Design and Manufacturing of Robust Textile Antennas ... - IEEE Xplore

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IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 60, NO. 9, SEPTEMBER 2012

Design and Manufacturing of Robust Textile Antennas for Harsh Environments Juha Lilja, Pekka Salonen, Member, IEEE, Tero Kaija, and Peter de Maagt, Fellow, IEEE

Abstract—Antennas made out of textile materials suffer from performance perturbing effects whose impact mainly depend on the mechanical properties of the fabrics. The soft and flexible nature of the fabrics is essential for user comfort in wearable systems, but makes the antenna performance sensitive to bending, stretching, compression, and the manufacturing process. Furthermore, water absorption into the woven textile structures can increase both the permittivity and the dielectric loss of the substrate materials. The potential performance reduction due to the material characteristics is addressed in this paper, and methods to improve performance robustness are introduced. Tests show that the use of a textile cover provides a rugged design which is insensitive to the effects of abrasion, saline water and varying climatic conditions. A dual frequency textile antenna is thoroughly tested and shown to be fully compliant with Iridium and GPS specifications. Index Terms—Antennas, patch antenna, textile antenna, wearable antenna.

I. INTRODUCTION

O

NE of the marvels of modern antenna technology is the application of naturally soft and flexible materials, such as fabrics and foams, as integral parts of planar antennas and microwave components. These materials open several possibilities for the creation of ingeniously engineered textile compounds where structural and electrical functionality are optimized simultaneously. The layered structure of such soft compounds gives the designer an opportunity to select each layer individually, and even combine different materials cost-efficiently within a single layer. Since the first public research reports on wearable antennas from the late nineties [1], the study on wearable antennas has received significant interest among university and industry researchers worldwide. The obvious development trend was the introduction of a flexible substrate [2], which was followed by textile substrate antennas around the turn of this century [3]–[5].

Manuscript received March 11, 2011; revised January 24, 2012; accepted March 23, 2012. Date of publication July 03, 2012; date of current version August 30, 2012. This work was supported in part by the European Space Agency under Contract AO/1-5500/07/NL/ST. J. Lilja and P. Salonen are with Nokia with Windows Phone Engineering, FI-33720 Tampere, Finland (e-mail: [email protected]; [email protected]). T. Kaija is with Patria Aviation Oy, FI-33100 Tampere, Finland (e-mail: tero. [email protected]). P. de Maagt is with Antennas and Submillimetre Wave Section, Electromagnetics and Space Environments Division, European Space Agency, NL 2200 AG Noordwijk, The Netherlands (e-mail: [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TAP.2012.2207035

Since then, the evolution of wearable antenna technology has taken quantum leaps in utilizing textile materials as antenna substrates, the latest embodiments taking advantages of metamaterials [6]–[8] for the antenna performance improvement. We are now at a stage where textile antennas can already compete in electrical performance with traditional rigid antennas. The countless application areas include lightweight wearable, body-worn antennas as well as man-carried inflatable or deployable antennas and arrays that can fit into a backpack of, for example, a trekker, rescue group, or an individual soldier. The flexibility enables the integration of low-frequency textile antennas into clothing [9] and thus the creation of a new generation of rescue apparel. Another fascinating application area arises from the imaginative use of conductive fabrics as microwave components. For example, conductive polymer fabrics can be exploited as lossy resistive layers within soft compound substrates [10]. Such layers can reduce the backward radiation in wearable antennas, or reduce the radar cross section (RCS) in low-observable platforms and can be used to design lightweight radar camouflage nets. Navigation and two-way communication seem to be an integral part of the next generation’s smart clothing. The latest advance was an Iridium phone field test, in which two-way satellite communication utilizing textile antennas (made of purely textile materials [11], [12]), was demonstrated. The next logical step is taking into account the effects of environmental conditions on antenna behavior. Textile antennas are believed to enable targeted sectors to enhance operations in harsh environments and operate year-round. These extreme climatic conditions lead to stringent requirements. Large temperature variations, including subzero temperatures, combined with high relative humidity and exposure to salt water affect the electrical performance as well as the lifetime of textile antennas. In [13] the dependency of permittivity and dielectric loss tangent of a Cordura textile on relative air humidity was presented, while in [11] and [14] the fluctuation of antenna resonance frequencies due to absorbed moistness was presented. This paper presents the design and fabrication methodology of a robust textile antenna for harsh environments. It should be stressed that many publications in this field relate to proof-ofconcept in a laboratory type environment, whereas the work described in this paper aims at production line antennas for practical applications. II. ANTENNA REQUIREMENTS The wearable antenna requirements are application specific and they are summarized in Table I for Iridium and GPS systems. However, common requirements for many applications

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LILJA et al.: DESIGN AND MANUFACTURING OF ROBUST TEXTILE ANTENNAS FOR HARSH ENVIRONMENTS

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TABLE I ANTENNA ELECTRICAL REQUIREMENTS

are: low-profile, light weight, low cost, low maintenance, and ease of use [15], [16]. Moreover, in demanding user scenarios at least the following two should be added: no damage from obstacles (robust), and material endurance to harsh climatic conditions. It has been shown that in urban environments in which multipath propagation is omnipresent, linearly polarized antennas can operate satisfactory in handset GPS receivers [17], and the axial ratio requirement for GPS can be somewhat relaxed. Therefore, the axial ratio requirement was set to be better than 10 dB for GPS as compared to the more stringent value of 5 dB for Iridium. III. DESIGN ASPECTS OF TEXTILE ANTENNAS The chosen patch antenna type for the demonstrator antenna was a probe-fed square patch with a polygon shaped slot, due to its proven wide band performance [18], [19]. The substrate was made out of a stack of woven fabrics with a high weaving density. The high dimensional accuracy that is required for the radiating element can be obtained with laser or water jet cutting. A polygon shaped slot provides the two orthogonally aligned electric field components with 90 phase difference in order to obtain the circularly polarized radiation. A. Material Selection Criteria In general, substrate and superstrate materials determine the electrical performance of a planar patch antenna. Characteristics, such as, bandwidth, operating frequency, and gain are driven by the electrical and mechanical parameters of the substrate [20], [21]. With textile antennas, additional requirements for the materials arise. These requirements include the temperature tolerance of materials, the durability against frequent bending, physical abrasion and the stress caused by the operational environment together with the structural characteristics of the fabric to maintain the electrical functionality under the environmental effects. The optimized performance can usually be reached with a combination of different functional layers, Fig. 1. The materials used for the conductive layers affect the electrical performance of the antenna [22]. Textiles can be made electrically conductive either by using conductive yarns to manufacture the fabric, or adding conductive materials to a nonconductive textile.

Fig. 1. Features of textile compound antennas.

Additive methods include conductive inks and pastes [23], as well as embroidery with conductive yarns [24]. Moreover, polyester fabrics doped with intrinsically conductive polymers, such as polypyrrole (PPy), can be tailored to have sheet resistances from a few per square up to several M per square. In contrast, the current state-of-the-art metal-based conductive textiles can reach sheet resistances lower than 20 . B. Uncertainties in the Antenna Modeling There are at least three factors that need to be considered when antennas made out of textile stacks are designed. Firstly, conductive fabrics should be modeled using impedance models that are capable to predict the reduced phase velocity that is caused by the nonideal conductivity [25], [26]. Secondly, knowing only the permittivity of plain sheets of the textile materials is usually not enough to accurately predict the resonance frequency of the fabricated antenna. The air gaps between stacked fabrics introduce some uncertainty in the modeling accuracy of complete antenna structures. Our microstrip measurements have indicated that, compared to the permittivity of plain sheets, the air layers between stacked fabrics lower the permittivity of the complete substrate pile. The third factor affecting the modeling uncertainty arises from the method of how the conductive layers are attached on the textile substrate. Microstrip resonator measurements have shown that when an etched resonating strip has been replaced with a dimensionally identical copper tape, a resonance shift of approximately 40 MHz has been measured at -band. When conductive fabrics are either sewn or glued onto the textile substrates, it is clear that accurate prediction of the antenna resonances has several uncertainties involved with. C. Protective Measures Against Environmental Effects An effective method to protect the antenna against many environmental effects is to design the antenna with a sufficient guard band allowing small resonance frequency fluctuations. A guard band refers to an operational bandwidth that is intentionally designed to exceed the required application band. Moreover, preshrinking of new fabrics may be used to prevent the resonance shift of fabricated antennas over time.

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Fig. 2. Temperature and relative humidity profiles during the test cycle.

Fig. 3. Measured permittivity with respect to relative humidity during the test cycle, at 1.626 GHz.

A textile cover has a crucial role of protecting the antenna in real-life systems. In addition to protection against abrasion, the cover fabric can provide a shield against water absorption as well as salt contamination. The effects of water and salt on the properties of the antenna materials are detailed in the next chapter. IV. EFFECTS OF ENVIRONMENTAL CONDITIONS PROPERTIES OF ANTENNA MATERIALS

ON THE

Absorbed water lowers the resonance frequencies of a textile antenna. Textile material relative permittivity is usually between one and three, whereas the permittivity of distilled water is 76.7 at 3 GHz [27]. The permittivity of water generally depends on factors such as frequency, temperature and salinity, but at and -bands the above value can be considered constant. An exception to this approximation is the situation when the water is frozen. The relative permittivity of ice is around 2.6–4.5 [28]–[31]. The remarkable change of water permittivity under freezing was verified in [32]. Therefore, material characterization in relevant circumstances is essential to incorporate this knowledge into the antenna design. A. Characterization of Nonconductive Textiles in Varying Climatic Conditions A transmission-line method based on the scattering parameter measurements of two microstrip lines [33]–[36] was used for material characterization. The microstrip samples were made out of a stack of Cordura sheets attached using a pressure sensitive adhesive (PSA). Both the signal trace and the ground plane were made out of copper foil. The temperature was varied from to and relative humidity from 10% to 95%. Fig. 2 shows the climatic test cycle where the measurement index (#1 #13) pairs the temperature and relative humidity profiles to the time instants the data was stored. The duration of the test was 168 hours, during which the scattering parameters were measured with a four-port vector network analyzer (VNA) Rohde & Schwarz ZVA24. The extracted permittivity at 1.626 GHz is shown in Fig. 3 with respect to relative humidity, whereas the loss tangent is

Fig. 4. Loss tangent with respect to the vapor density during the test cycle at 1.626 GHz.

shown in Fig. 4 as a function of vapor density (VD). The test cycle started when the samples were frozen at . During the first two instants (#1, #2) totaling 16 hours, the chamber was frozen, and the absolute humidity in the chamber was very low. Then the temperature was set to for 7.5 hours before the permittivity was calculated at instant #3. During this phase, ice in the chamber and the samples was melted, but any noticeable increase in permittivity was not observed. The amount of water in the chamber remained very small due to the relatively low temperature. After this, the amount of water in the chamber was gradually increased, and the observed permittivity followed closely to the changes in the relative humidity. The loss tangent behavior (see Fig. 4) shows a remarkable dependency on the vapor density. After samples were melted, the increased water in the chamber increased the loss tangent at instant #3. Between instants #3 and #4 the increase in relative humidity close to the dew point is observed to increase the loss tangent considerably, from 0.011 to 0.022. During this transition, the amount of water in the chamber increases. After instant #4, the temperature was increased to and relative humidity lowered to 75%, and after seven hours, at instant #5 the permittivity is seen to have decreased from


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Fig. 6. Iridium/GPS patch on a fabric compound substrate.

Fig. 5. Top: Penetration of salt water through a waterproof textile. The first DWR fabric has let strong salt water to penetrate it (left), whereas the second DWR fabric has kept all the salt water inside (middle).

1.92 to 1.89 and no noticeable change in loss tangent is observed. During this step, the absolute amount of water in the chamber [vapor density (VD)] is significantly increased as seen from Fig. 4. An increase in both loss tangent and permittivity could be expected. However, the results indicate that loss tangent remains nearly fixed, and there is a decrease in permittivity. The above phenomenon indicates that the evaporation rate from the fabrics is a factor that affects the behavior of the electrical material parameters in high humidity conditions. Increase in temperature increases the saturation vapor density (SVD), which is translated as decreased relative humidity when the water mass in the chamber is held constant. Decreased relative humidity, on the other hand, means increased evaporation rate. The measurements showed that both permittivity and loss tangent of nonconductive fabrics are dependent on the humidity conditions. The results indicated that permittivity changes follow closely the changes in the relative humidity. The loss tangent, on the other hand, was seen to have a better correlation with vapor density instead of relative humidity. In addition, it was observed that rapidly decreased relative humidity caused the permittivity to decrease despite that vapor density was simultaneously increased by increasing temperature. In other words, it seemed that the effect of increased water mass in the chamber was compensated by increasing the evaporation rate from the fabrics. This implies that the absorbed water mass in the fabrics would be the independent variable affecting the changes in permittivity and loss tangent. B. Antenna Protection With a Cover Fabric A comparative test with two other waterproof textiles was carried out. Pockets were made out of these fabrics and filled with 5% salt water and left to hang for several days. The first fabric has a Durable Water Resistant (DWR) coating and a water vapor flux of approximately 1000 g/m /24 h. This fabric is shown on the left of Fig. 5. The second waterproof textile had a Teflon finish and polyurethane membrane. Water vapor flux was 6000 g/m /24 h and water column over 2000 mm. The outer side of the pocket, that is, the inner layer of the textile, was observed to get wet very quickly after pouring the salt water into the pocket. The second fabric, shown on the right of Fig. 5, held all the salt water inside the pocket.

Fig. 7. Resonance fluctuation when patch edges are compressed.

It was seen that surface contamination can cause waterproof fabrics to fail in their primary function. There are two reasons for this. First, if the water repellency is chemically added to the fabric, a strong salt solution can wash out the added layer. Secondly, user comfort requires the fabrics to be breathable, and this is realized with small holes which let water vapor to evaporate. Water droplets cannot penetrate these small holes due to the surface tension. Salt, on the other hand, lowers the surface tension of water, and this is expected to be the reason why salt water can penetrate many waterproof fabrics. The above clearly shows that the environmental specifications should be taken into account from the very early stages of the antenna design. The search of low-loss substrate materials is just an initial step towards high-performance antennas, when it only assures high performance in nominal conditions. However, the protective measures are what provide reliable antenna performance and sufficient lifetime in real-life use. V. SEWING PROCESS OPTIMIZATION The thorough material characterization campaign under relevant environmental conditions allowed us to take this information into account in antenna design. To complicate things, mechanical deformation and substrate compression also produce continuous detuning. A. Substrate Compound Optimization The substrate is composed of Cordura layers combined with low-loss ballistic textiles as shown in Fig. 6 to reduce the water

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Fig. 8. Microstrip resonator at the three different stitching phases.

absorption of the substrate. The introduction of the ballistic textile was done in order to gain sufficient thickness to achieve wide impedance bandwidth as well as high radiation efficiency. The conductive elements were composed of commercially available woven conductive textiles. The fine-tuning of the antenna axial ratio was accomplished by adjusting the center slot of the antenna, and water jet cutting was used to cut the conductive elements. This ensured the accuracy and shape repeatability of the manufactured antennas. Conventional sewing techniques were used in antenna manufacturing. Thin adhesive layers were first tested in fabrication, but resulted in a bulky and rigid structure. Also, avoiding the adhesives was another step towards enhanced efficiency. After developing a more advanced and optimized sewing process, the antennas retained flexibility and maintained the optimum electrical performance as shown in the next sections. B. Identification of the Sensitive Parts of the Antenna The dependency of the antenna resonances on partial compression was determined using a patch antenna on the textile compound substrate. The radiating patch was attached using loose zig-zag sewing pattern. In order to prevent any dielectric loading a rod of Rohacell with dimensions of 10 and 120 mm was applied to compress the fabric antenna at specific locations. When compressed in this way, the thickness of the antenna was reduced by 0.7 mm under the Rohacell rod. First, the compression was localized on the substrate edges without compressing the radiating element. The return loss remained almost unaffected. This was a logical result because the electric fields are concentrated under the radiating element. Secondly, the patch radiator was compressed at all edges, Fig. 7. Compared to the uncompressed state, a significant shift of resonances is observed. It can be seen that the compression on the top and bottom edges reduce the coupling of the higher resonance, whereas compression on the left or the right patch edge has a similar effect on the lower resonance. The observation is logical, since there are two perpendicular resonating dimensions in the antenna. In each case the resonances are shifted downwards in frequency. This implies that compression causes localized increase in substrate relative permittivity, and the observations are consistent with the results shown in the next section, where a more quantitative test is carried out. The presented locations play an essential role in the optimization of the manufacturing techniques of robust textile antennas, because the electric field intensity is at its maximum at the patch edges.

TABLE II RESONANCE STABILIZATION

Similar behavior than with the edge compressed antennas was observed when the patch center was compressed, but this time both a downwards- and upwards-shift occurred. This was caused by the fact that the antenna feed area was not compressed symmetrically with respect to the two directions of compression used. When the compressing force was reduced, the resonances gradually returned to the state before compression. The most important observation is that the antenna resonances are most affected when the edges of the radiating element are compressed. This implies that the resonances are the most sensitive to the perturbation of the high-density electric fields at the patch edges. This is proved with the test shown in the next section where a method to stabilize the resonances with localized stitches is presented. C. Effect of Sewing Process on the Robustness of Resonance Frequencies A sewing parameter test with a microstrip resonator was performed to characterize the effect of sewing techniques on resonance stability. The measurement sample consisted of two layers of the ballistic textile, and two layers of Cordura. The sample in the measurements is shown in Fig. 8 at different phases of the test. The left hand side shows the initial structure, in which very loose sewing is used to attach the resonating strip. Later, precisely positioned stitches were added to high field density areas, and the transmission loss of the sample was measured. In each step, the transmission loss was measured five times, and between the measurements, the sample was manually deformed to test the structure stability. Table II summarize the effect of added stitches on the resonances. With the loose attachment, the resonances were highly unstable, and repeatability of measurements was unsatisfactory. By adding the three vertical stitches to the high field density areas, the resonance variation between different measurements was reduced from 104 to 9 MHz. When dense zig-zag stitches


Fig. 9. Convergence of resonances using few precisely located stitches. Stitch 1 shows the results of the initial pattern with loose stitches. Stitch 2 has three vertical stitches added, and Stitch 3 shows the converged resonances when the edge is properly sewn.

were added around the circumference of the resonator, this resonance variation was reduced to 2 MHz, and the bundle of the resonance peaks had shifted downwards in frequency. The transmission loss measurements are shown in Fig. 9. The black traces show the five measurements carried out when Stitch 1 only was added. Similarly, the blue and red traces show the corresponding measurements after Stitch 2 and Stitch 3 were added, respectively. The results showed that the regions of high field density below the resonating strip are the most critical areas where dense stitching should be applied. Moreover, it was seen from the results that the physical length of the conductive element is not the only factor defining the resonance length of the structure. The localized compression below the conductive element was shown to have an effect on the resonances. The more the substrate was compressed, the lower the resonances were seen to shift. The observation is consistent with our previous experience, where measurements have predicted that compression increases the permittivity of a textile material. Therefore, the sewing process has a pronounced effect on the resonance frequencies of a textile antenna, and in production-line antennas the final antenna tuning can be accomplished by a sewing process optimization. VI. OPTIMIZED GPS/IRIDIUM ANTENNA MEASUREMENTS The final antenna design iteration was performed based on the knowledge of sewing optimization and the effect of air gaps in the substrate stack. This was taken into account with an effective permittivity value. The effective value was obtained by using a homogenized substrate in Ansoft HFSS, for which the effective value was tuned to match the measured material parameters. This resulted an in-plane component of , and out-ofplane component of of the compound substrate. A square microstrip patch of 65 65 mm, fed with a soldered coaxial probe was used as the radiating element with a ground plane of 100 100 mm, Fig. 10. The polygon-shaped slot has been shown to provide good axial ratio performance in [18] and [19]. This final antenna version resonates at lower frequencies

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Fig. 10. Optimized patch antenna geometry (figure not in scale). Two 12 9-mm slots are centered to the patch and they intersect each by 3 2 mm area.

Fig. 11. Measured S11 (solid) compared with simulations.

than the one used in the antenna edge compression test due to two reasons. The sewing pattern lowered the resonances slightly and the polygon slot in the antenna radiator was modified to tune the resonances downwards in frequency. Fig. 11 shows a comparison of the measured reflection coefficient (S11) with simulated S11 in two different cases. In the Simulation 1 the substrate is modeled using the actual measured material parameters that were determined for plain textile sheets. Note that all the nonconductive textile layers were included in the model. Secondly, the substrate is modeled using the homogenized effective permittivity that was defined to take into account all mechanical factors that affect the antenna resonance (Simulation 2). The simulated S11 based on the effective permittivity shows a good agreement with measurements. The measured 10 dB S11 bandwidth is 72 MHz (4.5%). The radiation characteristics were measured with the Satimo Starlab measurement system in the frequency range from 1.54 to 1.66 GHz, and with a frequency resolution of 2.5 MHz. Table III shows the measured antenna parameters. The measured gain and axial ratio at zenith are presented in Fig. 12 as a function of frequency. The measured minimum axial ratio of 0.4 dB is at

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TABLE III MEASURED ANTENNA PARAMETERS

Fig. 13. Measured and simulated gain and axial ratio in 1.605 GHz.

Fig. 12. Measured and simulated gain and axial ratio at zenith

-plane at

.

1.6025 GHz, whereas the simulated minimum axial ratio is 0.3 dB at 1.605 GHz. The 3-dB axial ratio bandwidth is 23 MHz. Both the simulated and measured maximum gains are 6.5 dBic. The -plane radiation characteristics at 1.605 GHz are shown in Fig. 13. The measured gains and axial ratios show good agreement with simulations. The beamwidth for axial ratio less than 3 dB is 133 in -plane and 156 in -plane, respectively, while the 3-dB gain beamwidth is 82 in both - and -planes. The -plane parameters are seen in Table III. The effect of the protective textile cover on the resonance frequency is relatively small. Depending on the cover material, the resonance frequency shift around GPS frequencies was generally between 5–10 MHz, which is a small change compared to the effects of fabrication tolerances and the sewing process. The measured total efficiency of an antenna with waterproof textile cover is 75%. To investigate the effect of different waterproof textiles on antenna efficiency, an antenna covered with a durable Kevlar fabric was measured. This antenna showed a measured total efficiency of 73%, thus showing comparable efficiency to the benchmark antenna. The antenna behavior under bending conditions was studied as well, Fig. 14. The effects of bending on the antenna performance with a 65-mm bending radius was carried out with Ansoft HFSS ver. 12. The radius is comparable to a typical human

Fig. 14. Measured and simulated axial ratios at zenith. Both unbent and bent antennas are compared.

arm. The substrate material was modeled as an anisotropic dielectric. However, the anisotropic permittivity tensor is defined in the global coordinate axis of the simulator, whereas the yarns of the woven substrate are not aligned parallel to any rectangular coordinate axis under bending. When the antenna is bent, the permittivity tensor should follow the curvature of the dielectric, and thus cylindrical coordinates would be required for exact permittivity tensor definition. Despite this, the simulations have rather good agreement with measurements. The simulated minimum axial ratio of 2.2 dB at zenith under bending occurs around 1.6 GHz. The measured and simulated - and -plane axial ratio and gain at the given frequency are compared in Fig. 15. The measured minimum axial ratio at zenith under bending is 3.4 dB. The simulated and measured gains under bending are 6.3 and 6.6 dBic, respectively. Although all of these results were very encouraging and an excellent match was achieved between measurements and prediction, the final step was to apply environmental testing and field test the antenna.


Fig. 15. -plane gain and axial ratio under bending at 1.6 GHz. Antenna is -direction. bent along the

TABLE IV MEASURED ANTENNA PARAMETERS

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Fig. 16. Measured S11 of an uncovered antenna under strong salt water test.

seen in the return loss behavior at low frequencies. The salt contamination was performed by soaking the antennas in 5% salt water for a total of 94 hours. The presented results show the importance of environmental protection of the antenna, and the brutal influence that salt contamination can have on the electrical performance of textile antennas. If the water repellency of the cover textile is made using chemical additives, strong salt solution can dissolve the protective chemical and diminish the protection provided by the cover. VIII. TEXTILE ANTENNA FIELD TESTING

VII. SALT WATER TESTS FOR THE GPS/IRIDIUM ANTENNA Salt immersion tests were carried out in order to study how absorbed salt affects the electrical performance of the antenna. The DWR coated cover was the same that was shown to fail in salt contamination in Fig. 5. The antenna return loss was selected as a figure-of-merit and it was measured under the following conditions: 1) an uncovered antenna was sprayed with fresh water, and the shift in resonance frequency was measured; 2) an antenna with the DWR coated cover was measured at dry state and after fresh water spraying on the antenna cover; and 3) both the DWR coated antenna and the uncovered antenna was measured after a salt contamination experiment. The results are summarized in Table IV. It can be clearly seen how the waterproof coating prevents water absorption to the substrate fabric, and therefore resonance frequencies are only decreased by about 5 MHz due to the water droplets. In comparison, the corresponding frequency shift with the unprotected antenna is 140 MHz, which exceeds the operational bandwidth of the antenna. The measurements with the salt contaminated antennas showed that dry crystallized salt inside the antenna does not have noticeable effects on antenna resonances. However, when salt is dissolved in water, the conductivity of the substrate textile is considerably increased. The unprotected antenna showed a resonance frequency shift of 240 MHz (see Fig. 16). Moreover, the effect of increased substrate conductivity is

Finally, the antenna was subjected to field trials with both Iridium and GPS systems. Both covered and uncovered textile antennas were tested in different conditions. A commercial Iridium satellite phone Motorola 9505A was used in the tests. The original antenna was replaced with an adapter and the textile antenna. The GPS field tests were performed using a customized GPS test system. A. Iridium System Tests First the available link margin was tested with the Iridium phone. In the absence of quantitative data, the signal strength bar of the phone served as a visual indicator of the field strength. The signal bars are highlighted in each figure. An additional 9-dB coaxial attenuator was used between the antenna and the phone [see Fig. 17 (left)]. It is seen that the phone is still able to obtain a good signal quality. Secondly, another antenna prototype was frozen and tested with the phone [see Fig. 17 (right)]. Successful phone calls with the frozen antenna were made, and therefore, an extra attenuation was added by piling densely packed snow on top of the antenna. The visual indicator shows reduced signal strength, but successful phone calls were still made. A protected antenna was submerged in a water bucket while the Iridium phone call was active. There were several temporary immersions in which the whole antenna element was submerged. However, as the waterproof cover prevented the water from penetrating the antenna, the phone call remained active without disconnecting.

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Fig. 19. Antenna inside ice block (left), and partly melted ice (right).

Fig. 17. Antenna with additional 9-dB coaxial attenuators (left), and a layer of densely packed snow on antenna (right).

TABLE V MEASURED SNR IN DECIBELS WITH REFERENCE AND TEXTILE ANTENNA

Fig. 20. SNR values of obtained with the textile antenna.

Fig. 18. Bent reference case (left), water immersion (middle), and extreme bending after immersion (right).

B. GPS System Tests A custom designed GPS test setup was employed to analyze the signal reception quality in a more elaborate way. The Figure-of-Merit was the signal-to-noise-ratio (SNR) given by the GPS receiver. The SNR values were extracted from GVS messages provided by the NMEA communication protocol of the selected GPS receiver module based on SiRFstarIII chipset. Antennas were exposed to different environmental conditions while making observations of the signal quality in real-time. First, a comparative test between a commercial GPS receiver and the GPS test setup with textile antenna was performed. The commercial receiver employed a rigid antenna. Simultaneous SNR results (in dB) for these two configurations are shown in Table V. The textile antenna has excellent performance. The textile antenna test cases are: First, a covered antenna was bent in the -plane using a bending radius of 65 mm to set the benchmark for other tests. The bent antenna is shown on the left of Fig. 18, and the measured SNR values are shown in Fig. 18 (Bent). Next, the antenna was fully immersed to water for a short period of time, after which the antenna was pointed towards sky. The measured SNR values with the wet antenna show that the

antenna was unaffected by the immersion. Satellite ID 25 shows a satellite that was not properly visible to the test site. After this, the wet antenna was bent to a small radius, [see Fig. 18 (right)] and the measured SNR values reduced from 0 to 15 dB, depending on the satellite. Finally, an uncovered antenna was frozen inside a block of ice, Fig. 19, and the measurements were continued the next day. The ice block had a thickness of 6 mm, and fresh water was used. After storing the SNR of each satellite, the ice was partially melt, Fig. 19. The measurements with the frozen and partially melted antennas show degraded, but still good performance, when keeping in mind that a minimum visibility of three satellites is required for GPS positioning. Here, nine satellites with better than 20 dBHz SNR was seen with the melted antenna, and eight with the antenna inside an ice block. The measured SNR values are shown in Fig. 20. IX. CONCLUSION A textile antenna meeting real life requirements for dual-band SATCOM use was designed, manufactured and evaluated. An extensive set of extreme environmental conditions were taken into account into the design and the measures to minimize the effects were provided. Low moisture absorption of substrate fabrics and the use of an abrasion resistant waterproof cover are the key factors that are required to produce a robust textile antenna. Moreover, a provision of a guard band that exceeds the specified application frequency band offers extra protection that helps the


antenna to remain operational in high-humidity, saline, and low temperature conditions. In this paper, we have looked in detail into many possible manufacturing approaches and process parameters that should be optimized to obtain compliant designs and ultimately lead to high-yield production-lines of reproducible, long-lifetime textile antennas. This paper clearly shows for the first time that it is not only the electromagnetic properties of the materials used that define the resonance frequency of the antenna, but that the dependence on various manufacturing aspects is critical as well. Extensive real-life test cases were provided to show the ruggedness of the developed antenna. A reliable satellite link was maintained even when the antenna was submerged several times in water for short periods of time. Moreover, simultaneous signal reception from eight GPS satellites with over 20 dB SNR was provided with a frozen antenna inside an ice block, showing the high performance that can be reached with the state-of-the-art textile antenna. This paper provides a comprehensive set of design parameters that need to be taken into account for the design of an application specific antenna using textile materials. This serves as a benchmark for deriving design guidelines for future designs. REFERENCES [1] P. Salonen, L. Sydänheimo, M. Keskilammi, and M. Kivikoski, “A small planar inverted-F antenna for wearable applications,” in Proc. 3rd Int. Symp. Wearable Comput. (ISWC), San Francisco, CA, Oct. 18–19, 1999, pp. 95–100. [2] P. Salonen, M. Keskilammi, J. Rantanen, and L. Sydänheimo, “A novel Bluetooth antenna on flexible substrate for smart clothing,” in Proc. IEEE Conf. on Syst. Man Cybern., Tucson, AZ, Oct. 7–10, 2001, vol. 2, pp. 789–794. [3] P. Salonen and H. Hurme, “Modeling of a fabric GPS antenna for smart clothing,” in Proc. IASTED Int. Conf. Model. Simul., M. H. Hamza, Ed., Palm Springs, CA, Feb. 24–26, 2003, pp. 18–23. [4] P. Salonen and H. Hurme, “A novel fabric WLAN antenna for wearable applications,” in Proc. IEEE Antennas Propag. Soc. Int. Symp. (AP-S), Columbus, OH, Jun. 22–27, 2003, vol. 2, pp. 700–703. [5] M. Tanaka and J.-H. Jang, “Wearable microstrip antenna,” in Proc. IEEE Antennas Propag. Soc. Int. Symp. (AP-S), Columbus, OH, Jun. 22–27, 2003, vol. 2, pp. 704–707. [6] P. Salonen, F. Yang, Y. Rahmat-Samii, and M. Kivikoski, “WEBGA—Wearable electromagnetic band-gap antenna,” in Proc. IEEE Antennas Propag. Soc. Int. Symp. (AP-S), Monterey, CA, Jun. 20–25, 2004, vol. 1, pp. 451–454. [7] S. Zhu and R. Langley, “Dual-band wearable antennas over EBG substrate,” Electron. Lett., vol. 43, no. 3, pp. 141–142, Feb. 1, 2007. [8] S. Bashir, A. Chauraya, R. M. Edwards, and J. C. Vardaxoglou, “A flexible fabric metasurface for on body communication applications,” in Proc. IEEE Antennas Propag. Conf. (LAPC), Loughborough, U.K., Nov. 2009, pp. 725–728. [9] R. Seager, A. Chauraya, Y. Vardaxoglou, and P. de Maagt, “Towards a compact low-frequency woven antenna,” in Proc. IEEE Antennas Propag. Soc. Int. Symp. (APSURSI), Charleston, SC, Jun. 1–5, 2009, pp. 1–4. [10] J. Lilja and P. Salonen, “Making flexible resistors out of conductive polymer fabrics,” Electron. Lett., vol. 47, no. 10, pp. 602–604, May 12, 2011. [11] J. Lilja and P. Salonen, “Textile material characterization for SoftWear Antennas,” in Proc. IEEE Military Commun. Conf. (MILCOM), Boston, MA, Oct. 18–21, 2009, pp. 1–7. [12] E. Kaivanto, J. Lilja, M. Berg, E. Salonen, and P. Salonen, “Circularly polarized textile antenna for personal satellite communication,” in Proc. 4th Eur. Conf. Antennas Propag. (EuCAP), Barcelona, Spain, Apr. 12–16, 2010, pp. 1–4.

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[35] G. F. Engen and C. A. Hoer, “Thru-reflect-line: An improved technique for calibrating the dual six-port automatic network analyzer,” IEEE Trans. Microw. Theory Tech., vol. 27, no. 12, pp. 987–993, Dec. 1979. [36] A. M. Mangan, S. P. Voinigescu, M. T. Yang, and M. Tazlauanu, “Deembedding transmission line measurements for accurate modeling of IC designs,” IEEE Trans. Electron Devices, vol. 53, no. 2, pp. 235–241, 2006. Juha Lilja was born in Kaarina, Finland, in 1983. He received the M.Sc. degree in electrical engineering from the Tampere University of Technology, Tampere, Finland, in 2009. He is currently a Postgraduate Student at the Tampere University of Technology and a Senior Antenna Engineer in Nokia with Windows Phone Engineering, Tampere. From 2008 to 2012 he was with Patria, as an RF Design Engineer in Tactical Data Links product group. From 2006 to 2008, he was a Research Assistant with the Department of Electronics, Tampere University of Technology. The research interests involve soft and flexible RF components, textile antennas, and HPM countermeasures and effects on body tissues. Other research themes involve handset antennas, near field communication as well as material research, especially the application of lossy materials on passive RF components, attenuators, and low observable platforms.

Pekka Salonen (M’00) was born in Helsinki, Finland, in 1973. He received the M.Sc.(Tech.) and D.Sc.(Tech.) degrees from Tampere University of Technology, Tampere, Finland, in 1997 and 2001, respectively, both in electrical engineering. He is currently a Principal Antenna Engineer in Nokia with Windows Phone Engineering, Tampere, Finland. From 1997 to 2003, he was Researcher and Senior Researcher in Institute of Electronics, Tampere University of Technology. From 2003 to 2004, he was a visiting Postdoctoral Researcher in the Antenna Research, Applications, and Measurement Laboratory (ARAM), University of California, Los Angeles. From 2004 to 2011, he was with Patria in various R&D positions. He had a leading role in developing textile antennas for satellite communications ground systems such as Iridium, GPS, Galileo, and Cospas–Sarsat. He has made pioneering research contributions in the fields of wearable and textile antennas, RFID antennas and systems, and local area network antennas. He has published over 80 refereed journal articles and conference papers. He is a coauthor of book chapter “Wearable Antennas” in Antennas and Propagation for Body-Centric Wireless Communications (Artech House, 2006). His research interests include wearable and textile antennas, novel soft materials for antennas and microwave components, handset antennas, MIMO, adaptive antennas, antenna and RF systems for radar and EW systems, human/ antenna interaction, applied electromagnetics for HPM, EMP, ECM, and ECCM techniques, and bioelectromagnetism. Dr. Salonen is listed in Who’s Who in America, Who’s Who in Science and Engineering, and Who’s Who in the World.

Tero Kaija was born in Turku, Finland, in 1977. He received the M.Sc. degree in electrical engineering from the Tampere University of Technology, Tampere, Finland, in 2002. He is a Postgraduate Student at Tampere University of Technology and is currently with Patria Aviation Oy, Tampere, as Project Manager and RF Specialist in Tactical Data Links Product Group. From 2002 to 2006, he was a Research Engineer and a part-time Teacher with the Department of Electronics, Tampere University of Technology. From 2006 to 2009, he was a Senior RF Designer in the Radio Network Solutions Group, Elektrobit. His research interests include on-wafer microwave measurements and RF CMOS/SOI modeling. Other research interests involve intelligent body-worn systems, applications of active RF components in wearable systems.

Peter de Maagt (S’88–M’88–SM’02–F’08) was born in Pauluspolder, The Netherlands, in 1964. He received the M.Sc. and Ph.D. degrees from the Eindhoven University of Technology, Eindhoven, The Netherlands, in 1988 and 1992, respectively, both in electrical engineering. From 1992 to 1993, he was a Station Manager and Scientist with an INTELSAT propagation project in Surabaya, Indonesia. He is currently with the European Space Research and Technology Centre (ESTEC), European Space Agency (ESA), Noordwijk, The Netherlands. He spent the summer period of 2010 as a visiting Research Scientist with Stellenbosch University, Stellenbosch, South Africa. His research interests are in the area of millimeter and submillimeter-wave reflector and planar integrated antennas, quasi-optics, electromagnetic bandgap antennas, and millimeter- and submillimeter-wave components. Dr. de Maagt was corecipient of the H. A. Wheeler Award of the IEEE Antennas and Propagation Society (IEEE AP-S) for the Best Applications Paper of 2001 and 2008. He was granted an ESA Award for Innovation in 2002 and an ESA award for Corporate Team Achievements for the Herschel and Planck Programme in 2010. He was corecipient of Best Paper Awards at the Loughborough Antennas Propagation Conference (LAPC) 2006 and the International Workshop on Antenna Technology (IWAT) 2007. He served as an Associate Editor for the IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION from 2004 to 2010 and was Co-Guest Editor of the November 2007 Special Issue on Optical and Terahertz Antenna Technology.