Dual Polarized Textile Patch Antenna for Integration ...

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Abstract—In the context of wearable textile systems for rescue workers ... Index Terms—Dual polarized patch antenna, polarization diver- sity, protective ...
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IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 7, 2008

Dual Polarized Textile Patch Antenna for Integration Into Protective Garments Luigi Vallozzi, Hendrik Rogier, Senior Member, IEEE, and Carla Hertleer

Abstract—In the context of wearable textile systems for rescue workers, antennas are needed that exhibit robustness to channel fading and, in addition, are completely integrable into protective garments. Therefore, a dual polarized patch antenna, made out of textile materials, is proposed. The antenna is intended for operation in the ISM band [2 4 2 4835] GHz, permits to exploit polarization diversity and is fully integrable into protective garments. To our knowledge, it is the first textile antenna with dual polarization. Several prototypes have been realized and their performances were investigated by measurements and simulations, proving the effectiveness of the antenna in open-space and on-body operation. Index Terms—Dual polarized patch antenna, polarization diversity, protective garments, textile antenna, wearable textile system.

I. INTRODUCTION N the context of improved reliability, efficiency and safety during interventions in major disasters, wearable textile systems for rescue applications are becoming a very important field of research. In order to provide continuous monitoring of the activity and life signs of rescue workers active in the field, wearable textile systems are currently being developed for integration into protective garments [1]. In such systems a textile antenna, completely made out of textile and fully integrable into the garment, is required to provide reliable communication between a rescue worker and a command post, or between two or more rescue workers. Rescue workers often operate in environments characterized by severe multipath, resulting in fading of the received signal with consequent decrease of the transmission throughput. Thus, there is an obvious need to develop communication systems that exhibit robustness to channel fading. An effective way to reduce the undesired effects of multipath fading consists in exploiting antenna polarization diversity [2], [3], which requires antennas with multiple polarizations at the transmitting and/or receiving side. Textile antennas previously proposed in literature [4]–[7], already provide good performances. However, they possess a single-feed arrangement and consequently a single polarization which does not allow to exploit diversity. As improvement with respect to the above mentioned existing textile antennas, the proposed one introduces a dual-feed arrangement with high isolation between the feeds, resulting in a dual polarization. Moreover, the proposed antenna has the same dimensions of a single polarized antenna, resulting in a very compact diversity system.

I

Manuscript received March 17, 2008; revised April 17, 2008. First published May 16, 2008; current version published December 02, 2008. L. Vallozzi and H. Rogier are with the Information Technology Department, Ghent University, 9000 Ghent, Belgium (e-mail: [email protected]; [email protected]). C. Hertleer is with the Department of Textiles, Ghent University, 9052 Zwijnaarde, Belgium (e-mail: [email protected]). Digital Object Identifier 10.1109/LAWP.2008.2000546

Another important issue is that rescue workers are often active in harsh environmental conditions, being exposed to extreme temperature and humidity. Therefore, a fire-resistant water-repellent protective foam layer was chosen as substrate material together with breathable conductive textile material for patch and ground plane. The optimized design of the antenna was carried out by means of ADS Momentum, after which several prototypes were realized. The performance of the antenna was investigated by means of simulations and measurements, for the antenna placed in open space and in the presence of the human body and excellent results were obtained. The article is organized as follows: Section II gives a complete description of the antenna. Section III-A describes the S-parameters and Section III-B the gain patterns, both for the antenna in open space and in proximity of the human body. In Sections III-C and III-D the simulation and measurement results concerning polarization and efficiency are presented. II. ANTENNA GEOMETRY AND DESIGN In order to achieve the required dual polarization, the patch is nearly square with the two feed points positioned symmetrically with respect to the axis of the patch, as shown in Fig. 1. This ensures that two orthogonally polarized waves can be simultaneously radiated or detected by the antenna. The position of the 50 coaxial probe feeds is a crucial factor for the isolation between the two ports, which has to be as high as possible. In the design the following two design criteria were imposed:

(1) The rectangular slot in the patch improves the matching of the antenna to the feed lines. Patch and ground plane were realized in ShieldIt and FlecTron, respectively, two conductive lowcost electrotextile materials, with surface resistivity less than 0.1 and thickness less than 0.25 mm. The substrate material consists of a flexible protective foam which is commonly used in protective garments for rescue workers, to additionally protect vulnerable body parts such as shoulders, knees and elbows. This foam has a closed-cell structure and shows flexibility and ability to regain its original form after being exposed to compression. It has a density of and a thickness mm. The values of substrate permittivity and loss tangent, shown in Table I, were obtained, at GHz, by means of the experimental method described in [8]. The dimensions of the substrate and ground plane were 100 100 mm. An initial design was carried out, based on the theory in [9], after which the values for the parameters of the antenna were optimized with the ADS Momentum simulator. Table I shows the

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Fig. 2. Measured (prot. 1) and simulated S-parameters.

Fig. 1. Antenna scheme.

TABLE I PARAMETERS OF THE REALIZED ANTENNA

Fig. 3. Model of the body used in the simulations.

dimensions obtained after optimization. The design criteria in (1) were chosen as optimization goals. III. SIMULATION AND MEASUREMENT RESULTS A. S-Parameters Simulation and Measurement 1) Flat Antenna in Open Space: Several dual polarized patch antenna prototypes were realized, based on the final parameters in Table I. Then, measurements of the S-parameters on the prototypes were performed by means of an HP8510C Network Analyzer. Fig. 2 shows that the two design criteria in (1) are well satisfied, both by simulated (ADS) and measured (prototype 1) curves. Furthermore, a good agreement between the curves of the measured and simulated S-parameters is observed. However, the measured dB bandwidths of and are slightly larger than the simulated ones and the resonance peaks are less sharp, since additional dielectric losses in the substrate and, to a lesser extent, conductive losses in the patch, ground plane and connector solders, were not fully taken into account in the simulation. As shown in Fig. 2, the measured isolation is better than the simulated one, which is probably caused by the model used for the two coaxial feeds in ADS (not so close to reality, for the reason that Momentum is intended for the simulation of planar structures) and to little imperfections in the way the connectors were soldered onto the prototype. 2) Effect of the Presence of the Human Body: In normal working conditions the human body is present behind the ground plane of the patch antenna. The resulting effect on the

S-parameters was investigated first by means of simulations and then by measurements. The on-body situation was simulated with CST Microwave Studio, according to the model used in [10], as depicted in Fig. 3. The thicknesses of the layers used in the simulation were 1 mm for the skin, 3 mm for the fat and 18 mm for the muscle layer. The dimensions of the layers are 160 160 mm. The electrical properties ( and ), for each layer, at GHz, were obtained from [11]. The measurement was performed by placing the antenna on a real human body, with the ground plane at a distance of about 2 mm from the skin, in front of the area between the arm and the chest, as depicted in the drawing of Fig. 4. When placing the antenna in close proximity of the body, it is unavoidably subjected to bending, which affects the matching of the antenna, the port isolation and the resonance frequency. However, simulations showed that the resonance frequency only slightly shifts for a moderate degree of bending occurring on the chest, where the antenna is intended to operate, and the antenna performance remains acceptable. In particular, CST Microwave Studio simulations showed that, for a radius of curvacm, the return loss remains better than dB ture of dB. and the isolation is still lower than As shown in Fig. 5, at a distance of 2 mm between the ground plane and the skin, which can be considered as worst-case scenario, the presence of the body produces a shift in frequency of the minimum with respect to the open-space situation, which is about % in the simulation and % in the measurement. In the on-body situation the return loss remains lower dB in the simulation and lower than dB in the than measurement. The isolation remains better than dB over the entire ISM band, in both simulation and measurement. Similar results were obtained for the simulated and .

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

Fig. 4. On-body XZ gain pattern measurement scheme.

Fig. 6. Measured and simulated gain patterns, XZ plane, f

= 2:45 GHz.

Fig. 5. Effect of body on the return loss.

These results show that the antenna return loss and isolation are still acceptable when the body is present behind the ground plane.

Fig. 7. Antenna gain for port 1 and 2, broadside direction.

B. Gain Pattern Simulations and Measurements 1) Flat Antenna in Open Space: The active gain patterns, for the two ports, were simulated and measured, obtained by using a single port excitation and by terminating the other port with a 50 impedance. The ADS-simulated and measured active gain patterns, at 2.45 GHz, in the XZ plane of the antenna, for both ports, in open space, are shown in Fig. 6. The maximum gain is about 8.2 dBi in the simulation and, in the measurement, it is 5.8 dBi for port 1 and 6.7 dBi for port 2, which is more than sufficient for a reliable communication link. Similar results were obtained for the YZ plane. The small difference in the measured gain between the two ports is probably due to the differences in the way the probes were soldered onto the patch. The measured maximum gain is smaller than the ADS simulated gain because there are additional losses in the prototype, more specifically additional dielectric losses in the substrate and, to a lesser extent, conductive losses in the patch, ground plane and connector solders, which were not fully taken into account in the simulation, resulting in a larger value of the simulated gain. The gain as a function of frequency was measured for both ports and for broadside and backscattering directions. The resulting curves, for broadside, are shown in Fig. 7, from which one observes that the measured gain is more than sufficient over the entire ISM band; along the backscattering direction the gain remains lower than dBi over the entire band of interest. 2) Effect of the Presence of the Human Body: The effects produced by the presence of the body behind the ground plane

were investigated first by means of CST Microwave Studio simulations, followed by on-body measurements. The measurements were performed in an anechoic chamber, placing the antenna on a human body as shown in Fig. 4. The resulting simulated and measured XZ-plane gain patterns in open-space and on-body situation, for port 1, depicted in Fig. 8, show that the presence of the body behind the ground plane, at a distance of 2 mm, does not substantially affect the gain pattern and that the maximum gain is still more than sufficient for a reliable communications link. These results confirm that the presence of the body does not worsen the performance of the antenna. C. Polarization The polarization ellipses of the realized antenna were calculated from the data obtained with the transmission measurement on prototype 1. In particular the polarization angle (measured from the x-axis of the antenna), the eccentricity (ratio between minor and major axis of the ellipse) and the axial ratio for linear polarization as a function of the azimuth, for both ports, were GHz. The axial ratio for linear polarobtained at ization is defined as . In Table II the obtained polarization parameters, in the broadside direction, are presented, for open-space and on-body situations. In open-space and on-body conditions, one observes that the far field obtained by exciting each port and terminating

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TABLE III SIMULATED (CST) AND MEASURED RADIATION EFFICIENCY FOR ANTENNA IN OPEN SPACE

Fig. 8. Effect of body on the XZ measured gain pattern, f

= 2:45 GHz.

TABLE II MEASURED PARAMETERS OF POLARIZATION ELLIPSES IN THE BROADSIDE DIRECTION, f : GHZ

= 2 45

the other one with a 50 impedance, is almost linearly polarized and that the two polarization ellipses are almost orthogonal in space along the broadside direction. This means that the antenna is capable of independently transmitting and receiving two orthogonal linearly polarized waves, which are simultaneously radiated or detected by the antenna. The presence of the body slightly affects the orientation angles of the polarization ellipses which, however, continue to be nearly orthogonal, and has a moderate effect on the axial ratio, which becomes slightly larger with respect to the open-space configuration. D. Radiation Efficiency The radiation efficiency of the antenna in open space was obtained by means of CST simulations and radiation measurements in the anechoic chamber. It accounts for the dielectric and conductive losses and the power of surface modes flowing in the substrate. The results are shown in Table III, from which two conclusions are drawn: on the one hand, one observes that the measured radiation efficiency is different for the two ports, because of the small differences between the two feeds due to the solders. On the other hand, the values of radiation efficiency calculated by means of CST Microwave simulations are larger than the values obtained by measurement, because dielectric losses in the substrate, as well as conductive losses in the patch, ground plane and connector solders, were not fully taken into account in the simulation. However, the measured efficiency has a more than sufficient value in both simulation and measurement. IV. CONCLUSION A low cost, easy to realize, light weight, compact textile patch antenna, with a novel dual feed arrangement and dual polarization, fully integrable into protective garments, for application

in the ISM band, has been successfully simulated and realized. In the entire ISM band, a low return loss and excellent isolation between antenna ports is achieved, both for the simulated and the measured prototype, with excellent agreement between simulated and measured results. The proposed antenna exhibits a measured antenna gain of about 6 dBi along broadside direction, which is more than sufficient for reliable communication links. To our knowledge, it is the first textile patch antenna with dual polarization, which permits simultaneous transmission and reception of two orthogonally polarized signals which can independently be transmitted and received. In particular, the resulting two waves are almost linearly polarized and orthogonal in space. Thus, the antenna can be used to realize transmission links that exploit polarization diversity, which is an effective way to reduce the undesired effect of multipath fading on wireless communications. The presence of the human body behind the ground plane of the antenna has a moderate effect on the resonance frequency and on the performance of the antenna. However, the overall gain, return loss, polarization and radiation efficiency remain of sufficient quality for reliable communication. REFERENCES [1] C. Hertleer, H. Rogier, and L. Van Langenhove, “A textile antenna for protective clothing,” in Proc. IET Seminar on Antennas and Propag. for Body-Centric Wireless Commun., London, U.K., 2007, pp. 44–46. [2] J. J. A. Lempiainen and J. K. Lahio-Steffens, “The performance of polarization diversity schemes at a base station in small/micro cells at 1800 MHz,” IEEE Trans. Veh. Technol., vol. 47, no. 3, pp. 1087–1092, Aug. 1998. [3] T. Svantesson, M. A. Jensen, and J. W. Wallace, “Analysis of electromagnetic field polarizations in multiantenna systems,” IEEE Trans. Wireless Comm., vol. 3, no. 2, pp. 641–646, Mar. 2004. [4] A. Tronquo, H. Rogier, C. Hertleer, and L. Van Langenhove, “Robust planar textile antenna for wireless body LANs operating in 2.45 GHz ISM band,” IEE Electron. Lett., vol. 42, no. 3, pp. 142–143, Feb. 2006. [5] C. Hertleer, H. Rogier, L. Vallozzi, and L. Van Langenhove, “Aperturecoupled patch antenna for integration into wearable textile systems,” IEEE Antennas Wireless Prop. Lett., vol. 6, pp. 392–395, 2007. [6] P. Salonen, Y. Rahmat-Samii, H. Hurme, and M. Kivikoski, “Dualband wearable textile antenna,” in Proc. IEEE Antennas Propag. Society Int. Symp., Jun. 20–25, 2004, vol. 1, no. 4, pp. 463–466. [7] M. Klemm, I. Locher, and G. Tröster, “A novel circularly polarized textile antenna for wearable applications,” in Proc. 34th Eur. Microw. Conf., Oct. 2004, vol. 1, pp. 137–140. [8] F. Declercq, H. Rogier, and C. Hertleer, “Permittivity and loss tangent characterization for garment antennas based on a new matrix-pencil two-line method,” IEEE Trans. Antennas Propag., vol. 56, no. 8, pp. 2548–2554, Aug. 2008. [9] C. A. Balanis, Antenna Theory: Analysis and Design, 2nd ed. New York: Wiley, 1997. [10] M. Klemm, I. Z. Kov´cs, G. F. Pedersen, and G. Tröster, “Novel smallsize directional antenna for UWB WBAN/WPAN applications,” IEEE Trans. Antennas Propag., vol. 53, no. 12, pp. 3884–3896, Dec. 2005. [11] C. Gabriel and S. Gabriel, “Compilation of the dielectric properties of body tissues at RF and microwave frequencies,” AL/OE-TR-19960037 Report Prepared for U.S. Air Force Armstrong Laboratory, Brooks AFB. Jun. 1996.