4056 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 60, NO. 9, SEPTEMBER 2012
Passive UHF RFID Tag for Heat Sensing ApplicationsAbdul Ali Babar, Student Member, IEEE, Sabina Manzari, Student Member, IEEE,
Lauri Sydänheimo, Member, IEEE, Atef Z. Elsherbeni, Fellow, IEEE, and Leena Ukkonen, Member, IEEE
Abstract—A possible method of utilizing paraffin wax as a sub-strate material in developing a threshold heat sensing radio fre-quency identification (RFID) tag is discussed. A small narrowbandpassive UHF RFID tag is made on top of a multilayer substrate.Paraffin wax acts as the main heat sensitive layer of the substrate.The properties and characteristics of the paraffin layer change dueto heat. The narrowband tag on top of the substrate is designed tobe sensitive enough to detect any structural and physical changesof the substrate material. The changes in the properties of the sub-strate material will cause a shift in the operating frequency of thetag. This frequency shift will reduce the performance of the nar-rowband RFID sensor tag. The change in the properties of paraffinwax after being exposed to heat is irreversible under normal con-ditions and therefore, the proposed RFID tag can be referred toas a threshold heat sensing device. Such a low-cost solution canbe useful in detecting heat exposures in various supply chains andtransportation mishandling of heat sensitive items.
Index Terms—Heat sensor, low cost, paraffin wax, radio fre-quency identification (RFID).
I. INTRODUCTION
T HE rapid progress and developments in the field of wire-less communications and identification have made it pos-
sible to track and sense various materials wirelessly. The use ofradio frequency identification (RFID) technology as an effec-tive and reliable way for tracking and sensing is gaining muchimportance in recent years. There are two main components inan RFID system, the reader or scanner and the tag. The com-monly used passive RFID tag is composed of an IC chip and anantenna, which contains its own unique identification code (ID)[1]. This identification code is sent back to the reader when thetag is interrogated and energized through backscattered mod-ulation of the incident continuous wave [2]. The passive UHFRFID tag’s input impedance and reflectivity rely on the phys-ical properties of its substrate or materials it is attached to. Thedependence of an RFID tag on its substrate and material prop-erties can be exploited in sensing various behavioral changesusing analog processing of the physical signals [2]. Therefore,
Manuscript received June 29, 2011; revised October 21, 2011; acceptedFebruary 25, 2012. Date of publication July 03, 2012; date of current versionAugust 30, 2012. This work was supported in part by the Finnish FundingAgency for Technology and Innovation (TEKES), the Academy of Finland andthe Centennial Foundation for Finnish Technology Industries, Finnish CulturalFoundation, Nokia Foundation, and High Technology Foundation of Satakunta,Finland.A. A. Babar, S. Manzari, L. Sydänheimo, and L. Ukkonen are with the Rauma
Research Unit, Tampere University of Technology, Tampere 33720, Finland(e-mail: [email protected]).A. Z. Elsherbeni is with Electrical Engineering Department, The University
of Mississippi, University, MS 38677-1848 USA. (e-mail: [email protected]).Color versions of one or more of the figures in this paper are available online
at http://ieeexplore.ieee.org.Digital Object Identifier 10.1109/TAP.2012.2207045
the change in the substrate material properties due to several en-vironmental conditions can affect the RFID tag by changing itsoperating frequency. This eventually affects the read range, re-alized gain and threshold power required to turn on the IC, atthe desired frequency. This feature can be usefully utilized indeveloping several types of passive UHF RFID sensor antennasystems [3], [4].Heat is considered as an important environmental parameter
to be taken care of in many products, such as in pharmaceu-tical drugs and perishable food items. These products can bespoiled due to improper handling and exposure to excessiveheat in storage and in transit during supply chain operationsand transportation. There are several different types of batteryand memory operated sensors used to monitor excess heat andtemperatures for heat sensitive products. Some RFID IC chipshave integrated temperature sensor functionalities [5], [6].These types of temperature sensor tags need more power to turnon the IC, affecting the overall read range in the passive mode,due to its additional functionalities and complexity. In additionto that the tag needs continuous RF power or an external batteryto enable the IC chip to log the temperature. These types oftemperature sensors are expensive, need continuous power andcannot be used on every product or package in supply chain andother inventory systems [7]. This results in a risk of spoilingseveral consumer products.In this study, a low loss antenna and tag substrate is utilized
to develop a heat sensing RFID tag. The substrate is made ofdifferent layers, where paraffin wax behaves as the main heatsensitive part of the substrate. The main idea of this type of heatsensor is that the overall performance of the RFID tag and itsoperational frequency gets affected when exposed to high tem-peratures. Exposing this type of substrate to high temperatureswill change the physical and chemical properties of the paraffinwax. These changes will eventually result in changing the di-electric properties of the substrate, causing a shift in the tag’soperating frequency. The shift in the operating frequency is ir-reversible in normal conditions, unless the paraffin layer of thesubstrate is retreated and made, as mentioned in Section III-B.This feature makes the tag antenna more like a threshold tem-perature sensor, optimized for a certain frequency, avoiding theneed of a broadband interrogation system. To make this changemore prominent, a sensitive narrowband passive RFID tag is de-veloped. This type of sensitive narrowband tag can be useful indeveloping low-cost heat sensing tags, by sensing the change inthe dielectric properties of the substrate.These low-cost passive RFID tags are useful in detecting heat
exposures of several heat sensitive products like daily consumeritems, drugs and other perishable food items. In practical appli-cations the sensing tag should be carefully packaged with theproduct, to avoid any accidental damage to the tag. This can be
BABAR et al.: PASSIVE UHF RFID TAG FOR HEAT SENSING APPLICATIONS 4057
done in various ways such as, using some special type of casingfor the sensing tag, or placing the tag inside the package, de-pending on the type of product. However, in many applicationsthe use of external force can also damage the product inside thepackage, as well as the sensing tag. This will make the tag un-readable, indicating that the quality of the product has been low-ered or been damaged. This feature can also be useful in mon-itoring the quality of various food items in supply chain opera-tions. The readability of the tag after being heated can also becontrolled by limiting the transmitted power.In Section II, various characteristics and properties of paraffin
wax will be discussed. Sensor tag antenna design will be shownin Section III. This will be followed by the sensing principle ofthe sensor tag in Section IV. Section V will show the results andmeasurement methods of the tag antenna, followed by the heatmeasurements and their results on various consumer productsin Section VI. Conclusions will be stated in Section VII.
II. CHARACTERISTICS AND PROPERTIES OF PARAFFIN
Paraffins are straight-chains or branched saturated organiccompounds [8]. They are a form of hydrocarbons. Compositionof organic compounds in a paraffinic hydrocarbon is .The state of paraffin can be determined with its average molec-ular weight. Solid form of paraffin at room temperature, also re-ferred as paraffin wax, is considered to have a molecular weightfrom to . Paraffin wax is mainly a mixture ofparaffins and cyclo-alkanes [8], [9]. It is mostly found as whiteodorless, tasteless, waxy solid, at the room temperature.In the electrical industry paraffin waxes have been used
mainly for insulation. Among their characteristics are lowdielectric loss, high resistivity values, flexibility, ductility,and low thermal expansion coefficient [8]–[11]. Paraffin waxwith other additives can be used to make blocking layers forcapacitors, cable terminals and couplings, for impregnatingcable-insulation paper, filling the spaces between cables andaround the coupling [8].The paraffin waxes can be classified based on its molecular
weight, melting point and the mixture of other additives such asthe percent of oil content. The melting point of paraffin wax isdirectly proportional to its molecular weight. However the ad-dition of additives can also play a significant role in changingthe melting point and other characteristics of paraffin wax. Ac-cording to [12], the relationship from n-alkanes and meltingpoint is shown in (1)
(1)
where “ ” is the melting point in degrees Celsius and “ ”is the molecular weight of the paraffin wax. The melting pointof paraffin wax can vary approximately in the range from 40 Cto 80 C [8]–[11].According to [13], paraffin wax can acquire three different
forms of crystal shapes, plate crystal, mal crystals, and needlecrystals. The shape of the crystals can be transformed into oneanother depending on the temperature, molecular structure, andvarious heating and cooling techniques [8], [13].Temperature plays a major role in changing the characteris-
tics of paraffin wax. The characteristics of paraffin wax such as
TABLE IPHYSICAL CHARACTERISTICS [8]
density, dielectric constant, crystal shape, and molecular struc-ture can change with the change in temperature. The increase intemperature expands the volume of paraffin wax and decreasesits density, depending on its melting point with respect to theroom temperature. The decrease in density means fewer num-bers of molecules per unit volume, causing less interaction withthe electric fields and therefore, resulting in a decrease in thedielectric constant value [9], [14].According to [8], some of the physical characteristics of
paraffin wax are summarized in the following Table I.In Table I, the densities of paraffin waxes at two different
states are shown. Similarly the typical expansion coefficient insolid state and expansion at melting can also be seen.
III. UHF HEAT SENSOR TAG DESIGN
A. Tag Antenna Design
The tag antenna has been designed using Ansoft HFSS v. 12[15]. The tag was designed and fabricated on a multilayer sub-strate, constituting FR4, thin plastic bag and a layer of paraffinwax, as discussed in Section III-B. Alien Higgs 2 IC was usedin the tag antenna [16]. The sensitivity of the IC ( ) is equalto 14 dBm [16]. Parasitic capacitance of 0.2 pF was con-sidered in parallel with the nominal IC impedance, in orderto include the effect of IC strap impedance into account. Thetag was matched to a center frequency of 870 MHz using a-matching technique. The overall dimension of the tag is equalto 44 mm 30 mm. The geometrical parameters are shown inFig. 1.Fig. 1, shows a symmetric dipole structure type tag. The IC
of the tag antenna lies in the middle of the two “ ” lines. Thefeeding lines “ ” are connected to the two arms of the dipole-structure. -matching technique is used to miniaturize the
overall size of the antenna with the help of a shorted line “ ,”through lines “ .” The shorting line between the two dipolearms helps in tuning the tag antenna to the desired frequency of870 MHz, by introducing additional inductance to the antenna.-matching network works as an impedance transformer [17].The length and thickness of line “ ” and the distance from themain dipole antenna’s -structure “ ,” are of considerable im-portance in the matching of the tag antenna.To further reduce the size of the antenna, the antenna structure
is given a -structure by using lines “ ,” folded inwards withthe help of lines “ ” and “ .” This makes the tag antenna morecompact and gives the current a longer path to accommodate
4058 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 60, NO. 9, SEPTEMBER 2012
Fig. 1. Geometrical parameters of the designed tag antenna.
TABLE IIGEOMETRICAL PARAMETERS OF THE TAG
the desired wavelength. Slight tuning of the antenna can also beachieved by changing the length of line “ .”The small size and the use of thin microstrip lines contribute
in making the tag antenna narrowband. This feature is of con-siderable importance in designing the proposed sensor tag. Thenarrowband feature of the antenna makes it more sensitive toany structural or dielectric changes in the substrate. In the pro-posed sensor tag design, the temperature affects and changes thedielectric properties of the paraffin wax layer of the substrate,as mentioned in Section IV. Due to this change the dielectricconstant of the whole substrate is changed, which can easily beseen by the shift in the resonance of the narrowband tag antenna,making it detuned. The geometrical dimensions of the tag arelisted in Table II.
B. Substrate Design
The tag antenna is designed and fabricated on a 0.16-mm-thick FR4 substrate with 35- copper cladding using a millingmachine. It is then attached to a paraffin wax substrate of 1 mmthickness and placed in a 0.1-mm-thick vacuumed plastic bag,as shown in Fig. 2. A thin layer of commercially available glueis used in between the FR4 and plastic layer vacuuming paraffin
Fig. 2. Side view of the tag antenna substrate.
Fig. 3. Tag antenna.
wax layer. A thin transparent adhesive tape was used on top ofthe tag antenna to ensure the attachment of the layers, shownin Fig. 3. The effect of glue and adhesive tape is considerednegligibly small on the antenna’s performance.Paraplast type of paraffin wax is used to make the substrate
layer by melting the waxy pellets of paraffin into a cake frame[18]. According to the specification provided by the supplier, thesolid Paraplast paraffin wax is 99% pure with 1% of additives[18]. The properties of paraffin start changing at 36 C–40 C.In this study the paraffin wax was observed to start meltingaround 50 C–56 C. However, According to the literature, thedielectric constant value of paraffinwax can be between 2–2.6 atroom temperature, depending on the type of paraffin wax and itsmolecular weight [8], [9]. The relative dielectric constant valueof Paraplast paraffin wax in this experiment was approximately
at room temperature, as measured by the Agilent di-electric probe [19]. This was verified by various simulations andmeasured experimental results [20].The paraffin wax substrate was placed in a vacuumed plastic
bag to contain the paraffin and to avoid spilling during heat mea-surements. The dielectric constant of the plastic bag was ana-lyzed to be around 3, whereas the relative dielectric constant ofthin FR4 was set to 4.1.
IV. SENSING PRINCIPLE OF THE TAG
There are various types of heat sensors available to monitorthe heat behavior of a product. Currently many of these heat andtemperature sensors are battery powered with application pro-cessors, antennas and memory units [21]–[23]. These types ofsensors, equipped with discrete electronic components, increasethe price of the whole sensor unit. Therefore it is less feasible toinstall such sensor units on every consumer product.
BABAR et al.: PASSIVE UHF RFID TAG FOR HEAT SENSING APPLICATIONS 4059
A. Heat Sensing Tag Antenna
In the proposed heat sensing tag antenna design, the mainobjective is to develop a low-cost temperature threshold sensor.This type of sensor tag detects the exposure of various goods andproducts to high heat environments by detuning the sensor tag.The detuning affects the read range, required threshold power,and realized gain of the antenna at the operating frequency. Thesensor tag once exposed to heat remains detuned even after thetag is brought back to its previous temperature and state.To design such a sensor tag, a multilayer substrate approach is
utilized, as shown in Fig. 2. Due to the exposure of the sensor tagto heat, various physical characteristics of paraffin wax change,as explained in Section II. The change in the characteristics ofparaffin wax substrate layer affects the whole substrate of thesensor tag. This eventually affects the impedance of the tag an-tenna and results in changing the operating frequency of the tagantenna. Once exposed to heat, the paraffin wax somehow re-tains the changes in its physical characteristics, unless repro-cessed and formed, as stated in Section III-B [8]. This results ina detuned tag antenna, even if it is brought back to its previoustemperature. To maximize the effects of change in the resonancefrequency of the tag, a narrow band tag antenna is designed andfabricated, as shown in Fig. 1.
B. Electromagnetic Model
The variations in the electrical properties of any layer of amultilayer substrate can change the reflection coefficient be-tween the two layers, resulting in a change in the total effectivereflection coefficient of the multilayer substrate. Therefore wecan say that the effective reflection coefficient of amultiple layerdielectric substrate is a function of operating frequency, depthfrom the antenna, permittivity, and magnetic permeability of thelayers [7], [24].This change also affects the Poynting power, which relates
to the change in the signal strength of the backscattered powerfrom the tag and therefore a change in the threshold transmittedpower from the reader required to power up the tag [24].
V. RESULTS AND MEASUREMENTS
This section explains the results and measurement tech-niques used to measure the RFID sensor tag antenna. Thesensor RFID tag was measured using Tagformance RFIDmeasurement device, connected to a linearly polarized readerantenna [25]. Fig. 3 shows the fabricated tag antenna used forthe measurements.
A. Sensor Tag Antenna Simulated Results
The transfer of power between the complex sourceimpedance (tag antenna) and the complex load impedance(IC) is analyzed using the following (2). To analyze the powerreflection coefficient, we consider the definition provided in[26], [27] such that
(2)
In (2), is the impedance of the tag antenna,whereas is the impedance of the tag chip.
Fig. 4. Simulated power reflection coefficient of the RFID tag antenna.
Fig. 5. Simulated chip and tag impedance of the RFID sensor tag.
The superscript ( ) denotes the complex conjugate. To have anoptimal power transfer and maximum read range, it is desirableto have a lower value of the power reflection coefficient at theoperating frequency band.Fig. 4, shows the simulated power reflection coefficient of
the tag antenna, designed to operate at a center frequency of 870MHz. As illustrated in the figure the tag has a narrow bandwidth,which gives an advantage of sensing even a small change in thedielectric substrate.The input impedance of the sensor tag antenna and chip are
shown in Fig. 5. The figure shows how the tag antenna param-eters are carefully designed to provide a good conjugate matchwith the chip impedance at 870 MHz. This is required to mini-mize the reflection loss at this junction, and hence improve thepower transmission and maximize the read range.The change in the dielectric constant values of the paraffin
wax layer of the substrate, due to heating, changes the dielectricconstant of the whole substrate [8], [17]. This affects the oper-ating frequency of the tag antenna by a change in the impedanceof the tag antenna. The shift in the resonance frequency of thetag antenna, due to possible change in the dielectric constantvalues of the paraffin wax layer in the proposed design, due toheating, can be seen in Fig. 6 [8], [17]. The paraffin wax usedin the proposed antenna substrate shows a dielectric constantchange from around 2.1 to 1.8 ( ), due to heating, as shownfrom the following measured results in Section V-D. However,the dielectric constant values may also vary for other types ofparaffin products.
4060 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 60, NO. 9, SEPTEMBER 2012
Fig. 6. Simulated power reflection coefficients of the sensing tag due to variousdielectric constant values of the paraffin wax layer.
Fig. 7. Measurement setup.
B. Measurement Setup
The heat sensor tag was measured in an EMC chamber witha custom made measurement setup environment, as shown inFig. 7. The measurement setup has a linearly polarized readerantenna connected to the Tagformance measurement device, ab-sorbing panels, hot air blower, and an RFID tag to be measured.
C. Sensor Tag Antenna Measurement Results
The read range of the tag was calculated by using the mea-sured results from the Tagformance, with the help of the fol-lowing equation [28]:
(3)
In the above equation, “ ” is the read range of the tag an-tenna. “ ” is the measured path loss from the generator’soutput port to the input port of a hypothetic isotropic antennaplaced at the tag’s location. Forward path loss was achievedfrom the measured calibration data using the Tagformance mea-surement. European effective radiated power “ ” value wasconsidered equal to 2 W (33 dBm) according to [29]. The mea-sured threshold transmitted power in the forward direction fromthe transmitter to the tag is represented as “ .” The thresholdpower is the minimum continuous wave power transmitted to
Fig. 8. Calculated read range of the tag antenna at “ direction,” usingmeasured results, as shown in (3).
Fig. 9. Measured threshold transmitted power at “ direction.”
enable the tag to send a response to EPC Gen 2 protocol’s querycommand. The resulting read range of the tag antenna, calcu-lated based on the measured results, is shown in Fig. 8.In the above figure, the maximum read range is in between
6.5–7 m, at a frequency of 870 MHz. The threshold transmittedpower required to power up the tag can be seen in the followingFig. 9.The measured realized gain of the tag antenna is analyzed
using the path loss measurement data from the Tagformancemeasuring equipment. This can be described as [28]
(4)
where “ ” refers to the sensitivity of the IC, which in ourcase is set to 14 dBm. “ ” is the forward path loss fromthe transmitter to the tag antenna, and “ ” represents thethreshold power. The maximum simulated and measured real-ized gain at axis direction of the tag antenna can be seen inFig. 10.In the above figure, themeasured and simulated realized gains
are in good agreement with each other.
D. Heat Measurement Results
The heat measurements are done using the same measure-ment setup, shown in Fig. 4. The RFID sensor tag is placed hor-
BABAR et al.: PASSIVE UHF RFID TAG FOR HEAT SENSING APPLICATIONS 4061
Fig. 10. Simulated and measured realized gain at “ direction.”
Fig. 11. Calculated read range of the tag antenna at “ direction” usingmeasured results, as shown in (3).
izontally, facing upwards on top of the Styrofoam boxes. Theaxis direction of the RFID tag was facing a linearly po-
larized reader antenna, connected to the Tagformance measure-ment equipment [25].With the help of a hot air blower, the RFIDtag was heated, which resulted in melting the paraffin wax of thesubstrate. After heating the RFID tag antenna the tag was remea-sured. Due to the placement of the tag and the use of a vacuumbag surrounding the paraffin wax substrate layer, the shape ofthe paraffin substrate layer was kept almost the same. The readrange before the heating of the RFID sensor tag antenna andafter the heating of the tag antenna is shown in Fig. 11.According to the figure above, themaximum read range of the
tag antenna has been shifted to a higher frequency with a shiftof about 15–20 MHz. Due to the use of a narrowband RFID tagantenna design, the read range on the desired frequency of 870MHz have been reduced to more than half. This also affects thethreshold power required to power up the IC of the tag antenna,as shown in Fig. 12.Similarly, according to the figure above, there is a frequency
shift in the minimum threshold power required to power up theIC. According to the results above, after heating the tag antenna,around 6 dBm of more power is required to power up the IC onthe desired frequency of 870 MHz. Fig. 13 shows the measuredthreshold transmitted power to power up the tag at 870 MHzwith respect to the temperature change, during the heating andcooling of the tag. The measurements were carried out in the
Fig. 12. Measured threshold transmitted power at “ direction.”
Fig. 13. Measured threshold transmitted power with respect to temperature at870 MHz, while heating and cooling of the tag, at “ direction.”
measurement environment similar to as stated in Section V-B.The measurement inaccuracies can be from 0.5 to 1 dBm.According to Fig. 13, the transmitted power required by the
tag, increase with the increase in temperature at 870 MHz, untilthe paraffin wax is fully melted. After heating, the tag is left tocool down slowly in room temperature. Slight increase in therequired transmitted power can be seen during the cooling andresolidification process of the paraffin wax layer. Fig. 13 showsthat the tag once detuned, due to melting of paraffin wax layercannot return to its original state, even if brought back to itsprevious temperature and solid form. If the temperature doesnot reach the melting point of the wax, and the cooling processstarts, the threshold transmitted power will follows a new curvesimilar to the solid (blue) curve in Fig. 13
VI. HEAT MEASUREMENTS ON CONSUMER PRODUCTS
A. Measurement Setup
To demonstrate the use of this type of heat sensing tag, somemeasurements were made on some consumer products andboxes. Tagformance RFID measurement device was used tomeasure the tag antenna in an anechoic chamber by VoyanticLtd. [25]. The anechoic chamber is designed to measure theUHF RFID tag antennas. Chamber contains a linearly polarizedreader antenna, with a rotating table. A linearly polarized reader
4062 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 60, NO. 9, SEPTEMBER 2012
Fig. 14. Anechoic chamber’s measurement setup.
Fig. 15. Measured packages with a heat sensing tag.
antenna is connected to the Tagformance measurement device,as shown in Fig. 14.In our measurements, the tag antenna was attached to the ex-
terior of the packages, with the -axis of the tag facing the readerantenna, as shown in Fig. 15. In this article, the performanceof the RFID heat sensor tag will be analyzed on two differenttypes of packages. First the RFID heat sensing tag is attachedand measured on an empty white paper box, and then a similarRFID tag is attached on top of a box containing metallic cans offood items. During the measurements, the tag is first measuredwithout being exposed to heat. The sensor tag attached on thebox later is exposed to hot air for less than a minute and then re-measured in the anechoic chamber. The temperature of the hotair was around 30–60 C. Therefore, a thinner layer of paraffinwax will require less heat exposure to change its properties.
B. Measurement Results
The change in the read range and threshold power of thesensor tag due to heat on top of an empty paper box is shown inFigs. 16 and 17.In Fig. 16 the maximum read range on the desired frequency
of 865 MHz is reduced from almost 5 m to less than 1.8 m. Theshift in the frequency of themaximum read range is almost equalto 25 MHz. Similarly the minimum power required to power upthe IC on the respective frequency of 865 MHz, increases from7 dBm to almost 16 dBm.The maximum read range and threshold power of the tag
placed on top of a box of metallic cans are shown in Figs. 18
Fig. 16. Calculated read range variation due to heat on an empty paper box ataxis, using measured results, as shown in (3).
Fig. 17. Measured threshold power variation on an empty paper box at axis.
Fig. 18. Calculated read range variation on a box of metallic cans at axis,using measured results, as shown in (3).
and 19. The read range of the tag is affected due to the metallicbackground of the tag.In Fig. 18, the maximum read range of the tag has a frequency
shift of almost 30 MHz. According to the figure, the tag at-tached to the package of metallic cans cannot be read by themeasurement equipment on the desired frequency of 862 MHzafter heating. The same behavior can also be seen in Fig. 19,where the sensor tag cannot be read up to 28 dBm of suppliedpower.
BABAR et al.: PASSIVE UHF RFID TAG FOR HEAT SENSING APPLICATIONS 4063
Fig. 19. Measured threshold power variation of sensor tag attached to on a boxof metallic cans at axis.
Therefore, it can be observed that these types of sensor tagsare more affected due to heat, with a wider frequency shift whenplaced on top of a package containing metallic cans.
VII. CONCLUSIONS
A heat sensing RFID tag was presented using a multilayersubstrate. Paraffin wax acts as the heat sensitive layer of thesubstrate, which changes its properties when exposed to hightemperatures. This affects the dielectric properties of the wholemultilayer substrate, causing a change in the impedance of thetag antenna and mismatching the tag antenna from its desiredoperating frequency. These changes are irreversible even if thetag is brought back to the same temperature and therefore the tagcan be referred to as a threshold passive heat sensing tag. Thistype of heat sensing tag can be useful in many applications in-cluding supply chain operations and transportations of variousheat sensitive drugs and food items. As the proposed sensor tagis low cost, it can be placed on large variety of common pack-ages to qualify and display the quality status of these products.
ACKNOWLEDGMENT
The authors would like to thank all the reviewers and editorialboard for their contribution, help and positive feedback, whichimproved the manuscript and made it possible to be published.
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Abdul Ali Babar (S’09) received the M.Sc. in radiofrequency electronics (electrical engineering) fromthe Tampere University of Technology, Tampere,Finland, in 2009, and is currently working towardthe Ph.D. degree at the same university.He is currently working as a Research Scientist
with the Wireless Identification and Sensing Sys-tems Research Group, Department of Electronics,Tampere University of Technology. His area ofresearch includes RFID systems, sensors, readersand tag antennas, miniaturized antennas, and radio
frequency systems and their integration with wireless systems.
4064 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 60, NO. 9, SEPTEMBER 2012
Sabina Manzari (S’12) received the M.Sc. inmedical engineering from the University of Rome“Tor Vergata,” Rome, Italy, in 2010, and is currentlyworking toward the Ph.D. degree in informatics,systems, and production at the same university.Her research interests include electromagnetism,
hypertermic treatments, wireless health monitoring,and sensing by means of radio frequency identifi-cation techniques. From 2010 to 2011, she was aVisiting Researcher in the Wireless Identificationand Sensing Systems Research Group, Tampere
University of Technology, Tampere, Finland. Her research was mainly focusedin passive RFID sensors development for temperature and heat monitoring.
Lauri Sydänheimo (M’97) received the M.Sc. andPh.D. degrees in electrical engineering from TampereUniversity of Technology (TUT), Tampere, Finland.He is currently Professor and Head of the Depart-
ment of Electronics, TUT. He is the Research Di-rector of Tampere University of Technology’s RaumaResearch Unit. Dr. Sydänheimo holds Adjunct Pro-fessorship position at the University of Mississippi,Electrical Engineering Department. He has authoredover 120 publications in the field of RFID tag andreader antenna design and radio frequency identifi-
cation (RFID) system performance improvement. His research interests are fo-cused on wireless data communication and RFID, especially RFID antennas andsensors.
Atef Z. Elsherbeni (S’84–M’86–SM’91–F’07) iscurrently a Professor of electrical engineering andAssociate Dean of Engineering for Research andGraduate Programs, the Director of the School ofEngineering Computer-Aided Design (CAD) Lab-oratory, and the Director of The Center for AppliedElectromagnetic Systems Research (CAESR) atThe University of Mississippi, University. In 2004he was appointed as an adjunct Professor, at TheDepartment of Electrical Engineering and ComputerScience of the L.C. Smith College of Engineering
and Computer Science at Syracuse University. In 2009 he was selected asFinland Distinguished Professor by the Academy of Finland and TEKES. Dr.Elsherbeni is the coauthor of the books The Finite Difference Time DomainMethod for Electromagnetics With MATLAB Simulations (SciTech, 2009),Antenna Design and Visualization Using Matlab (SciTech, 2006), MATLABSimulations for Radar Systems Design (CRC Press, 2003), ElectromagneticScattering Using the Iterative Multiregion Technique (Morgan & Claypool,2007), Electromagnetics and Antenna Optimization Using Taguchi’s Method(Morgan & Claypool, 2007), and the main author of the chapters “HandheldAntennas” and “The Finite Difference Time Domain Technique for MicrostripAntennas” in Handbook of Antennas in Wireless Communications (CRC Press,2001). He is the Editor-in-Chief for ACES Journal.Dr. Elsherbeni a Fellow of The Applied Computational Electromagnetic So-
ciety (ACES).
Leena Ukkonen (M’03) received the M.Sc. andPh.D. degrees in electrical engineering from theTampere University of Technology (TUT), Tampere,Finland, in 2003 and 2006, respectively.She is currently leading the Wireless Identification
and Sensing systems Research Group at the Depart-ment of Electronics, RaumaResearch Unit, TUT. Shealso holds Adjunct Professorship in Aalto UniversitySchool of Science and Technology, Aalto, Finland,and the Electrical Engineering Department, Univer-sity of Mississippi, University. She has authored over
110 scientific publications in the fields of RFID antenna design and industrialRFID applications. Her research interests are focused on RFID antenna devel-opment for tags, readers, and RFID sensors.
