A Planar H-Shaped Directive Antenna and its ... - IEEE Xplore

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IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 61, NO. 9, SEPTEMBER 2013

Communications A Planar H-Shaped Directive Antenna and its Application in Compact MIMO Antenna Yu Luo, Qing-Xin Chu, Jian-Feng Li, and Yu-Ting Wu

Abstract—a compact multiple-input multiple-output (MIMO) antenna with planar H-shaped directive antenna elements for WLAN/WiMAX application is presented. A printed H-shaped antenna without any reflectors and directors is applied as antenna element of the proposed MIMO antenna, but the radiation pattern of it exhibits high front-to-back ratio in the azimuthal plane. Four those H-shaped antennas with compact size and directive radiation pattern are placed in a square loop array to obtain nearly orthogonal patterns. Therefore, a compact MIMO antenna with high isolation among those four elements is achieved. A laboratory model has been characterized experimentally, and the effectiveness of the proposed design in terms of theoretical achievable capacity is demonstrated. Measurement agrees well with simulation. Index Terms—Antenna array, directive antenna, multipath channels, multiple-input multiple-output (MIMO) systems.

I. INTRODUCTION Multiple-input multiple-output (MIMO) system is beneficial for improving channel capacity and reliability of wireless communication systems [1], [2] . Current implementations often employ uniform linear arrays (ULA) of dipoles or monopoles (i.e., nearly omnidirectional radiators) with quite electrically large spacing (i.e., large fractions of the free space wavelength) to reduce the mutual coupling and to minimize the correlation between signals at the antenna ports. To this aim, a half-wavelength inter-element spacing is quite common. For the design of a compact MIMO antenna, coupling among antenna elements can no longer be neglected because antenna elements are closely spaced in a limited room, and the displacement between antennas will influence both in space diversity and in induced pattern diversity. Various approaches have been proposed to reduce mutual coupling. Some typical designs employed RF circuits [3], [4] or parasitic elements [5], [6] to decrease mutual coupling among the radiation elements. Pattern orthogonality is a key factor to achieve low correlation and improve the channel capacity in a rich multipath environment. More sophisticated designs achieved pattern orthogonality in different ways, for example polarization diversity [7]–[9], exciting orthogonal modes within the same geometrical structure in co-located patch antennas [10] or in spirals [11]. Directive antenna has been investigated to improve channel capacity through channel measurement [12]. Reconfigurable planar arrays made of combined Landstorfer and Yagi-Uda antennas [13] and a printed Yagi-Uda antenna with integrated balun [14] have been pro-

Fig. 1. Schematic of the proposed planar H-shaped antenna. Unit: mm. (a) top layer. (b) bottom layer.

posed, and one reflector and at least one director were employed in each directive element. In this communication, a MIMO antenna with four planar directive H-shaped antenna elements is presented, and each of the H-shaped antenna elements consists of two dipoles. Without any reflector and director, the H-shaped antenna element exhibits a 15 dB front-to-back ratio due to the radiation patterns of its two dipoles cancel out each other in one direction and superimpose with each other in the reverse direction. Those four H-shape antennas placed in square loop compose a compact MIMO antenna, the high front-to-back ratio is useful to lower the mutual coupling caused by radiation pattern, and high isolation for the proposed MIMO antenna is achieved while elements are close to each other (distance between two neighbor elements is 0.07 ( is wavelength in free space)). A laboratory model has been realized and experimentally characterized in terms of reflection coefficients, transmission coefficients and active element pattern across the operation band of 5.1–6.0 GHz, which covers 5.2/5.8-GHz WLAN and 5.5-GHz WiMAX band. In the desired band, reflection coefficients of level and transmission coefficients is the antenna is below level while four elements achieved nearly orthogonal below patterns and 360-degree coverage in the azimuthal plane. The gain of the proposed antenna is about 5 dBi and efficiency is more than 80% in the desired band. The effectiveness of the proposed layout is assessed against the resulting channel capacity with IEEE 802.11n propagation models [15]. The proposed array offered a theoretical achievable capacity that is remarkably close to an ideal (i.e., uncorrelated) receiver and transmitter. II. H-SHAPED SINGLE ANTENNA DESIGN

Manuscript received April 10, 2012; revised December 14, 2012; accepted June 03, 2013. Date of publication June 07, 2013; date of current version August 30, 2013. This work was supported by the National Natural Science Foundation of China (61171029) and Guangzhou Science and Technology Project (12C42081659). The authors are with the School of Electronic and Information Engineering, South China University of Technology, Guangzhou, China (e-mail: [email protected]; [email protected]; [email protected]; [email protected]). Color versions of one or more of the figures in this communication are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TAP.2013.2267193

An H-shaped antenna printed on a RF4 ( ) substrate of height is shown in Fig. 1. The H-shaped antenna consists of two dipoles which are connected by coplanar stripline (CPS). A 50 ohm impedance microstrip line is used to feed the CPS by a via hole. The microstrip-to-CPS transition also plays a role as a balun. If currents distributing on two dipoles are equal in amplitude and have a 90 degree phase contrast, while the distance between the two , the antenna will be achieve a high front-to-back ratio dipoles is due to the radiation patterns of its two dipoles cancel out each other in one direction and superimpose with each other in the reverse direction.

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Fig. 2. Average current distribution of H-shaped antenna. (a) .

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. (b)

Fig. 3. Simulation of an isolated proposed H-shaped antenna radiation patterns. a) -plane. (b) -plane. (c) -plane.

Fig. 4. Simulation reflection coefficients of an isolated proposed H-shaped antenna. TABLE I COMPARISON OF SEVERAL DIRECTIVE ELEMENTS Fig. 5. Relationship between placement and coupling of two antennas. (a) Two directive antennas with relative main radiation direction. (b) Two omni-directional antennas. (c) Two directive antennas with opposite main radiation direction. (d) Two antennas vertical placement.

The first step of design is to determine the size of dipoles ( and ) to make sure dipoles are in the best states when the port impedance is 100 ohm. Secondly, the distance of two dipoles ( ) should be selected ( , is the speed of light in vacuum, is center ( frequency). Thirdly, the position of feed point should be is waveguide length at 5.5 GHz) offset ( ) from the midpoint of the two dipoles, and the offset leads to a 90 degree phase contrast of currents on the two dipoles. The dielectric constant of the

substrate should be high enough in order to have enough places for the simple balun. The last step consisted in optimizing the size of the CPS ( ) and microstrip line in simulation program, Ansoft HFSS12 [16] to achieve a broadband match. The characteristic impedance of CPS should be 100 ohm, which guarantees equally power dividing. The (0.4 0.29 ). Fig. 2(a) and (b) size of the antenna is 22 16 and illustrate the current distributions of antenna at time respectively, where represents the period of time at 5.5 GHz. It is found that, when the phase of excitation source equals to 0 degree, the current densities are very high at the one dipole, whereas the current

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Fig. 6. Proposed array configuration (Unit: mm).

Fig. 7. Photo of a laboratory model of the proposed MIMO array. (a) Top layer. (b) Bottom layer.

densities are high at another dipole when the phase equals to 90 degree. It can be observed a 90 degree phase contrast of currents on the two dipoles. The magnitudes of currents on two dipoles are nearly equal, which shows equally power dividing of the two dipoles. The simulated results of an isolated H-shaped antenna are shown in Figs. 3 and 4. From the simulated results, the proposed antenna has a 15 dB front-to-back ratio at center frequency and the reflection level across the band of interest (5.1 coefficients is below GHz–6 GHz).Table I shows a comparison of several directive elements in terms of size and front-to-back ratio.

Fig. 8. Simulation and measurement active radiation pattern of a proposed H-shaped antenna in MIMO array (normalized values). (a) 5.2 GHz -plane. (b) 5.2 GHz -plane. (c) 5.2 GHz -plane. (d) 5.5 GHz -plane. (e) 5.5 GHz -plane. (f) 5.5 GHz -plane. (g) 5.8 GHz -plane. (h) 5.8 GHz -plane. (i) 5.8 GHz -plane.

III. ARRAY BEHAVIOR The H-shaped antenna with compact size and high directivity is suitable to be an antenna element of MIMO antenna. In order to reduce coupling among antenna elements, every element should be placed in a right way. Fig. 5 shows several ways to place two antenna elements with different distances. It is concluded from Fig. 5 that displacement in (c) and (d) can always achieve a better isolation than (a) and (b) when the distance between two antennas changed. Based on above discussion, a MIMO antenna is composed of four H-shaped antennas placed in a square loop configuration (Fig. 6). The proposed MIMO antenna has been fabricated and tested, and the photos of the fabricated antenna are shown in Fig. 7. The measured and the simulated results are in a good agreement. Because of symmetrical , , and radiation placement of antenna elements, only pattern of element 1 are showed in this communication. As expected, the sectorial radiating properties of each element results in an almost undistorted active pattern and 360-degree coverage in the azimuthal plane ( -plane) (Fig. 8) (Fig. 9). The active element pattern (Fig. 9) exhibit a weak overlap in azimuthal plane, i.e., they are nearly orthogonal, that is very low mutual coupling is achieved. Moreover, there is no need for additional matching line as the scattering parameter of each antenna is well below the level, along with low values

Fig. 9. Simulated active element pattern for H-shaped antenna 1, 2, 3 and 4 (normalized values) in the azimuthal plane ( -plane). (a) 5.2 GHz. (b) 5.5 GHz. (c) 5.8 GHz.

(below level) of the transmission coefficients in the desired 5.1-6 GHz band (Fig. 10) (Fig. 11). The gain and efficiency of the proposed MIMO antenna are shown in Fig. 12. In the desired band, the efficiency is more than 80% and the gain is about 5 dBi. Table II shows a comparison of several antenna arrays in terms of size, relative bandwidth of reflection coefficients, number of element and gain. Therefore, as shown in the following, a high channel capacity is expected, in a rich multipath environment. IV. MIMO BEHAVIOR The proposed array is intended to be used in rich multipath scenarios. Its performance is evaluated in terms of the correlation coefficient and the ergodic channel capacity. The correlation coefficient between the


COMPARISON

OF

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TABLE II SEVERAL ANTENNA ARRAYS

Fig. 10. Measured and simulated reflection coefficients of proposed array configuration.

Fig. 11. Measured and simulated transmission coefficients of proposed array configuration.

generic th and th antenna of the array, at either transmitter or receiver side, is given by [17] (1) where (2) and are, respectively, the components and of the electric field radiated by the th antenna, in the azimuthal plane, intended as active element pattern in the far-field region [18]. The co-polar component is assumed to be the component of the field, which lays on the plane parallel to the substrate. is the cross-polar and are power azimuth spectrum (PAS) of discrimination, denotes the two polarizations, in the azimuthal plane, and finally, : this means that the the complex conjugate. It is assumed that

Fig. 12. Measured and simulated gains and efficiencies of the proposed array configuration.

scenario provides half of the received power of the received power with the polarization and half of the received power with the po. These larization. In addition, it is considered assumptions are widely accepted in the literature for transmissions in a rich multipath environments [19]. Since a careful evaluation of the correlation between antennas requires the complex-valued active element patterns, simulated values have been used in the computation of (1) and (2). in the interval It is considered a uniform PAS and a flat Rayleigh fading channel with a single-cluster. The correlation and for the transmitter and the receiver have elematrices given by (1), with parameters depending on the ments of position array adopted respectively at either the transmitter or the receiver side. At both receiver and transmitter sides, the proposed array of H-shaped elements are compared, in terms of channel capacity, with an ideal receiver and an ideal transmitter, whose correlation matrix is the identity matrix. is calculated as The channel matrix (3) here a 4 4 matrix is whose entries are independent and identically distributed complex Gaussian random variables with unitary for given average signal-topower. The ergodic channel capacity noise ratio (SNR) value results (4) denotes expectation with respect to different channel realwhere , is the 4 4 identity matrix, is the number izations denotes Hermitian transposition.Fig. 13 of transmit antennas, and shows that the proposed array offers a theoretical achievable capacity that is remarkably close to ideal (i.e., uncorrelated) receiver and transmitter case for a uniform PAS scenario.

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Fig. 13. Ergodic capacity over a 2000 channel realization as a function of SNR, in the range (single-cluster flat-fading for a uniform PAS channel model).

V. CONCLUSION In this communication, a planar compact MIMO antenna with four H-shaped antennas as sectorial radiators has achieved almost orthogonal patterns. The proposed design has made a reduction in signal correlation among radiating elements. Therefore, channel capacity has ). It is been maintained high even in such a small array (42 42 particularly attractive for very compact devices employed in indoor applications. Numerical evaluation of the array performance in a typical MIMO scenario, by means of IEEE 802.11n channel models, confirmed the validity of the considered approach.

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[12] C. Hermosilla, R. Feick, R. Valenzuela, and L. Ahumada, “Improving MIMO capacity with directive antennas for outdoor-indoor scenarios,” IEEE Trans. Wireless Commun., vol. 8, no. 5, pp. 2177–2181, May 2009. [13] A. C. K. Mak, C. R. Rowell, and R. D. Murch, “Low cost reconfigurable Landstorfer planar antenna array,” IEEE Trans. Antennas Propag., vol. 57, no. 10, pp. 3051–3061, Oct. 2009. [14] A. D. Capobianco, F. M. Pigozzo, A. Assalini, M. Midrio, S. Boscolo, and F. Sacchetto, “A compact MIMO array of planar end-fire antennas for WLAN applications,” IEEE Trans. Antennas Propag., vol. 59, no. 9, pp. 3462–3465, Sep. 2011. [15] TGn Channel Models, IEEE 802.11-03/940r4, May 2004. [16] AnsoftCorp HFS. [Online]. Available: http://www.ansoft.com/products/hf/hfsss [17] R. G. Vaughan and J. B. Andersen, “Antenna diversity in mobile communications,” IEEE Trans. Veh. Technol., vol. 36, pp. 149–172, Nov. 1987. [18] D. M. Pozar, “The active element pattern,” IEEE Trans. Antennas Propag., vol. 42, pp. 1176–1178, Aug. 1994. [19] R. Vaughan and J. B. Adnersen, Channel, Propagation and Antennas for Mobile Communications. London, U.K.: Inst. Elect. Eng., 2003, vol. 50, IEE Electromagnetic Waves.

Compact Multi-Band PIFAs on a Semi-Populated Mobile Handset With Tunable Isolation Kasra Payandehjoo and Ramesh Abhari

Abstract—In this communication, miniaturized tunable two-antenna systems composed of printed inverted-F antennas (PIFAs) are developed for a semi-populated mobile phone handset. The PIFAs are loaded with a series combination of an inductor and a varactor to simultaneously achieve miniaturization and tunability. The compact 32 mm-long PIFAs demonstrate tuning range of more than 240 MHz covering personal telecommunication bands from LTE-band13 to GSM900 MHz. As well a miniaturized tunable parasitic element is integrated in the handset to efficiently suppress coupling between the PIFAs to below 28 dB across the entire operational bandwidth of the antennas. Simulation and measurement results demonstrate the successful implementation of a tunable MIMO system with passive adjustable coupling reduction mechanism for mobile handsets and achievement of a channel capacity profile close to that of an un-correlated system. Index Terms—Antenna isolation, antenna mutual coupling, diversity antennas, GSM900 MHz, long term evolution (LTE), multiple-input and multiple-output (MIMO), miniaturization, mobile handsets, printed inverted-F antennas (PIFAs), tunable antennas.

I. INTRODUCTION The spectral crowding and increasing user demand for fast and reliable wireless services has pushed for the prevalent adoption of long term evolution (LTE) and multiple-input and multiple-output (MIMO) technologies in the next generations of mobile devices [1], [2]. The emerging smart handsets operate at the LTE 700 MHz frequency bands Manuscript received November 29, 2012; revised April 08, 2013; accepted June 03, 2013. Date of publication June 11, 2013; date of current version August 30, 2013. The authors are with the Department of Electrical and Computer Engineering, McGill University, Montreal, QC H3A 0E9, Canada (e-mail: [email protected]; [email protected]). Color versions of one or more of the figures in this communication are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TAP.2013.2267718

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