Research Article A Passive Harmonic Tag for Humidity Sensingdownloads.hindawi.com/journals/ijap/2014/670345.pdfLeakage signal Backscattered signal RFID reader F : Block diagram of

Research ArticleA Passive Harmonic Tag for Humidity Sensing

Antonio Lazaro Ramon Villarino and David Girbau

Department of Electronics Electrical and Automatics Engineering Universitat Rovira i VirgiliAvinguda Paisos Catalans 26 43007 Tarragona Spain

Correspondence should be addressed to Antonio Lazaro antonioramonlazarourvcat

Received 18 March 2014 Accepted 3 July 2014 Published 23 July 2014

Academic Editor Ananda Sanagavarapu Mohan

Copyright copy 2014 Antonio Lazaro et alThis is an open access article distributed under the Creative CommonsAttribution Licensewhich permits unrestricted use distribution and reproduction in any medium provided the original work is properly cited

This paper describes a passive harmonic tag for radio frequency identification (RFID) and wireless sensor applicationsThe tag usesa dual polarized UHF patch antenna as an input antenna One of the outputs is connected to a frequency doubler which consistsof a Schottky diode with its output connected to a patch tuned at twice the input frequency The other output of the input antennafeeds a DC power harvested converter that drives an oscillator whichmodulates its output signal by controlling the bias point of theSchottky diode The antennarsquos output is also used as a humidity sensor To achieve this the antenna is loaded with an interdigitalcapacitor with humidity-dependent capacitance The antenna is consequently detuned when humidity varies and therefore thesecond harmonic power is received The tag is manufactured using standard fiberglass substrate The basic theory of harmonictag operation is described and compared with the standard backscattering approach Experimental results with a proof of conceptusing commercial components are presented

1 Introduction

Passive RFID systems have experienced tremendous growthin recent years [1] A passive RFID system operates as follows(see Figure 1) The RFID reader transmits a modulated RFsignal to the RFID tag It consists of an antenna and anintegrated circuit (IC) The IC receives power from theantenna and responds by varying its input impedance andthus modulating the backscattered signal In these systemsthe continuous-wave (CW) signal that powers up the passivetags is inevitably coupled to the receiver input as a strongself-interference which presents a major challenge to thereaderrsquos receiver design Several cancellation techniques toovercome this problem have been proposed [2 3] Analternative approach based on harmonic radar can be usedto avoid it This consists of a frequency multiplier thatgenerates a harmonic or subharmonic of the interrogationsignal [4ndash9] providing a unique response signal among theleakages from the transmitter Harmonic radars have beenused to track insects for several years [6] using maritimeradar based technologies at X-band fundamental frequenciesThe transponder typically consists of a simple lightweightantenna structure directly matched to a Schottky diode In

this application it is possible to detect the insect at longdistances within an environment with strong clutter Thisfeature is exploited to find avalanche victims [4 10] Acommercial avalanche detector is available fromRECCO [10]and it uses 917MHz fundamental frequency to achieve verygood ground penetration capability Depending on the finalapplication transponders that operate at different frequencyranges have been presented in the literature UHF and L-bandCW hand-held harmonic radars were used to track groundinsects [11 12] Other transponders have been designed at 245or 58GHz [13 14] A millimeter wave transponder based ona nonlinear transmission line (NTL) has been reported in [7]with a read range less than 1m

Potential applications of passive RFID technology requireits functionality to be increased by adding sensing capa-bility to the tags These could be used to sense differentenvironmental conditions that is temperature and humiditySome commercially available sensors are usually based onactive tags which make them quite expensive Passive orsemipassive tags are thus a preferred option for low-costsensor applications [15] Consequently a great interest todevelop passive UHF sensors has arisen in the literatureThese tags are often based on detuning the antenna by the

Hindawi Publishing CorporationInternational Journal of Antennas and PropagationVolume 2014 Article ID 670345 11 pageshttpdxdoiorg1011552014670345

2 International Journal of Antennas and Propagation

Transmitter

Receiver

Directionalcoupler

Band passfilter

TagLeakagesignalBackscattered

signal

RFID reader

Figure 1 Block diagram of a UHF passive system

sensorHence both the required power to turn-on the tag andthe received power depend on the physical magnitude to beread [16ndash18]

Harmonic scattering from passive UHF RFID tags hasbeen used to characterize the radiation pattern diagram ofthe tag [19] In a recent work [20] the residual harmonicsignal from commercial UHF tags has been investigatedand it is proposed as useful tool for the developmentof future sensors These works show that standard UHFtags generate backscattered harmonics However since theantenna is matched at the fundamental frequency it filtersthese components Therefore the backscattered power of theharmonics is too low In this sense standard tags can beconsidered as harmonic tags due to nonlinear behaviour Onepractical problem when a harmonic sensor is designed isthe selection of the working frequency In order to enhancethe nonlinearities high power should be used to achievea great read range of meters thus UHF RFID band isa good candidate However the second harmonic can fallwithin other band dedicated to other services that can beinterfered by the backscattered harmonics near the reader Ata few meters the harmonic level is small and therefore theinterference will be small too and comparable to the level ofharmonics in conventional UHF RFID systems In any casethere is a lack of regulation about the frequency band forharmonic radars However sensors based on harmonic tagshave begun to be studied in the literature [21] For instance atemperature harmonic tag has been proposed in the literatureworking at 15 GHz fundamental frequency [22]

In this work a proof of concept for a novel humiditysensor based on harmonic tag working at UHF RFID bandis presented An input rectenna is used to power a low-fre-quency oscillator that modulates the frequency doublerbased on a Schottky diodeThismodulation allows increasingthe read range because it reduces the problems associatedwith the harmonic leakage in the reader The RF signalreceived by the other polarisation of the input antenna isused as input signal towards the doubler A simple low-costinterdigital capacitor that loads the output patch antenna isused as sensor The effective permittivity changes with therelative humidity changing the capacitance and detuning theoutput antenna The paper is structured as follows Section 2presents the humidity sensor tag and describes the basictheory of operation A link-budget study is done in thissection in order to study the read range and the level ofharmonic A comparison between the read range that is

Power amplifier

x2

Detector

Low-noiseamplifier

x2

Reader

UHFoscillator

f0

f0

BPF f0

TXantennaf0

RXantenna2f0

2f0BPF

2f0

Figure 2 Block diagram of the system

achievable with the proposed harmonic tag and conventionalbackscattering RFID systems will be studied in this sectionSection 3 presents the experimental results of the systemFinally some conclusions are drawn in Section 4

2 RFID System Based on Harmonic Radar

21 System Description and Theory of Operation A blockdiagram of the RFID system based on a harmonic tag isshown in Figure 2 comprising the reader and the tagThe tagcan be observed in Figure 3 It consists of a dual-polarizedpatch reception (Rx) antenna that collects the interrogationsignal at aUHF frequency of119891

0(865ndash868MHz)This antenna

is connected to the transmission (Tx) patch antenna bymeansof a frequency doubler The Tx antenna is loaded with aninterdigital capacitor with a capacitance dependent on thehumidity so that when the relative humidity changes theantenna is detuned A variation in the received power at thereader is thus obtainedThe experimental results given in thenext section show that despite using filters at the transmitteroutput the amplitude of the leakage signal at 2119891

0is larger

than the tag reflected amplitude at the second harmonicModulation of the tag is therefore required to exploit thereceiverrsquos sensitivity To that end the second port of theRx antenna is connected to a RF to DC converter modulewhich supplies a low-frequency oscillator This oscillatormodulates the output signal at 2119891

0by controlling the bias

point of the frequency doubler In consequence the taganswers the reader modulating the second harmonic Thereaderrsquos mixer demodulates the backscattered signal usingthe second harmonic of the transmitted signal as the localoscillator The tag is manufactured using standard fiberglasssubstrate FR4 The Rx antenna is a square patch antenna79mmwide and an inset line length of 14mm inset linewidthof 1mm and an inset gap of 1mmThe frequency conversionin harmonic tags is generally accomplished by a Schottkydiode which should have zero-bias high voltage sensitivityand a low barrier in order to maximize the conversion ofsmall signals In this study the frequency doubler is basedon a zero-bias Schottky diode (Avago Technologies modelHSMS-2850) The fundamental signal at 119891

0is filtered using

International Journal of Antennas and Propagation 3

(a)

RF to DCconverter

HumiditysensorHSMS-2850Dual

polarizationinput antenna

f0

Outputantenna

2f0

1205824f0 12058242f012058242f0

12058242f0Rbias

R2 (0Ω)R1 (750kΩ) C1 (1nF)

100 pF

(b)

Figure 3 Photograph of the implemented tag (a) and block diagram (b)

a high pass filter implemented with a 1205824 stub at 21198910 The

frequency doubler blocks the second harmonic at the inputusing a short-circuited 1205824 stub at119891

0using a bypass capacitor

The output of doubler can be modulated by changing thediode DC bias point To this end a bias resistor (119877bias)is inserted at the output of the oscillator The end of thequarter wavelength line is connected to the ground at RF bya bypass capacitor The high value of the resistor (1 kΩ) alsoblocks the RF signal and limits the DC current consumptionwhile the DC return is allowed through the resistor Themodulating capability is shown in Figure 4This figure showsthe measured conversion loss of the designed frequencydoubler at 868MHz for 0V and a 05 V DC bias With aninput power under minus10 dBm a difference larger than 12 dBbetween the two states is observed

The RF to DC converter is based on a diode voltage mul-tiplier (see Figure 5) using series-connected zero-bias diodes(Avago Technologies model HSMS-2852) Figure 6 showsthe measured detected voltage as a function of input powerat 868MHz The low-frequency oscillator is implementedwith two NAND logic gates (74AUP1G00) This oscillatorneeds 045V to start oscillating The minimum input powerto obtain this voltage is thus about minus19 dBm which is closeto the value achieved by commercial UHF RFID tags (egImpinj Monza Gen 5 or Alien Higgs-3 ICs) The output ofthis oscillator is connected to the input stub of the doublerto modulate its conversion loss

22 Loaded Patch Antenna as a Humidity Sensor The Txpatch antenna is tuned to the center frequency of the secondharmonic and loaded with a variable capacitorThis structureis used as a humidity sensor The capacitor is an interdigitalcapacitor connected to the ground Fiberglass is known tochange its permittivity depending on the water content Alow-cost integrated humidity sensor is therefore designedusing fiberglass (relative permittivity 120576

119903= 44 substrate

height 16mm) When the relative humidity increases thesurface permittivity of the substrate between the interdigi-tal electrodes also increases The capacitance consequentlyincreases and the resonant frequency of the patch antenna

0 510

20

30

40

50

60

Input power (dBm)

Con

vers

ion

loss

(dB)

minus35 minus30 minus25 minus20 minus15 minus10 minus5

Measured 0VMeasured 05V

Figure 4Measured conversion loss of the frequency doubler for 0Vand 05 V DC bias voltage at 868MHz

VDCout

Frominput

antennaHSMS-2852

HSMS-2852

HSMS-2852100 pF

100 pF 100 pF 100 pF

100 pF

100 pF

75nH

68pFmiddot middot middot

⋱

Figure 5 RF to DC converter module based on a 5-level voltagemultiplier

therefore changes Figure 7 shows the layout of the antennawith its main dimensions

A model based on a transmission line to understand theloaded antenna operation is shown in Figure 7 119871 is the length


00

2

4

6

8

Input power (dBm)

Det

ecte

d vo

ltage

(V)

minus30 minus25 minus20 minus15 minus10 minus5

Figure 6 Measured detected voltage as a function of input powerat 868MHzThe dashed line shows the minimum voltage needed topower up the oscillator (045V)

of the patch and Δ119871 is the extension length in order to takeinto account the end capacitance of the patch119877rad models theradiation resistance of the patchThe input match is obtainedby adjusting the inset length 119871 inset From this model it isevident that the resonant frequency of the patch can be tunedwith the capacitance C that is a function of the humidity Thepatch and input matching network are adjusted using AgilentMomentum for an interdigital capacitor with the nominalpermittivity of the substrate The patch dimensions are119882 =51mm 119871 = 375mm and 119871 inset = 95mm with an inset gapof 1mm An 18-finger interdigital capacitor with a length of5mm a width of 06mm and a gap between fingers of 300microns has been designed

Figure 8 shows the capacitance obtained from the mea-sured reflection coefficient for different humidity percent-ages As expected the capacitance is proportional to the effec-tive permittivity that increases with the relative humidityFigure 9 shows the variation of capacitance and quality factoras a function of humidity

Figure 10 compares the input reflection of the antennaloaded with the sensor and without the sensor (antennaunloaded) In the case of the antenna unloaded the variationof the resonant frequency is small because the patch protectsthe substrate from the humidity However a small change dueto the humidity in the fringing capacitance modeled with theeffective length incrementΔ119871 is observedThevariation of theresonance frequency is 32MHz (017) Figure 10 shows theinput reflection coefficient of the loadedpatch antennawhichis close tominus10 dB for the entire relative humidity range As themodel of Figure 7 predicts the main effect of the interdigitalcapacitor is a reduction of the resonance frequency Figure 11shows the measured resonance frequency of the loadedantenna as a function of relative humidity The frequencyrange is between 1740MHz and 1728MHz (069) whichis approximately within double the frequency range of theEuropean RFID band A small shift between the nominal(1730ndash1736MHz) and measured resonance frequency rangecan be caused by the tolerance between the gaps and the linesof the interdigital capacitor in the fabrication process Highersensitivity can be obtained increasing the capacitance of theinterdigital capacitor These figures show that the sensor hasgreater sensitivity for relative humidities larger than 40ndash50

As expected the quality factor decreases with the watercontent due to the water losses and the antenna suffersa small mismatch when the relative humidity approaches100 The temperature dependence of the FR4 substratecauses changes on its permittivity Consequently the sensormust be characterized as function of the temperature Themeasured sensitivity of the resonance frequency with respectto the temperature in the range between 25 and 100∘C wasminus0847MHzK

23 Link Budget The aim of this section is to study theperformance of the harmonic radar as a function of thedistance A comparison with the conventional backscatteringmethod using a single antenna is made for reference ForPassive RFID the read range is limited by two factors [23] theneed for very strong power to feed the tag limiting the readerto the tag range and the backscattered power from the tag torespond to the reader limiting the tag to the reader range

The power transferred from the reader to the tag 119875119903tag

under matching conditions can be calculated from the Friisequation However propagation in RFID environments dif-fers from free-space conditions An approach for predictingaverage power level in indoor environments for mobilecommunications is the dual slope model [24] This modelis derived from a two-ray path model where there are adirect ray and a reflected ray This model assumes that forlarge distances the angle of incidence of the reflected rayapproaches 90∘ and the reflection coefficient is minus1 Physicallythis corresponds to the case where the propagation path iswithin the first Fresnel zone boundary for a distance greaterthan the breaking distance 119877B1 as follows

119877B1 =4ℎreaderℎtag

120582in (1)

where ℎreader and ℎtag are the transmitter and receiver antennaheights (reader and tag antenna heights in this case) and 120582in iswavelength at frequency 119891in Using this model 119875

119903tag is givenby the following expression [24]

119875119903tag =

119875119879119866119879

4120587(1

1198891198991sdot

1

(1 + 119889119877B1)119899119861minus1198991

)119860119890tag (2)

where the effective area of the input tag antenna is given by

119860119890tag =

1205822in4120587119866tagin (3)

where 119866tagin is the gain of the input antenna 119875119879is the

transmitted power and119860119890tag is the effective area at frequency

119891in For distances 119889 ≪ 119877B1 the path loss exponent is 1198991 and

for distances 119889 ≫ 119877B1 the path loss exponent is 119899B The free-space model is recovered when the antenna heights are large(119877B1 very large) and 1198991 is set to 2 Using the flat earth modelat the limit the path loss exponent tends to 119899B = 4 Valuesof 1198991gt 2 correspond to radio environments that include

multiple path propagation effects A typical range of 1198991in

RFID environments is 2 lt 1198991lt 25 [25] and depends on

the polarization and the antenna heights


L inset = 95mm

L = 375mm

W = 51mm

Width = 1mmGap = 1mm

18 fingersLength = 5mmWidth = 06mmGap = 03mm

08mm via holes

(a)

Input

L + 2ΔL

L inset

RradRrad

(b)

Figure 7 (a) Antenna layout (b) Antenna loaded with a humidity sensor model

0 500 1000 1500 2000 2500 3000

0

5

10

15

20

25

Frequency (MHz)

Capa

cita

nce (

pF)

10 RH50 RH

80 RH100 RH

minus20

minus15

minus10

minus5

Figure 8 Measured capacitance as a function of the frequency fordifferent relative humidity contents

The power received by the tag must be greater thanthe tag sensitivity or the minimum power (also known aspower-up threshold 119875th) that is needed for the rectificationof the incident RF power to feed the tag and to generate theresponse by modulating the radar cross section The powerbackscattered by the tag can be calculated using the conceptof a differential cross section described in [23] and the two-slope propagation model as follows

119875119903reader

= 119875119903tag119866doubler sdot

119866tagout

4120587(1

1198891198992sdot

1

(1 + 119889119877B2)119899119861minus1198992

)1198701205822out4120587

119866119877

(4)

where 119866tagout is the gain in the tag output antenna 119866119877is the

reader receiver antenna gain and the wavelength at frequency2119891in and119866doubler is the average gain of the frequency doublerIn (4) 119899

2is the path loss exponent for 119889 ≪ 119877B2 and

the breaking distance 119877B2 is calculated from (1) but usingthe wavelength at the frequency 119891out = 2119891in The classicalbackscattering case can be recovered setting 120582in = 120582out119866tagin = 119866tagout and 119866doubler = 1

In (4) 119870 is the modulation loss factor [23] In conven-tional backscattering systems 119870 can be estimated as

119870 =1

4

10038161003816100381610038161205881 minus 120588210038161003816100381610038162 (5)

The factor 119870 for a BPSK modulation with a 50 duty cycleis 14 and 120588

1and 120588

2are the complex reflection coefficients

for the two tag states In passive UHF tags the typical choiceof modulating states is matched and short which results ina minus6 dB modulation loss In practice the modulation lossis lower because the reflection coefficient module does notreach the unit and depends on the incident power Typicalvalues of minus12 to minus23 dB were experimentally found [23] Amodulation loss of minus12 dB is considered in the simulations forboth systems

In the case of the harmonic tag the output amplitude atthe output frequency ismodulated by a 50 duty cycle squarewaveform The time domain signal can be approximated by

119909 (119905) = (1198601 + (1198602 minus 1198601)infin

sum119899=1

prod(119905 minus 119899119879

1198792)) cos (2120587119891out119905)

(6)

where119879 is the period of themodulating signal and1198601and119860

2

are the amplitudes for the two stateswhich are proportional tothe square of conversion loss of the doubler given by Figure 4From the Fourier transform of (6) themodulation factor loss119870 in a harmonic radar case can be obtained as

119870 =2

1205872(1198602minus 1198601

1198601+ 1198602

)2

(7)

Figure 12 shows the calculated modulation factor119870 using (7)and the conversion loss of Figure 4 Values between minus12 andminus10 dB are observed for input powers under minus10 dBm


10 20 30 40 50 60 70 80 90 10036

38

4

42

44

46

Relative humidity ()

Capa

cita

nce (

pF)

(a)

10 20 30 40 50 60 70 80 90 100


0

20

40

60

Qua

lity

fact

or

(b)

Figure 9 Measured capacitance (a) and quality factor (b) as a function of relative humidity at 868MHz

1700 1720 1740 1760 1780 1800 1820 1840 1860 1880 1900

0

Frequency (MHz)

(dB)

10 RH50 RH

80 RH100 RH

Loadedantenna

Unloadedantenna

minus2

minus4

minus6

minus8

minus10

minus12

minus14

Figure 10 Measured input reflection coefficient as a function offrequency for patch antennawithout the sensor and the loaded patchantenna with the sensor

Equation (2) is only valid under matched conditionsThis is approximately true for the harmonic tag because theimpedance that loads the antenna does not change muchbetween states However in a conventional backscatteringtag the average mismatch coefficient should be taken intoaccount Consider

119872 = 1 minus

1003816100381610038161003816120588110038161003816100381610038162+1003816100381610038161003816120588210038161003816100381610038162

2 (8)

Under the previous conditions (matching and shortstates)119872 is in the order of 05 A value of119872 = 05 is chosenin the simulation for the two systems

In order to study the read range limiting factors thethreshold power and reader sensitivity for both systemsare required For instance for the commercial tag ImpinjMonza Gen 5 the threshold power 119875th is about minus20 dBmFor a commercial UHF reader a signal-to-noise ratio (SNR)of 116 dB is required to guarantee 0001 of BER [26]The typical reader sensitivity for a commercial reader islimited by the leakage power to approximately minus70 dBm

10 20 30 40 50 60 70 80 90 1001726

1728

1730

1732

1734

1736

1738

1740

1742


Reso

nanc

e fre

quen

cy (M

Hz)

Figure 11 Measured resonance frequency as a function of relativehumidity

0 5

Input power (dBm)minus35 minus30 minus25 minus20 minus15 minus10 minus5

minus22

minus20

minus18

minus16

minus14

minus12

minus10

minus8

Mod

ulat

ion

fact

orK

(dB)

Figure 12 Calculatedmodulation factor119870 as a function of the inputpower for the designed harmonic tag

[23 26] The required receiver noise floor for commercialreaders is thereforeminus816 dBm [26]The receiver sensitivity ofharmonic radar can be improved because the leakage signal atthe transmitted frequency can be easily filtered in the receiverand the leakage at the second harmonic can be reduced byfiltering the signal at the output of the transmitter The noisefloor can thus be reduced below minus138 dBm using a receiverwith a 6 dB of noise figure and a spectrum analyzer resolutionbandwidth of 1 KHz The phase noise is consequently themost limiting factor The typical phase noise of minus120 dBcHzat 100 kHz offset can be obtained using commercial UHFphase-locked oscillators A reader sensitivity better thanminus120 dBm can thereby be achieved


Tag input power

Power-up threshold

0 5 10 15

Distance (m)

Received power by harmonic tag at 2f0

(dBm

)

minus120

minus100

minus80

minus60

minus40

minus20

0

20

Received power by tag with backscattering at f0

(a)

0 5 10 15

Distance (m)

(dBm

)

Tag input power

Power-up threshold


minus140

minus120

minus100

minus80

minus60

minus40

minus20

0

20


(b)

Figure 13 Simulated tag input power and received power by a harmonic tag and conventional tag using a free-space model (a) and using atwo-slope model with 119899

1= 1198992= 25 and ℎreader = ℎtag = 15m (b) The power-up threshold (minus20 dBm) is also shown

Harmonic tag Required SNR

0

20

40

60

80

Sign

al-to

-noi

se ra

tio (d

B)

Tag by backscattering at f0

0 5 10 15

Distance (m)

(a)

0

20

40

60

80

Sign

al-to

-noi

se ra

tio (d

B)

Harmonic tag Required SNRTag by backscattering at f0

0 5 10 15

Distance (m)

minus20

(b)

Figure 14 Simulated signal-to-noise ratio by a harmonic tag and convention tag by backscattering at 1198910using a free-space model (a) and

using a two-slope model with 1198991= 1198992= 25 and ℎreader = ℎtag = 15mThe required SNR limit of 116 dB for a 0001 of BER is also shown

The following figures show a comparison between the twosystems based on the conventional backscattering methodand harmonic radar The simulation assumes that the trans-mitted power complies with the European RFID band regu-lation where the equivalent isotropic radiated power (EIRP)is limited to 33 dBm This value is taken into account in thesimulations of both systems Figure 13(a) shows the powerreceived at the tag and the power received at the reader inboth systems as a function of the distance using a free-spacemodel (119899

1= 1198992= 2 119877B1 = 119877B2 = infin) whereas Figure 13(b)

shows the simulation using a more realistic propagationmodel (119899

1= 1198992= 25 and the antenna heights ℎreader = ℎtag =

15m) Figure 14 shows the signal-to-noise ratio (SNR) inboth systems as a function of distance for the two propagationmodels The gains of the reader antennas are assumed to

be equal to 6 dB and the antenna gain corresponds to thesimulated gain for the patch antennas used in the harmonictag design 16 dB for the input patch and 26 dB for theoutput patch In the case of the tag by backscattering theantenna gain is assumed to be the same as the input antennafor the harmonic tag A power-up threshold of minus20 dBm isconsidered in the simulations

Some conclusions can be obtained from these figuresFirst the maximum range considering only the reader-to-tag communication is the same in the two RFID systems ifthe same antenna and mismatch coefficient are taken intoaccount However whereas the read range in conventionalsystems is limited by the tag-to-reader communication inthe harmonic radar it is limited by the reader-to-tag radio-link Depending on the radiopropagation model considered


Power amplifierminicircuits ZHL-

3010

Spectrumanalyzer

RampS FSP-30

LNAminicircuitsZX60-3018G

x2

LNAminicircuitsZRL-3500+

Commercialhumidity

sensor

Multimeter

Dryair

Wetair

Chamber2f0

TX

poynting PATCH A0025-01

antennaf0

BPF f0

Signal generatorat f0

RampS SMF-10A

2f0BPF

RX

GeozondasAu-05G25G-1

antenna2f0

f0

Figure 15 Block diagram and photography of the experimentalsetup used

the simulated harmonic tag read range varies between 10m(free space) and 58m (using a two-slope model with 119899

1=

1198992= 25 and ℎreader = ℎtag = 15m) In conventional RFID

systems the simulated read range varies between 88m (freespace) and 5m (using a two-slope model with 119899

1= 1198992= 25

and ℎreader = ℎtag = 15m) A better signal-to-noise ratio(SNR) can thus be achieved using harmonic radars becauseof their greater sensitivity although the received power atthe reader is lower than in conventional RFID systems dueto the tag conversion loss This SNR improvement is greaterthan 10 dB at a distance of 5m Harmonic RFID systems areconsequently more robust in front multipath fading effectsbecause they allow larger fading margins than conventionalRFID systems

3 Experimental Results

Some experiments were performed in order to validate theprototype designed The experimental setup is similar tothe one proposed in Figure 2 and is described in Figure 15The oscillator is implemented using a Rohde amp SchwarzSMF-100A signal generator Its output signal is amplified bya Minicircuits ZHL-3010 power amplifier A notch filter isconnected between the power amplifier and theUHF antennain order to reduce the spurious emissions at the second har-monic The commercial UHF RFID antenna model PATCHA0025-01 from Poynting with circular polarisation and again of 6 dB is used The level of the signal generator isadjusted to obtain an EIRP of 33 dBm at the antennarsquos outputA Rohde amp Schwarz FSP-30 spectrum analyser model is

17295

17296

17297

17298

17299

1730

17301

17302

17303

17304

17305

Frequency (MHz)

(dBm

)

minus110

minus100

minus90

minus80

minus70

minus60

minus50

minus40

Figure 16 Measured spectrum at a distance of 7m between tagand reader The antenna height is 15m and the input frequency is865MHz The resolution bandwidth of the spectrum analyzer is setto 1 KHz

830 835 840 845 850 855 860 865 870 875

Input frequency (MHz)

10 RH60 RH

100 RH

(dBm

)

minus85

minus80

minus75

minus70

minus65

Figure 17 Measured variation of the received power as a functionof the input frequency for different relative humidity contents Thetag-to-reader distance was 15m

used as receiver A low-noise amplifier (LNA) composedof the cascade connection of a ZX60-3018G-S+ and ZRL-3500+ from Minicircuits is used to reduce the overall noisefactor A band pass filter centered at the second harmonic isconnected to the input of the amplifier to reduce the levelof the fundamental UHF signal and is coupled between thetransmitter and receiver antennas to prevent the saturationof the amplifiers The overall noise factor is approximately5 dB A broadband antenna model AU-05G25G-1 fromGeozondas with a gain of 6 dBi at 17 GHz is used as a receiverantenna A custom climatic chamber is made A wet air flowis used to increase the relative humidity while a dry air flowis injected to reduce the humidity The chamber is madewith glass to prevent excessive attenuation of radiofrequencysignals A commercial humidity sensor (model HIH-5030fromHoneywell) is used as a reference tomeasure the relativehumidity

In order to check the read range predicted by the linkbudget Figure 16 shows the spectrumat the reader-to-tag dis-tance of 7m in an indoor environmentThis distance is within


0 50 100 150 200 250 300 350 400

Time (s)

(dB)

minus8

minus6

minus4

minus2

0

(a)

0 50 100 150 200 250 300 350 400

Time (s)

0

50

100

150

RH (

)

(b)

Figure 18 Measured variation of the received power (a) and relative humidity measurement of a commercial sensor located near the tagInput frequency of 865MHz and tag-to-reader distance of 15m

0 10 20 30 40 50 60 70 80 90 1000

20

40

60

80

100


Nor

mal

ized

pow

er ch

ange

()

Figure 19 Normalized power level change as a function of relativehumidity measured by the commercial sensor Input frequency of865MHz and tag-to-reader distance of 15m

the range predicted in Figure 13Thehighest peak in Figure 16corresponds to the coupling between transmitter and receiverantennas due to the leakage signal at the second harmonicof the transmit signal generated by the nonlinearities inthe power amplifier Despite using filters at the transmitteroutput the amplitude of this signal is larger than the tagreflected amplitude at the second harmonic Modulation ofthe tag to exploit the receiver sensitivity is therefore neededThe other peaks around the second harmonic are due to themodulation of the tag The received power at the modulatedfrequency is 30 dB above the noise floor so a good SNR isobserved (about 30 dB) In consequence the uplink does notlimit the read range The tag is not powered up for distancesgreater than 7m This result demonstrates that the mainlimitation is the reader-to-tag link

Figure 17 shows the received power at the modulatedfrequency when the carrier frequency is swept between830MHz and 875MHz for different relative humidity levelsIt can be shown that the peak power corresponds to thefrequency where the antenna is best matchedThis frequencydepends on the relative humidity For relative humidity under40ndash50 the change is difficult to detect as explained abovebecause the change in the sensor capacitance is very smallBased on these results an alternative measurement methodcan be used avoiding the sweep of the oscillator frequencyThe major changes in the detected power take place at a

frequency of 865MHz The received power at the first peakof the modulated frequency is therefore used to detect therelative humidity

Figure 18 shows the variation of the received power at865MHz as a function of timeThe output of the commercialsensor is also compared in Figure 18(b) The chamber isexposed to several cycles of humidity (low-high-low) byinjecting dry or wet air as required The sensor output givesvalues greater than 100 when the chamber is saturated withwater vapour and the sensors are wet The receiver powerreturns to the same value when the antenna is dry

Figure 19 shows the normalized received power levelchange (variation of the received power with respect to themaximum variation of the received power) as a function ofthe relative humidity measured by the commercial sensorused for reference The relationship between relative humid-ity and the normalized power level change is approximatelylinear from 10 to 90 A good correlation coefficientbetween the twomagnitudes is found close to 09 Below 10the measurement is too noisy due to the small capacitancechange of the interdigital capacitor For values greater than90 the capacitance changes very quickly as can be seenin Figure 9 Figure 19 can thus be used to obtain the relativehumidity from the received power measurements at themodulated frequency A drawback of this approach is thesensibility of the DC voltage at the output of the rectennain relation to the distance A change in the distance variesthe input RF power and therefore the rectified DC voltagechanging the oscillator frequency Therefore this approachcan be used when the tag-to-reader distance is fixed Theproblem can be solved integrating the rectifier and theoscillator including an integrated regulator similar to thatused in commercial UHF tags [27] A simplemethod consistsin measuring the antenna detuning so that it is independentof the distance as shown in Figure 17

4 Conclusions

In this study we present an RFID humidity sensor based ona passive harmonic tag The sensor is based on the change inthe response of a patch antenna loaded with an interdigitalcapacitor with a capacitance that is a function of the relativehumidity The basic theory of operation of harmonic tagsis introduced A read range of 7m is experimentally found


consistent with link budget simulations when a 2W EIRPis transmitted This read range is limited by the power-up distance in the reader-to-tag link A larger signal-to-noise ratio can be achieved providing higher robustnessagainst channel fading thanks to better receiver sensitivity ofharmonic radars than in conventional backscattering RFIDExperimental results with a proof of concept using commer-cial components are presented Two methods for obtainingthe relative humidity of the sensor have been presented Thefirst is based on the measurement of the resonant frequencyof the antenna by sweeping the transmitter frequency Usingthis method the response for humidities under 40ndash50 ispoor due to the finite quality factor of the antenna andsmall capacitance change in this humidity range The secondmethod is based on the relative change in the receivedpower level at one frequency A good correlation coefficientis achieved with commercial sensors Using this secondmethod the sensor can work at 10 to 100 of RHwith goodlinearity up to 90 Further developments will focus on thesize reduction of the tag antennas In conclusion the proof-of-conceptmeasurements open the door to harmonic tags forRFID and wireless sensor applications

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

Acknowledgment

This work was supported by the Spanish Government ProjectTEC2011-28357-C02-01

References

[1] K FinkenzellerRFIDHandbook JohnWileyamp Sons NewYorkNY USA 2nd edition 2003

[2] P Pursula M Kiviranta and H Seppa ldquoUHF RFID readerwith reflected power cancellerrdquo IEEE Microwave and WirelessComponents Letters vol 19 no 1 pp 48ndash50 2009

[3] S-C Jung M-S Kim and Y Yang ldquoA reconfigurable carrierleakage canceler for UHF RFID reader front-endsrdquo IEEE Trans-actions on Circuits and Systems I Regular Papers vol 58 no 1pp 70ndash76 2011

[4] M Bouthinon J Gavan and F Zadworny ldquoPassive microwavetransponder frequency doubler for detecting avalanche vic-timsrdquo inProceedings of the 10th EuropeanMicrowaveConferencepp 579ndash583 September 1980

[5] CW Pobanz and T Itoh ldquoMicrowave noncontact identificationtransponder using subharmonic interrogationrdquo IEEE Transac-tions on Microwave Theory and Techniques vol 43 no 7 pp1673ndash1679 1995

[6] B G Colpitts and G Boiteau ldquoHarmonic radar transceiverdesignminiature tags for insect trackingrdquo IEEETransactions onAntennas and Propagation vol 52 no 11 pp 2825ndash2832 2004

[7] F Yu K G Lyon and E C Kan ldquoA novel passive RFIDtransponder using harmonic generation of nonlinear trans-mission linesrdquo IEEE Transactions on Microwave Theory andTechniques vol 58 no 12 pp 4121ndash4127 2010

[8] S M Aguilar and T M Weller ldquoTunable harmonic re-radiatorfor sensing applicationsrdquo in Proceeding of the IEEE MTT-SInternational Microwave Symposium (IMS 09) pp 1565ndash1568Boston MassUSA June 2009

[9] I T Nassar T M Weller and J L Frolik ldquoA compact 3-D har-monic repeater for passive wireless sensingrdquo IEEE Transactionson Microwave Theory and Techniques vol 60 no 10 pp 3309ndash3316 2012

[10] RECCORescue System a system for locating avalanche victimsRecco AB Liding Sweden httpwwwreccocomabout 2014

[11] M OrsquoNeal D Landis E Rothwell L Kempel and D ReinhardldquoTracking insects with harmonic radar a case studyrdquo AmericanEntomologist pp 212ndash218 2004

[12] G L Lovei I A N Stringer C D Devine and M CartellierildquoHarmonic radarmdasha method using inexpensive tags to studyinvertebrate movement on landrdquo New Zealand Journal ofEcology vol 21 no 2 pp 187ndash193 1997

[13] A Singh and V M Lubecke ldquoRespiratory monitoring andclutter rejection using aCWdoppler radarwith passive RF tagsrdquoIEEE Sensors Journal vol 12 no 3 pp 558ndash565 2012

[14] H Aumann E Kus B Cline and N W Emanetoglu ldquoA low-cost harmonic radar for tracking very small tagged amphibiansrdquoin Proceedings of the IEEE International Instrumentation andMeasurement Technology Conference (I2MTC rsquo13) pp 234ndash237Minneapolis Minn USA May 2013

[15] G Marrocco and F Amato ldquoSelf-sensing passive RFID fromtheory to tag design and experimentationrdquo in Proceedings ofthe European Microwave Conference (EuMC rsquo09) pp 1ndash4 IEEERome Italy October 2009

[16] J Virtanen L Ukkonen T Bjorninen A Z Elsherbeni andL Sydanheimo ldquoInkjet-printed humidity sensor for passiveUHF RFID systemsrdquo IEEE Transactions on Instrumentation andMeasurement vol 60 no 8 pp 2768ndash2777 2011

[17] L Yang R Zhang D Staiculescu C P Wong and M MTentzeris ldquoA novel conformal RFID-enabled module utiliz-ing inkjet-printed antennas and carbon nanotubes for gas-detection applicationsrdquo IEEE Antennas and Wireless Propaga-tion Letters vol 8 pp 653ndash656 2009

[18] R Vyas V Lakafosis H Lee et al ldquoInkjet printed self poweredwireless sensors for environmental gas and authentication-based sensingrdquo IEEE Sensors Journal vol 11 no 12 pp 3139ndash3152 2011

[19] P Nikitin and K Rao ldquoHarmonic scattering from passiveUHF RFID tagsrdquo in Proceedings of the IEEE Antennas andPropagation Society International Symposium (APSURSI rsquo09)pp 1ndash4 Charleston SC USA June 2009

[20] G A Vera Y Duroc and S Tedjini ldquoRedundant backscatteringmodulation of passive UHF RFID tagsrdquo in Proceedings of theIEEE MTT-S International Microwave Symposium Digest (IMSrsquo13) pp 1ndash3 Seattle Wash USA June 2013

[21] F Alimenti and L Roselli ldquoTheory of zero-power rfid sensorsbased on harmonic generation and orthogonally polarizedantennasrdquo Progress in Electromagnetics Research vol 134 pp337ndash357 2013

[22] B Kubina J Romeu C Mandel M Schuszligler and R JakobyldquoQuasi-chipless wireless temperature sensor based on harmonicradarrdquo Electronics Letters vol 50 no 2 pp 86ndash88 2014

[23] P V Nikitin and K V S Rao ldquoAntennas and Propagation inUHF RFID Systemsrdquo in Proceedings of the IEEE InternationalConference on RFID pp 277ndash288 Las Vegas Nev USA April2008


[24] S C M Perera A G Williamson and G B Rowe ldquoPredictionof breakpoint distance in microcellular environmentsrdquo Elec-tronics Letters vol 35 no 14 pp 1135ndash1136 1999

[25] A Lazaro D Girbau and D Salinas ldquoRadio link budgets forUHF RFID on multipath environmentsrdquo IEEE Transactions onAntennas and Propagation vol 57 no 4 pp 1241ndash1251 2009

[26] I Kwon Y Eo H Bang et al ldquoA single-chip CMOS transceiverfor UHF mobile RHD readerrdquo IEEE Journal of Solid-StateCircuits vol 43 no 3 pp 729ndash738 2008

[27] R Barnett G Balachandran S Lazar et al ldquoA passive UHFRFID transponder for EPC gen 2 with-14 dBm sensitivityin 013 120583m CMOSrdquo in Proceedings of the IEEE InternationalSolid-State Circuits Conference (ISSCC 07) pp 582ndash623 SanFrancisco Calif USA February 2007

International Journal of

AerospaceEngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

RoboticsJournal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014


Active and Passive Electronic Components

Control Scienceand Engineering

Journal of



RotatingMachinery


Hindawi Publishing Corporation httpwwwhindawicom

Journal ofEngineeringVolume 2014

Submit your manuscripts athttpwwwhindawicom

VLSI Design



Shock and Vibration


Civil EngineeringAdvances in

Acoustics and VibrationAdvances in



Electrical and Computer Engineering

Journal of

Advances inOptoElectronics


Volume 2014

The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014

SensorsJournal of


Modelling amp Simulation in EngineeringHindawi Publishing Corporation httpwwwhindawicom Volume 2014


Chemical EngineeringInternational Journal of Antennas and

Propagation




Navigation and Observation



DistributedSensor Networks



Transmitter

Receiver

Directionalcoupler

Band passfilter

TagLeakagesignalBackscattered

signal

RFID reader

Figure 1 Block diagram of a UHF passive system

sensorHence both the required power to turn-on the tag andthe received power depend on the physical magnitude to beread [16ndash18]

Harmonic scattering from passive UHF RFID tags hasbeen used to characterize the radiation pattern diagram ofthe tag [19] In a recent work [20] the residual harmonicsignal from commercial UHF tags has been investigatedand it is proposed as useful tool for the developmentof future sensors These works show that standard UHFtags generate backscattered harmonics However since theantenna is matched at the fundamental frequency it filtersthese components Therefore the backscattered power of theharmonics is too low In this sense standard tags can beconsidered as harmonic tags due to nonlinear behaviour Onepractical problem when a harmonic sensor is designed isthe selection of the working frequency In order to enhancethe nonlinearities high power should be used to achievea great read range of meters thus UHF RFID band isa good candidate However the second harmonic can fallwithin other band dedicated to other services that can beinterfered by the backscattered harmonics near the reader Ata few meters the harmonic level is small and therefore theinterference will be small too and comparable to the level ofharmonics in conventional UHF RFID systems In any casethere is a lack of regulation about the frequency band forharmonic radars However sensors based on harmonic tagshave begun to be studied in the literature [21] For instance atemperature harmonic tag has been proposed in the literatureworking at 15 GHz fundamental frequency [22]

In this work a proof of concept for a novel humiditysensor based on harmonic tag working at UHF RFID bandis presented An input rectenna is used to power a low-fre-quency oscillator that modulates the frequency doublerbased on a Schottky diodeThismodulation allows increasingthe read range because it reduces the problems associatedwith the harmonic leakage in the reader The RF signalreceived by the other polarisation of the input antenna isused as input signal towards the doubler A simple low-costinterdigital capacitor that loads the output patch antenna isused as sensor The effective permittivity changes with therelative humidity changing the capacitance and detuning theoutput antenna The paper is structured as follows Section 2presents the humidity sensor tag and describes the basictheory of operation A link-budget study is done in thissection in order to study the read range and the level ofharmonic A comparison between the read range that is

Power amplifier

x2

Detector

Low-noiseamplifier

x2

Reader

UHFoscillator

f0

f0

BPF f0

TXantennaf0

RXantenna2f0

2f0BPF

2f0

Figure 2 Block diagram of the system

achievable with the proposed harmonic tag and conventionalbackscattering RFID systems will be studied in this sectionSection 3 presents the experimental results of the systemFinally some conclusions are drawn in Section 4

2 RFID System Based on Harmonic Radar

21 System Description and Theory of Operation A blockdiagram of the RFID system based on a harmonic tag isshown in Figure 2 comprising the reader and the tagThe tagcan be observed in Figure 3 It consists of a dual-polarizedpatch reception (Rx) antenna that collects the interrogationsignal at aUHF frequency of119891

0(865ndash868MHz)This antenna

is connected to the transmission (Tx) patch antenna bymeansof a frequency doubler The Tx antenna is loaded with aninterdigital capacitor with a capacitance dependent on thehumidity so that when the relative humidity changes theantenna is detuned A variation in the received power at thereader is thus obtainedThe experimental results given in thenext section show that despite using filters at the transmitteroutput the amplitude of the leakage signal at 2119891

0is larger

than the tag reflected amplitude at the second harmonicModulation of the tag is therefore required to exploit thereceiverrsquos sensitivity To that end the second port of theRx antenna is connected to a RF to DC converter modulewhich supplies a low-frequency oscillator This oscillatormodulates the output signal at 2119891

0by controlling the bias

point of the frequency doubler In consequence the taganswers the reader modulating the second harmonic Thereaderrsquos mixer demodulates the backscattered signal usingthe second harmonic of the transmitted signal as the localoscillator The tag is manufactured using standard fiberglasssubstrate FR4 The Rx antenna is a square patch antenna79mmwide and an inset line length of 14mm inset linewidthof 1mm and an inset gap of 1mmThe frequency conversionin harmonic tags is generally accomplished by a Schottkydiode which should have zero-bias high voltage sensitivityand a low barrier in order to maximize the conversion ofsmall signals In this study the frequency doubler is basedon a zero-bias Schottky diode (Avago Technologies modelHSMS-2850) The fundamental signal at 119891

0is filtered using


(a)

RF to DCconverter



f0

Outputantenna

2f0

1205824f0 12058242f012058242f0

12058242f0Rbias

R2 (0Ω)R1 (750kΩ) C1 (1nF)

100 pF

(b)










0 510

20

30

40

50

60

Input power (dBm)

Con

vers

ion

loss

(dB)




VDCout

Frominput

antennaHSMS-2852

HSMS-2852

HSMS-2852100 pF

100 pF 100 pF 100 pF

100 pF

100 pF

75nH


⋱





00

2

4

6

8

Input power (dBm)

Det

ecte

d vo

ltage

(V)











120582in (1)



119875119903tag =

119875119879119866119879

4120587(1

1198891198991sdot

1

(1 + 119889119877B1)119899119861minus1198991

)119860119890tag (2)


119860119890tag =

1205822in4120587119866tagin (3)









L inset = 95mm

L = 375mm

W = 51mm



08mm via holes

(a)

Input

L + 2ΔL

L inset

RradRrad

(b)


0 500 1000 1500 2000 2500 3000

0

5

10

15

20

25

Frequency (MHz)

Capa

cita

nce (

pF)

10 RH50 RH

80 RH100 RH

minus20

minus15

minus10

minus5



119875119903reader


119866tagout

4120587(1

1198891198992sdot

1

(1 + 119889119877B2)119899119861minus1198992

)1198701205822out4120587

119866119877

(4)






119870 =1

4

10038161003816100381610038161205881 minus 120588210038161003816100381610038162 (5)


1and 120588




119909 (119905) = (1198601 + (1198602 minus 1198601)infin

sum119899=1

prod(119905 minus 119899119879

1198792)) cos (2120587119891out119905)

(6)


2


119870 =2

1205872(1198602minus 1198601

1198601+ 1198602

)2

(7)



10 20 30 40 50 60 70 80 90 10036

38

4

42

44

46


Capa

cita

nce (

pF)

(a)

10 20 30 40 50 60 70 80 90 100


0

20

40

60

Qua

lity

fact

or

(b)


1700 1720 1740 1760 1780 1800 1820 1840 1860 1880 1900

0

Frequency (MHz)

(dB)

10 RH50 RH

80 RH100 RH

Loadedantenna

Unloadedantenna

minus2

minus4

minus6

minus8

minus10

minus12

minus14



119872 = 1 minus

1003816100381610038161003816120588110038161003816100381610038162+1003816100381610038161003816120588210038161003816100381610038162

2 (8)



10 20 30 40 50 60 70 80 90 1001726

1728

1730

1732

1734

1736

1738

1740

1742


Reso

nanc

e fre

quen

cy (M

Hz)


0 5


minus22

minus20

minus18

minus16

minus14

minus12

minus10

minus8

Mod

ulat

ion

fact

orK

(dB)




Tag input power

Power-up threshold

0 5 10 15

Distance (m)


(dBm

)

minus120

minus100

minus80

minus60

minus40

minus20

0

20


(a)

0 5 10 15

Distance (m)

(dBm

)

Tag input power

Power-up threshold


minus140

minus120

minus100

minus80

minus60

minus40

minus20

0

20


(b)




0

20

40

60

80

Sign

al-to

-noi

se ra

tio (d

B)


0 5 10 15

Distance (m)

(a)

0

20

40

60

80

Sign

al-to

-noi

se ra

tio (d

B)


0 5 10 15

Distance (m)

minus20

(b)












3010

Spectrumanalyzer

RampS FSP-30


x2


Commercialhumidity

sensor

Multimeter

Dryair

Wetair

Chamber2f0

TX


antennaf0

BPF f0


RampS SMF-10A

2f0BPF

RX


antenna2f0

f0



1=



1= 1198992= 25




17295

17296

17297

17298

17299

1730

17301

17302

17303

17304

17305

Frequency (MHz)

(dBm

)

minus110

minus100

minus90

minus80

minus70

minus60

minus50

minus40


830 835 840 845 850 855 860 865 870 875


10 RH60 RH

100 RH

(dBm

)

minus85

minus80

minus75

minus70

minus65





0 50 100 150 200 250 300 350 400

Time (s)

(dB)

minus8

minus6

minus4

minus2

0

(a)

0 50 100 150 200 250 300 350 400

Time (s)

0

50

100

150

RH (

)

(b)


0 10 20 30 40 50 60 70 80 90 1000

20

40

60

80

100


Nor

mal

ized

pow

er ch

ange

()







4 Conclusions






Acknowledgment


References































RoboticsJournal of





Journal of



RotatingMachinery





VLSI Design



Shock and Vibration







Journal of



Volume 2014


SensorsJournal of





Propagation










(a)

RF to DCconverter



f0

Outputantenna

2f0

1205824f0 12058242f012058242f0

12058242f0Rbias

R2 (0Ω)R1 (750kΩ) C1 (1nF)

100 pF

(b)










0 510

20

30

40

50

60

Input power (dBm)

Con

vers

ion

loss

(dB)




VDCout

Frominput

antennaHSMS-2852

HSMS-2852

HSMS-2852100 pF

100 pF 100 pF 100 pF

100 pF

100 pF

75nH


⋱





00

2

4

6

8

Input power (dBm)

Det

ecte

d vo

ltage

(V)











120582in (1)



119875119903tag =

119875119879119866119879

4120587(1

1198891198991sdot

1

(1 + 119889119877B1)119899119861minus1198991

)119860119890tag (2)


119860119890tag =

1205822in4120587119866tagin (3)









L inset = 95mm

L = 375mm

W = 51mm



08mm via holes

(a)

Input

L + 2ΔL

L inset

RradRrad

(b)


0 500 1000 1500 2000 2500 3000

0

5

10

15

20

25

Frequency (MHz)

Capa

cita

nce (

pF)

10 RH50 RH

80 RH100 RH

minus20

minus15

minus10

minus5



119875119903reader


119866tagout

4120587(1

1198891198992sdot

1

(1 + 119889119877B2)119899119861minus1198992

)1198701205822out4120587

119866119877

(4)






119870 =1

4

10038161003816100381610038161205881 minus 120588210038161003816100381610038162 (5)


1and 120588




119909 (119905) = (1198601 + (1198602 minus 1198601)infin

sum119899=1

prod(119905 minus 119899119879

1198792)) cos (2120587119891out119905)

(6)


2


119870 =2

1205872(1198602minus 1198601

1198601+ 1198602

)2

(7)



10 20 30 40 50 60 70 80 90 10036

38

4

42

44

46


Capa

cita

nce (

pF)

(a)

10 20 30 40 50 60 70 80 90 100


0

20

40

60

Qua

lity

fact

or

(b)


1700 1720 1740 1760 1780 1800 1820 1840 1860 1880 1900

0

Frequency (MHz)

(dB)

10 RH50 RH

80 RH100 RH

Loadedantenna

Unloadedantenna

minus2

minus4

minus6

minus8

minus10

minus12

minus14



119872 = 1 minus

1003816100381610038161003816120588110038161003816100381610038162+1003816100381610038161003816120588210038161003816100381610038162

2 (8)



10 20 30 40 50 60 70 80 90 1001726

1728

1730

1732

1734

1736

1738

1740

1742


Reso

nanc

e fre

quen

cy (M

Hz)


0 5


minus22

minus20

minus18

minus16

minus14

minus12

minus10

minus8

Mod

ulat

ion

fact

orK

(dB)




Tag input power

Power-up threshold

0 5 10 15

Distance (m)


(dBm

)

minus120

minus100

minus80

minus60

minus40

minus20

0

20


(a)

0 5 10 15

Distance (m)

(dBm

)

Tag input power

Power-up threshold


minus140

minus120

minus100

minus80

minus60

minus40

minus20

0

20


(b)




0

20

40

60

80

Sign

al-to

-noi

se ra

tio (d

B)


0 5 10 15

Distance (m)

(a)

0

20

40

60

80

Sign

al-to

-noi

se ra

tio (d

B)


0 5 10 15

Distance (m)

minus20

(b)












3010

Spectrumanalyzer

RampS FSP-30


x2


Commercialhumidity

sensor

Multimeter

Dryair

Wetair

Chamber2f0

TX


antennaf0

BPF f0


RampS SMF-10A

2f0BPF

RX


antenna2f0

f0



1=



1= 1198992= 25




17295

17296

17297

17298

17299

1730

17301

17302

17303

17304

17305

Frequency (MHz)

(dBm

)

minus110

minus100

minus90

minus80

minus70

minus60

minus50

minus40


830 835 840 845 850 855 860 865 870 875


10 RH60 RH

100 RH

(dBm

)

minus85

minus80

minus75

minus70

minus65





0 50 100 150 200 250 300 350 400

Time (s)

(dB)

minus8

minus6

minus4

minus2

0

(a)

0 50 100 150 200 250 300 350 400

Time (s)

0

50

100

150

RH (

)

(b)


0 10 20 30 40 50 60 70 80 90 1000

20

40

60

80

100


Nor

mal

ized

pow

er ch

ange

()







4 Conclusions






Acknowledgment


References































RoboticsJournal of





Journal of



RotatingMachinery





VLSI Design



Shock and Vibration







Journal of



Volume 2014


SensorsJournal of





Propagation










00

2

4

6

8

Input power (dBm)

Det

ecte

d vo

ltage

(V)











120582in (1)



119875119903tag =

119875119879119866119879

4120587(1

1198891198991sdot

1

(1 + 119889119877B1)119899119861minus1198991

)119860119890tag (2)


119860119890tag =

1205822in4120587119866tagin (3)









L inset = 95mm

L = 375mm

W = 51mm



08mm via holes

(a)

Input

L + 2ΔL

L inset

RradRrad

(b)


0 500 1000 1500 2000 2500 3000

0

5

10

15

20

25

Frequency (MHz)

Capa

cita

nce (

pF)

10 RH50 RH

80 RH100 RH

minus20

minus15

minus10

minus5



119875119903reader


119866tagout

4120587(1

1198891198992sdot

1

(1 + 119889119877B2)119899119861minus1198992

)1198701205822out4120587

119866119877

(4)






119870 =1

4

10038161003816100381610038161205881 minus 120588210038161003816100381610038162 (5)


1and 120588




119909 (119905) = (1198601 + (1198602 minus 1198601)infin

sum119899=1

prod(119905 minus 119899119879

1198792)) cos (2120587119891out119905)

(6)


2


119870 =2

1205872(1198602minus 1198601

1198601+ 1198602

)2

(7)



10 20 30 40 50 60 70 80 90 10036

38

4

42

44

46


Capa

cita

nce (

pF)

(a)

10 20 30 40 50 60 70 80 90 100


0

20

40

60

Qua

lity

fact

or

(b)


1700 1720 1740 1760 1780 1800 1820 1840 1860 1880 1900

0

Frequency (MHz)

(dB)

10 RH50 RH

80 RH100 RH

Loadedantenna

Unloadedantenna

minus2

minus4

minus6

minus8

minus10

minus12

minus14



119872 = 1 minus

1003816100381610038161003816120588110038161003816100381610038162+1003816100381610038161003816120588210038161003816100381610038162

2 (8)



10 20 30 40 50 60 70 80 90 1001726

1728

1730

1732

1734

1736

1738

1740

1742


Reso

nanc

e fre

quen

cy (M

Hz)


0 5


minus22

minus20

minus18

minus16

minus14

minus12

minus10

minus8

Mod

ulat

ion

fact

orK

(dB)




Tag input power

Power-up threshold

0 5 10 15

Distance (m)


(dBm

)

minus120

minus100

minus80

minus60

minus40

minus20

0

20


(a)

0 5 10 15

Distance (m)

(dBm

)

Tag input power

Power-up threshold


minus140

minus120

minus100

minus80

minus60

minus40

minus20

0

20


(b)




0

20

40

60

80

Sign

al-to

-noi

se ra

tio (d

B)


0 5 10 15

Distance (m)

(a)

0

20

40

60

80

Sign

al-to

-noi

se ra

tio (d

B)


0 5 10 15

Distance (m)

minus20

(b)












3010

Spectrumanalyzer

RampS FSP-30


x2


Commercialhumidity

sensor

Multimeter

Dryair

Wetair

Chamber2f0

TX


antennaf0

BPF f0


RampS SMF-10A

2f0BPF

RX


antenna2f0

f0



1=



1= 1198992= 25




17295

17296

17297

17298

17299

1730

17301

17302

17303

17304

17305

Frequency (MHz)

(dBm

)

minus110

minus100

minus90

minus80

minus70

minus60

minus50

minus40


830 835 840 845 850 855 860 865 870 875


10 RH60 RH

100 RH

(dBm

)

minus85

minus80

minus75

minus70

minus65





0 50 100 150 200 250 300 350 400

Time (s)

(dB)

minus8

minus6

minus4

minus2

0

(a)

0 50 100 150 200 250 300 350 400

Time (s)

0

50

100

150

RH (

)

(b)


0 10 20 30 40 50 60 70 80 90 1000

20

40

60

80

100


Nor

mal

ized

pow

er ch

ange

()







4 Conclusions






Acknowledgment


References































RoboticsJournal of





Journal of



RotatingMachinery





VLSI Design



Shock and Vibration







Journal of



Volume 2014


SensorsJournal of





Propagation










L inset = 95mm

L = 375mm

W = 51mm



08mm via holes

(a)

Input

L + 2ΔL

L inset

RradRrad

(b)


0 500 1000 1500 2000 2500 3000

0

5

10

15

20

25

Frequency (MHz)

Capa

cita

nce (

pF)

10 RH50 RH

80 RH100 RH

minus20

minus15

minus10

minus5



119875119903reader


119866tagout

4120587(1

1198891198992sdot

1

(1 + 119889119877B2)119899119861minus1198992

)1198701205822out4120587

119866119877

(4)






119870 =1

4

10038161003816100381610038161205881 minus 120588210038161003816100381610038162 (5)


1and 120588




119909 (119905) = (1198601 + (1198602 minus 1198601)infin

sum119899=1

prod(119905 minus 119899119879

1198792)) cos (2120587119891out119905)

(6)


2


119870 =2

1205872(1198602minus 1198601

1198601+ 1198602

)2

(7)



10 20 30 40 50 60 70 80 90 10036

38

4

42

44

46


Capa

cita

nce (

pF)

(a)

10 20 30 40 50 60 70 80 90 100


0

20

40

60

Qua

lity

fact

or

(b)


1700 1720 1740 1760 1780 1800 1820 1840 1860 1880 1900

0

Frequency (MHz)

(dB)

10 RH50 RH

80 RH100 RH

Loadedantenna

Unloadedantenna

minus2

minus4

minus6

minus8

minus10

minus12

minus14



119872 = 1 minus

1003816100381610038161003816120588110038161003816100381610038162+1003816100381610038161003816120588210038161003816100381610038162

2 (8)



10 20 30 40 50 60 70 80 90 1001726

1728

1730

1732

1734

1736

1738

1740

1742


Reso

nanc

e fre

quen

cy (M

Hz)


0 5


minus22

minus20

minus18

minus16

minus14

minus12

minus10

minus8

Mod

ulat

ion

fact

orK

(dB)




Tag input power

Power-up threshold

0 5 10 15

Distance (m)


(dBm

)

minus120

minus100

minus80

minus60

minus40

minus20

0

20


(a)

0 5 10 15

Distance (m)

(dBm

)

Tag input power

Power-up threshold


minus140

minus120

minus100

minus80

minus60

minus40

minus20

0

20


(b)




0

20

40

60

80

Sign

al-to

-noi

se ra

tio (d

B)


0 5 10 15

Distance (m)

(a)

0

20

40

60

80

Sign

al-to

-noi

se ra

tio (d

B)


0 5 10 15

Distance (m)

minus20

(b)












3010

Spectrumanalyzer

RampS FSP-30


x2


Commercialhumidity

sensor

Multimeter

Dryair

Wetair

Chamber2f0

TX


antennaf0

BPF f0


RampS SMF-10A

2f0BPF

RX


antenna2f0

f0



1=



1= 1198992= 25




17295

17296

17297

17298

17299

1730

17301

17302

17303

17304

17305

Frequency (MHz)

(dBm

)

minus110

minus100

minus90

minus80

minus70

minus60

minus50

minus40


830 835 840 845 850 855 860 865 870 875


10 RH60 RH

100 RH

(dBm

)

minus85

minus80

minus75

minus70

minus65





0 50 100 150 200 250 300 350 400

Time (s)

(dB)

minus8

minus6

minus4

minus2

0

(a)

0 50 100 150 200 250 300 350 400

Time (s)

0

50

100

150

RH (

)

(b)


0 10 20 30 40 50 60 70 80 90 1000

20

40

60

80

100


Nor

mal

ized

pow

er ch

ange

()







4 Conclusions






Acknowledgment


References































RoboticsJournal of





Journal of



RotatingMachinery





VLSI Design



Shock and Vibration







Journal of



Volume 2014


SensorsJournal of





Propagation










10 20 30 40 50 60 70 80 90 10036

38

4

42

44

46


Capa

cita

nce (

pF)

(a)

10 20 30 40 50 60 70 80 90 100


0

20

40

60

Qua

lity

fact

or

(b)


1700 1720 1740 1760 1780 1800 1820 1840 1860 1880 1900

0

Frequency (MHz)

(dB)

10 RH50 RH

80 RH100 RH

Loadedantenna

Unloadedantenna

minus2

minus4

minus6

minus8

minus10

minus12

minus14



119872 = 1 minus

1003816100381610038161003816120588110038161003816100381610038162+1003816100381610038161003816120588210038161003816100381610038162

2 (8)



10 20 30 40 50 60 70 80 90 1001726

1728

1730

1732

1734

1736

1738

1740

1742


Reso

nanc

e fre

quen

cy (M

Hz)


0 5


minus22

minus20

minus18

minus16

minus14

minus12

minus10

minus8

Mod

ulat

ion

fact

orK

(dB)




Tag input power

Power-up threshold

0 5 10 15

Distance (m)


(dBm

)

minus120

minus100

minus80

minus60

minus40

minus20

0

20


(a)

0 5 10 15

Distance (m)

(dBm

)

Tag input power

Power-up threshold


minus140

minus120

minus100

minus80

minus60

minus40

minus20

0

20


(b)




0

20

40

60

80

Sign

al-to

-noi

se ra

tio (d

B)


0 5 10 15

Distance (m)

(a)

0

20

40

60

80

Sign

al-to

-noi

se ra

tio (d

B)


0 5 10 15

Distance (m)

minus20

(b)












3010

Spectrumanalyzer

RampS FSP-30


x2


Commercialhumidity

sensor

Multimeter

Dryair

Wetair

Chamber2f0

TX


antennaf0

BPF f0


RampS SMF-10A

2f0BPF

RX


antenna2f0

f0



1=



1= 1198992= 25




17295

17296

17297

17298

17299

1730

17301

17302

17303

17304

17305

Frequency (MHz)

(dBm

)

minus110

minus100

minus90

minus80

minus70

minus60

minus50

minus40


830 835 840 845 850 855 860 865 870 875


10 RH60 RH

100 RH

(dBm

)

minus85

minus80

minus75

minus70

minus65





0 50 100 150 200 250 300 350 400

Time (s)

(dB)

minus8

minus6

minus4

minus2

0

(a)

0 50 100 150 200 250 300 350 400

Time (s)

0

50

100

150

RH (

)

(b)


0 10 20 30 40 50 60 70 80 90 1000

20

40

60

80

100


Nor

mal

ized

pow

er ch

ange

()







4 Conclusions






Acknowledgment


References































RoboticsJournal of





Journal of



RotatingMachinery





VLSI Design



Shock and Vibration







Journal of



Volume 2014


SensorsJournal of





Propagation










Tag input power

Power-up threshold

0 5 10 15

Distance (m)


(dBm

)

minus120

minus100

minus80

minus60

minus40

minus20

0

20


(a)

0 5 10 15

Distance (m)

(dBm

)

Tag input power

Power-up threshold


minus140

minus120

minus100

minus80

minus60

minus40

minus20

0

20


(b)




0

20

40

60

80

Sign

al-to

-noi

se ra

tio (d

B)


0 5 10 15

Distance (m)

(a)

0

20

40

60

80

Sign

al-to

-noi

se ra

tio (d

B)


0 5 10 15

Distance (m)

minus20

(b)












3010

Spectrumanalyzer

RampS FSP-30


x2


Commercialhumidity

sensor

Multimeter

Dryair

Wetair

Chamber2f0

TX


antennaf0

BPF f0


RampS SMF-10A

2f0BPF

RX


antenna2f0

f0



1=



1= 1198992= 25




17295

17296

17297

17298

17299

1730

17301

17302

17303

17304

17305

Frequency (MHz)

(dBm

)

minus110

minus100

minus90

minus80

minus70

minus60

minus50

minus40


830 835 840 845 850 855 860 865 870 875


10 RH60 RH

100 RH

(dBm

)

minus85

minus80

minus75

minus70

minus65





0 50 100 150 200 250 300 350 400

Time (s)

(dB)

minus8

minus6

minus4

minus2

0

(a)

0 50 100 150 200 250 300 350 400

Time (s)

0

50

100

150

RH (

)

(b)


0 10 20 30 40 50 60 70 80 90 1000

20

40

60

80

100


Nor

mal

ized

pow

er ch

ange

()







4 Conclusions






Acknowledgment


References































RoboticsJournal of





Journal of



RotatingMachinery





VLSI Design



Shock and Vibration







Journal of



Volume 2014


SensorsJournal of





Propagation











3010

Spectrumanalyzer

RampS FSP-30


x2


Commercialhumidity

sensor

Multimeter

Dryair

Wetair

Chamber2f0

TX


antennaf0

BPF f0


RampS SMF-10A

2f0BPF

RX


antenna2f0

f0



1=



1= 1198992= 25




17295

17296

17297

17298

17299

1730

17301

17302

17303

17304

17305

Frequency (MHz)

(dBm

)

minus110

minus100

minus90

minus80

minus70

minus60

minus50

minus40


830 835 840 845 850 855 860 865 870 875


10 RH60 RH

100 RH

(dBm

)

minus85

minus80

minus75

minus70

minus65





0 50 100 150 200 250 300 350 400

Time (s)

(dB)

minus8

minus6

minus4

minus2

0

(a)

0 50 100 150 200 250 300 350 400

Time (s)

0

50

100

150

RH (

)

(b)


0 10 20 30 40 50 60 70 80 90 1000

20

40

60

80

100


Nor

mal

ized

pow

er ch

ange

()







4 Conclusions






Acknowledgment


References































RoboticsJournal of





Journal of



RotatingMachinery





VLSI Design



Shock and Vibration







Journal of



Volume 2014


SensorsJournal of





Propagation










0 50 100 150 200 250 300 350 400

Time (s)

(dB)

minus8

minus6

minus4

minus2

0

(a)

0 50 100 150 200 250 300 350 400

Time (s)

0

50

100

150

RH (

)

(b)


0 10 20 30 40 50 60 70 80 90 1000

20

40

60

80

100


Nor

mal

ized

pow

er ch

ange

()







4 Conclusions






Acknowledgment


References































RoboticsJournal of





Journal of



RotatingMachinery





VLSI Design



Shock and Vibration







Journal of



Volume 2014


SensorsJournal of





Propagation













Acknowledgment


References































RoboticsJournal of





Journal of



RotatingMachinery





VLSI Design



Shock and Vibration







Journal of



Volume 2014


SensorsJournal of





Propagation
















RoboticsJournal of





Journal of



RotatingMachinery





VLSI Design



Shock and Vibration







Journal of



Volume 2014


SensorsJournal of





Propagation











RoboticsJournal of





Journal of



RotatingMachinery





VLSI Design



Shock and Vibration







Journal of



Volume 2014


SensorsJournal of





Propagation









Documents

Research Article A Passive Harmonic Tag for Humidity Sensingdownloads.hindawi.com/journals/ijap/2014/670345.pdfLeakage signal Backscattered signal RFID reader F : Block diagram of