SARComplianceAssessmentof PMR446andFRSWalkie-Talkies

G€unterVermeeren,*Wout Joseph, and LucMartensDepartment of InformationTechnology, iMinds/UGent, Ghent, Belgium

The vast amount of studies on radiofrequency dosimetry deal with exposure due to mobile devicesand base station antennas for cellular communication systems. This study investigates complianceof walkie-talkies to exposure guidelines established by the International Commission on Non-Ionizing Radiation Protection and the Federal Communications Committee. The generic walkie-talkie consisted of a helical antenna and a ground plane and was derived by reverse engineering ofa commercial walkie-talkie. Measured and simulated values of antenna characteristics andelectromagnetic near fields of the generic walkie-talkie were within 2% and 8%, respectively. Wealso validated normalized electromagnetic near fields of the generic walkie-talkie against acommercial device and observed a very good agreement (deviation <6%). We showed that peaklocalized specific absorption rate (SAR) induced in the oval flat phantom by the generic walkie-talkie is in agreement with four commercial devices if input power of the generic walkie-talkie isrescaled based on magnetic near field. Finally, we found that SAR of commercial devices is withincurrent SAR limits for general public exposure for a worst-case duty cycle of 100%, that is, about 3times and 6 times lower than the limit on the 1 g SAR (1.6W/kg) and 10 g SAR (2W/kg),respectively. But, an effective radiated power as specified by the Private Mobile Radio at 446MHz(PMR 446) radio standard can cause localized SAR exceeding SAR limits for 1 g of tissue.Bioelectromagnetics. 36:517–526, 2015. © 2015 Wiley Periodicals, Inc.

Key words: modeling; exposure; two-way radio transceiver; absorption assessment; helicalantenna

INTRODUCTION

Although a vast amount of studies on dosimetrydeal with exposure due to mobile devices and basestation antennas for cellular communication systems,the mobile phone is not the only device operated inproximity of the human head. Using the case study ofthe walkie-talkie, we draw attention to a device thatgained interest as an alternative to cellular phones forshort-range communications. Walkie-talkies, or two-way radios, operate close to the human head as is thecase for mobile phones. Typical operating positions ofa walkie-talkie are in front of the face, whereas amobile phone is typically operated next to the ear.When operating a walkie-talkie, its antenna could bejust in front of the eye. The International Commissionon Non-Ionizing Radiation Protection (ICNIRP)restricts local-averaged specific absorption rate (SAR)in 10 g (applicable in Europe) based on cataract in theeye of a rabbit [ICNIRP, 1998]. As opposed to mobilephones, no or little attention is paid to walkie-talkiesin dosimetry, although compliance tests also apply tothese devices. Cecil et al. [2014] and Dimbylow et al.[2003] investigated numerically peak 10 g localizedSAR induced by Terrestrial Trunked Radio (TETRA)transmitters positioned around the human body.

In recent years, walkie-talkies are more fre-quently encountered as a short-range communicationdevice (e.g., adults give a walkie-talkie to theirchildren when they go to play outside their house;parking lot attendants employ a walkie-talkie to directthe traffic inside a parking lot to assist drivers infinding a free parking spot, etc.) Advantages ofwalkie-talkies with respect to cellular phones are theirease-of-use (push-to-talk), their inexpensiveness, andfree-of-charge communications.

In Europe, a walkie-talkie operates according tothe Private Mobile Radio at 446MHz (PMR 446)standard; in the United States, Family Radio Service(FRS) is authorized for license-free short-range voicecommunication [ERC, 1998; ETSI, 2001a, 2015;

Conflicts of interest: None.

*Correspondence to: G€unter Vermeeren, iMinds/UGent-INTEC,Gaston Crommenlaan 8 box 201, Ghent B-9050, Belgium.E-mail: gunter.vermeeren@intec.ugent.be

Received for review 21 August 2014; Accepted 13 August 2015

DOI: 10.1002/bem.21935Published online 6 September 2015 in Wiley Online Library(wileyonlinelibrary.com).

Bioelectromagnetics 36:517^526 (2015)

� 2015Wiley Periodicals, Inc.

FCC, 2015]. FRS uses 14 channels within frequencybands 462.5625–462.7125MHz and 467.5625–467.7125MHz. PMR 446 specifies eight channelswithin frequency range 446.0–446.1MHz. Each chan-nel has a bandwidth of 25 and 12.5 kHz for FRS andPMR 446, respectively. Maximum allowed EffectiveRadiated Power (ERP) is 500mW for both systems.Frequency modulation (FM) has been adopted asmodulation scheme. Relative low frequency, effectiveradiated power of 500mW, use of walkie-talkies bychildren, position in front of the face, and limitednumber of exposure studies for walkie-talkiesattracted our attention to these devices.

The objective of this study was to develop andevaluate an accurate helical antenna of a genericwalkie-talkie for compliance assessment with ICNIRP[1998] and FCC [2001] guidelines. We designed ageneric walkie-talkie and evaluated it in free spaceand below a flat phantom against a real device. Thisvalidation between model and real device will allowus to use the walkie-talkie model in future studies(e.g., to evaluate absorption in anatomical humanbody models.) To our knowledge, compliance ofwalkie-talkies operating according to PMR 446 andFRS standards under worst-case conditions has notyet been investigated in literature. In addition, com-pliance of four commercial walkie-talkies was eval-uated. The model is obtained by reverse engineeringand is validated by reflection, near-field, and far-fieldmeasurements. The use of helical monopole antennasas a model for portable handheld devices have beenmainly discussed for mobile-phone technologies[Lazzi and Gandhi, 1998; Koulouridis and Nikita,2004] operating in frequency bands around 900MHzand 1800MHz. In this study, a helical monopoleantenna model of a walkie-talkie operating at446MHz was designed to study electromagneticfields induced in the human head by walkie-talkies. Amodel is preferred over a real walkie-talkie devicemainly for two reasons: firstly, antenna characteristics(e.g., reflection at input terminals of a model can bemeasured accurately); secondly, input power is easilyadjustable as one can feed using a signal generatorcontrary to a real device. The model also allows studyof influence of dimensions and material parameters onantenna characteristics and absorption or SAR assess-ment in realistic human head models.

MATERIALS AND METHODS

Generic Walkie-Talkie Design

We developed a generic walkie-talkie by reverseengineering of a commercially available PMR 446

radio, that is, the COBRA MT500 (Cobra Electronics,Chicago, IL) shown in Figure 1a. We dismantled theCOBRA MT500 and developed a generic walkie-talkie (Fig. 1b) based on physical dimensions of theantenna structure (including coating around antenna)and Printed Circuit Board (PCB). The model consistedof a helical antenna mounted on a rectangular groundplane. Terminals of the source connected the helicalantenna and ground plane of the model. To limitcomplexity of the model, the PCB of the walkie-talkiewas replaced by a perfectly conducting ground planeand the case was removed. The helical antenna fits ina dielectric cover. This cover around the helicalantenna influences antenna characteristics of themodel. Dielectric properties of this cover wereunknown and we derived properties by tuning simu-lated antenna characteristics and near-field distribu-tions to measured results.

SAR Assessment

We tested SAR compliance for the genericwalkie-talkie as well as for four commercially avail-able walkie-talkies (Table 1) according to IEC stand-ard 62209-2 [IEC, 2005]. We placed the walkie-talkieat a distance (d) below an oval flat phantom as shownin Figure 2. We did not take the user’s hand intoaccount. IEC standard 62209-2 [IEC, 2005] for com-pliance testing does not specify hand position becausedosimetric studies [Balzano et al., 1995; Kuster et al.,1997a, b; Meyer et al., 2001] suggest that excludingthe hand in modelling constitutes a conservative casescenario for SAR in head.

Measurement Setup

Measurements of reflection and input impedancewere performed in an anechoic room using a vectornetwork analyzer (VNA) (type HP8710, AgilentTechnologies [formerly Hewlett Packard], Palo Alto,CA). The far-field characteristics in terms of TotalRadiated Power (TRP) and the Effective RadiatedPower (ERP) were measured. TRP measurementswere performed in a reverberation chamber accordingto TCO’01 Certification of Mobile Phones [TCODevelopment, 2008]. Measurement uncertainty was1 dB. ERP measurement was performed in ananechoic room according to European Telecommuni-cations Standards Institute (ETSI) standard EN 300296-2:2001-03 [ETSI, 2001b]. Measurement uncer-tainty was 3.3 dB, which is within the uncertaintyboundary of 6 dB specified by the ETSI standard. Thislarge uncertainty is due to simplifications built intomeasurement methodology to reduce time and costsof tests. For walkie-talkies, ERP was only measuredin eight different directions in the azimuth plane

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resulting in an uncertainty of 3.3 dB. Near fields weremeasured in an indoor environment using a robot andDASY3 measurement system with the followingprobes: E-field probe ER3DV6 and H-field probeH3DV6 (SPEAG, Zurich, Switzerland). Reflectionsof the environment and robot arm were minimized byplacing absorbers.

Measurement setup for SAR compliance testingconsisted of a robot (Staubli Type Rx90B L, Staubli,Pf€affikon, Switzerland), a DASY4 measurement sys-tem (SPEAG), a power meter (Agilent E4419B),power sensors (8482H, Agilent), a directional coupler(HP775D Dual Directional Coupler 450-940 MC,Agilent), an RF termination (Meca 480-1, Agilent), a

generator (HP8647A, Agilent), a network analyzer(HP8753E, Agilent), and the flat phantom (ELI4,SPEAG) filled with head simulating liquid (HSL450,SPEAG). The DASY4 measurement system consistedof data acquisition electronics (DAE3 from SPEAG)and a dosimetric probe (ET3DV6 from SPEAG).Figure 2 shows setup for performing dosimetricmeasurements. The dosimetric system (probe and dataacquisition electronics) was attached to a robot. Robotand data acquisition electronics were connected to aserver and managed through the graphical user inter-face of a laptop or desktop computer. Walkie-talkiesand the generic walkie-talkie were placed at short

TABLE 1. Four Commercially Available Walkie-Talkies

Walkie-talkie (manufacturer)

COBRA MT 500 (Cobra Electronics, Chicago, IL)TwinTalker 3300 (Topcom, Tilburg, The Netherlands)TwinTalker 1300 (Topcom, Tilburg, The Netherlands)Alecto FR-20 (Alecto Electronics, ’s-Hertogenbosch,The Netherlands)

Fig. 1. (a) COBRA MT500 (Cobra Electronics); (b) back of derived model with helical antennaenclosed by a dielectric cover (bazooka balun is not shown).

Fig. 2. Drawing of setup for dosimetric measurements.

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distance (d) below oval flat phantom. Oval flatphantom was filled with head simulating liquid(HSL450 from SPEAG) as suggested by IEC 62209[IEC, 2001]. Dielectric properties of the liquid at450MHz were s¼ 0.87 S/m and er¼ 43.5. Separa-tions between liquid and walkie-talkie were 2mm(touch position), 15, 30, and 50mm. For the genericwalkie-talkie we did not measure in touch position(we did not want the ground plane of the genericwalkie-talkie to make contact with the shell of thephantom) and at 50mm (due to antenna’s lowefficiency, we could not measure at this distance). Inthe case of the generic walkie-talkie, it was connectedto a radiofrequency (RF) generator. Input power ofantennas was measured by a power meter via adirectional coupler.

The worst-case uncertainty of the DASY4 meas-urement system was mentioned in the manual[SPEAG, 2008] provided with the system. Combinedstandard uncertainty on the local-averaged SAR in 1 gand 10 g equals 10.9% and 10.7%, respectively;expanded standard uncertainty on local-averaged SARin 1 g and 10 g equals 21.9% and 21.4%, respectively.This worst-case uncertainty budget for DASY4 wasassessed according to IEEE 1528 [IEEE, 2003]. Forspecific tests and configurations, uncertainty can beconsiderably smaller [SPEAG, 2008].

The commercially available walkie-talkies wereplaced below the flat phantom with their push-to-talkbutton fixed during measurement in order to transmitcontinuously. Before every measurement, fully-charged batteries were placed in walkie-talkies toensure transmission at maximum power. A walkie-talkie uses half-duplex communication resulting in avarying duty cycle from 0% to 100%. A continuouslytransmitting device has a duty-cycle of 100%. Hence,a worst-case duty cycle of 100% was selected forcompliance testing.

Besides four commercially available walkie-talkies, we also assessed peak local-averaged SAR in1 g and 10 g for the hand-made generic walkie-talkie.Measured peak local-averaged SAR values werecompared with simulations.

Numerical Methods

We employed the finite-difference time-domain(FDTD) solver available in the three-dimensional full-wave electromagnetic software package SEMCAD-X(SPEAG) for performing numerical analysis in freespace and below the oval flat phantom. In FDTDcalculations, simulation domain is finite and boundaryconditions are used to mimic free space. We applieduni-axial perfectly matched layers (UPML) at theboundaries. The number of layers was automatically

set by the FDTD solver to obtain a selected efficiencyof 99.9%. Padding (minimum distance betweenabsorbing boundaries [UPML] and the bounding boxaround the walkie-talkie or combination of walkie-talkie and the flat phantom) was a quarter of awavelength. Grid step for the helical antenna equaled0.5mm (133 times smaller than a tenth of thewavelength in free space at 450MHz), and maximumgrid step in the flat phantom was 2mm (5 timessmaller than a tenth of the wavelength in the tissuesimulating liquid at 450MHz). Based on thesesettings, we estimated uncertainty on peak local-averaged SAR from the study of Bakker et al. [2010,2011]. Expanded uncertainty U (k¼ 2) on SAR10g inthe flat phantom filled with tissue-simulating liquidequaled 11.9% (there is no uncertainty on dielectricproperties of tissue-simulating liquid because thevalues are specified by standards).

RESULTS

Free-Space Evaluation of the GenericWalkie-Talkie

Figure 3 shows dimensions of the derivedgeneric walkie-talkie operating at 446MHz and thecoordinate system used in this study. Model dimen-sions and dielectric parameters are listed in Table 2.These values can be modified to specific dimensionsof different types of walkie-talkies. The helicalantenna fits in a dielectric cover with a relativepermittivity (er) of 3.9 and a conductivity (s) of12mS/m.

Fig. 3. Generic walkie-talkie in (a) xz-plane and (b) yz-plane.Measurement line (y-axis) and plane (y¼ 20mm) for nearfields are also shown.

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We realized a balanced feed current at terminalsof the measurement model with a bazooka or a sleevebalun [Balanis, 1982]. Operating frequency of thebazooka balun was 446MHz. (The bazooka balun isnot shown in Fig. 1b).

Return loss S11j j in dB of the model with respectto 50V is shown in Figure 4. A very good agreementis observed between simulations and measurements.Resonance frequency fres calculated by the FDTD toolis 436.5MHz and deviates only 0.3% of the measuredvalue fres, that is, 435.2MHz. Simulated input impe-dance at resonance Zin,res equaled 20.8V, measuredinput impedance equaled 24.5V. At operating fre-quency of 446MHz, input impedance of model Zinwas 28.6þ i30.4V and 23.1þ i29V for measurementand FDTD simulation, respectively.

We already mentioned that dielectric parametersof the cover (relative permittivity of 3.9, conductivityof 12mS/m.) around the antenna were derived bytuning these parameters until simulations agreed withmeasurements in term of resonance frequency, returnloss, and near fields.

Radiation efficiency (h) of an antenna is definedas ratio of radiated power (Prad) and input antennapower (Pin), or h¼Prad/Pin [Balanis, 1982]. Radiationefficiency was calculated using FDTD simulations.For the generic walkie-talkie, we obtained an effi-ciency of 53% at 446MHz. This low efficiency wasdue to the reflection at input terminals of the helicalantenna and ohmic losses in housing around thehelical antenna introduced by the conductivity (s) of12mS/m.

Far-field behavior of the generic walkie-talkiehas been investigated numerically in terms of gain(G). Using FDTD, a gain of 1.07 for simulation modelwas obtained. This value is lower than the standardgain of a half-wave dipole antenna (Gd), mainly dueto losses in the dielectric cover around the helicalantenna of the model. ERP can be calculated asfollows [FCC, 1997]:

ERP ¼ GGd

Pin ð1ÞEffective radiated power was measured for the

generic walkie-talkie with coating. For an input powerof 10mW, measured maximum ERP was 4.9mW(measurement uncertainty was 3.3 dB). This agreedwell with simulated ERP of 4.8mW (relative errorwas 2%). Based on simulated ERP for an input powerof 10mW, we calculated that input power of thehelical antenna (or output power of generator) mustbe set to 1042mW to obtain maximum allowed ERPof 500mW as specified by the PMR 446 [ERC,1998] and FRS standard [FCC, 2015]. This inputpower is used below to determine compliance of themodel with safety guidelines [ICNIRP, 1998; FCC,2001].

Near fields of the model were simulated andmeasured in the y-plane at 20mm from the model(y¼ 20mm) according to the setup shown in Figure 3.This plane was parallel to the ground plane of thegeneric walkie-talkie (or PCB of real devices) at adistance that might be considered a typical separationdistance value when a walkie-talkie is operated infront of the face. ERP was set to 500mW. Figure 5shows simulated ([a] and [b]) and measured ([c] and[d]) RMS electric Erms ([a] and [c]) and RMSmagnetic Hrms ([b] and [d]) near fields of the model aty¼ 20mm. One observes that distribution of theelectric and magnetic field as well as absolute fieldvalues of the simulations agree well with measure-ments. Dots in Figure 5 show the position of themaximum field values. The position of the maximumRMS E-field and H-field was situated near the helicalantenna and terminals of the model, respectively. Inthe plane y¼ 20mm, simulated Erms,max (Fig. 5a)

TABLE 2. Dimensions of a Walkie-Talkie Model

Helical antenna Ground plane

a 4.2mm l 80mmb 9.8mm w 45mmc 10mm t 0.5mme 5mm Antenna housing

f 2mm ri 3.5mmg 8.54mm ro 7mmh 42mm er 3.9k 7.2mm s 12mS/mnturns 21 hi 51mmrh 2.4mm ho 53mmrw 0.5mm m 1mm

400 420 440 460 480 500−10

−8

−6

−4

−2

0

Frequency (MHz)

|S11

| (dB

)

model (meas)model (FDTD)

Fig. 4. Return loss S11j j of model.

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equals 762V/m in the point (x¼ 10mm, z¼ 55mm)and deviates only 1.6% from measured field value,that is, Erms,max of 750V/m in point (x¼ 5mm,z¼ 50mm). In the same plane y¼ 20mm, simulatedHrms,max is 1080mA/m in (x¼ 5mm, z¼ 15mm) anddeviates 7.7% from measured Hrms,max of 1003mA/min (x¼ 5mm, z¼ 15mm).

Free-Space Evaluation of Four CommerciallyAvailable Walkie-Talkies

Besides the COBRA MT500, three other PMR446 radios were acquired. In this section, the radiationcharacteristics of all the walkie-talkies (Table 1) andnear-field behavior of the COBRA MT500 are inves-tigated.

Radiation characteristics of real devices weremeasured at SP Technical Research Institute ofSweden (Borås, Sweden). Table 3 lists TRP, maxi-mum ERP, and averaged ERP. Maximum effective

radiated power varied from 51mW to 138mW. Thisis about 9.8–3.6 times below specification of allowedERP for PMR 446 radios.

Near fields of COBRA MT500 were measuredand compared to the model of the walkie-talkie(which has dimensions based on dimensions of theCOBRA MT500). We positioned the walkie-talkie inthe same way as the model (see Fig. 3) such that thefeed point coincided with the origin of the coordinatesystem. We normalized fields to the maximum fieldvalue in the plane y¼ 20mm, because we were notable to determine input power for the real walkie-talkie. Figure 6 shows normalized electric and mag-netic field in a plane for the real walkie-talkie (onlymeasurements). These distributions agree well withsimulated (see Fig. 5a and c) and the measured (seeFig. 5b and d) near field distributions of the genericwalkie-talkie. Compared to the real walkie-talkie,maximum relative error for the measured and simu-lated model on the normalized electric field were only1.2% and 5.2%, respectively. For normalized mag-netic near field, maximum relative error for themeasured and simulated model was 5.2% and 5.5%,respectively. These low deviations show that themodel behaves electromagnetically like a real walkie-talkie and can be used for analysis of interaction withthe human body.

SAR Assessment of Real Walkie-Talkies

Figure 7 shows peak local-averaged SAR in1 g and (SAR1g) and 10 g (SAR10g) of the four

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TABLE 3. Total Radiated Power and Effective RadiatedPower of Four Commercial Walkie-Talkies

Walkie-talkieTRP(mW)

Max. ERP(mW)

Avg. ERP(mW)

COBRAMT500 105 138 117TwinTalker3300

59 79 68

TwinTalker1300

51 69 63

Alecto FR-20 44 51 47

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commercial walkie-talkies for a duty-cycle of100% (i.e., continuously speaking for at least6 min.) The Alecto was not measured at a distanceof 15mm between the device and the liquid insidethe flat phantom. Except for the touch position, theCOBRA MT500 showed a higher SAR in 1 g and10 g than the walkie-talkies from Topcom (Tilburg,The Netherlands) and Alecto (’s-Hertogenbosch,The Netherlands).

Highest values of the SAR1g and SAR10g occurredwhen the walkie-talkie touched the flat phantom:SAR1g¼ 0.52W/kg (Alecto) and SAR10g¼ 0.35W/kg(Alecto). These values for the touch position comply

with the SAR-limits for 1 g (i.e., 1.6W/kg) and 10 g(i.e., 2W/kg), respectively, even for a worst-case dutycycle of 100%.

Cecil et al. [2014] and Dimbylow et al.[2003] reported peak 10 g SAR values up to about50% of ICNIRP limit for general public exposure forTETRA transmitters. This is higher than the maxi-mum of 0.35W/kg observed in our study. Besides thedifference in phantom (human body model vs. flat)and distance between the walkie-talkie and thephantom, the difference was mainly due to the differ-ence in applied power: Cecil et al. as well asDimbylow et al. used a transmitted power of 0.25Wfor the hand-held TETRA devices whereas in ourstudy, SAR values were for typical TRP-values (infree space) of walkie-talkies ranging from 44mW to105mW (Table 3).

SAR Assessment of Generic Walkie-Talkie

The input power of the generic walkie-talkiewas set to 10mW during measurement taking intoaccount reflection losses. We validated use of thegeneric walkie-talkie as a substitute for real devicesby comparing peak local-averaged SAR of realdevices with ones of the model for the same ERPof the real devices and for the same magnetic nearfield. Above, we discussed ERP values of the fourwalkie-talkie devices. We observed that ERP ofconsidered devices was at least 3.6 times smallerthan maximum allowed ERP for PMR 446 radios.We rescaled ERP of the generic walkie-talkie(measured value of 4.9mW and simulated ERPvalue of 4.8mW for an antenna input power of10mW) to values listed in Table 3. The antennainput power needed to reach ERP of the walkie-talkies was applied during dosimetric measurement.Figure 8 shows the SAR1g and SAR10g of thegeneric walkie-talkie with the same ERP of theCOBRA MT500. We observed that the genericwalkie-talkie overestimated peak local-averagedSAR of the real devices (Fig. 7). Rescaling input

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power based on ERP (a far-field characteristic) wasinadequate for assessment of SAR in human tissuedespite the use of the helical antenna with coatingfrom the COBRA MT500. The ratio for SAR1g andSAR10g (for ERP equal to ERP of the devices) ofthe model and device was 2.5 and 2.3, respectively.The deviation originates from uncertainties on ERPand SAR measurements, simplifications of themodel with respect to the real device and fromdifferences in coupling when the device or modeloperates at close distance to the phantom. Ratio forthe other real walkie-talkies was maximum 2.8(Topcom tt3300) and 2.5 (Topcom tt3300) forSAR1g and SAR10g, respectively. For further numer-ical analysis of the SAR10g in the flat phantomusing the generic walkie-talkie and based on arescaling of ERP, we will take into account acorrection factor of two for SAR10g value.

A better quantity for validating the genericwalkie-talkie as a substitute for a real device forassessing peak local-averaged SAR is the magneticnear field. Kuster and Balzano [1992] showed thatthe SAR is mainly proportional to incident mag-netic near field. We rescaled the maximum of themagnetic near field in a plane at a distance of20mm of the ground plane of the generic

walkie-talkie to the maximum value of the magneticfield in the same plane above the real device. Peak-local averaged SAR of the generic walkie-talkie fora rescaling based on magnetic field is shown inFigure 9. We observed that maximum deviation onthe peak local-averaged SAR was less than 26%.Thus, magnetic near field is a better quantity thanfar-field parameter ERP for estimating peak local-averaged SAR of a real device.

Finally, we estimated worst-case peak local-averaged SAR for the walkie-talkies. We deter-mined peak local-averaged SAR in 1 g and 10 g foran input power that gives an ERP of 500mW infree space (we assumed a duty cycle of 100% forworst-case evaluation) for the generic walkie-talkieand taking into account a correction factor of two(see above). Figure 10 shows that the genericwalkie-talkie is not compliant to the FCC limit of1.6W/kg in a cube of 1 g at 15mm. The worst-casepeak local-averaged SAR in 1 g of tissue exceededthe FCC limit by up to 7% (based on simulatedvalue). If we compared peak local-averaged SAR in10 g of tissue with ICNIRP basic restriction of2W/kg, then we observed that the walkie-talkiesare compliant.

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CONCLUSIONS

An accurate model for a walkie-talkie has beendeveloped and very good agreement reported fromsimulations and measurements in terms of reflectionand transmission characteristics. Near fields of themodel have been validated with measured near fieldsof a real walkie-talkie. A relative error of less than5.5% has been observed on the near electric andmagnetic field indicating that the walkie-talkie modelbehaves electromagnetically as a real walkie-talkie.Therefore, the model can be used to determineelectromagnetic interaction with the human body andto test compliance with safety limits. Total radiatedpower and effective radiated power were measuredfor four real or commercially available walkie-talkiesand a generic walkie-talkie. Measured effective radi-ated power of the real walkie-talkies is about 3.6–9.8times lower than allowed effective radiated power forPMR 446 radios.

We found that rescaling the input power of thegeneric walkie-talkie based on the ERP (a far-fieldcharacteristic) is inadequate for assessment of SAR inhuman tissue: we observed a ratio for SAR1g andSAR10g between the model and the device of 2.5 and2.3. A better quantity for validating the genericwalkie-talkie as a substitute for a real device for

assessing peak local-averaged SAR is the magneticnear field with deviations of less than 26%.

We investigated compliance to SAR safety limitsof a walkie-talkie. An effective radiated power asspecified by the PMR 446 radio standard can cause alocal-averaged SAR, which exceeds limits. However,based on effective radiated power of four real walkie-talkies, we showed that peak local-averaged SARvalues are unlikely to exceed current SAR limits.

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touch 15 20 30 500

0.5

1

1.5

2

d (mm)

SA

R1g

(W

/kg)

M2 (meas)M2 (sim)

touch 15 20 30 500

0.5

1

1.5

d (mm)

SA

R10

g (W

/kg)

M2 (meas)M2 (sim)

(a)

(b)

Fig. 10. Worst-case assessment of peak local-averaged SARin (a) 1g and (b) 10 g induced by generic walkie-talkie forERP¼ 500 mW (100% duty cycle).

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