. There is good agreement with each other, and the minimum return losses are below 30 dB at 5.2 GHz. The measured radiation patterns in - and - planes are also similar to those of the simulation. From Fig. 3(b) and (c), the radiation pattern is omnidirectional in the - plane, and there

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Fig. 1. Current distribution. (a) Half-wavelength dipole. (b) One-and-a-halfwavelength dipole. (c) Bent dipole. (d) Spiral dipole. (e) Top view of the spiral dipole.

are nulls along the -axis. It means that the radiation patterns are analogous to the half-wavelength dipole. However, it is insufficient to satisfy the CP conditions by only comparing the magnitudes. The phase difference condition should be met for CP as well. Fig. 4(a) and (b) shows the simulation setup to verify the phase difference, and the simulated result shown in Fig. 4(c). At first, the scattering parameter of the V-Pol is simulated with a dipole antenna which is located 173 from the center of the spiral-dipole antenna. Then, mm the scattering parameter of the H-Pol is simulated following the procedure shown in Fig. 4(a) and (b). Fig. 4(c) shows the phase , and it is close to difference of the scattering parameters 90 at 5.2 GHz. From the simulation results in Figs. 3 and 4, the proposed antenna produces the omnidirectional CP in the x-y plane, and the obtained magnitude and the phase difference results represent LHCP. For MIMO systems, more than two antennas are required, and an orthogonal antenna is preferred for high isolation. If the upper spiral is replaced with the lower spiral in Fig. 2(b) and vice versa, the orthogonal RHCP can be obtained. It is similar to the

Fig. 2. (a) Spiral. (b) Top view. (c) Side view of the proposed antenna.

TABLE I DESIGN PARAMETERS UNIT (IN MILLIMETERS)

mirror image of Fig. 2(b), and it is a theoretically perfect polarization mismatched antenna. However, the radiation patterns and the return-loss characteristic will be the same as the counterpart of the LHCP, while achieving high isolation. Fig. 5 shows the collocated two antennas spacing with half-wavelength. The

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IM et al.: A SPIRAL-DIPOLE ANTENNA FOR MIMO SYSTEMS

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Fig. 4. Scattering parameter simulation setup of the proposed antenna. (a) Vertical polarization. (b) Horizontal polarization. (c) Phase difference.

Fig. 3. (a) Return loss. (b) Radiation patterns in the x-y plane (c) and in the y -z plane.

left antenna acts similar to an LHCP antenna, and the right antenna looks like an RHCP antenna. Since both antennas’ radiation patterns are orthogonal at 5.2 GHz, the scattering paramin Fig. 6(a) is the minimum around the operating freeter quency. The mutual coupling effect is below 20 dB overall, especially below 30 dB at 5.2 GHz, and below 40 dB at the minimum point in the simulation and measurement results. While retaining the high isolation characteristic, the radiation patterns of the two antennas are measured with the conditions of inphase and out of phase by 180 . The simulated and measured radiation patterns of an inphase condition in Fig. 6(b) agree with each other. Null axes are both -directional and -directional, because the field cancellation occurs when two orthogonal polarizations (RCHP and LHCP) propagate in the same direction. The radiation pattern of the out-of-phase condition in Fig. 6(c) is

Fig. 5. Collocation of the LHCP and RHCP antennas.


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Fig. 7. Fabricated antenna. (a) Spiral. (b) Dipole.

TABLE II MAXIMUM GAINS AND THE BANDWIDTH

Fig. 6. (a) Mutual coupling. (b) Inphase radiation pattern. (c) Out-of-phase radiation pattern.

similar to the inphase radiation pattern, which means omnidirectional. However, the vertical and horizontal field patterns have been interchanged. Table II summarizes the maximum gains and and are close to 0 dBi, bandwidth. The simulated gains is 2.9 dBic. This is a and the circular polarization gain good agreement between the simulated and measured results. Fig. 7 shows the fabricated antenna for the measurement. III. CONCLUSION A spiral-dipole antenna has been proposed and realized for MIMO systems. The design of the proposed CP antenna is based on the combination of a dipole and spiral antenna. The tapered balun and the impedance transformer are used for easy fabrication and matching of the antenna input impedance. It shows

pattern diversity by adjusting phase difference between two orthogonal antennas. Also, it can be used for applications of polarization diversity by switching each antenna and spatial diversity while maintaining good isolation characteristics. The proposed spiral-dipole antenna has similar performance to that of the half-wavelength dipole, but it is more versatile in the points of the diversity techniques and isolation characteristics. REFERENCES [1] C. Waldschmidt and W. Wiesbeck, “Compact wide-band multimode antennas for MIMO and diversity,” IEEE Trans. Antennas Propag., vol. 52, no. 8, pp. 1963–1968, Aug. 2004. [2] C.-Y Chiu, J.-B Yan, and R. D. Murch, “Compact three-port orthogonally polarized MIMO antennas,” IEEE Antennas Wireless Propag. Lett., vol. 6, pp. 619–622, Dec. 2007. [3] A. B. Constantine, Antenna Theory: Analysis and Design, 3rd ed. Hoboken, NJ: Wiley, 2005. [4] W. L. Stutzman and G. A. Thiele, Antenna Theory and Design, 2nd ed. Hoboken, NJ: Wiley, 1998. [5] J. D. Morrow, “Polarization-Adjustable omnidirectional dipole array,” IEEE Antennas Wireless Propag. Lett, vol. 2, pp. 223–225, 2003. [6] R. D. Bogner and N. Y. Roslyn, “Omnidirectional circularly polarized antenna,” USA U.S. Patent no. 3 474 452. [7] R. T. Klopach and J. Bohar, “Broadband Circularly Polarized Omnidirectional Antenna,” U.S. Patent no. 3 656 166. [8] J. D. Dyson and P. E. Mayes, “Circularly polarized omnidirectional cone mounted spiral antenna,” USA U.S. Patent no. 3 188 643.
