User Interaction with Handset Antennas

Transcription

1 1 User Interaction with Handset Antennas Rita Marques dos Santos Departamento de Engenharia Electrotécnica e de Computadores Instituto Superior Técnico rita.ims@gmail.com Abstract In this work the effects of human interaction with handset antennas are discussed, for both antenna performance and specific absorption in human. A comparative study is made for three antennas, each with specific characteristics in terms of structure, radiation pattern and input matching. Firstly, the effect of the antenna on the user s body is analyzed in terms of specific absorption, or SAR. Human models are studied, and some possible alternatives to the whole-body homogeneous model are considered. The effects of nearby objects on the SAR in the human model are also investigated. For the human influence on the antenna parameters study, the performance of the three mentioned antennas in the presence of the human model is compared and analyzed. The input refection coefficient, the radiation pattern and the antenna s efficiency are evaluated, as they are the most affected by the user s presence. These parameters are computed in the presence of a complete human model, as well as with a single hand and forearm model, so that the individual effects of each one can be analyzed. Finally, measurements were made for the input reflection coefficient, using one of the studied antennas in the presence of real users in several positions. Results are presented, and highlight the vulnerability of the measurement to its configuration and human model. Index Terms SAR, human model, user interaction, handset antennas, nearby objects, human exposure. I. INTRODUCTION IN the past century the general population has been increasingly exposed to man-made electromagnetic fields, mainly because of the massive use of wireless devices, and particularly mobile devices. With mobile phones becoming almost omnipresent, the concerning about possible health effects caused by electromagnetic exposure has been increasing. This concern, sometimes excessive, is also motivated by the lack of reliable information. It s important to notice that all kind of radiation exposure has effects on human, including the solar radiation, and the question should not be about if it has effects or not, but what are the exposure limits that guarantee the users safety. In order to answer this request for guidelines and recommendations, international organizations like the International Commission on Non-Ionizing Radiation Protection (ICNIRP) or the Comité Européen de Normalization Electrotechnic (CENELEC), have worked to establish guidelines and limits for electromagnetic field exposure. In Europe, this effort resulted on the publication of Council recommendation of 12 July 1999 on the limitation of exposure of the general public to electromagnetic fields (0 Hz to 300 GHz) in the Official Journal of the European Communities, which contains basic restrictions and reference levels, in terms of SAR, that should be applied to all radiations emitted by electromagnetic fields. SAR, or Specific Absorption Rate, is the standard criteria to measure the amount of electromagnetic energy absorbed by the body. Beyond the consequences to the human body, the User-Antenna interaction also affects the antenna performance. The user s body, specially hand and head, influence the antenna input reflection coefficient, radiation patterns, efficiency and gain. This study is focused on the use of mobile terminals working in the 900/, and it s mainly divided in two stages. First, the antenna effects in the user are investigated. For better characterization of the human model, several models are studied, including a layered model, and some simplifications to the whole-body model. A study on the way nearby objects, like glasses with metallic frames, affect the SAR measurements is also done. Second, the effects on the antenna performance are analyzed for several scenarios using a complete human model, and a hand and forearm model. These effects are studied and analyzed via electromagnetic simulations using Computer Simulation Technology (CST) Microwave Studio software package. It should be notice that, unless stated otherwise, all results presented were obtained by means of simulation. A. Multiband Antenna II. STUDIED ANTENNAS This is a planar antenna (PIFA) of a mobile phone designed to work in dual-band over the and, which can be found among the CST simulation examples. Its structure is represented in Fig.1. Fig. 1. Multiband Antenna structure. The antenna s free-space input reflection coefficient curve and far field total gain radiation pattern can be seen in Fig.2 and Fig.3.

2 2 reflection coefficient curve and the far field total gain radiation pattern, respectively. Fig. 2. Free-space input reflection coefficient. Fig. 5. Free-space input reflection coefficient. Fig. 3. (a) XY (b) YZ Free-space far field gain radiation pattern for and. As it is shown, it has a good matching at both 900 and, and an almost omnidirectional radiation pattern. The simulated efficiency results are presented in table I. TABLE I FREE-SPACE EFFICIENCY RESULTS Total Efficiency B. Wideband Triangular Monopole Based on [1], this antenna was design to integrate services like public safety radio at the frequency of 700MHz to, GSM900/1800 bands, GPS, WLAN, and Bluetooth in a single element. In its original context it provides a bandwidth from 700MHz to 3000MHz, with -7.36dB of simulated input reflection coefficient. It s structure is illustrated in Fig.4. Fig. 6. (a) XY (b) YZ Free-space far field gain radiation pattern for and. Results show a good matching for 900MH, and a poorer one for. Efficiency computed results are presented in II. TABLE II FREE-SPACE EFFICIENCY RESULTS Total Efficiency C. Ultra-Wideband Monopole This antenna, like the one before, is intended to operate in a vast group of services. However, this covers from 1.76GHz to 8.76GHz with a input reflection coefficient under -7dB. Its structure is shown in Fig.7[2]. Fig. 4. Wideband Triangular Monopole strusture. Figures 5 and 6 represent simulated the free-space input Fig. 7. Ultra-Wideband Monopole strusture. The results for the input reflection coefficient and radiation patterns are provided in Fig.8 and 9.

3 3 B. Exposure Limits The International Committee for Non-ionizing Radiation Protection (ICNIRP) sets guidelines for limiting exposure to electromagnetic radiation. These guidelines are generally accepted around the world. However, some countries have more conservative SAR limits. Table IV gives a comparison between the adopted limits in several regions [3][4]. TABLE IV SAR LIMITS COMPARISON. Fig. 8. Free-space input reflection coefficient. Europe USA Japan Whole body Average SAR [W/kg] Local SAR (head and trunk) [W/kg] Local SAR (limbs) [W/kg] Averaging time [min] Averaging mass [g] Fig. 9. (a) XY (b) YZ Free-space far field gain radiation pattern for e. It is worthy to note that, apart from the numerical difference between the exposure standards, the averaging mass used to define the SAR in these standards have a significant influence on the actual quantity of energy allowed to be absorbed by the tissue. The 1-g SAR gives a more precise representation of energy absorption and a more biologically significant measure of SAR distribution. It should be notice that the antenna shows too much return loss to be considered resonant at. However, SAR measurements will also be performed for this GSM band. Results for free-space efficiency are presented in table III. TABLE III FREE-SPACE EFFICIENCY RESULTS Total Efficiency III. ANTENNA INFLUENCE ON HUMAN A. Specific Absorption Rate - SAR SAR is the standard criteria to measure the amount of electromagnetic energy absorbed in the human body, and is given by the Equation 1. SAR = σ E2 (W/kg) (1) ρ Where ρ is the mass density in [kg/m 3 ], σ is the electrical conductivity in [S/m] and E is the root mean square of electrical field strength in [V/m]. Local SAR values are usually averaged over 1 gram or 10 gram of contiguous tissue. C. Human Model A human body is an electromagnetically complex structure, which consists of various biological tissues (such as skin, bones, internal organs, etc.) modeled by their permittivity and conductivity (or loss tangent). These dielectric properties are frequency dependent, and are available at [5] in a frequency range of 10 Hz GHz. High-resolution human models obtained from magnetic resonance imaging scans can provide accurate results, however such detailed model require high computational time and resources. In this paper, a whole-body homogeneous humanlike model, generated by Poser Software tool, is used. Its representation can be seen in Fig.10. 1) Simplified Model Study: In order to find the best tradeoff between model detail and computational resources needed for numerical methods, a comparative study on the model simplification is made using the models shown in Figures 10, 11a and 11b. For this study the Multiband Antenna has been placed between the model s hand and head, as in a normal use position. SAR simulations are performed and compared for and. The results can be seen in tables V and V I. Results show that, aside from total SAR measure, the simplifications made had very little influence on the SAR

4 4 2) Layered Model Study: As mentioned, the human body is a complex system composed of a diversity of tissues and organs. In the existing human model were introduced a skull and, inside it, a sphere to simulate a brain, in order to evaluate its effect on the SAR measures. The resulting inhomogeneous model is presented in Fig.12. (a) Vertical cut to evidence layers (b) Skeleton inside the model Fig. 10. Whole body homogeneous model. Fig. 12. Layered human model. Also for this study, the Multiband Antenna was considered, and placed between the model s hand and head. The simulated SAR results are shown in tables V II and V III. TABLE VII SAR COMPARISON BETWEEN MODELS, FOR 900MHZ. Homogeneous Layered model model Model mass [kg] Total SAR [W/kg] Max. 1-g SAR [W/kg] Solver time [h]:[m]:[s] 02:46:07 02:52:15 Fig. 11. (a) Half model Human model simplifications. (b) Quarter model TABLE VIII SAR COMPARISON BETWEEN MODELS, FOR 1800MHZ. TABLE V SAR COMPARISON BETWEEN MODELS, FOR 900MHZ. Whole Body Half Quarter model model model Model mass [kg] Total SAR [W/kg] Max. 1-g SAR [W/kg] Solver time [h]:[m]:[s] 02:46:07 01:34:51 00:50:46 TABLE VI SAR COMPARISON BETWEEN MODELS, FOR 1800MHZ. Homogeneous model Layered model Model mass [kg] Total SAR [W/kg] Max. 1-g SAR [W/kg] Solver time [h]:[m]:[s] 04:09:02 04:42:11 Results show a slight increase on the maximum SAR results. However, due to the lack of accuracy of the layered model one cannot conclude about the accuracy of the related results, but only that the presence of the different layers does influence the results and the way the energy is absorbed by the model. Whole Body Half Quarter model model model Model mass [kg] Total SAR [W/kg] Max. 1-g SAR [W/kg] Solver time [h]:[m]:[s] 04:09:02 02:28:19 01:26:25 measures, with a significant reduction in the computation time. This allows to infer that this simplified models can be use without affecting the maximum SAR results, however total SAR should not be considered out of its simulation context. D. SAR Estimation In this section, simulated SAR results are presented for the three introduced antennas in terms of 1-g SAR, 10-g SAR and total SAR. These results correspond to the whole-body absorption, and the individual absorption on the models head. To accomplish this, the homogeneous whole-body model is used, and the antenna is placed between the head and hand, as before (fig.13, 15 and 17). It s important to notice that the simulated SAR results are normalized to 1W peak stimulated power and in continuous wave, while GSM () and

5 5 DCS () have maximum output power limits of 2W and 0.25W respectively, and radiate in a non-continuous way, occupying 1/8 of the emission time. Also, in [6] the simultaneous exposure to fields of different frequencies is considered, and some basic restrictions are indicated. So, for thermal effects, specific absorption rates (between 100kHz and 10GHz) and power densities (between 10GHz and 300GHz) should be added. 2) SAR Estimation with Wideband Triangular Monopole: Figure 16 illustrates the superficial 1-g SAR distribution, obtained with the Wideband Triangular Monopole. 1) SAR Estimation with Multiband Antenna: Figure 14 represents 1-g average SAR distribution on the human body surface, corresponding to the Multiband Antenna. Fig. 15. Monopole. SAR simulation configuration using the Wide-band Triangular Fig. 13. SAR simulation configuration using the Multiband Antenna. (a) (b) Fig g SAR distribution on model surface with the Wide-band Triangular Monopole. As previously, the highest SAR values occur in the hand. Though in this case, there are some areas of the hand that are not affected. Maximum SAR values (for 1-g or 10-g averaging mass), and total SAR values can be seen in table X Fig. 14. (a) (b) 1-g SAR distribution on model surface with the Multiband Antenna. Predictably, the maximum peak SAR occurs in the nearest part of the model to the antenna, the hand, being much more intense in the than in. Peak SAR values for 1 and 10 averaging mass, as well as total SAR, are given in table IX. TABLE IX SAR RESULTS FOR THE MULTIBAND ANTENNA. Whole 1-g SAR [W/kg] Body 10-g SAR [W/kg] Head 1-g SAR [W/kg] g SAR [W/kg] Total SAR [W/kg] TABLE X SAR RESULTS FOR THE WIDE-BAND TRIANGULAR MONOPOLE. Whole 1-g SAR [W/kg] Body 10-g SAR [W/kg] Head 1-g SAR [W/kg] g SAR [W/kg] Total SAR [W/kg] So, considering the use of the GSM service, and the recommendations for simultaneous exposure, these results correspond to: i) In the head: SAR (1g) = 0.212W/kg and SAR (10g) = 0.133W/kg; ii) for the whole body: SAR (1g) = W/kg and SAR (10g) = W/kg; iii) SAR T otal = W/kg. Although results are all below the safety limits, SAR (1g) it s close to the USA limits, since it is located on the users hand. Regarding GSM and DCS mentioned specifications, and [6] recommedations, the resultant SAR values are:i) in the head: SAR (1g) = 0.204W/kg and SAR (10g) = 0.111W/kg; ii) for the whole body: SAR (1g) = 2.098W/kg and SAR (10g) = 0.345W/kg; and at last iii) SAR T otal = W/kg. When compared with the safety limits presented, these SAR values are much below the safety limits for any represented region.

6 6 3) SAR Estimation with Ultra-Wideband Monopole: The superficial 1-g SAR distribution corresponding to the Ultra- Wideband Monopole usage is present in Fig.18. measurement, a pair of eyeglasses were designed to have a metallic pin of 13cm (0.4λ at and 0.8λ at ) placed between the antenna and the model. Its location relative to the head of the human model is shown in Fig.19. Fig. 17. SAR simulation configuration using the Ultra Wide-band Monopole. Fig. 19. SAR simulation configuration for the model with glasses with metallic frame. Simulations were made using the Multiband Antenna, for GSM Bands and, and its results are presented in table XII and XIII. (a) (b) Fig g SAR distribution on model surface with the Ultra Wide-band Monopole. As in both cases shown before, the hand is the most affected by SAR area. However, the main difference between this and the previous cases, is the difference on the obtained results for and, as it reflects the poor matching at. Maximum SAR and total SAR results are shown in table XI. TABLE XI SAR RESULTS FOR THE ULTRA WIDE-BAND MONOPOLE. Whole 1-g SAR [W/kg] Body 10-g SAR [W/kg] Head 1-g SAR [W/kg] g SAR [W/kg] Total SAR [W/kg] TABLE XII SAR RESULTS CONSIDERING MODEL WITH METALLIC GLASSES, FOR 900MHZ. Without Glasses With Glasses Whole 1-g SAR [W/kg] Body 10-g SAR [W/kg] Head 1-g SAR [W/kg] g SAR [W/kg] Total SAR [W/kg] TABLE XIII SAR RESULTS CONSIDERING MODEL WITH METALLIC GLASSES, FOR 1800MHZ. Without Glasses With Glasses Whole 1-g SAR [W/kg] Body 10-g SAR [W/kg] Head 1-g SAR [W/kg] g SAR [W/kg] Total SAR [W/kg] For the GSM service, and multiple exposure, the results are: i) for the head: SAR (1g) = W/kg e SAR (10g) = W/kg; ii) for the whole body: SAR (1g) = W/kg e SAR (10g) = 0.342W/kg; and iii) SAR T otal = W/kg. These results are much below the safety limits for any indicated region. It s interesting to note that the majority of the energy absorption is located on the hand of the user, and not on the head. Even though, the head is considered to be the most critical area, as it contains human vital organs. E. Wearable Objects The energy absorption study is very complex, and its results may vary with each simulation context. In the attempt of studying the effects of metallic nearby objects in SAR For both frequencies the introduction of metallic objects between the head and the antenna slightly increases the 1-g and 10-g SAR values registered on the model s head. However, despite this local SAR increasing, results show lower values for the Whole-body maximum SAR, and Total SAR. IV. HUMAN INFLUENCE ON ANTENNA PERFORMANCE For better understanding the human influence on the antenna performance, each antenna was simulated and compared in three different scenarios: i) antenna in free space, already presented in section II; ii) antenna in the presence of the user, placed between the user s head and hand; iii) antenna in the presence of user s hand and forearm (without user s head and body). These last two simulation setups can be seen in Figures 20, 24 and 28, for the three studied antennas.

7 7 A. Human Effect on Multiband Antenna The comparison of the antenna input reflection coefficient for the different setups is shown in Fig.21. Fig. 23. far field gain radiation pattern comparison for. Fig. 20. (a) Whole-body (b) Hand and forearm Configuration for the human influence simulation. TABLE XIV RADIATION EFFICIENCY COMPARISON FOR THE Antenna in Antenna with Antenna with free-space hand and forearm whole-body model Results show a strong decay on the radiation efficiency with the user s presence, being the hand responsible for almost 80% of the power absorption. In important to note that, the radiation efficiency is defined as the ratio between the Radiated Power and the Accepted Power, and so does not take into account the antenna input mismatch loss. Fig. 21. S11 curve comparison for the Multiband Antenna. As one can see, the user s presence has major consequences on the input reflection coefficient, for the multiband antenna. In fact, the antenna loses its lower frequency resonance at, and at the return loss increases depending upon the user representation. The radiation pattern comparison for and are represented in Figures 22 and 23 respectively. B. Human Effect on Wideband Triangular Monopole Figure 25 shows the S1, 1 curves for the three studied configurations, with the Wideband Triangular Monopole. Fig. 24. (a) Whole-body (b) Hand and forearm Configuration for the human influence simulation. Fig. 22. far field gain radiation pattern comparison for. Total gain radiation pattern results confirm that the radiation characteristic are also affected due to the human presence, with either the complete human model or the hand and forearm models. However, in the complete model, where the user s head is present, a strong blocking effect can be observed caused by the closure of the antenna in between the hand and head. Table XIV presents the radiation efficiency for the Multiband Antenna in the presence of the complete human model and in the presence of hand and forearm. Fig. 25. S11 curve comparison for the Wideband Triangular Monopole.

8 8 Results show that the global antenna matching is not as affected as in the Multiband Antenna. Despite the resonance shifting to lower frequencies, the S1, 1 parameter remains below -5dB in all bandwidth range. Figures 26 and 27 give a radiation pattern results comparison. Fig. 28. (a) Whole-body (b) Hand and forearm Configuration for the human influence simulation. Fig. 26. far field gain radiation pattern comparison for. Fig. 29. S11 curve comparison for the Ultra-Wideband Monopole. Fig. 27. far field gain radiation pattern comparison for. the S1, 1 parameter to the human presence, ie, the wider the resonant band the less affected by the human presence is the antenna. Radiation patterns are presented in Figures 30 and 31. As before, it s evident the distortion caused by the user s hand and head. Radiation efficiency results are compared in table XV. TABLE XV RADIATION EFFICIENCY COMPARISON FOR THE WIDEBAND TRIANGULAR MONOPOLE. Antenna in Antenna with Antenna with free-space hand and forearm Fig. 30. far field gain radiation pattern comparison for. These results support those obtained for the Multiband Antenna, where can be seen the successive decay of the efficiency for both frequencies. C. Human Effect on Ultra-Wideband Monopole The input reflection coefficient results for the three scenarios considered for simulation are present in Fig.29. Fig. 31. far field gain radiation pattern comparison for. These results show little influence of the human presence n the antenna input matching, specially when compared with those obtained for the Multiband Antenna or the Wideband Triangular Antenna. Given this, it is possible to establish a connection between the bandwidth and the vulnerability of Concerning the radiation pattern, this antenna has a similar behavior to the previous ones, and here the distortion caused by the human presence is also remarkable. Radiation efficiency results are shown in table XV I.

9 9 TABLE XVI RADIATION EFFICIENCY COMPARISON FOR THE Antenna in Antenna with Antenna with free-space hand and forearm whole-body model As before, the efficiency is also strongly affected by the user s presence, and decreases for less than 3% when the whole-body model is considered. Fig. 34. Comparison between simulated and measured S1, 1 results, for the antenna in right hand far from the head. V. EXPERIMENTAL TESTS A. S1, 1 Measurements S1, 1 measurements were performed using the Ultra- Wideband Monopole in several different scenarios with the help of two real users, which will be referred as user A and user B. The measurement scenarios are the following: i) free-space; ii/iii) user A/B with the antenna near the head and hand; iv/v)user A/B with the antenna in hand away from the head. This setups were applied for both left and right sides of the user and an example of each can be seen in Figures 32 and 33. The experimental results and its comparison with the simulated, are presented in Figures 34, 35, 36 and 37. Fig. 35. Comparison between simulated and measured S1, 1 results, for the antenna in left hand far from the head. (a) simulation setup (b) measurement setup (User A) Fig. 32. Example of measurement and simulation setup for the antenna in the (right) hand far from the head. Fig. 36. Comparison between simulated and measured S1, 1 results, for the antenna in left hand near the head. (a) simulation setup (b) measurement setup (User B) Fig. 33. Example of measurement and simulation setup for the antenna in the (right) hand near the head. Results show that, for any of the simulated settings, there are significant deviations between simulated and tested results. These discrepancies are easily justified by the disparity of the models used in simulating and measuring environments, and by the difficulty in replicating the setup used for simulation. However, these deviations between simulated and tested (especially between user A and B) highlight the measurements sensitivity to the context in which they are made, and that in fact it s important to define rules and procedures to unify results.

10 10 Fig. 37. Comparison between simulated and measured S1, 1 results, for the antenna in right hand near the head. VI. CONCLUSION In this paper a study on the mutual influence between handset antennas and the user has been presented for the use of GSM service, ie, this study was performed considering the use of the and GSM bands. In the human model study, it was possible to conclude that the simplification of the model is a good approximation when available resources are limited. However, since there is a significant reduction in the model, the total SAR result becomes meaningless, and it should not be considered out of its simulations context. The inhomogeneous model, which consider various human tissues, has no significant impact on global measures of SAR, and, since it requires a lot of computational resources, may not be a viable option. Even though, it may be interesting for studying the effects of absorption in specific human tissues. The SAR analysis led to relate some parameters of the antennas, such as the input matching, with the SAR distributions and maximum values. It was also possible to compare the results averaged over 1g and 10g of tissue, and confirm the 1g averaged results are a more precise representation of localized SAR. Finally, within the study of the effects on the user, the presence of glasses on the model revealed some changes in the results of SAR, when compared with the model without glasses, which suggest that the presence of metal objects, the size in the order of the wavelength, between the antenna and the user, affect the SAR distribution in the human model. Afterwards, a study on the effects on the antenna caused by the user s presence has been presented, where some of the antenna s parameters where evaluated in the presence of the human model. The human model were represented either by a complete human model, either by a hand and forearm model. It was possible to see changes in all examined parameters. Regarding the input reflection coefficient, or S1, 1 parameter, in the Multiband Antenna case, it has lost it s lower resonate frequency at, compromising the antenna operation. However, this parameters proved to be much more robust to human presence for wideband antennas. For radiation pattern and efficiency, the results where similar for all studied antennas. Radiation pattern distortion caused by the presence of the entire model, or just the hand and forearm, is evident. However, it has been seen that when the whole-body model is considered, the radiation pattern suffers a strong blocking effect caused by the closure of the antenna in between the hand and head. Which means that representing the user only by it s hand and forearm leads to an underestimation of the effects of the user and therefore the results unrealistic. Radiation efficiencies are strongly affected by the user s presence, and it s evident a successive decline as more elements are in close proximity to the antenna. Finally, the experimental results were presented, and there were significant deviations between simulated and tested results, which were easily justified by the disparity of the models used in simulating and measuring environments, and by the difficulty in replicating the setup used for simulation. ACKNOWLEDGMENTS Rita Santos would like to thank Professor António Alves Moreira for his guidance and support during this work; to Mr. António Almeida for the help and patience in measurements in the laboratory and to Dr. Jerzy Guterman for the generosity and availability demonstrated in the clarification of CST software handling doubts. REFERENCES [1] B. Koh, S. Ooi, and K. Tan, Wideband antenna for portable radio, Proceedings of iwat, Keyaki-kaikan, Nishi-Chiba Campus, Chiba University, Japan. [2] N. G. do Serro, Projecto de antena compacta de banda muito larga para terminais móveis, Master s thesis, Instituto Superior Técnico, Novembro [3] Telecommunications Technology Council, Measurement of SAR from Mobile Phone Terminals and Other Terminals that are Intended for Use in Close Proximity to the Side of the Head, Deliberation No.118. [4] 3GPP, Specific absorption rate (sar) requirements and regulations in different regions, tech. rep., 3rd Generation Partnership Project, G TR V3.0.0 ( ) Technical Report. [5] An internet resource for the calculation of the dielectric properties of body tissues in the frequency range 10 hz ghz. Italian National Research Council, Institute for Applied Physics, Nello Carrara - Florence (Italy). Visitado em Julho [6] Council Of The European Union, Council Recommendation of 12 July 1999 on the limitation of Exposure of the general public to electromagnetic fields (0 Hz To 300 Ghz), Julho 1999.