手机SAR相关内容

SAR Calculation of a dual band helical antenna mounted in a mobile handsetusing a SAM homogeneous phantomIntroductionThe objective of this application note is to show how Microstripes Version 6.0 is used to evaluate the performance of a generic dual band helix antenna designed to operate in the GSM band when mounted inside a typical handset enclosure. The key parameters of interest are1.Influence of the handset enclosure on the antenna performance2.Influence of the presence of the head phantom on the antennaradiation pattern performance3.Evaluation of the 1g and 10 g SAR performance of the handset whenoperating in the GSM band. (Upcoming functionality Part of Microstripes 6.1 Release)The SAR modelling is performed in such a way as to recreate the measurements used during SAR qualifications of mobile handsets.The standard procedures are as described in CENELEC EN50360, IEC 62 209 and IEEE SCC34,SC2 WG1, document Std P1528-200 X " Recommended practice for determining the spatial-peak specific absorption rate (SAR) in the human body due to wireless communications devices: Experimental techniques".PART 1. Evaluation of the antenna performance when mounted inside a handset enclosureIt is often the case these days that a complete solid model of a piece of hardware exists within the manufacturer's organisation owing to the ubiquitous use of CAD tools. The different departments will use these models as digital prototypes to perform simulations in order to check and confirm the performance of the finished product against the design specification.In our example, the different constituent parts of a typical mobile handset such as the phone enclosure, the PCB and the antenna were available and were easily imported into the geometry modeller BUILD software tool as 3 D solid models. The BUILD module supports the import and export of the following native CAD file formats SAT, IGES, STEP, STL and DXF. Figure 1, shows a typical solid model of a handset enclosure imported into BUILD from several IGES files representing each component part.Figure 1: A typical handset model as imported into Build(Note that this is not the handset antenna being analyzed in thisdocument)Since these models have a multi-purpose use and are primarily a true representation of the final product. It is often judicious to exploit the features of each analysis tool and construct a model that will give you a faithful engineering answer in the shortest amount of time.In this study we are concerned with a handset equipped with a dual band helical antenna and the following model simplification are used to fully exploit some of the most powerful features of Micro-Stripes.e of the wire compact model to represent the helical antenna2.Representation of the enclosure by a "sheeted " box combined withthe use the thin film compact modelThe antenna itself was constructed using the "Laws" function in BUILD which allows an analytic formula to represent the helical geometry of the antenna; Two pitches are used to represent the dual band nature of the antenna which is designed to operate in the 900 and 1800 MHZ frequency band.Figure 2: Mounting configuration of the dual band helical antennaon the phone PCB.Inset a zoomed view of the antenna and its representation as acompact wire.From Figure 1, it is obvious that to recreate a detailed discretized model of the enclosure a fine mesh will be necessary to give a faithful spatial representation. The enclosure is simplified and represented by a box, which has been operated upon with the "Sheet" tool. The "Sheet" operation preserves the sides of the box and allocates to them a value of zero thickness. During the material assignment process, the enclosure is given the properties of a "thin film", whose dielectric permittivity corresponds to ABS plastic (DK=3.0) in this case and a physical thickness of 2 mm. This simplified model of the enclosure is sufficient to preserve the influence of the enclosure on the antenna and still yield an efficient EM model capable of delivering very good EM results.Figure 3: Simplified enclosure model for the handsetThe influence of the enclosure on the antenna return loss performance is shown in Figure 4.Figure 4: Return loss performance of the dual band helicalantenna(Black: Free Space), (Red : Embedded within the handset enclosure)The presence of the enclosure has shifted the frequency down for both the low and high band. The antenna used in this example was tuned to give its optimum performance when mounted within the enclosure.It is also worth noting that the presence of the head and the hand will also have an influence on the return loss performance of the antenna and will cause the resonant frequency to shift.PART 2. Influence of the presence of the head phantom on the antenna radiation pattern performanceTo evaluate the influence of the presence of a human head on the antenna performance, the antenna model enclosed in the handset was brought close to a Specific Anthropomorphic homogeneous phantom (SAM) at location (x=0,y=0 and z=0). This reference point is chosen to indicate the location of the user's ear; the head was also tilted backwards by 300 to ensure that the line joining the ear to the mouth is coincident with the X-axis.Figure 5: Handset positioned in its operational attitude withrespect to the head phantomIn this analysis, the head was assigned the dielectric properties of the "Brain (White Matter)" extracted from the "tissues.sat" database, which is delivered as part of the software. The file "tissues.sat" contains a 4 poles Debye representation of most human tissues which describes the variation of the dielectric properties of tissue material with frequency. Figure 6 shows the standard 4 pole Debye representation for Brain (White matter)Figure 6: 4 Pole Debye representation of the "Brain (WhiteMatter)"Tissue as extracted from "Tissues.SAT" database fileTo check the permittivity values used in the model are as expected, it is advisable to plot the ".prop" file, which is generated by the solver at run time and holds the variation of the electrical properties of the dielectric materials used in the model with frequency. Figure 7, shows the variation of permittivity with frequency for Brain tissue.Figure 7: Permittivity versus frequency plot for Brain tissue asextracted from .prop fileFrom the plot, a value for the relative permittivity of 44 can be read for a frequency of 1.8 GHz as expected and corresponds to the value of permittivity that a liquid simulant used during SAR measurements would have.Figure 8Figure 8, Shows the influence of the presence of the head on the radiation pattern performance of the antenna at 900 MHz (Cut at 00 from theE-plane). (Red: No head present, Black: With head)1g and 10 g SAR evaluation (Microstripes V6.1 Beta)The head phantom was finely meshed to give Voxels of 8 mm3, corresponding to a sampling mesh of 2mm along each Cartesian axis. The quantitative SAR output can be accessed through a drop down menu form within "Field plotter". The point SAR, 1 g and 10g values can be chosen and by setting the scale to linear scale the peak value can be read off directly from this dialogue in the data range boxes. This value is also displayed on the color bar at the bottom of the SAR intensity plot.Figure 9, shows a sample dialogue window and where the Peak SAR value is displayed.Figure 9: SAR dialogue selection menu within Field plotter Figure 10: shows the 10 g SAR distribution at 1.8 GHz for the dual band antenna.Figure 10: 10 g SAR distribution, a spatial peak value of 0.0031W/Kgis read from the color scale.Figure 11: 2 dimensional 1g SAR distribution selected in Fieldplotter,The peak value of 0.0049 W/Kg can be read off the colour scalebarIt is important to note that in order to calibrate the SAR values, the delivered power to the antenna should be known and quantified. This is the value of power delivered at the output of the antenna and includes for the reflected power due to the antenna mismatch. The delivered power into the system is accessed through the (.io) file and is tagged as "Power IN". A plot of this quantity is shown in Figure 12, the values of power IN at 900 and 1800 MHz can easily be read off this graph.Figure 12: A plot of power in versus frequencyTable 1: Summarizes the SAR performance of this antenna handset combination at 900 MHz and 1800 MHz as follows.It is now important to calibrate these values of SAR to those that would be measured during tests. A typical handset would have a delivered power of 23 dBm at 900 MHz and 21 dBm at 1800 MHz to meet its operational link budget. Assuming these values for delivered power Table 1, can bere-written to correspond to the values that would be measured during SAR compliance tests and is shown belowWe can see that the simulations show that this phone will be compliant to the SAR specification limits. However the simulated values for SAR are too close to the limits and a better margin would be preferable to ensure compliance during tests.。