EFFECTS OF HAND ON EM ABSORPTION AND ANTENNA PERFORMANCES FOR INTERNAL HANDSET PIFA

Preliminary communication The aim of this investigation is to analyse the effects of hand on electromagnetic (EM) absorption and antenna performances. The EM absorption is evaluated by using the specific absorption rate (SAR) in the human head and total absorbed power by the user in this paper. The antenna performance’s parameters comprising radiation efficiency, total efficiency, gain and bandwidth are considered in this investigation. The analysis was performed using mobile phone with a human head and hand model in both cheek and tilt positions. The main part of the investigation is based on the finite-difference timedomain (FDTD) method. The results show that the SAR values are decreased and total absorbed power by user increased rapidly due to insertion of hand model. The user’s hand also leads to degrade antenna’s communication performance considerably. Moreover, the presented analysis provides some useful indication to design handset antenna considering hand effects.


Introduction
The use of portable terminal devices like cellular phone is being widespread gradually and hence the number of cellular phone user is increasing dramatically.The antennas of the cellular devices receive and radiate electromagnetic (EM) energy of different frequency depending on the type of applications.The emitted EM energy from portable terminal device's antenna may lead to serious health risks [1,2] since the EM energy can penetrate human body tissues and a part of this energy is absorbed in it [3,4].The power absorption by human body tissue is usually evaluated in terms of specific absorption rate (SAR) [5].The safety limits of the SAR values are imposed by the American National Standards Institute (ANSI) and International Commission on Non-Ionizing Radiation protection (ICNIRP) [6].Inclusion of the hand model affects SAR values as well as antenna performances [7÷10].The hand effects on antenna performances highly depend on the hand grip and palmhandset gap [11,12].The user's hand causes shifting the resonance frequency [13], reducing the channel's data rate [14] and detuning the antenna input impedance [7,13].This also contributes to degrade the radiation efficiency and increase absorption loss [13].In [14], an investigation of effects of hand phantom on planar inverted-F antenna (PIFA) and helical antenna was presented.The results indicated that hand phantom leads to affect antenna performances depending on the position of hand with respect to mobile phone antenna.In [15], an analysis of hand effects on the resonance characteristics of dual band PIFA was presented.The results showed that the hand phantom brought slight impedance matching deterioration for both GSM frequency bands.Additionally, presence of the user's hand leads to less conservative SAR values to the human head [16].The human hand leads to reduce the SAR values to the head tissues, as it dispels a part of antenna radiated energy [17,18].The effects of hand holding position on the EM interaction between mobile phone and human was investigated in [19].The obtained results indicated that the SAR values remarkably change in consequence of the positioning of the mobile phone against the human head at different hand levels.
The objective of this investigation is to find out the impacts of the human hand on PIFA EM absorption characteristics and performances for two distinctive GSM frequency bands.Two parameters -SAR and total absorbed power are used to explain the EM absorption characteristic.The commonly used cheek and tilt position of talk mode are used to simulate the handset antenna with the user's head and hand phantoms.Moreover, the effects on the antenna performance parameters comprising radiation efficiency, bandwidth, and gain were investigated to evaluate the effects of hand phantom on antenna performances.

Models and methods
A dual band GSM (0.9 GHz and 1.8 GHz) planar inverted-F antenna (PIFA) [20] was used as handset antenna.The antenna consists of patch, ground, feed and shorting point.The antenna operated over dual GSM band has been selected to evaluate SAR in favour of talk mode of real use.A complete handset model consists of circuit board, display, battery, and plastic mobile casing was utilized.Tab. 1 shows the properties of mobile phone Technical Gazette 24, Suppl.2(2017), 459-467 components.In this investigation, three-dimensional structural models were considered for head and hand phantom.The head phantom contains of several types of tissues: bone, brain, skin, muscle, fat, and eye (whole brain atlas website).The electrical properties of head and hand model are listed in Tab. 2.  The head and hand dielectric properties were set in accordance with the frequency of exposure.The head and hand dielectric properties were set in accordance with the frequency of exposure.The values relative permittivity and conductivity of dielectric material are chosen based on human tissue measurement data as described in [21].To precisely illustrate the performance over broad frequency range dispersive models for all the dielectric materials were used for simulation [22].The numerical simulation of SAR values and other antenna performance parameters were assessed espousing the Finite Integration Technique (FIT) method based on the Computer Simulation Technology (CST) Microwave Studio.FIT connected to Cartesian matrices in the time domain is computationally proportionate to the standard Finite Difference Time Domain (FDTD) strategy.FIT joined with Perfect Boundary Approximation (PBA) can keep up the helpful organized Cartesian lattices and allow an exact displaying of bended structures [23].Fig. 1 demonstrates the simulation setup showing the relative positions of the portable handset, human head and hand.In lossy-Drude simulation model, 128 × 128 × 128 cells domain was used with cell size Δx = Δy = Δz = 0.5 mm.Here, the Drude model is more competent for the FDTD simulation for both the permeability and permittivity functions.This approach gives a substantially more extensive data transmission [25].For the calculation of SAR values, the following standard formula was used: Here E expresses induced electric field strength (V/m), ρ and σ present density (kg/m 3 ) and conductivity (S/m) of the tissue respectively.SAR values averaged over 1 g and 10 g of body tissues were evaluated espousing IEEE standard algorithm using the stimulated power 0.5 W (rms).

Effects of hand on SAR
Two different talking position, cheek and tilt were studied in this investigation.The SAR and total absorbed power in the head was evaluated in all cases so that the effect of hand could be separated clearly.

Cheek position analysis
In cheek position analysis, three different distances (16 mm, 18 mm and 20 mm) of cellular phone from head axis were taken both for without the hand and with hand configurations.The SAR values along with its 3D surface distribution are shown in Tab. 3 for different cheek positions.Moreover, Fig. 2 indicates the total absorbed power values by the user.In a cheek position of the cell phone with 16 mm apart from the user's head axis, the SAR reduction occurs 27.2 % for 1 g averaging (26.7 % for 10 g of tissue) at lower GSM frequency band and 2.7 % for 1 g (2.1 % for 10 g of tissue) at the upper GSM frequency band.It is observed that the SAR values at 1.8 GHz are less affected than those at 0.9 GHz.Moreover, cheek position of 18 mm contributes to reduce the SAR 27.6 % for 1 g, averaging (27.7 % for 10 g of tissue) at 0.9 GHz and 6.21 % for 1 g (5.7 % for 10 g) at 1.8 GHz.In addition, cheek position of 20 mm contributes to reduce the SAR 29.8 % for 1 g, averaging (29.7 % for 10 g of tissue) at 0.9 GHz and 0.94 % for 1 g (5.97 % for 10 g) at 1.8 GHz.Increase in phone distance from the user's head reduces the SAR values in every case.It is also important to mention that the percentage of SAR reduction due to user's hand is not affected greatly for lower frequency band for three different configurations.But the SAR reduction due to user's hand increases with increasing phone distance at 1.8 GHz.The inclusion of hand model prompts to enlarge total absorbed power in all cases.In case of cheek position with 16 mm phone distance, power absorbed by the user was 32.2.% for with hand configuration and 14.6 % for without hand configuration at 0.9 GHz.On the other hand, the power absorbed by the user 66.6 % and 26.6 % for with and without hand configuration, respectively at 1.8 GHz considering 16 mm cell phone distance.The results from 18 mm and 20 mm cell phone distance also provide similar tendency as compared with the results from 16 mm distance configuration.The hand phantom causes more than two times increment of total absorbed power compared to without hand configurations in all cases.

Tilt position analysis
In this section, 15 degree and 30 degree tilt position of phone were considered for both with hand and without hand configuration.Three distance configurations (16 mm, 18 mm and 20 mm) were also used in tilt position analysis.The results of the SAR values have been listed in Tabs.4, 5 and 6.In case of different tilt position, the SAR values decline due to the presence of the user's hand at 0.9 GHz, but the SAR values rise at 1.8 GHz.For 15 degree tilt position of 16 mm distance, 27.2 % for 1 g (27 % for 10 g) SAR reduction can be found at 0.9 GHz.Instead, the SAR rises 5.14 % for 1 g (2.15 % for 10 g) at 1.8 GHz.In addition, 30 degree tilt position of 16 mm distance configuration contributes to decrease the SAR 26.5 % for 1 g (31.8 % for 10 g) at 0.9 GHz and increase the SAR 5.8 % for 1 g (4.4 % for 10 g) at 1.8 GHz.
Sequentially, 28.2 % reduction and 3 % (for 1 g) increment in the SAR are marked at 0.9 GHz and 1.8 GHz respectively for 15 degree tilt position of 18 mm configuration.30 degree tilt position of 18 mm distance configuration contributes to decrease the SAR 22.4 % for 1 g at 0.9 GHz and increase the SAR 5.8 % for 1 g at 1.8 GHz.The results from tilt positions of 20 mm distance configuration show identical tendency as two other configurations.The results indicate that the SAR values decrease significantly with increasing tilt angle.Moreover, 8.5 % (1 g) reduction in the SAR is obtained from 16 mm to 18 mm distance and also 18 mm to 20 mm distance configuration in case of 15 degree tilt position at 0.9 GHz.The SAR decreases 14.4 % (1 g) from 16 mm to 18 mm and 11.2 % (1 g) from 18 mm to 20 mm in case of 15 degree tilt position at 1.8 GHz respectively.Figs. 3, 4, and 5 indicate the total absorbed power by the user for 16 mm, 18 mm, and 30 mm distance configurations, respectively.Tilt position analysis shows that the hand phantom causes more than two times increment of total absorbed power compared to without hand configurations.For 15 degree tilt position of 16 mm, the total power absorbed by user increases 17.6 % at 0.9 GHz and 38.8 % at 1.8 GHz.In addition, 30 degree tilt position with hand of 16 mm contributes to increase the total absorbed power 17.4 % at the lower frequency band and 36.6 % at the upper frequency band.The 18 mm and 20 mm distance configuration also show same tendency as 16 mm distance configuration.

Effects of hand on Antenna Performances
The effects of hand on antenna performances were evaluated comprising different performance parameters: radiation efficiency, total efficiency, bandwidth, and gain.Also, the results related to the antenna performance have been presented in two categories in this paper.

Cheek position analysis
The performance parameters of mobile phone were calculated for three different cheek positions (16 mm, 18 mm and 20 mm).The radiation efficiency, total efficiency, gain and bandwidth decrease due to the insertion of hand model for all configurations of cheek position.Fig. 6 and Fig. 7 represent the values of radiation efficiency and total efficiency respectively for cheek positions.At 0.9 GHz, the radiation efficiency decreases 44.4 % for 16 mm phone distance, 47.3 % for 18 mm phone distance and 47.5 % for 20 mm phone distance.Additionally, the degradation of radiation efficiencies is 40.1 %, 40.4 % and 40.9 % for 16 mm, 18 mm and 20 mm cell phone distance respectively at 1.8 GHz.Moreover, the antenna gain degrades 2.83 dB, 2.82 dB, 2.78 for 16 mm, 18 mm and 20 mm phone distance configuration as shown in Table 7 at the lower frequency band respectively.Sequentially, 5.1 dB, 4.98 dB, 4.97 dB antenna gain degradation occur for 16 mm, 18 mm and 20 mm phone distance at the upper frequency band respectively.The frequency bandwidth of the antenna for 16 mm distance is reduced 3.6 MB at 0.9 GHz and 13 MB at 1.8 GHz after using the user's hand.For other distance configurations, the bandwidth is also reduced by few MB.The bandwidth reduction is more significant at the upper frequency band as compared with the lower frequency band.

Tilt position analysis
The results from tilt position analysis indicate that the radiation efficiency, total efficiency, gain and bandwidth degrade owing to insertion of the hand phantom as the cheek position analysis.The radiation efficiency and total efficiency are reduced 41.5 % and 8.7 % respectively for 15 degree tilt configuration at 0.9 GHz in case of 16 mm phone distance (as shown in Figs. 8 and 9).Sequentially, 40.8 % and 35.4 % degradation can be found for the radiation efficiency and total efficiency respectively at 1.8 GHz for 15 degree tilt position of 16 mm distance configuration.The tilt position analysis with 18 mm (as indicated in Figs. 10 and 11) and 20 mm (as shown in Figs. 12 and 13) cell phone distance provide identical tendency as 16 mm distance configuration.Additionally, the antenna gain degrades 1.6 dB at 0.9 GHz and 3.95 dB at 1.8 GHz for 15 degree tilt position of 16 mm as indicated in Tab. 8.The bandwidth of antenna falls 2.28 MB at the lower frequency band and 13.6 MB at the upper frequency band for 15 degree tilt position of 16 mm configuration.Likewise, the antenna bandwidth degrades 16.7 MB at 0.9 GHz and 13.6 MB at 1.8 GHz considering 30 degree tilt position of 16 mm distance.The radiation efficiency, total efficiency and gain improve little bit due to increase in cell phone distance.The results of gain and bandwidth from tilt position analysis with 18 mm (as indicated in Tab. 9) and 20 mm (as shown in Tab.10) cell phone distance provide identical tendency as 16 mm distance configuration.With the variation of tilt angle, antenna performance's parameters show little variation.

Conclusion
In this study, the effects of the human hand on the SAR values towards human head have been presented using mobile handset of the dual band PIFA antenna.The effects of hand phantom on the antenna performances have been investigated also.The cheek position results show that the user's hand causes the SAR reduction greatly at the lower frequency band and slightly at the upper frequency band.But the results from the tilt position analysis indicate that the SAR values decrease considerably at 0.9 GHz and increase slightly at 1.8 GHz.The maximum SAR reduction was 27.2 % due to user's hand in cheek and tilt position.Moreover, the hand phantom contributed to increase the total power absorption more than two times in both cheek and tilt position talk mode.
The hand phantom degrades the antenna performances greatly.The degradation in radiation efficiency is more than 40% both for cheek and tilt positions.In addition, gain degradation has occurred where the gain at the upper frequency band decreases extremely in both cheek and tilt position talk mode.To design a handset antenna, hand phantom effect should be taken into consideration.Further study may be extended to design a mobile phone antenna with minimal hand effects by using adaptive matching of the antenna input impedance.This can cancel the additional reflection loss caused by the detuning of the input impedance.

Figure 1
Simulation setup (a) front view (b) side view 3 Results and discussions 3.

Figure 3 Figure 4 Figure 5
Figure 3 Total absorbed power by head for different tilt position (16 mm) Figure 4 Total absorbed power by head for different tilt position (18 mm)

Figure 6
Figure 6 Radiation efficiency of mobile phone for cheek positions

Figure 7
Figure 7 Total efficiency of mobile phone for cheek positions.

Figure 8 Figure 9
Figure 8 Radiation efficiency of mobile phone for tilt positions (16 mm)

Figure 10 Figure 11
Figure 10 Radiation efficiency of mobile phone for tilt positions (18 mm)

Figure 12 Figure 13
Figure 12 Radiation efficiency of mobile phone for tilt positions (20 mm)

Table 1
Electrical properties of the handset

Table 2
Dielectric properties of the head and hand tissues

Table 3
SAR values to the human head for different cheek positions Figure 2 Total absorbed power by head for different cheek position

Table 4
SAR values to the human head for different tilt positions(16 mm)

Table 5
SAR values to the Human Head for Different Tilt Positions (18 mm).

Table 6
SAR values to the human head for different tilt positions(20 mm)

Table 7
Gain and Bandwidth of mobile phone for cheek positions

Table 8
Gain and Bandwidth of mobile phone for tilt positions(16 mm)

Table 9
Gain and Bandwidth of mobile phone for tilt positions(18 mm)

Table 10
Gain and Bandwidth of mobile phone for tilt positions(20  mm)