Low-Profile Dual-Band Textile Antenna With Artificial Magnetic Conductor Plane

> REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) < Low-Profile Dual-Band Textile Antenna with Artificial Magnetic Conductor Plane  Sen Yan, Ping Jack Soh, Member, IEEE, Guy A. E. Vandenbosch, Fellow, IEEE Abstract—A dual-band textile antenna loaded with an artificial magnetic conductor (AMC) plane is proposed for WLAN applications. Its dual-band operation is enabled by a rectangular patch in the 2.4 GHz band and a patch-etched slot dipole in the 5 GHz band. Since the AMC approaches a perfect magnetic conductor (PMC) in the 5 GHz band, the slot dipole can be located close to the ground. The proposed antenna is fully fabricated using textiles except for a feeding connector used for testing purposes and a via. Simulations and experiments agree well and validate that this low profile antenna operates with a good reflection coefficient and a high front-to-back ratio (FBR) within the desired bands. Index Terms—dual-band antenna, textile antenna, artificial magnetic conductor (AMC), metamaterial. I. INTRODUCTION T HE rapid development in wearable communication devices is making high performance textile antennas more attractive. In line with these developments, the requirements of the antennas in terms of performance are becoming more demanding. The main additional requirement for a textile antenna compared to a traditional design is its ability to minimize the interaction between antenna and human body, despite of the fact that they are located close to each other. Moreover, irradiation of the human body over prolonged periods of time is controversial, since it may present a health risk factor. The power densities available in the near field region of an antenna are much higher (and roll-off more rapidly) than in the far field. Although the front-to-back ratio (FBR) and the specific absorption rate (SAR) are quantities which are related to the near and far field of an antenna, respectively, they are interrelated to a certain degree, since the far field is the Fourier transform of the near field. This means that the FBR can already provide a rough idea concerning the SAR [1]. Another potential problem of textile antennas is related to robustness. Since a textile antenna may curve and deform when worn, it is crucial that it is designed in such a way that it is not too sensitive to deformations. Due to the high FBR required, microstrip patch antennas are widely used for wearable devices. However, their inherent narrowband property poses a challenge [2], especially when a Manuscript submitted May 15, 2014. S. Yan, and G. A. E. Vandenbosch are with the ESAT-TELEMIC Research Div., Dept. of Electrical Eng., KU Leuven, Kasteelpark Arenberg 10, Box 2444, 3001 Leuven, Belgium (e-mail:sen.yan@esat.kuleuven.be). Ping Jack Soh is with the Advanced Communication Engineering (ACE) CoE, The School of Computer and Communication Engineering, Universiti Malaysia Perlis (UniMAP), Pauh Putra Campus, 02600 Arau, Perlis, Malaysia. Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. doi: 1 single antenna is needed to cover both the 2.4 and 5 GHz bands for WBAN/WLAN applications. A dipole or monopole antenna can be easily tuned to reach wideband operation, but their omnidirectional patterns should be amended for on-body application. Moreover, electric dipoles / monopoles are less efficient when operating closely to a parallel piece of metal due to the anti-phase image induced [2]. Conversely, magnetic dipoles / monopoles, in practice realized by slots or apertures in a ground plate, are also not suitable for placement near any metal planes because of the generation of parallel plate modes between the two metal planes, which considerably distorts the characteristics [3]. An artificial magnetic conductor (AMC) plane is a kind of two-dimensional metasurface. It has been widely used already in the design of planar antennas [4,5]. One of its main advantages is that it can be located close to a parallel electric current forming a reflector, since the current on the AMC corresponds to an in-phase image [5-11]. It is also possible to suppress any parallel plate modes so that this structure may be used as a low-profile reflector for a magnetic dipole. Such concept has been demonstrated in literature via the design of several AMC-based antennas for wideband or dual-band operation [3]. The dual-band characteristic is usually realized by exciting the higher modes of the structure, or by integrating different resonator shapes. These methods typically reduce the bandwidth of the AMC, or result in a more complex optimization and fabrication process [12-15]. The small dimensional tolerances of such a design make it unsuitable for textile antennas. In a WLAN application, the antenna needs to operate with a narrow lower band (from 2.4 GHz to 2.484 GHz) and a wide upper band (from 5.15 GHz to 5.85 GHz). The main idea of the proposed antenna is to utilize the patch as the radiator for the lower WBAN/WLAN band, and a slot dipole etched on the patch for the upper WLAN band. The AMC plane replaces the normal metallic ground. In the lower band this AMC plane approaches a perfect electric conductor (PEC) forming the ground for the patch antenna. In the upper band, it approaches a perfect magnetic conductor (PMC), allowing an efficiently-radiating magnetic dipole without exciting the parallel plate mode in between the antenna plane and the ground plane [3]. To our best knowledge, this is the first textile antenna to utilize such concept. Compared with other designs, the proposed antenna is advantageous in two aspects. Firstly, the AMC layer is very simple. Since only a single resonance is required, the unit cell can be designed either using square or rectangular patches. This simplifies the fabrication using textile materials and the manual fabrication procedure. There is more risk for manufacturing inaccuracies if more complex shapes are incorporated in the AMC layer. Secondly, the wide bandwidth in the upper band enables the AMC to easily cover the whole WLAN communication band at 5 GHz, even when a slight frequency shift is expected when worn on the human body. The organization of the paper is as follows. The structure and characteristics of the AMC plane are first introduced and analyzed prior to the evaluation of the overall integrated design in Section III. Next, the on body performance of the antenna is > REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) < 2 Fig. 1 Topology (a) and reflection coefficient (b) of the AMC plane. discussed in Section IV, before drawing the conclusions. II. CHARACTERISTICS OF AMC PLANE Various types of AMC plane have been studied in literature, [3-7, 10-14]. In this design, the square patch structure is chosen due to its ease in obtaining a large bandwidth [13], besides its fabrication simplicity when using textile materials. The topology of a single AMC unit cell is shown in Fig. 1 (a). The dielectric constant and loss tangent of the 1.5 mm thick felt substrate are 1.2 and 0.044 [16], respectively. ShieldIt Super conductive textile from LessEMF Inc. [17,18] with an estimated conductivity of 1.18 × 105 S/m is used to form the metallic layers. The complete AMC plane consists of a periodic square patch structure on the top layer, a layer of substrate and a layer of ground at the reverse side. Each square patch is 18.2 mm in size and the period is 20 mm. Another layer of substrate is placed on top to separate the AMC plane from the antenna. CST Microwave Studio [19] is used to simulate the structure, using unit cell boundaries and Floquet ports to mimic the infinite planar periodicity. Fig. 1 (b) depicts the reflection coefficient (S11) of the AMC with a plane wave incident from the normal direction. It can be clearly seen that the reflected wave is nearly in anti-phase at 2.45 GHz, where it is supposed to behave as a PEC. On the contrary, the phase of the reflected wave is about 0o in the 5 GHz band, which exhibits a PMC-like characteristic. The operating bandwidth of the proposed AMC plane, defined by the phase of the reflected wave lying between 0o ± 90o, is from 5.04 GHz to 5.935 GHz. It is clear from the calculations that this proposed structure is capable of operating within the desired WLAN bands. III. ANTENNA ON AMC PLANE The overall topology of the antenna is illustrated in Fig. 2. A coaxially-fed, rectangular patch antenna functions as a 2.45 GHz radiator. Due to the PEC-like behavior of the AMC plane in this band, the antenna performs similarly as a conventional microstrip patch antenna. Next, two slots in y-direction are introduced onto the rectangular patch. Note that different slot widths and lengths are selected to enable a wide impedance bandwidth. A coplanar waveguide (CPW) line which is about a quarter wavelength at 5.5 GHz is used to connect these two Fig. 2 Topology of the proposed antenna. (a) 3-D view, (d) side view, (c) top view, (d) fabricated prototype slots. The CPW line is connected with the structure's ground by a via, between the patch and the CPW strip. Since the slots cause perturbations of the current distribution on the patch in the lower 2.45 GHz band, the size of the patch has to be adjusted upon the integration of the slots. The antenna is optimized using the genetic algorithm in CST MWS with the aim of obtaining the best S11 in both frequency bands. The final dimensions of the proposed antenna are given in Fig. 2 (c). The fabricated antenna prototype is shown in Fig. 2 (d). A comparison between simulated and measured S11 is shown in Fig. 3 (a). In the lower band, the maximum S11 is -16.2 dB (simulated) and -8.3 dB (measured), whereas this is -11.7 dB (simulated) and -12.5 dB (measured) in the upper band. Note that the slot dipole enables a much wider bandwidth than a common microstrip patch antenna. A small disagreement between the simulated and measured curves is observed. This is explained by the uncertainty concerning the textile substrate properties and the mechanical inaccuracies caused by the manual fabrication procedure with simple tools. Note that using machine dimensioning or laser cutting does not necessary lead to better results in practice, due to the unavoidable deformations of the antenna during practical operation. The most important is that the antenna performance still fulfills the requirements for WLAN communication. A noticeable point is that the lower band splits into two resonances. This is related to the coupling between the large patch and the center CPW patch. At the first resonance, the currents on these two patches are in phase while at the second resonance they are out of phase. More details about mode > REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) < 3 Fig. 4. Simulated results for several ground sizes. TABLE I SIMULATED SAR Fig. 3 Measured results of the antenna, (a) reflection coefficient, radiation pattern at 2.45 GHz in x-z plane (b) and y-z plane (c), radiation pattern at 5.2 GHz in x-z plane (d) and y-z plane (e). Solid (red) line: co-polarization, dashed (blue) line: cross-polarization. coupling can be found in [20]. The S11 dip around 4 GHz represents the second mode (TM20) of the patch. The measured radiation patterns at 2.45 GHz and 5.2 GHz are shown in Fig. 3 (b)-(e). The FBR is higher than 12 dB in both bands. The high cross polarization in y-z plane is mainly caused by the CPW line which is perpendicular to the radiated slot, but that is not a concern in WLAN communication systems. The realized gain of the antenna is about 2.5 dB at 2.45 GHz, and between 0 to 4 dB in the higher band. The total efficiency is above 40 % throughout the whole operating band. This value is typical for textile antennas fabricated using this type of materials [21]. IV. PERFORMANCE ON BODY The fabricated antenna prototype is measured on the chest of a male volunteer, who weighs 90 kg and is 178 cm in height. The result of this evaluation performed in an anechoic chamber is also shown in Fig. 3 (a). Due to the effectiveness of the ground plane, the S11 is only slightly affected by the coupling between the human body and the antenna. The maximum S11 is -9.8 dB and -8.4 dB in the lower and upper band, respectively. Thanks to the large area available on the human chest and back, it is possible to enlarge the ground plane in order to further reduce the backward radiation. If this additional ground Freq. (GHz) Antenna with 100x100 mm2 ground (W/kg) Antenna with 200x200 mm2 ground (W/kg) 2.45 5.2 5.8 0.0464 0.0232 0.0300 0.00424 0.00130 0.00168 layer is a thin conductive textile layer, this may not affect the antenna conformability or the users' comfort. Simulations for ground planes up to 200 × 200 mm2 show that the S11 is nearly not affected while the back radiation is further reduced by a value between 5 and 10 dB, see Fig. 4. The large ground is also expected to significantly reduce the SAR value. To analyze the contribution of the metasurface placed between the antenna and the human body, a series of SAR simulations was performed using a simplified human model in CST MWS. This model is defined behind the antenna, at 10 mm distance from the antenna ground layer in order to emulate practical antenna to skin distances in clothing. This distance is also larger than λ/4 at the lower operating frequency of 2.4 GHz, ensuring a proper distance from the edges of the combined antenna / metasurface. The model combines a 3 mm thick layer of skin, a 7 mm thick layer of fat, and a 60 mm thick muscle layer. This model has been validated in [22]. The input power to the antenna for SAR calculations in this work is set at 0.5 W (rms). SAR values were calculated based on the IEEE C95.1 standard and averaged over 10 g of biological tissue. The SAR distributions in both bands are displayed in Fig. 5. The calculated results are summarized in Table 1. Intuitively, measuring SAR at the exact same position using a common antenna will result in higher SAR with increasing frequency. This is due to the conductivity of the human tissues, which increases with frequency [23]. However, in this case, it is clear from the simulated SAR values that the AMC is functioning well as a PMC at the higher frequencies. The SAR values observed in the 5 GHz band are lower compared to the 2.4 GHz band. Using an enlarged 200 x 200 m2 ground plane, see Table 1, the SAR values were reduced with a factor of more than 10. The antenna was also simulated when bent over a cylinder along the two main axes and for different radii. For the cylinder oriented along the x-axis, no significant S11 changes are observed. This is due to the direction of bending, which is parallel to the current on the patch, in this way not affecting the current distribution considerably. Conversely, the antenna S11 changes are more profound when it is bent along the y-axis, see > REPLACE THIS LINE WITH YOUR PAPER IDENTIFICATION NUMBER (DOUBLE-CLICK HERE TO EDIT) < 4 Fig. 5. SAR distributions at different frequencies. (a) 2.45 GHz, (b) 5.2 GHz, (c) 5.8 GHz. [5] [6] [7] [8] Fig. 6 Simulated reflection coefficients for bent topology. Fig. 6. In the lower band, the resonant frequency shifts upwards as the radius of the cylinder is decreased. Nonetheless, even when bent the -10 dB operational bandwidth still covers the whole 2.4 GHz WLAN band. Meanwhile, the antenna operation in the upper band remains quasi unchanged. Only a slight S11 degradation can be seen at 5.85 GHz (with S11 = -9.32 dB). [9] [10] [11] [12] V. CONCLUSION In this paper, an AMC-integrated all-textile dual band antenna for WLAN applications is proposed. This novel structure combines a patch antenna and a slot dipole to enable its dual-band property, whereas the AMC is used to effectively suppress back radiation and, consequently, electromagnetic coupling with the body. The simplicity of the topology used enables its realization using flexible textile materials and a simple manual fabrication procedure. Both simulations and measurements performed in free space and on body indicated a satisfactory performance in terms of bandwidth and radiation properties for dual-band WBAN/WLAN applications. [13] [14] [15] [16] [17] [18] [19] REFERENCES [1] [2] [3] [4] Z. Jiang, D. E. Brocker, P. E. Sieber, D. H. Werner, "A compact, low-profile metasurface-enabled antenna for wearable medical body-area network devices," IEEE Trans. Antennas Propag., vol. 62, no. 8, pp. 4021-4030, Aug. 2013. C. A. Balanis, Antenna theory: analysis and design. John Wiley & Sons, 2012. J. Joubert, J. C. Vardaxoglou, W. G. Whittow, J. W. 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