IEEE SENSORS JOURNAL, VOL. 15, NO. 8, AUGUST 2015 4393
Compact Personal Distributed Wearable ExposimeterPeter Vanveerdeghem, Patrick Van Torre, Arno Thielens, Jos Knockaert, Member, IEEE,
Wout Joseph, Senior Member, IEEE, and Hendrik Rogier, Senior Member, IEEE
Abstract— A compact wearable personal distributedexposimeter (PDE) is proposed, sensing the power densityof incident radio frequency (RF) fields on the body of a human.In contrast to current commercial exposimeters, our PDE,being composed of multiple compact personal wearableRF exposimeter sensor modules, minimizes uncertainties causedby the proximity of the body, the specific antenna used, and theexact position of the exposimeter. For unobtrusive deploymentinside a jacket, each individual exposimeter sensor module isspecifically implemented on the feedplane of a textile patchantenna. The new wearable sensor module’s high-resolutionlogarithmic detector logs RF signal levels. Next, on-board flashmemory records minimum, maximum, and average exposuredata over a time span of more than two weeks, at a one-secondsample period. Sample-level synchronization of each individualexposimeter sensor module enables combining of measurementscollected by different nodes. The system is first calibrated inan anechoic chamber, and then compared with a commerciallyavailable single-unit exposimeter. Next, the PDE is validatedin realistic conditions, by measuring the average RF powerdensity on a human during a walk in an urban environmentand comparing the results to spectrum analyzer measurementswith a calibrated antenna.
Index Terms— Dosimetry, radio frequency exposure, textileantennas, wearable electronics.
I. INTRODUCTION
NATIONAL legislation and the international Commissionon Non-Ionizing Radiation Protection (ICNIRP) [1]
impose limits in terms of whole-body averagedSAR (SARwb) [2]. Since these SARwb levels can onlybe evaluated by numerical simulations [3], equivalentreference levels, which can be measured and compared to theinternational guidelines issued by INCIRP, have been definedon the incident power density. Such exposure measurementsare currently performed with commercially available PersonalExposiMeters (PEMs) [2], [4]–[7]. These measurementsare compromised by large measurement uncertainties,
Manuscript received December 15, 2014; revised February 13, 2015 andMarch 25, 2015; accepted March 27, 2015. Date of publication April 6,2015; date of current version June 17, 2015. This work was supported bythe Interuniversity Attraction Poles Program–Belgian Science Policy Office.The associate editor coordinating the review of this paper and approving itfor publication was Dr. Francis P. Hindle.
P. Vanveerdeghem is with the Department of Information Technology,iMinds/Ghent University, Ghent B-9000, Belgium, and also with theDepartment of Industrial System and Product Design, Ghent University,Kortrijk B-8500, Belgium (e-mail: [email protected]).
J. Knockaert is with the Department of Industrial System andProduct Design, Ghent University, Kortrijk B-8500, Belgium (e-mail:[email protected]).
Color versions of one or more of the figures in this paper are availableonline at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/JSEN.2015.2420583
due to shadowing by the test person’s body [8], [9],dependence on polarization [10] and out-of-band detection.Furthermore, conventional PEMs cannot be unobtrusivelydeployed on the human body, whereas a fully integratedwearable on-body PEM enables continuous monitoring oflong-term RF exposure, without hindering the test person’sdaily activities. Moreover, an on-body Personal DistributedExposimeter (PDE), composed of multiple PEMs, increasesthe measurement accuracy [11].
We present a fully autonomous on-body PDE, composed ofmultiple independent RF-exposure modules, each integratedon a textile antenna feed plane. Thanks to their groundplane [12], the applied patch antennas minimize capacitiveantenna loading by the body. Without loss of generality, wespecifically configure the exposimeter for the Global Systemfor Mobile Communications (GSM) worldwide down-link[925 MHz-960 MHz] frequency band [2].
Each wearable exposimeter module makes use of astate-of-the-art logarithmic-detector to pair accuracy with adynamic range of 80 dB. The on-board flash memory logsfor over two weeks of measurement data, thereby eliminatinga permanent Personal Computer (PC) connection. A similaron-body device was documented in [11], yet without datalogging or unobtrusive integration potential. To the authors’knowledge, this is the first fully tested wearable PDE.
In Section II, the wearable PDE system is described,followed by its validation on a human body in Section III.The calibration procedure is explained in Section IV. A reallife measurement is outlined in Section V. Conclusions arelisted in Section VI.
II. SYSTEM OVERVIEW
The PDE is composed of multiple newly designed wearableexposimeter modules, of which the construction is describedbelow.
A. Antenna
Due to the large wavelength of the frequency of theGSM downlink band, an aperture-coupled shorted patchantenna is selected, as displayed in Fig. 1. The antennafeatures a compact size and excellent antenna performancein proximity of the human body, while avoiding fragile probefeed connections [13], [14]. Therefore, this antenna is moresuitable for garment integration than conventional antennas.Further improvement of the coupling between the antenna andthe exposimeter module is obtained by employing an H-shapedcoupling slot, thereby also minimizing the backward radiationinto the human body [15]. The textile material is flexibleand lightweight, without sacrificing antenna performance,in comparison to rigid antennas. A flexible polyurethane
4394 IEEE SENSORS JOURNAL, VOL. 15, NO. 8, AUGUST 2015
Fig. 1. Top view (left) and bottom view (right) of the patch antenna.On the bottom view, the active circuit and antenna feed are shown separately.In the actual implementation, the exposimeter’s electronics are mounted ontothe feed plane of the antenna.
Fig. 2. Top view (left) and cross-section (right) of the patch antenna.
protective foam (thickness = 11 mm permittivity εr = 1.16and tan δ = 0.010), commonly used in protective garments forrescue workers is applied as a substrate material, protectingthe electronic circuitry from external factors, such as heatand humidity. A low-cost, conductive, electro-textile material,called Flectron, is used to construct the ground plane andradiating element. The material has a thickness less than0.25 mm and surface resistivity less than 0.1 �/sq., minimizingconductor losses. The influence of the body, which is in closeproximity to the antenna, is limited, thanks to the groundplane structure. This feature makes the PDE’s performancenearly independent from the test person’s body morphology.Measurements performed on another person produced similarresults [16], [17]. The feed substrate is constructed using twostacked Aramid textile layers, a material which is frequentlyused as outer layer in firefighter jackets (thickness = 0.95 mm,permittivity εr = 1.97 and tan δ = 0.02). The feed lineis realized by means of copper foil. The antenna covers thecomplete GSM 935 MHz to 960 MHz downlink band. The3 dB beam width of the antenna approximately equals 110°,with an antenna gain of 2.9 dBi and an antenna efficiencyof 76.6 %. The top view of the antenna and its dimensionsare shown in Fig. 1, with the electronic circuitry integratedonto the feed plane of the antenna. A detailed constructiondiagram of the antenna is shown in Fig. 2.
By placing the electronics on the feed plane of the patchantenna, each very compact module may be unobtrusivelyintegrated into garments or clothing [18]. In addition, each unit
Fig. 3. Bock diagram of single autonomous exposimeter module, withintegrated band-pass filter (BPF), logarithmic RF detector and temperaturecompensation (Temp Comp). The PC connection is only used to configure themicrocontroller (μC) and to download logged data from the 4 MB onboardflash memory.
can be encapsulated together with the textile antenna intoa breathable Thermoplastic PolyUrethane coating (TPU),implementing a washable system [19].
B. System Design
The block diagram of the proposed exposimeter module,integrated on the antenna feed-plane, is presented in Fig. 3.The selection of the key components is motivated as follows.
The RF-signal received by the antenna is filtered by aSurface Acoustic Wave (SAW) Bandpass filter (BPF), toremove undesired out-of-band signals. In this configurationfor the GSM down-link, the TriQuint 856528 SAW filter ischosen, with a bandwidth of 35 MHz and passband insertionloss of 2.5 dB. Outside the passband, an attenuation of 35 dBis quickly achieved. Thanks to the filter architecture, noadditional impedance matching network is required, helpingto reduce the physical size of the circuit. The filtered signal ismeasured by an Analog Devices ADL5513 [20] LogarithmicRF Detector, providing an output voltage proportional to theinput RF level in dBm over a very large dynamic range. Theoutput voltage of the RF detector is measured by a 16 bitAnalog-to-Digital Converter (ADC) and transported on thePDE board to the C8051F921 microcontroller (μC), by SiliconLabs, through the high speed Serial Peripheral Interface (SPI)data bus. To correct the small temperature-dependent variationof the output level of the Logarithmic RF Detector,an automatic frequency-dependent temperature compensa-tion is implemented by a Digital-to-analog-converter (DAC)connected to the on-board microcontroller and its built-intemperature sensor. The micro controller is programmedin C. The software is uploaded to its memory through theIn-Circuit-Programming interface.
The digitized measurement data are further processed bythe on-board software at a data rate of 1000 samples/sec. Theminimum, maximum, as well as arithmetic- and geometricaverages of the received RF signal power, over a one-secondtime slot, are stored into the on-board flash memory. The4 MB flash memory provides up to two weeks of non-volatilestorage space. To guarantee the data integrity, at each timeslot, a check sum of the measurement data is calculated andstored into the memory together with the measured data.
Besides the storage of the averaged measurement data at aone-second rate, the system may store raw sampled data at afull sample rate of 1000 samples per second for analysis ofhigh-speed measurements in post processing. The maximumsample rate of the exposimeter is 250 ksps, as determined
VANVEERDEGHEM et al.: COMPACT PERSONAL DISTRIBUTED WEARABLE EXPOSIMETER 4395
Fig. 4. Top view of the system [57 mm × 33 mm].
TABLE I
SPECIFICATIONS OF THE SINGLE PEM NODE
by the ADC specifications. After the logging period, themeasurement data are easily transferred to the personalcomputer (PC) over the USB-link for data-analysis.
Owing to the flexible and lightweight design of the system,the PDE is comfortably wearable by the test persons withoutrestricting their movements. A top-view of the exposimetercircuit is shown in Fig. 4. Its planar circuit board’s size of35 mm by 55 mm is smaller than the antenna, allowing easyintegration onto its feed-plane. An overview of the technicalspecifications is shown in Table I.
C. Frequency Selection of the Personal Exposimeter
The measured frequency band is selected by the bandpassSurface Acoustic Wave (SAW) filter. For each desiredfrequency band, a filter with the appropriate response may beinserted, without needing to adjust the circuit design. In thisapplication, the full GSM 900 frequency band is measured,without further adjustments to the circuit or antenna, allow-ing to measure the incident power density in this particularfrequency band.
D. Calibration
The digitized output voltage of the logarithmic-detector is afunction of the corresponding RF input power. This functionis accurately determined by means of calibration.
The calibration of the RF input level of the exposimeterwithout antenna is performed in an anechoic chamber.Calibration datasets are constructed for each unit sepa-rately and stored into its flash memory for use during theactual exposure measurements. Logarithmic detection resultsin an accurate measurement over a large dynamic range,stored in a limited number of bits per measurement value.
TABLE II
CURRENT CONSUMPTION OF THE MAIN COMPONENTS
OF THE EXPOSIMETER NODE
The Logarithmic RF Detector exhibits 80 dB dynamic range,with a minimum RF input level of −70 dBm. By employingthe calibration data, a 1 dB resolution is achieved. To com-pensate the temperature-dependent offset, the output voltageof the DAC (Analog Devices AD5641 [21]) is automaticallyadjusted as a function of the operating temperature.
E. Exposimeter Synchronization
In the proposed PDE setup, where more than oneexposimeter node is employed on the human test person,synchronization of all the exposimeter nodes is required toachieve an exposure measurement with accurate timestamps.All the exposimeter nodes are equipped with the same24.576 MHz crystal with a frequency stability of 10 PPM.The sample period is directly derived from this main on-boardclock, thereby minimizing the influence of frequency insta-bility over a long time. Synchronization is achieved byconnecting each individual exposimeter node to the PC. ThePC will initialize the timing registers of the microcontroller forderiving the sample period. In addition, a time stamp is placedinto the flash memory based on the PC clock. This ensuresthat all modules composing the exposimeter will sample at thesame time instant within the defined sample period, with onlya minimal deviation. After synchronization, each individualexposimeter immediately starts capturing exposure data. Whenthe measurement campaign is terminated by the user, the dataare transferred to the PC, including the time stamps in the flashmemory. Based on the timestamps, the processing softwarealigns the data samples and starts further data processing.
F. Power Consumption
The exposimeter is powered by a 1-cell Lithiumpolymer (Li-po) battery and a low-drop linear voltageregulator. From the technical data sheets of the integratedcomponents, an estimation of the power consumption is made.An overview of that current consumption is given in Table II.
The microcontroller will consume an average current of4 mA at 3.3 V and at a clock frequency 24.576 MHz. In sleepmode, the current consumption can be reduced to 600 nA.
4396 IEEE SENSORS JOURNAL, VOL. 15, NO. 8, AUGUST 2015
Fig. 5. Free-space frequency response of the vertically polarized exposimeter.
The flash memory consumes on average 12 mA at 3.3 V whileoperating (reading or writing), whereas in standby mode, thecurrent consumption is reduced to 25 μA, or to 5 μA in deeppower-down mode.
The most current-consuming device on the exposimeternode is the RF detector, consuming 31 mA in full operation.In power down mode, its current consumption is lowered toless than 200 μA. The current consumption of the ADC andtemperature compensation circuits is 550 μA and 60 μA,respectively.
The average current consumption is estimated to be 40 mAat 3.3 V, taking into account that the detector is always inpower on mode, while the memory is accessed only once everysecond.
In full operation, the measured average power consumptionof one exposimeter node equals 131 mW (39.6 mA currentconsumption, at 3.3 V supply voltage). This enables the sensornetwork to operate for many hours, without the need forcharging the battery. The power consumption can further bereduced by employing the sleep mode of the system whenthere is no need to continuously operate at high speed.
III. VALIDATION
A. Free-Space Performance
To measure the frequency response at the input ofthe exposimeter, the complete sensor (including verticallypolarized antenna) is placed in an anechoic chamber at4.34 m from the radiating antenna. The TX standard gainhorn (NSI-RFSG975, with a gain of 14 dBi at 942.5 MHz),connected to a signal source with an output power of 10 dBm(cable losses = 3.75 dB), swept over a frequency range from840 MHz to 1040 MHz, is radiating along both horizontal andvertical TX polarizations. The response is shown in Fig. 5,indicating a large attenuation for out-of-band signals. Clearly,the attenuation is very steep on the bottom side of theGSM downlink band, resulting in a rejection by at least 35 dBof 880-915 MHz GSM-900 uplink signals. For frequenciesslightly above the GSM-900 downlink band, a better than23 dB attenuation is also sufficient, considering that, accordingto the band planning, no strong signals are expected adjacent tothe upper end of the GSM downlink band. Since the measuredreceived power on the exposimeter in this anechoic measure-ment is significantly larger than the signals that will actually
Fig. 6. Positions of the four personal exposimeter (PEM) modulescomposing the personal distributed exposimeter (PDE), shown togetherwith the position of the EME Spy 140 onto the body of the test-person.
be measured during a real-world measurement, out-of-bandsignals will be below the noise floor of the exposimeter thanksto the band-pass filtering characteristics. They will not affectthe actual measurements in the desired frequency band.
The clearly visible difference of approximately 12 dBbetween both TX polarizations is due to the vertically polar-ized receive antenna of the node. An ideal exposimeter hasno dependence of the polarization. In order to minimize theinfluence of the received polarizations, several exposimetersare placed onto the body, oriented along orthogonal polariza-tions, as further described and evaluated.
While the transmitted signal of the base station is verticallypolarized, the received polarization will vary due to the angleof arrival on the nodes and the different paths followed by thesignal in the environment. By orienting the nodes along bothpolarizations, all signals received from the base station can becaptured. Furthermore, these linearly polarized antennas areeasier to construct, in comparison to textile patch antennaswith a circular polarization, thereby reducing the cost and thesize of the nodes.
B. On-Body Performance
The PDE is configured for the GSM-900 down linkfrequency band, which is present in most environments. Fourseparate nodes of the PDE are distributed at optimal positionsover the front and rear sections of the torso of a 1.85 m largetest person having a weight of 80 kg, as shown in Fig. 6.The polarizations of the individual nodes are also chosen forcomplementarity.
The person with the four nodes distributed over the bodystands on the rotor inside the anechoic chamber, in the far-fieldof the standard gain horn, radiating at 942.5 MHz, being thecenter frequency of the GSM-900 down-link band, connectedto a signal generator with a transmit power of 10 dBm(cable losses = 3.75dB). The person wearing the PDE isrotated in the azimuth plane over an azimuth angle of 360°.These measurements are repeated for Horizontal (H) andVertical (V) TX polarizations. During these measurements,
VANVEERDEGHEM et al.: COMPACT PERSONAL DISTRIBUTED WEARABLE EXPOSIMETER 4397
Fig. 7. Electric field strength [dB] on all four nodes in the azimuth plane,worn on-body as shown in Fig. 6. 180° = front side of the body, verticallypolarized TX antenna.
Fig. 8. Electric field strength [dB] on all four nodes in the azimuth plane,worn on-body as shown in Fig. 6. 180° = front side of the body, horizontallypolarized TX antenna.
a commercially available secondary exposimeter(EME Spy 140) is worn at waist-height. The distancefrom the middle of the human test subject to the aperture ofthe horn antenna is 4.34 m.
These measurements are plotted in a logarithmic scale,shown in Figs. 7 and 8, for TX Vertical and Horizon-tal polarizations, respectively. Besides the field strength oneach single node, the averaged field strength over the fournodes is calculated in the azimuth plane for each azimuthangle ϕ, for both horizontal and vertical polarizations. Theaverage field strength of the proposed PDE, calculated foreach angle in the azimuth plane, is approximately constant,making the exposimeter output independent of the transmitpolarization.
In order to determine the dependence on the polarizationwhen worn on-body, the standard deviation σpol is calculatedbased on the difference in the received field strength of bothpolarizations. It is determined for both the “EME Spy 140”
Fig. 9. Normalized electric field strength [dB] on the PDE and“EME Spy 140”. 180° = front side of the body, TX vertical polarization.
and for the PDE, by averaging the logarithmic field strengthover the four nodes. This results in
σP DE di f f H/V = 2.32 d B
σE M E Spy 140 di f f H/V = 3.73 d B
This allows us to conclude that the PDE is less polarizationdependent than the “EME Spy 140”. To compare the PDEand the “EME Spy 140” in an on-body scenario, the standarddeviation σ of the field strength over different azimuth anglesis calculated for both horizontal as well as vertical polarization.σ indicates how circular the pattern is in azimuth angle.A pattern that is perfectly omnidirectional results in a standarddeviation of 0 dB. σϕ is derived from the (Logarithmic) fieldstrength on the four nodes, resulting in
σP DE Hor = 2.70 d B
σP DE V ert = 3.35 d B
σE M E Spy 140 Hor = 6.94 d B
σE M E Spy 140 V ert = 9.13 d B
We clearly obtain a better performance for the PDE incomparison to the commercial exposimeter. In Figs. 9 and 10,the normalized field strengths, for both the PDE and the“EME Spy 140”, are plotted for a vertically and horizontallypolarized transmitted signal, respectively. This visually verifiesthe above results. The PDE clearly achieves a more uniformdistribution of the field strength over the azimuth plane.
IV. CALIBRATION
As discussed above, the power received by the PDE isalmost constant, independent of polarization or azimuth anglefor a given transmit power. To obtain an accurate measurementresult of the actual RF field strength at the location ofthe human body in a real environment, the PDE requirescalibration, which is performed in an anechoic chamber.The calibration eliminates the influence of the body on thePDE measurement.
4398 IEEE SENSORS JOURNAL, VOL. 15, NO. 8, AUGUST 2015
Fig. 10. Normalized electric field strength [dB] on the PDE and“EME Spy 140”. 180° = front side of the body, TX horizontal polarization.
The measurements in the anechoic chamber, for bothhorizontal and vertical TX polarizations, used earlier tovalidate the exposimeters, are now employed to cali-brate the PDE when performing on-body measurements.PH
geom (ϕ) and PVgeom (ϕ) are the geometric average received
powers for the horizontally and vertically transmit polarizationas a function of the azimuth angle, respectively. Furthermore,the free-space incident powers SH
inc and SVinc are measured
using the NBM-550 broadband probe, for both polarizationsat the TX horn antenna, as described in section III.B for thePDE, but now with the broadband probe at exactly the samelocation.
From the calibration measurements, the geometric averageAntenna Aperture (AAgeom) of the total PDE is determined,given by
AAgeom (ϕ,ψ) = P Hgeom (ϕ)
SHinc
cos2 (ψ) + PVgeom (ϕ)
SVinc
sin2 (ψ)
where SVinc = 0.541 mW/m2 and SH
inc = 0.394 mW/m2,and with ψ the polarization of an incident electric field.AAgeom (ϕ, ψ) is calculated for 103 ϕ-samples, located inthe interval [0−2π] radians. The ψ samples are drawnfrom a Gaussian distribution in an “Urban Macro cell”scenario [3], [22], in order to take into account a realistic polar-ization of the incident electric field. This scenario provides thebest correspondence to earlier measurements performed in thecity of Ghent [6].
The set of values resulting from this procedure provides thedistribution of AAgeom for realistic angles of arrival. Fromthis distribution, the median is chosen as the value of theAntenna Aperture (AAgeom) of the total PDE. In addition,the full calibration procedure is repeated 100 times, and theresults are averaged in order to improve accuracy. Based onthis calibration process, the average value of AAgeom is foundto be 6.58 cm2.
Once the value of AAgeom is determined, a real world mea-surement can start. After this measurement, the incident power
received on the body of the test person can be determined by
Sinc = Pgeom
AAgeom
where Pgeom is the geometric average received power onthe four nodes of the PDE, observed during the real worldmeasurement.
V. REAL-WORLD MEASUREMENTS
To perform a real-world measurement, the same test personas in previous measurements, equipped with the PDE and the“EME Spy 140”, walks along a predefined path in the city-center of Ghent (Belgium). During this walk, the receivedpower is recorded on all the exposimeter nodes as well as onthe “EME Spy 140”, in a time interval of 1 second. Fig. 11shows the total power density received by the test person withthe calibrated PDE during the complete walk, as well as thepower density on the “EME Spy 140”. In Fig. 12, the 2.6 kmoutdoor trajectory through Ghent, followed by the test person,is shown. On this map, the position markers corresponding tothe numbers in Fig. 11, as well as the position of the nearbyGSM-900 base-stations [23], are shown.
The total power density, received during the walk, is deter-mined based on the active exposimeter nodes of the PDE, afterapplying the calibration procedure described in Section IV.The power density of the “EME Spy 140” is extractedfrom the measurement logging file. The measurement resultsclearly show that the received powers of both measurementdevices exhibit the same trend, but with short-term differencesin power density levels. The shadowing by the body hasa significant influence on the measurement results by thecommercial “EME Spy 140”. As stated earlier in Section III,the signals received on the “EME Spy 140” are dependenton the angle of arrival of the signals in the azimuth plane.The omnidirectional receive pattern, which is obtained withthe PDE, ensures a more accurate estimation of the powerdensity levels in comparison to a non-distributed device suchas the “EME Spy 140”. To verify whether the “EME Spy 140”indeed underestimates the RF field exposure levels, two addi-tional experiments were performed. To demonstrate that theunderestimation due to body shadowing occurs in a generalcase, the experiments were performed in a different partof the city. First, the “EME Spy 140” was deployed intwo setups. On the one hand, an on-body measurement wasperformed by wearing the exposimeter near the waist of theuser. On the other hand, the exposimeter was held above thehead, carrying out a measurement that is certainly is lessinfluenced by the body. During the on-body measurement, anaverage power density level of −45.5 dBW/m2 was measured.In contrast, the second measurement yielded an average powerdensity level of −43 dBW/m2. Hence, a difference of 2.5 dBwas obtained between both situations, clearly showing theinfluence of the body on the exposimeter, resulting in anunderestimation of the exposure. Next, static measurementswere performed by the “EME Spy 140” on a tripod, bya spectrum analyzer with a calibrated reference antenna(Rohde & Schwarz TSEMF-B1), and by our PDE,worn on-body. All devices were positioned in the line
VANVEERDEGHEM et al.: COMPACT PERSONAL DISTRIBUTED WEARABLE EXPOSIMETER 4399
Fig. 11. Received power density [dBW/m2] on the body of the test person, measured with the active nodes of the PDE versus the commercially availableexposimeter. The position markers corresponding to the numbers in Fig. 12.
Fig. 12. Map of Ghent with the path walked by the test person,with numbered position markers and GSM-900 basestation symbols,(source: Google Earth).
of sight of a GSM base station. The spectrum analysermeasured an average power density of −37.4 dBW/m2,whereas the “EME Spy 140” yielded an average power densityof −41.9 dBW/m2. The 4.5 dB difference in the resultssuggests that the EME slightly underestimates the fieldstrengths, even without the presence of the human body.We conclude that the total difference of 7.0 dB, betweenthe spectrum analyser and the “EME Spy 140” on the body,approximately corresponds to the difference between the PDEand the on-body “EME Spy 140”. The average power densitylevel measured by the on-body PDE equals −38.0 dBW/m2,yielding only a slight measurement difference of 0.6 dBwith respect to the spectrum analyser. Table III summarizesthese average power density levels, obtained during the staticmeasurements campaign.
The maximum instantaneous power density measured bythe PDE during the experiment is 28 mW/m2. As a result,
TABLE III
AVERAGE POWER DENSITY LEVELS OBTAINED
DURING STATIC MEASUREMENT
the average power density levels over a 6-minute time frameare well below the ICNIRP reference level of 4.8 W/m2 [1].Furthermore, from these measurements, the SARwb can bedetermined, as described in [16] and [17].
VI. CONCLUSION
A compact wearable Personal Distributed Exposimeteris proposed, which increases the measurement accuracyin comparison to conventional Personal Exposimeters,including the dependency on the polarization and the anglein the azimuth plane. The Personal Distributed Exposimeteris composed of several newly designed on-body exposimetermodules, which are integrated onto the feed plane of a textileantenna. The different modules apply synchronous exposuredata sampling, while being unobtrusively integrated inside agarment and being distributed over the body of the test person.Therefore, this new compact exposimeter is a step forwardtowards user-friendly Personal Distributed Exposimeters inmultiple frequency bands, integrated into a single garment formeasuring exposure data in a convenient way. Validation ofthe Personal Distributed Exposimeter shows that the systemexhibits less dependence of the received polarization or theangle of the azimuth plane, compared to commercial avail-able exposimeters. A fast and accurate calibration process isproposed, to eliminate the influence of the body onto the PDE.
To validate the measurements performed by the PersonalDistributed Exposimeter, a real world exposure measurement
4400 IEEE SENSORS JOURNAL, VOL. 15, NO. 8, AUGUST 2015
was carried out for the GSM-900 downlink band.The measurement is performed in the city center ofGhent, whose propagation characteristics correspond to anUrban Macro Cell. The measurement was also carried outemploying an “EME Spy 140” commercial exposimeter.This experiment clearly illustrates that the PDE provides amore accurate estimation of the power density levels on thehuman body. The commercial, non-calibrated exposimeterdeployed on the body influences the measurement resultsdue to shadowing by proximity of the body, leading to anunderestimation of the power density levels on the humanbody. The maximum instantaneous power density measuredby the PDE during the experiment equals 28 mW/m2, whichis well below the ICNIRP reference level of 4.8 W/m2 for anaverage power density level in a 6-minute time frame. Basedon these measurement data, the whole-body SAR is readilydetermined.
Besides for verifying compliance of RF field exposurewith ICNIRP reference levels, the proposed modules can alsoserve as sensor nodes to evaluate the potential of RF energyharvesting [24]–[27] and wireless power transfer [28].
REFERENCES
[1] International Commission on Non-Ionizing Radiation Protection,“Guidelines for limiting exposure to time-varying electric, magnetic, andelectromagnetic fields (up to 300 GHz),” Health Phys., vol. 74, no. 4,pp. 494–522, 1998.
[2] W. Joseph, G. Vermeeren, L. Verloock, and L. Martens, “Estimation ofwhole-body SAR from electromagnetic fields using personal exposuremeters,” Bioelectromagnetics, vol. 31, no. 4, pp. 286–295, May 2010.
[3] G. Vermeeren, W. Joseph, C. Olivier, and L. Martens, “Statistical mul-tipath exposure of a human in a realistic electromagnetic environment,”Health Phys., vol. 94, no. 4, pp. 345–354, 2008.
[4] D. Urbinello, A. Huss, J. Beekhuizen, R. Vermeulen, and M. Röösli,“Use of portable exposure meters for comparing mobile phone basestation radiation in different types of areas in the cities of Baseland Amsterdam,” Sci. Total Environ., vols. 468–469, pp. 1028–1033,Jan. 2014.
[5] G. Thuróczy et al., “Personal RF exposimetry in urban area,” Ann.Telecommun.-Ann. Télécommun., vol. 63, nos. 1–2, pp. 87–96, 2008.
[6] W. Joseph, G. Vermeeren, L. Verloock, M. M. Heredia, and L. Martens,“Characterization of personal RF electromagnetic field exposure andactual absorption for the general public,” Health Phys., vol. 95, no. 3,pp. 317–330, 2008.
[7] A. Thielens et al., “On-body calibration and processing for a combina-tion of two radio-frequency personal exposimeters,” Radiat. ProtectionDosimetry, vol. 163, no. 1, pp. 58–69, Jan. 2015.
[8] S. Iskra, R. McKenzie, and I. Cosic, “Factors influencing uncertaintyin measurement of electric fields close to the body in personal RFdosimetry,” Radiat. Protection Dosimetry, vol. 140, no. 1, pp. 25–33,Feb. 2010.
[9] J. F. Bolte, G. van der Zande, and J. Kamer, “Calibration and uncertain-ties in personal exposure measurements of radiofrequency electromag-netic fields,” Bioelectromagnetics, vol. 32, no. 8, pp. 652–663, 2011.
[10] J. F. Bolte and T. Eikelboom, “Personal radiofrequency electromagneticfield measurements in the Netherlands: Exposure level and variabilityfor everyday activities, times of day and types of area,” Environ. Int.,vol. 48, pp. 133–142, 2012.
[11] A. Thielens et al., “Personal distributed exposimeter for radio frequencyexposure assessment in real environments,” Bioelectromagnetics, vol. 34,no. 7, pp. 563–567, 2013.
[12] P. Salonen, L. Sydanheimo, M. Keskilammi, and M. Kivikoski, “A smallplanar inverted-F antenna for wearable applications,” in 3rd Int. Symp.Wearable Comput. Dig. Papers, Oct. 1999, pp. 95–100.
[13] F. Declercq, A. Georgiadis, and H. Rogier, “Wearable aperture-coupledshorted solar patch antenna for remote tracking and monitoring applica-tions,” in Proc. 5th Eur. Conf. Antennas Propag. (EUCAP), Apr. 2011,pp. 2992–2996.
[14] C. Hertleer, A. Tronquo, H. Rogier, L. Vallozzi, and L. Van Langenhove,“Aperture-coupled patch antenna for integration into wearable textilesystems,” IEEE Antennas Wireless Propag. Lett., vol. 6, pp. 392–395,2007.
[15] V. Rathi, G. Kumar, and K. P. Ray, “Improved coupling for aperturecoupled microstrip antennas,” IEEE Trans. Antennas Propag., vol. 44,no. 8, pp. 1196–1198, Aug. 1996.
[16] A. Thielens et al., “Whole-body averaged SAR assessment using a per-sonal, distributed exposimeter,” in Proc. Joint Meeting Bioelectromagn.Soc. Eur. BioElectromagn. Assoc. (BioEM), 2014, pp. 417–422.
[17] A. Thielens et al., “Whole-body averaged specific absorption rateestimation using a personal, distributed exposimeter,” IEEE AntennasWireless Propag. Lett., to be published.
[18] P. Vanveerdeghem, P. Van Torre, C. Stevens, J. Knockaert, and H. Rogier,“Flexible dual-diversity wearable wireless node integrated on a dual-polarised textile patch antenna,” IET Sci., Meas. Technol., vol. 8, no. 6,pp. 452–458, May 2014.
[19] M. L. Scarpello, I. Kazani, C. Hertleer, H. Rogier, and D. Vande Ginste,“Stability and efficiency of screen-printed wearable and washable anten-nas,” IEEE Antennas Wireless Propag. Lett., vol. 11, pp. 838–841, 2012.
[20] ADL5513 Datasheet, Analog Devices, Norwood, MA, USA, 2008.[21] AD5641 Datasheet (Rev. D), Analog Devices, Norwood, MA, USA,
2008.[22] A. Thielens, G. Vermeeren, W. Joseph, and L. Martens, “Stochastic
method for determination of the organ-specific averaged SAR in real-istic environments at 950 MHz,” Bioelectromagnetics, vol. 34, no. 7,pp. 549–562, 2013.
[23] Antenna Site Register, Belgian Institute for Postal Services andTelecommunications, Brussels, Belgium, 2014.
[24] T. Le, K. Mayaram, and T. Fiez, “Efficient far-field radio frequencyenergy harvesting for passively powered sensor networks,” IEEE J.Solid-State Circuits, vol. 43, no. 5, pp. 1287–1302, May 2008.
[25] J. Zbitou, M. Latrach, and S. Toutain, “Hybrid rectenna and monolithicintegrated zero-bias microwave rectifier,” IEEE Trans. Microw. TheoryTechn., vol. 54, no. 1, pp. 147–152, Jan. 2006.
[26] G. Andia Vera, A. Georgiadis, A. Collado, and S. Via, “Design of a2.45 GHz rectenna for electromagnetic (EM) energy scavenging,” inProc. IEEE Radio Wireless Symp. (RWS), Jan. 2010, pp. 61–64.
[27] A. Nimo, D. Grgic, and L. M. Reindl, “Optimization of passive lowpower wireless electromagnetic energy harvesters,” Sensors, vol. 12,no. 10, pp. 13636–13663, Oct. 2012.
[28] N. Carvalho et al., “Wireless power transmission: R&D activitieswithin Europe,” IEEE Trans. Microw. Theory Techn., vol. 62, no. 4,pp. 1031–1045, Apr. 2014.
Peter Vanveerdeghem was born in 1986.He received the M.Sc. degree in electronics engi-neering from the University College West-Flanders,Belgium, in 2008, and the M.Sc. degree in electricalengineering and the Ph.D. degree from GhentUniversity, Ghent, Belgium, in 2011 and 2015,respectively. His research interests are the designof embedded systems and transceivers on textile-antenna platforms and body-centric wirelesscommunication.
Patrick Van Torre received the Electrical Engineer-ing and Ph.D. degrees from Ghent University, Ghent,Belgium, in 1995 and 2012, respectively. He iscurrently a Lecturer and a Post-Doctoral Researcherwith the Department of Information Technology,iMinds/Ghent University. He lectures theory coursesin electronics and ICT, organizes project-oriented labsessions and is involved in public relations activitiesand hardware development projects for third parties.As a Researcher, he is involved in the field ofwireless communication, focusing on body-centric
multiple-input multiple-output and beam-forming systems, the design ofradio-frequency exposimeters, and the integration of embedded systems andtransceivers on textile-antenna platforms.
VANVEERDEGHEM et al.: COMPACT PERSONAL DISTRIBUTED WEARABLE EXPOSIMETER 4401
Arno Thielens was born in Brasschaat, Belgium,in 1987. He received the M.Sc. degree inapplied physics from Ghent University, Ghent,Belgium, in 2010. Since 2010, he has been aResearch Assistant with iMinds/Ghent University.His scientific work focuses on numerical dosimetryand exposure assessment of radio frequencyelectromagnetic fields.
Jos Knockaert received the Industrial Engineerdegree in electrotechnics from the UniversityCollege of Bruges-Ostend, in 1996, the M.Sc. degreein electronic system design from Leeds MetropolitanUniversity, in 2001, and the Ph.D. degree in engi-neering science from the Katholieke UniversiteitLeuven, in 2009. He was an EMC-Design Engineerin the industry and became an Assistant with theUniversity College of Bruges-Ostend. Since 2010,he has been an Assistant Professor with GhentUniversity, teaching electrical machines, electro-
magnetic compatibility, and power electronics. He is a member of theResearch Group Lemcko with a focus on electromagnetic compatibility, powerelectronics, and high-frequency problems in industrial systems.
Wout Joseph (M’05–SM’12) was born in Ostend,Belgium, in 1977. He received the M.Sc. degreein electrical engineering and the Ph.D. degreefrom Ghent University (UGent), Ghent, Belgium,in 2000 and 2005, respectively. From 2000 to 2005,he was a Research Assistant with the Departmentof Information Technology (INTEC), iMinds/UGent.His scientific work was focused on electromagneticexposure assessment. His research work dealt withmeasuring and modeling of electromagnetic fieldsaround base stations for mobile communications
related to the health effects of the exposure to electromagnetic radiation.Since 2005, he has been a Post-Doctoral Researcher with INTEC,iMinds/UGent. From 2007 to 2013, he was a Post-Doctoral Fellow of ResearchFoundation-Flanders. Since 2009, he has been a Professor in the domain ofexperimental characterization of wireless communication systems. His pro-fessional interests are electromagnetic field exposure assessment, propagationfor wireless communication systems, antennas, and calibration. Furthermore,he specializes in wireless performance analysis and quality of experience.
Hendrik Rogier was born in 1971. He receivedthe Electrical Engineering and Ph.D. degreesfrom Ghent University, Ghent, Belgium,in 1994 and 1999, respectively. From 2003 to 2004,he was a Visiting Scientist with the MobileCommunications Group, Vienna University ofTechnology. He is a currently a Full Professorwith the Department of Information Technology,iMinds/Ghent University, a Guest Professor withIMEC, Heverlee, Belgium, and a Visiting Professorwith the University of Buckingham, U.K. He has
authored or co-authored about 90 papers in international journals and about110 contributions in conference proceedings. He serves as a member ofthe Editorial Board of IET Science, Measurement Technology and actsas the URSI Commission B representative for Belgium. Within the IEEEMicrowave Theory and Techniques Society, he is a member of TechnicalCommittee 24 on RFID technology, and within the European MicrowaveAssociation, he is a member of the Governing Board of Topical GroupMAGEO on Microwaves in Agriculture, Environment and Earth Observation.His current research interests are antenna systems, radiowave propagation,body-centric communication, numerical electromagnetics, electromagneticcompatibility, and power/signal integrity.
