Simulation of Mobile Phone Antenna Performance
Tough technical requirements are being put on handsets. Mobilephones have to deal with an ever increasing number of services, whileat the same time the cost of the systems is being reduced. R&D inthe mobile phone industrycopes with this situation by continuously improving the mobile phoneefficiency in order to be able to comply with the grade of service inthe mobile network. Thus, we are moving towards mobile designs, whichare not only becoming thinner, smaller and more complex with everygeneration but also have to perform with the same or even betterperformance, and at more frequency bands.
In addition to maximizing the antenna-accepted power of thehandsets, the effects on the antenna performance from surroundingobjects such as the human body have to be studied and considered.Homogeneous models are used when measuring the effects on antennaperformance and represent a conservative estimation of the antennalosses and dissipated power. The performance of the antenna and theentire system may be quantified using sets of technical requirementsfor both passive and active modes.
In passive mode the antenna performance is often measured by theantenna efficiency, which is subdivided into the radiation efficiencyand the return-loss efficiency. In active mode the entire systemefficiency is defined by the Total Radiated Power (TRP) on transmitting(Tx), and the Total Isotropic Sensitivity (TIS) on receiving (Rx). Theactive performance of the handset is often measured using exact andtime consuming procedures. These have to be conducted several timesduring the development phase of the device. Furthermore, the producthas to be developed to a certain stage before any measurement orreliable prediction of antenna performance is possible.
Figure 1: Analysis phases of the mobile phone study.
The results of simulation for the several system complexity levelsand its comparison to measurements are compared for the recentlyreleased Sony Ericsson M600 mobile phone. All measurements were conducted at Sony Ericsson’stest facilities. In Figure 1 the models are shown, from left to rightwith the least complex structures such as the simple antenna design andPCB, to the complete phone structure containing several hundredcomponents, and finally the entire system using the head of theStandard Anthropomorphic Model (SAM), a homogeneous hand model and the full phone.
SIMULATION AT ANTENNA LEVEL
At the antenna level the design and optimization of the antennaitself is the prime goal. Results of interest are typically the returnloss, radiation efficiency and radiation pattern of the antenna at various frequencies. Even though the transient solver of CST MICROWAVE STUDIO® (CST MWS)is used for the simulations, frequency quantities like near and farfields can be evaluated easily at numerous frequencies during onetransient run due to field monitors based on discrete Fouriertransforms (DFT). Realistic loss values were chosen for both themetallic and dielectric objects.
A convergence study indicates that the model converges to anaccurate solution. Starting with a relatively coarse mesh of 221.000mesh cells, and refining it using an energy based criterion, the finalsolution is achieved in only three steps. The criterion for stoppingthe study is that the maximum difference of an S-parameter between tworuns is less then 0.02 over the complete band (0 – 3 GHz).Additionally, the convergence of the radiation efficiency at the twobands was evaluated.
The total convergence took 38 minutes. For further simulations andstructure optimizations, the third run can be skipped, as the meshsetup in the second run with around 383,000 mesh cells, delivers wellconverged results. This means that all further simulations in theoptimization process will have a run time of only 12 minutes.
Figure 2: The converged mesh for the antenna simulation, just phone (left) and including SAM phantom (right).
The converged mesh is shown in Figure 2. The bent planar parts ofthe PIFA antenna, the antenna carrier and the PCB can be seen clearly.As the bendings are not aligned with the Cartesian mesh, this wouldcause significant problems in simple staircase methods, however, due tothe thin sheet technique, mesh cells can be intersected by metallicsheets. Together with the PBA technique, the shown grid, although itmight look relatively coarse, delivers fully converged results.
Figure 3: Simulationversus measurements at antenna level: the return-loss (left) and theantenna efficiency (right) are compared.
In Figure 3 the converged simulation results of the antenna arecompared with measurements. The results are in agreement for both thereturn loss (left) and the radiation efficiency (right). Finally, inFigure 4, the radiation pattern is shown for two GSM frequencies. Sincethe plastic housing is not considered for this study, the resonancesare slightly shifted to ~1 GHz and ~2 GHz.
Figure 4: The radiation pattern of antenna for two GSM bands.
SIMULATION AT PHONE LEVEL
After the antenna design is complete, the next step is to includeit in the complete phone. This enables the evaluation of the couplingeffects of neighboring objects such as the battery, camera, flashcapacitors, etc., as well as the influence of dielectric materials suchas the housing and display screen.
The phone is subdivided into roughly 60 components, eachconsisting of hundreds or even thousands of individual facets; seeFigure 5 (the back cover and battery lid are hidden for the picture).The components used for the simulation are chosen based on theirinfluence on electromagnetic fields(which is controlled by both dimensions and location), in order to givean accurate simplification of the phone geometry for the investigatedfrequencies.
Figure 5: Full phone model containing phone plastics (red + blue) and metallic parts (copper + gold).
The structure was imported into CST MWS using the STEP interface.No additional healing was necessary before conducting the simulations,which is an important pre-requisite for efficient industrial design andworkflow. The simulation of the full mobile phone consisted of 594,000mesh cells (again after a convergence study as described in theprevious section). The total simulation for the converged model took 19minutes.
The simulation of the return loss and radiation efficiency of thefull phone is compared with measurements in Figure 6. As the completephone is now considered, the frequencies are shifted down to thewell-known mobile phone bands. The results again agree well in respectto resonance frequencies, bandwidths and radiation efficiency; but some differences occur in the return loss for the upper frequencies.
These can be explained by the uncertainty regarding the antenna’sexact feeding point during measurement and measurement de-embedding.Additionally the properties of the various materials used in the phonemodel may be inexact.
Figure 6: Comparisonof simulation and measurement for the full phone simulation: thereturn-loss (left) and the antenna efficiency (right) are compared.
Alongside radiation efficiency and return loss, global quantitiessuch as the TRP and the TIS are relevant for the simulation of thecomplete phone. These values require the consideration of not only theantenna characteristics but also the amplifier, the signal transmissioninside the printed circuit boardand the matching network. Figure 7 shows the simplified setup of such acircuit in CST DESIGN STUDIO™ (CST DS) including an idealized source,touchstone file describing the PCB transmission, matching network and amicrostripline to feed the antenna. The simulation delivers systemS-parameters, system near and far fields, and from these, the TRPvalue. The TRP value of this idealized setup gives 23.27 dBm at 1.8 GHzand amplifier power of 0.25 W.
Figure 7: Network simulation including matching network and the 3D results of the antenna.
The near field is also of significance for the complete phone, asinteraction with other electromagnetic devices such as hearing aids(hearing aid compatibility, HAC) might occur. Near field informationcan be predicted very accurately by means of simulation. Figure 8compares the normalized E and H near fields at a distance of 10 mm fromthe back of the phone in a free space configuration. The blue outlinerepresents the position of the phone.
Figure 8: Electric and magnetic near fields on the back of the phone, simulation (left) and measurement (right).
SIMULATION AT BODY LEVEL
A final test for the mobile phone is to evaluate it in thepresence of a human body, with particular emphasis on the head andhand. In accordance with IEEE standards, such as 1528, the StandardAnthropomorphic Model is used as the head model. The frequencydependent dielectric propertiesof the tissue simulating liquid are also defined by this standard andcan be modeled as a dispersive material in the simulation tool.
The simulation of this phone, with head and hand modelsrequired 4.24 million mesh cells and had a simulation run time of 1.58hours on a PC (dual core dual CPU, 2 GHz, 8 GB RAM). The number of meshcells can be reduced using the sub-gridding scheme. When applied, avery fine mesh is created inside the phone, a coarser one in the headand a very basic one in the vacuum (see Figure 2 right). Using thesub-gridding reduces the number of mesh cells to only 922,000 and thesimulation time to 44 minutes.
Such a simulation can give important insight into how the SAMphantom or the homogeneous body models affect the performance of themobile phone. The radiation pattern is obviously affected, but also theradiation efficiency is influenced by the head and hand. Figure 9 showsthe radiation pattern of the phone; a significant difference is visiblein comparison to the plain antenna far fields from Figure 4.
Figure 9: The modified radiation pattern for the two GSM bands in presence of head and hand.
As mentioned, the radiation efficiency is influenced by the bodymodels. Figure 10 shows the antenna efficiency calculations for onlythe phone, the phone placed at the right cheek of SAM, and the phoneheld at SAM’s right cheek with a hand model present.
Figure 10: Antenna efficiency calculations for only the phone, the phone and SAM, and the phone with SAM and hand model.
Finally, full SAM simulations are very useful to predict thedissipated power. This quantity – as a measured value – is anotherimportant design issue and a requirement for certification. However, asimulation allows the designer to control this power at a much earlierdesign stage.
CONCLUSION
This article has shown what is currently possible in the world ofadvanced 3D EM simulation. Throughout all steps of a mobile phone’sterminal antenna development – from antenna design, through full phoneoptimization up to investigating the influence of, and impact on bodytissues – simulation and measurement have been compared and shown verygood agreement. In addition to measurable data, numerical simulationgrants insight into previously unseen electromagnetic detail.
Within one simulation run, all-important quantities such asreturn-loss, radiation efficiency, near and far fields, loss monitors(all at various frequencies) can be obtained. Advanced mesh technologysignificantly reduces the simulation time, bringing it down to a fewminutes for an antenna simulation. Even complex automatic optimizationruns become feasible. The increasing efficiency and reliability ofsimulation, which reduces design cost risk is recognized asindispensable in the industry.
ACKNOWLEDGMENT: This article is the result of a common study of CST and Sony Ericsson Mobile Communications. CST likes to thank especially Dr. Omid Sotoudeh from Sony Ericsson forproviding the models, the measurement data and many fruitfuldiscussions regarding the simulation results. A full version of thisarticle was published in the January 2008 issue of Microwave Journal (www.mwjournal. com) and can also be found here
