Circularly Polarized Wearable Antennas

CHAPTER 8 Circularly Polarized Wearable Antennas Emmi Kaivanto 1, Erkki Salonen 1 & Haider Khaleel 2 1 Department of Communications Engineering, Centre for Wireless Communications, University of Oulu, Finland. 2 Department of Engineering Science, Sonoma State University, USA. Abstract By employing circularly polarized antennas, it is possible to prevent polarization mismatch losses which in turn increases the connection reliability between communication devices. This chapter focuses on circularly polarized wearable antennas. The Introduction section presents the motives behind this particular antenna type and discusses the advantages of circular polarization in wearable applications, not forgetting the challenges involved in the implementation of such antennas. To understand the nature of circular polarization, and to avoid design errors, a brief theory section follows. The next section is dedicated to survey the scientific publications related to this vital topic, providing a review of the latest developments. Finally, a case study of a dual-band circularly polarized wearable antenna for personal satellite communication and navigation at L1 and Iridium frequency bands is presented. The section goes through the design procedure from material selection to antenna performance evaluation by means of return loss and radiation measurements. Keywords: Wearable antennas, circular polarization. 1 Introduction One of the fastest developing wireless technologies currently is Wireless Body Area Network (WBAN), which offers countless new applications for both professional and leisure activities. Typical applications may include smart clothing for firefighters and rescue workers, military and space personnel, health and activity monitoring for outdoor enthusiast like hikers and cyclists. Battery life is often a concern and limiting factor in wireless communication systems; moreover, antenna performance plays an essential role in battery consumption. Hence, it is sensible to pay extra attention in antenna design to achieve an optimal performance. doi:10.2495/978-1-84564-986-9/008

146 Innovation in Wearable and Flexible Antennas In such communication systems where the antenna is not only receiving but also transmitting, especially when an adaptive power control is present, an under-performing antenna may increase power consumption and hence shorten the battery life remarkably. Sometime unnecessary losses might still occur when the polarizations of the transmitting and receiving antennas are not matched. It follows from the reciprocity theorem that a transmitter antenna behaves the same as an identical receiving antenna and vice versa. Fortunately, power losses due to polarization mismatch can be avoided by employing circularly polarized antennas. The polarization type is independent on the mutual orientation of the transmitting and receiving antennas and is relatively easy to implement in wearable applications as the size of the antenna is not as strictly limited than in handheld devices. There are mainly two cases when circularly polarized antennas are, if not essential, at least necessary to consider in wearable communication systems. The first is in satellite systems that are used for navigation and communication. Since satellite antennas often transmit circularly polarized waves, by employing circularly polarized receiving antenna, 3 db better power level can be achieved compared with linearly polarized antennas. The second case where circularly polarized antennas are beneficial includes off-body communication systems in which the power level is very low and the user is moving and hence the orientation of transmitting and receiving antennas varies. If polarizations of both transmitting and receiving antennas are close to linear, the connection might be totally lost when a user moves in such a position where one antenna s orientation is perpendicular with respect to the other from a polarization perspective. As usual, there is no such thing as a free lunch. Circular polarization is sensitive to distortion due to undesired reflections and hence a system employing circularly polarized antennas is at its best in a line of sight connection. This usually is the case in satellite communication systems; however, it has been shown that Global Positioning System (GPS), for example, may also operate in multipath environment with linearly polarized antennas [1,2]. On the other hand, a circularly polarized receiving antenna is blind to reflected, interfering signals because the handedness of the circular polarization is often changed in reflection. Another downside is that designing a circularly polarized antenna may be more challenging compared with a design process in which the polarization preference/ requirement is ignored. Depending on the structure and the chosen antenna topology, generating a circularly polarized wave may require a special feeding technique. Furthermore, additional measurements are needed in order to ensure circularly polarized operation over the bandwidth and coverage angle under consideration. As depicted in Fig. 1, if the antenna beam is very narrow, the line-ofsight connection may be possible only when the antenna s bore-sight is almost perpendicular to the satellite antenna. By placing several antennas around the user s clothing, probability for line of sight communication can be increased. The wider the coverage angle, the fewer antennas are needed for full coverage.

Circularly Polarized Wearable Antennas 147 Figure 1: The effect of antenna beam-width on coverage angle. 2 A Brief Theory on Antenna Polarization 2.1 Polarization types Polarization of an antenna is defined by the orientation of the electric field component of the electromagnetic plane wave transmitted by the antenna with respect to the earth s surface. Polarization of a plane wave is said to be linear, elliptical, or circular depending on the pattern in which the electric field vector of the wave traces while propagating. As the electric field of a propagating plane wave is always oriented perpendicular to the propagation direction, the shape of the polarization pattern can be seen when observing the wave from the xy-plane as the wave is travelling along the z-axis. On the other hand, when the plane wave is linearly polarized, the electric field vector is oscillating along a line in xy-plane. The direction of this line depends on the position of the antenna. However, either horizontal or vertical position is usually considered. In the case where the electric field vector of the plane wave is rotating and traces out a circle, the wave is said to be circularly polarized. The electric field has now two perpendicular components, which have equal magnitude and 90 phase difference. The most common polarization type, however, is elliptical polarization. Equation (1) describes the polarization in such a way that the wave is linearly polarized when the phase difference is 0 or p, and circularly polarized when E x = E y and b = ±p/2. Otherwise, the wave is elliptically polarized. E = E x e -jωt + E y e -jωt+β (1) In fact, both linear and circular polarizations are special cases of elliptical polarization. The three different polarization types are depicted in Fig. 2.

148 Innovation in Wearable and Flexible Antennas Figure 2: The three polarization types: Linear, circular and elliptical. Figure 3: Left-hand and right-hand circular polarization. 2.2 The handedness of circular polarization When applying a circularly polarized antenna into a communication system, it is essential to account for the direction in which the electric field vector is rotating. By convention, when the thumb is pointing in the direction of wave propagation, the curled fingers show the direction, and the polarization is referred as right-hand circular polarization (RHCP) when the rotation is counter clockwise, while it is a lefthand circular polarization (LHCP) when the rotation is clockwise. Figure 3 shows the two rotation directions when the wave is travelling perpendicularly through the paper, moving away from the observer. The desirable handedness depends on the system in which the antenna is to be applied in. The satellites in GPS system, for example, utilize RHCP waves. The handedness of circularly polarized plane wave can be determined by measuring both LHCP and RHCP gains of the antenna. When the antenna is circularly polarized, either LHCP or RHCP gain is remarkably greater than the other. The difference between these two gains also indicates the purity of the circular polarization. The greater the difference, the better the purity. If LHCP and RHCP gain values are equal, the wave is linearly polarized. In the case of wearable antennas, especially circularly polarized ones, it is important to take into consideration all possible operational circumstances which may have an effect on the antenna performance. Hence, RHCP and LHCP gain

Circularly Polarized Wearable Antennas 149 measurements need to be repeated for each situation/position, such as bending in different directions. It should also be noted that antenna gain measurement alone does not take into account the handedness of circular polarization. It is often advisable to investigate the direction of antenna gain as well, especially in the case where line-of-sight communication is desirable. Maximum gain presented over a frequency range without angle information shows the maximum gain value from the three-dimensional perspective, but it does not give information about the direction. If the pattern changes in shape at different frequencies, the direction of the maximum gain may vary significantly. Furthermore, and as explained below, the direction of circular polarization may vary too. 2.3 Axial ratio As explained in the previous section, the purity of circular polarization can be evaluated on the basis of LHCP and RHCP gain measurements. However, in practice, it is often more accurate and convenient to use axial ratio as a measure of the purity. Axial ratio can be defined either as the ratio between RHCP and LHCP electric fields or by the means of absolute value gains of the antenna as follows: AR log E RHCP + E LHCP RHCP = RHCP LHCP = G + 20 20log E E G G LHCP GRHC P LHCP (2) When the wave is perfectly circularly polarized, the value for axial ratio is 0 db. As the value increases, the polarization turns elliptical and keeps flattening until it turns to a perfectly linear polarization at infinity. Depending on a given standard, the purity limit for circular polarization varies from 0 to 5 db. The equation above gives axial ratio at a certain frequency and at a certain azimuth and elevation angles. Figure 4 depicts the space coordinate system of a (patch) antenna, in which q is the elevation angle between the z-axis and xy-plane and j is the azimuth angle at xy-plane. To ensure circularly polarized operation, axial ratio needs to be measured over the desired bandwidth and coverage angle. In addition to the fact that axial ratio Figure 4: Antenna measurement coordinate system.

150 Innovation in Wearable and Flexible Antennas tends to degrade towards antenna beam edges, changes in antenna shape followed from operating situations may accordingly affect the purity of circular polarization and its handedness. After all measurements are conducted, the worst case situations can be eluded by adjusting the antenna position in such a way that the negative effects of bending are minimized. 3 State-of-Art Designs A circularly polarized antenna is typically a crossed dipole, a helix, or a patch antenna. As the wearable antenna is preferred to beas less obtrusive as possible, a planar, low profile patch is the most widely used antenna topology for circularly polarized applications. To transmit a circularly polarized wave, a ±90 phase difference between orthogonal current modes has to be created. A common way to achieve the difference in phase is to use asymmetric structures in the patch. A patch antenna usually consists of two or three separate layers. In the three-layered structure, a dielectric substrate is sandwiched between a conductive patch radiator and a ground plane as pictured in Fig. 5. The ground plane makes the antenna less sensitive to the effects of human body proximity and also prevents the user from harmful radiation (reduced SAR). The first circularly polarized textile antennas were presented in 2004 by Klemm et al. [3] and Salonen et al. [4]. As the development of wearable antennas had just started, the main focus in many antenna design processes was on considering suitable textile materials. In Salonen s paper, the authors discuss the characteristics and suitability of different dielectric fabrics for circularly polarized GPS antenna use. Moreover, patch antennas with different substrate materials were designed and analysed. The operational band of Klemm s antenna extends from 2.29 to 2.36 GHz with axial ratio less than 3 db. The reported antenna is a 6-mm thick patch antenna with a ground plane and a microstrip feed. Polyamide is used as the substrate material while nickel-plated woven textile is chosen for the conductive parts. Circular polarization is achieved by cutting the two corners of the radiator in such a way that the length and width of the patch slightly differ from each other, which gives Figure 5: The structure of a three-layered patch antenna.

Circularly Polarized Wearable Antennas 151 rise to two orthogonal current modes. The width of the microstrip feed depends on the relative permittivity of the substrate material where the strip becomes wider as lower permittivity values are used. Hence, for very low permittivity substrates, a 50 Ω feed may be impractical since it becomes too wide compared with the dimensions of the radiator. 3.1 Circularly polarized wearable antennas for ISM band applications A plethora of circularly polarized wearable antenna designs has been reported in the literature recently aimed for (2.4 2.4835 GHz) industrial scientific medical (ISM) band applications. Hertleer et al. [5] introduced a truncated patch antenna to be exploited in protective garments of firefighters. The thickness of the antenna is 3.94 mm using foam as a substrate. Circularly polarized radiation is generated by cutting the corners of the patch radiator and deflecting the feed point slightly off the patch diagonal. The geometry is simple and hence easy to realize utilizing textile materials. To achieve an optimal performance, the authors recommend to use a ground plane at least 1.5 times larger than the patch. It is also found that some bending directions cause more deterioration to the antenna performance than others, hence, such positioning must be avoided. Another design of a wearable circularly polarized ISM band antenna is reported by Lui et al. [6]. The design is aimed for power transmission in a wirelessly powered, battery-less temperature sensor system to communicate over a distance of 1.7 m with 50 mw transmitted off a base station. The reported antenna is based on a two-layered patch with no ground plane. To achieve a reasonable performance in the vicinity of human body, the optimal height is selected as 10 mm. The handedness of circular polarization is found to be depending on the bending direction, which must be taken into account when aligning the antenna. Tronquo et al. [7] have developed a nearly circularly polarized wearable patch antenna for ISM band applications. Instead of truncating the corners, a rectangular slot in the centre of the radiator is introduced to excite the two orthogonal current modes. Fleece is used as a substrate material and FlecTron fabric is chosen for the ground plane and the radiator. All layers are sewn together while the antenna is fed by a SubMiniature version A (SMA) connector. The effect of bending over 3.75 and 6 mm radii is analysed in terms of return loss measurements. Kuhjani and Neshati [8] have proposed a dual-band circularly polarized wearable antenna for 2.4 and 5.7 GHz bands. The antenna is fed by a microstrip via an aperture in the ground plane. The substrate under the patch radiator is based on a 2.56-mm thick fleece fabric with a permittivity close to 1. Altogether, the antenna consists of four textile layers and the total thickness is approximately 3.7 mm. The dimensions of the radiator are 43 mm43 mm. The two frequency bands with circular polarization are achieved by truncating the corners and cutting several symmetrical grooves on the rectangular radiator. The authors found that the higher band of the antenna is more sensitive to bending than the lower band and that bending increases back lobe radiation. However, SAR values remained below EU limit levels even when the antenna is bent.

152 Innovation in Wearable and Flexible Antennas Locher et al. [9] have investigated the design of wearable Bluetooth antennas for the 2.4 2.4835 GHz ISM band. The authors presented an elaborate analysis on the properties of substrate and conductive materials in addition to attachment methods. The advantages and drawbacks of probe and microstrip feeding methods are also presented. Detailed measurement results and analysis are provided for a truncated corner circularly polarized antenna on a 3.5-mm thick felt substrate and a linearly polarized antenna on a 6-mm thick spacer fabric. Measurement results show that the circularly polarized antenna is more sensitive to bending over a radius of 37.5 mm than in a linear one. It is worth noting that bending changes the effective dimensions of the radiator and consequently affects the two orthogonal current modes of the circularly polarized antenna. The thickness might change slightly during operation which in turns affects the effective permittivity of the substrate. Consequently, the beam-width of the circularly polarized antenna is narrowed down, and the purity of the circular polarization is degraded. The degradation level depends on the bending direction extent. As Kellomaki et al. noted in [10], the main lobe direction may vary with frequency, and hence the angle information is needed. 3.2 Circularly polarized wearable antennas for personal satellite communication and navigation In addition to ISM-band antennas, other popular applications of circularly polarized wearable antennas are GPS and satellite communication. Some solutions include dual-band operation for both navigation and personal communication. Elliot et al. [11] used a hybrid with two fabric feed lines to generate circular polarization in their wearable dual-band antenna deign. The design consists of a stacked patch antenna for GPS at 1,575 MHz and global navigation satellite systems (GNSS) at 1,227 MHz. The thickness of the antenna is 3.9 mm, length and width are 91 mm 91 mm. The authors found that multilayer textile patch and especially the feed-line structure is challenging to realize. However, it allows placing a low noise amplifier (LNA) for the receiver end closer to the antenna and hence losses and noise due to long cabling can be minimized. Circular polarization is estimated by LHCP and RHCP gain radiation patterns. A patch antenna with a central rectangular slot aimed for Iridium satellite phone communication is reported by Kaivanto et al. [12]. The substrate of the antenna is based on Cordura fabric sheets, while the radiator and the ground plane are based on woven conductive textiles with electric conductivity close to that of copper foil. Circular polarization is generated by adjusting the dimensions of the slot and the radiating element. Deflecting the feed point slightly from the diagonal of the radiator is another design parameter. The antenna is fed by an SMA connector. The effect of bending is studied by measuring the antenna bent on a cylinder of 50-mm radius in four layouts. Results show that changing the bending directions affect the antenna performance differently. Hence in practice, the worst cases can be avoided by placing the antenna in a way where bending in the undesirable direction is improbable.

Circularly Polarized Wearable Antennas 153 Another design reported by Kaivanto et al. [13] based on a circularly polarized dual-band antenna for personal satellite communication at iridium band (1621.35 1626.50 MHz), and for navigation at L1 band (1,575 MHz). To excite a wideband circular polarization, the slot at the centre of the rectangular patch is shaped as a polygon. The substrate layer consists of two different dielectric textile sheets: Cordura and a ballistic textile. The ground plane and patch radiator are woven using silver and copper plated, low-loss nylon fabric. To keep the structure as bendable as possible and to avoid losses caused by adhesives, the layers are sewn together. The effect of four defined bending directions on the antenna performance is investigated as described in the previous section. It is worth mentioning that no significant changes in total efficiency or RHCP gain were observed, whereas one bending direction remarkably degraded the circular polarization purity. An active wearable dual-band antenna for L1 GPS (1,575 MHz) and iridium (1616 1626.5 MHz) satellite phone is reported by Dierck et al. [14]. Enhanced signal reception with over 25 dbi gain is achieved by applying a LNA underneath the radiating element. The authors proposed a method of enlarging the bandwidth of circular polarization by using a discrete hybrid coupler, which fixes 90 phase difference between feed lines. As the circular polarization is generated by the coupler instead of radiator reshaping, the antenna is found to be less sensitive to bending effects and fabrication inaccuracies. The reported probe-free planar patch antenna is fed by two perpendicular feed lines, which are coupled to the radiator via an aperture. A GPS antenna intended for rescue workers is reported by Vallozzi et al. [15]. As the antenna is required to tolerate harsh environmental conditions, fire-resistant and water-repellent materials are chosen. The antenna is based on a truncated corners patch with a ground plane. In addition to free space, an evaluation of a reallife scenario performance of the antenna integrated within a rescue worker jacket is conducted. Kellomaki et al. [10] have conducted a comparative study by investigating bending effects on different GPS antenna topologies. In addition to circularly polarized truncated corner patch antenna fed by a microstripline, a linearly polarized dipole, an elliptically polarized inverted-f antenna, and a wideband circular slot antenna are compared. According to return loss measurements, the last two types are found to be virtually immune to bending effects, whereas the dipole antenna is found to be the most sensitive. 4 Case Study: A Wearable Circularly Polarized Antenna for Personal Satellite Communication and Navigation In this section, the design procedure of a circularly polarized wearable antenna is presented and discussed in details. The first step after deciding on the antenna type is to select a suitable material for the antenna based on the design requirements. When selecting a dielectric substrate, one needs to characterize the electrical and mechanical properties of the material.

154 Innovation in Wearable and Flexible Antennas Table 1: Electrical parameters of the dielectric substrate fabrics. e x e y e z tan d x tan d y tan d z Ballistic 1.46 1.46 1.38 0.003 0.003 0.002 textile Cordura 1.88 1.91 1.67 0.009 0.010 0.005 For some common electro-textiles, the electrical properties are documented and available. In many other cases, however, the permittivity and loss tangent of the textile are unknown and hence need to be characterized. When determining the electrical parameters, it must be taken into account that textiles are often anisotropic, and hence the parameters of interest need to be determined separately in three orthogonal directions. The dielectric materials used for the antenna discussed here are Cordura and a ballistic textile. Their electrical parameters are listed in Table 1. The conductive material is a low-loss, woven nylon fabric, plated with copper. The surface resistance of this fabric is 0.03 Ω/ 2. The next step is to optimize the antenna dimensions. Nowadays, numerical simulations are widely used, as most simulation packages provide a fast and reliable solution. However, in the case of a textile antenna, the designer needs to be more careful as the anisotropic textiles are often challenging to model. Moreover, inaccuracies are inevitable in the manufacturing process of textile antennas, and hence it is often common to end up with measurement results diverged from simulation ones. It is also worth noting that a 100% realistic setups cannot be simulated such as wrinkles, air gaps, or stretching that might occur during bending. Hence, it is always advisable to confirm the simulated results by measurements. The proposed antenna is first simulated using CST Microwave Studio and followed by prototype fabrication once the optimized dimensions are obtained. The required operational bandwidth is 51 MHz to cover a 10 db return loss between the start of L1 band and the upper end of iridium band. It is also recommended to achieve larger bandwidth than the required one to compensate for possible frequency shifts caused by operational and environmental variables. The geometry and layout of the proposed antenna is depicted in Fig. 6. The dimensions of the radiator is approximately 65 mm 65 mm, and the coordinates of the feed point in millimetres are x = 3, y = 11.5, where the origin is at the centre of the antenna. To avoid any additional losses due to glue or other adhesives and to ensure durable attachment of the antenna structures, all fabric layers are sewn together. The fabricated prototype is shown in Fig. 7. The next step is to determine the return loss and bandwidth via (S11) measurements. The antenna is first measured in the flat setup in free space, then bent in x- and y-direction over a Rohacell half cylinder with a radius of 50 mm. The bending directions and measurement setup are depicted in Figs 8 and 9, respectively. As can be seen from the measurement curves depicted in Fig. 10, bending in x-direction shifts the lower resonance to the left whereas bending in the y-direction shifts the resonance to the right.

Circularly Polarized Wearable Antennas 155 Figure 6: The structure of the proposed circularly polarized antenna. Figure 7: The manufactured antenna prototype. Figure 8: The antenna bent in y-direction (left) and in x-direction (right). Satimo Starlab measurement system is used for radiation pattern measurements. Figure 11 shows the total efficiency curves of the antenna. It should be noted that the total efficiency value of 70% is relatively reasonable for a textile antenna. The RHCP gain over the desired frequency range at antenna bore-sight (z-direction) is shown in Fig. 12. It can be seen that bending in x-direction has only a minor effect on RHCP gain, whereas bending in y-direction decreases the gain around resonance.

156 Innovation in Wearable and Flexible Antennas Figure 9: Antenna measurement set-up. Figure 10: Measured S11 curves of the flat and the bent antenna.

Circularly Polarized Wearable Antennas 157 Figure 11: Measured total efficiency of the flat and bent antenna. Figure 12: Measured RHCP gain of the flat and bent cases as a function of frequency. To acquire the polarization behaviour when the antenna is bent, LHCP gain needs to be examined as well. Figure 13 shows both LHCP and RHCP gains for 1,575 MHz resonance as a function of theta angle when j = 0. As the difference between LHCP and RHCP curves is small in all cases, the antenna is considered as elliptically polarized. An interesting observation is that although the RHCP gain in x-direction bending is almost similar to that of the flat case, the LHCP gain level is higher and hence, the purity of circular polarization is degraded in x-direction bending.

158 Innovation in Wearable and Flexible Antennas Figure 13: Measured RHCP and LHCP gains at L1 frequency. Figure 14: Measured RHCP and LHCP gains at iridium frequency. Figure 14 shows the same previous analysis but for iridium (1,625 MHz). The difference between right- and left-hand gains is quite big, which indicates that the antenna is circularly polarized. Axial ratio curves at the bore-sight direction of the antenna, in which both theta and phi angles are zero, are depicted in Fig. 15. As the pre-set limit for axial ratio in the case of this antenna was 5 db, it can be seen in the graph that the antenna is circularly polarized around the iridium frequency even when the antenna is bent in the x-direction. Bending in the y-direction has a lower effect in terms of return loss and gain measurements. However, the polarization at the L1 band is elliptical.

Circularly Polarized Wearable Antennas 159 Figure 15: Axial ratio at bore-sight direction (q = f = 0 ) as a function of frequency. Figure 16: Axial ratio versus theta angle at iridium frequency (1,625 MHz). Finally, axial ratio as a function of theta angle is investigated in order to find the coverage angle for which the antenna maintains circular polarization. It can be seen from Fig. 16 that the purity of circular polarization is best when the antenna is flat. It was also found that bending in x-direction narrows the conical width remarkably. Hence, the best placement for the antenna is on the flattest parts of the body, such as shoulders, back, and thighs. Bending in y-direction can be avoided by attaching the antenna in such a way that the y-axis runs along the arm. This is also an advantage of circularly polarized antennas over linearly polarized ones, since the position of the antenna can be changed without the risk of increased polarization mismatch losses.

160 Innovation in Wearable and Flexible Antennas 5 Conclusion Wearable wireless systems and applications are generally aimed at maximizing the quality of life. One aspect of achieving that is by providing freedom of movement, hands-free devices, and a more flexible platform. However, mobility causes many challenges in wearable communication systems. For example, arbitrary alignments of transmitting and receiving antennas may cause polarization mismatch losses. On the other hand, the user s mobility may impose some garment deformations/misalignment, which often change the characteristics of the integrated flexible antenna (i.e. based on textile), and in turn leads to performance deterioration. Since utilizing a circularly polarized antenna is one way of preventing mismatch losses, the current research is aiming at developing such antennas with capability of integration within wearable devices. In this chapter, the advantages of exploiting circularly polarized antennas in wearable applications and the related challenges are discussed. Moreover, an extensive review of state-of-the-art circularly polarized antenna designs is provided. Finally, a case study of a dual band circularly polarized wearable antenna for personal satellite communication and navigation at L1 and iridium frequency bands is presented. Design procedure starting with material properties and ending with return loss and radiation pattern measurements are discussed in detail. It is concluded that in addition to the total efficiency and gain, analysing the purity and handedness of circular polarization is of great importance. Moreover, to characterize the circular polarization over a desired operational bandwidth and a sufficient conical coverage, radiation pattern measurements for LCHP and RHCP gains are required. References [1] Serra, A.A., Nepa, P., Manara, G. & Massini, R., A low-profile linearly polarized 3D PIFA for handheld GPS terminals. IEEE Transactions on Antennas and Propagation, 58(4), pp. 1060 1066, April 2010. [2] Pathak, V., Thornwall, S., Krier, M., Rowson, S., Poilasne, G. & Desclos, L., Mobile handset system performance comparison of a linearly polarized GPS internal antenna with a circularly polarized antenna. IEEE Antennas and Propagation Society International Symposium, Vol. 3, pp. 666 669, 22 27 June 2003. [3] Klemm, M., Locher I. & Troster, G., A novel circularly polarized textile antenna for wearable applications. 34th European Microwave Conference, Vol. 1, pp. 137 140, 14 14 October 2004. [4] Salonen, P., Rahmat-Samii, Y., Schaffrath, M. & Kivikoski, M., Effect of textile materials on wearable antenna performance: a case study of GPS antennas. IEEE Antennas and Propagation Society International Symposium, Vol. 1, pp. 459 462, 20 25 June 2004. [5] Hertleer, C., Rogier, H., Vallozzi, L. & Van Langenhove, L., A textile antenna for off-body communication integrated into protective clothing for firefighters.

Circularly Polarized Wearable Antennas 161 IEEE Transactions on Antennas and Propagation, 57(4), pp. 919 925, April 2009. [6] Lui, K.W., Murphy, O.H. & Toumazou, C., A wearable wideband circularly polarized textile antenna for effective power transmission on a wirelesslypowered sensor platform. IEEE Transactions on Antennas and Propagation, 61(7), pp. 3873 3876, July 2013. [7] Tronquo, A., Rogier, H., Hertleer, C. & Van Langenhove, L., Robust planar textile antenna for wireless body LANs operating in 2.45 GHz ISM band. Electronics Letters, 42(3), pp. 142 143, 2006. [8] Kuhjani, A. & Neshati, M.H., Design investigation of a dual-band circularlypolarized wearable antenna. Second Iranian Conference on Engineering Electromagnetics (IECCM), 8 9 January 2014. [9] Locher, I., Klemm, M., Kirstein, T. & Troster, G., Design and characterization of purely textile patch antennas. IEEE Transactions on Advanced Packaging, 29(4), pp. 777 788, November 2006. [10] Kellomaki, T., Heikkinen, J. & Kivikoski, M., Effects of bending GPS antennas. Asia-Pacific Microwave Conference APMC, pp. 1597 1600, 12 15 December 2006. [11] Elliot, P.G., Rosario, E.N., Rama Rao, B., Davis, R.J. & Marcus, N.M., E-textile microstrip patch antennas for GPS. IEEE/ION Position Location and Navigation Symposium (PLANS), pp. 66 73, 23 26 April 2012. [12] Kaivanto, E., Lilja, J., Berg, M., Salonen, E. & Salonen, P., Circularly polarized textile antenna for personal satellite communication. Proceedings of the Fourth European Conference on Antennas and Propagation (EuCAP), pp. 1 4, 12 16 April 2010. [13] Kaivanto, E., Berg, M., Salonen, E. & de Maagt, P., Wearable circularly polarized antenna for personal satellite communication and navigation. IEEE Transactions on Antennas and Propagation, 59(12), pp. 4490 4496, December 2011. [14] Dierck, A., Rogier, H. & Declercq, F., A wearable active antenna for global positioning system and satellite phone. IEEE Transactions on Antennas and Propagation, 61(2), pp. 532 538, February 2013. [15] Vallozzi, L., Vandendriessche, W., Rogier, H., Hertleer, C. & Scarpello, M.L., Wearable textile GPS antenna for integration in protective garments. Proceedings of the Fourth European Conference on Antennas and Propagation (EuCAP), pp. 1 4, 12 16 April 2010.