IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 7, 2008 803 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) sys- tems. A dipole antenna is loaded with spirals at both of its ends to generate omnidirectional left-hand or right-hand circular po- larization. The sense of polarization [left-hand circular polariza- tion (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 between collocated LHCP and RHCP an- tennas 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 R 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 mu- tual 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 Commu- nications University, Daejeon 305-732, Korea (e-mail: [email protected]; [email protected]).This work was supported by the Korea Science and Engi- neering 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 pro- posed 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 op- posite current occurs at the upper and lower parts of the dipole. Generally, this case looks undesirable due to the opposite cur- rents. However, the bent dipole antenna in Fig. 1(c) will have similar radiation performances as that of the simple half-wave- length dipole antenna, because the radiations due to the opposite current distributions at the end parts are cancelled. Then, the an- tenna 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 for- mation 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 an- tenna. 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 ta- pered 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 1536-1225/$25.00 © 2008 IEEE Authorized licensed use limited to: Korea Advanced Institute of Science and Technology. Downloaded on June 12, 2009 at 00:09 from IEEE Xplore. Restrictions apply.