IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 7, 2008 803

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

Abstract—A new circular polarization spiral-dipole antenna hasbeen proposed for multilple-input–multiple-output (MIMO) sys-tems. A dipole antenna is loaded with spirals at both of its endsto 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)] dependson the orientation of the spirals. The measured bandwidth of theantenna is 8% at the center frequency of 5.2 GHz with 3.6-dBicgain. 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 promisehigh channel capacity and improved overall performance [1].The capacity of an MIMO system depends on the correlationbetween received signals. The concept of diversity is used inorder to decorrelate the received signals. Spatial, pattern, andpolarization diversities are commonly used to achieve betterMIMO performances. The polarization diversity technique ispromising when collocated antennas are required with a highdegree of isolation. High isolation between antenna elementsalso ensures low mutual coupling among them. Strong mu-tual coupling adversely affects the antenna efficiency whichresults in low signal-to-noise-ratio (SNR) leading to degradedMIMO system capacity [1], [2]. Ideally, only two cases canbe considered for perfect polarization mismatch. The firstcase is that of vertical polarization (V-Pol) versus horizontalpolarization (H-Pol). The second case can be that of right-handcircular polarization (RHCP) and left-hand circular polarization(LHCP). Both cases require H-Pol that can be realized fromsmall antennas of which the total length should be less thanone-tenth of a wavelength. However, a small-loop antenna isquite reactive and difficult to match, so it is hardly used toproduce H-Pol. In this regard, a simple combination of a dipoleand 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 publishedJune 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: ytimicu@icu.ac.kr;sopark@icu.ac.kr).This work was supported by the Korea Science and Engi-neering Foundation (KOSEF) through Acceleration Research Program fundedby 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 onlineat 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 forthe V-Pol and H-Pol, respectively. The proposed antenna isa much simpler and more practical configuration than that ofthe previous studies [2], [5]–[8]. All simulations are verifiedthrough computer simulation technology (CST) microwavestudio (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 andlower parts of the dipole. If we increase the total dipole lengthbeyond one wavelength, the current distribution is different fromthe 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 havesimilar radiation performances as that of the simple half-wave-length dipole antenna, because the radiations due to the oppositecurrent distributions at the end parts are cancelled. Then, the an-tenna can maintain the omnidirectional V-Pol pattern which isthe same as that of the half-wavelength dipole. In Fig. 1(d), thespiral-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 thepoints A, B, C, and D in Fig. 1(e), we can find that each pointhas the same magnitude and the phase that looks like the for-mation of the virtual small-loop current. Since points A and Cof the upper spiral are of the same magnitude and phase, pointsB and D also have the same magnitude and phase. So we candraw another virtually uniform loop current by connecting eachpoint. Fig. 1(e) shows only four branches, but the addition ofmore spiral branches will result in a much smoother horizontalradiation pattern.

Fig. 2 shows the whole geometry of the spiral-dipole an-tenna. The upper spiral is shown in Fig. 2(a), and the lowerspiral is exactly the opposite shape of the upper one as shown inFig. 2(b) and (c). The combined antenna, consisting of a dipoleand spirals shown in Fig. 2(c), is fabricated on the substratewith the dielectric constant 2.2 and thickness of 0.508 mm. Thedipole antenna consists of two layers—the top and the bottomlayers—connected through the vias of radius 0.25 mm. The ta-pered balun and the input impedance transformer are used foreasy fabrication. The optimized dimensions are listed in Table I.

Fig. 3 shows the simulated and measured results of return lossand 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 alsosimilar to those of the simulation. From Fig. 3(b) and (c), theradiation pattern is omnidirectional in the - plane, and there

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

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

are nulls along the -axis. It means that the radiation patternsare analogous to the half-wavelength dipole.

However, it is insufficient to satisfy the CP conditions byonly comparing the magnitudes. The phase difference conditionshould be met for CP as well. Fig. 4(a) and (b) shows the sim-ulation setup to verify the phase difference, and the simulatedresult shown in Fig. 4(c). At first, the scattering parameter of theV-Pol is simulated with a dipole antenna which is located 173mm from the center of the spiral-dipole antenna. Then,the scattering parameter of the H-Pol is simulated following theprocedure shown in Fig. 4(a) and (b). Fig. 4(c) shows the phasedifference of the scattering parameters , and it is close to90 at 5.2 GHz. From the simulation results in Figs. 3 and 4, theproposed antenna produces the omnidirectional CP in the x-yplane, and the obtained magnitude and the phase difference re-sults represent LHCP.

For MIMO systems, more than two antennas are required,and an orthogonal antenna is preferred for high isolation. If theupper spiral is replaced with the lower spiral in Fig. 2(b) and viceversa, 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 IDESIGN PARAMETERS UNIT (IN MILLIMETERS)

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

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IM et al.: A SPIRAL-DIPOLE ANTENNA FOR MIMO SYSTEMS 805

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

left antenna acts similar to an LHCP antenna, and the right an-tenna looks like an RHCP antenna. Since both antennas’ radi-ation patterns are orthogonal at 5.2 GHz, the scattering param-eter in Fig. 6(a) is the minimum around the operating fre-quency. The mutual coupling effect is below 20 dB overall,especially below 30 dB at 5.2 GHz, and below 40 dB at theminimum point in the simulation and measurement results.

While retaining the high isolation characteristic, the radiationpatterns of the two antennas are measured with the conditions ofinphase and out of phase by 180 . The simulated and measuredradiation patterns of an inphase condition in Fig. 6(b) agree witheach other. Null axes are both -directional and -directional,because the field cancellation occurs when two orthogonal po-larizations (RCHP and LHCP) propagate in the same direction.The radiation pattern of the out-of-phase condition in Fig. 6(c) is

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

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

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

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

similar to the inphase radiation pattern, which means omnidirec-tional. However, the vertical and horizontal field patterns havebeen interchanged. Table II summarizes the maximum gains andbandwidth. The simulated gains and are close to 0 dBi,and the circular polarization gain is 2.9 dBic. This is agood 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 forMIMO systems. The design of the proposed CP antenna is basedon the combination of a dipole and spiral antenna. The taperedbalun and the impedance transformer are used for easy fabri-cation and matching of the antenna input impedance. It shows

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

TABLE IIMAXIMUM GAINS AND THE BANDWIDTH

pattern diversity by adjusting phase difference between two or-thogonal antennas. Also, it can be used for applications of po-larization diversity by switching each antenna and spatial diver-sity while maintaining good isolation characteristics. The pro-posed spiral-dipole antenna has similar performance to that ofthe half-wavelength dipole, but it is more versatile in the pointsof the diversity techniques and isolation characteristics.

REFERENCES

[1] C. Waldschmidt and W. Wiesbeck, “Compact wide-band multimodeantennas 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 orthog-onally 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, 2nded. 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 polarizedantenna,” USA U.S. Patent no. 3 474 452.

[7] R. T. Klopach and J. Bohar, “Broadband Circularly Polarized Omnidi-rectional Antenna,” U.S. Patent no. 3 656 166.

[8] J. D. Dyson and P. E. Mayes, “Circularly polarized omnidirectionalcone mounted spiral antenna,” USA U.S. Patent no. 3 188 643.

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