This letter introduces a planar antenna layout suited to Sensor-RFID fabrication. The geometry is based on a meandered-slot profile on a suspended patch and permits to host sensors and electronics in a small space. The available geometrical parameters are optimized by means of a Genetic Algorithm (GA) procedure aimed to maximize the antenna realized gain. The antenna performances are discussed through examples and prototypes

Calabrese, C

Marrocco, G

Cineca Institutional Research Information System

Meandered-slot antennas for sensor-RFID tags

Calabrese C., Marrocco G.

Archivio della Ricerca - Università di Roma Tor vergata

IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 7, 2008 5
Meandered-Slot Antennas for Sensor-RFID Tags
Claudio Calabrese and Gaetano Marrocco, Member, IEEE
Abstract—This letter introduces a planar antenna layout suited
to Sensor-RFID fabrication. The geometry is based on a mean-
dered-slot profile on a suspended patch and permits to host sensors
and electronics in a small space. The available geometrical param-
eters are optimized by means of a Genetic Algorithm (GA) proce-
dure aimed to maximize the antenna realized gain. The antenna
performances are discussed through examples and prototypes.
Index Terms—Genetic algorithm (GA), miniaturization, radio
frequency identification (RFID), sensor, slot-line.
I. INTRODUCTION
ONE of the most fascinating evolutions of the Radio Fre-quency IDentification (RFID) technology is the Sensor
RFID family (S-RFID) where the tag is augmented with sensing
and signal processing capabilities in addition to more traditional
labelling operations [1]. Some applications have been already
envisaged such as the monitoring of environmental parameters
as well as of the human activity (temperature, blood pressure,
heartbeat, glucose content, motion) [2], [3]. Such devices re-
quire a higher degree of integration when compared with stan-
dard RFID tags, since the antenna, the chip (IC), and all the rel-
evant sensing electronics need to be efficiently packaged within
a small space.
In a recent paper [4], a versatile tag antenna configuration,
having impedance tuning capability and the possibility to
host electronics packaging, has been proposed for application
in biomonitoring. The antenna consists of a nested H-slot
suspended patch, wherein the external size determines the
maximum gain while the H-aspect ratio affects the complex
impedance matching. This layout is now generalized by in-
troducing a larger number of design parameters so that, when
properly optimized, the antenna could permit to fulfill several
electrical and geometrical constraints, such as the impedance
matching to a particular IC, the integration of a given size
sensor, the space miniaturization, and a stable response over a
large variety of tagged dielectrics and multiband performances.
II. ANTENNA LAYOUT
The considered antenna layout (Fig. 1) comprises a planar
patch (with no ground plane) provided with a nonuniform
slot-line which occupies just a portion of the metallization.
The choice of the slot, as main radiating element, is expected
Manuscript received September 22, 2007; revised November 27, 2007.
The authors are with the Dipartimento di Informatica, Sistemi e Produzione,
Università di Roma Tor Vergata, 00133 Rome, Italy (e-mail: marrocco@disp.
uniroma2.it).
Digital Object Identifier 10.1109/LAWP.2007.914123
Fig. 1. General layout of the proposed tag family and distributed model
of the slot-line sections including radiation from step discontinuity (shunt
conductance).
to simplify the synthesis of the inductive reactance required to
match the generally capacitive IC’s impedance.
As discussed in [4], the H-slot may radiate, depending on the
slot’s aspect ratio, as a broadband patch, a doubly folded dipole
or as a dumbbell slot.
By generalization of the circuit model of the dumbbell
slot in [5], it can be assumed that each short slot-line section
mainly provides electric energy storage and radiation, while the
discontinuity (tooth) between two slot-lines having different
section adds an inductive reactance besides producing radiation.
The whole slot profile, hereafter denoted to as Meander-Slot
Antenna (MSA) can be, therefore, considered as a slot-line
impedance transformer, with many degrees of freedom to
convert the terminating short circuit to the complex conjugate
of the IC impedance.
III. OPTIMIZATION PROBLEM
The antenna’s shape optimization problem can be cast as the
selection of the geometrical profile of the slot-line transformer,
under sensor space constraints, such to achieve the maximum
activation distance of the RFID tag, when it is placed onto a
particular dielectric object.
Having fixed the effective power EIRP transmitted by the
reader and the sensitivity of the tag’s IC (i.e., the RF
1536-1225/$25.00 © 2008 IEEE
6 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 7, 2008
Fig. 2. Geometric parameters of the slot-line tag to be optimized, for the par-
ticular case of 2N = 6 (three independent teeth). Only half of the antenna
layout is shown.
power required to the microchip electronics to turn on and to
perform backscattering modulation), the maximum activation
distance of the tag is given [6] by
EIRP (1)
where is the maximum tag’s gain, and the factor
(2)
is the power transmission coefficient which accounts for the
impedance mismatch between the antenna
and the chip . The ultimate antenna’s
feature to be maximized is, therefore, the realized gain .
To reduce the design complexity, the shaped slot is assumed
to be symmetric with respect to the -plane. The geometrical
constraints are: tag’s external size , slot-over-tag ratio
, depending on the space to allocate sensors, and the
number of slot’s teeth. The parameters to optimize are
(see Fig. 2) the position of the chip and the vertical exten-
sion of the independent teeth given in terms of their upper
and lower coordinates . The length of
each slot-line section is instead regular and fixed by the teeth’s
number and the size. The teeth’s coordinates are
subjected to some constraints for geometrical consistency, e.g.
(3)
where is a safeguard distance from the patch edges.
The fitness function to maximize is
(4)
where are the frequencies at which the RFID should be best
matched to the chip, and are arbitrary weights.
The optimization problem is solved using a Genetic Algo-
rithm (GA) [7] procedure, already successfully applied to me-
ander-line tag design [8]. To simplify the electromagnetic com-
putation, required to calculate the gain and the impedance of
each antenna of the GA population in presence of the tagged
body, a canonical reference geometry is assumed for the target,
consisting of a homogeneous dielectric slab, of permittivity ,
orthogonal to the axis and bounded within . The
tag is placed on . Configurations with represent the
free space and hence is useful to model a tag sensing some en-
vironmental parameters, such as humidity or temperature. The
limiting case with (half-space) could instead model
large and deep targets, such as walls or containers. Intermediate
thickness could account for the tag response over finite objects
like pallets or blood sacs. The evaluation of the fitness in (4) for
each antenna of the GA population is accomplished by a planar
Moment Method solver [9], using Sommerfeld integral to model
the dielectric slab.
IV. DESIGN EXAMPLES
Some design examples at single and dual frequency, are here
reported for an RFID chip having high-angle-phase impedance
(which represents one of the most chal-
lenging matching condition). All the presented results have been
obtained by the GA procedure using 4 bit coding of the
geometrical parameters to optimize.
A. Tag Miniaturization
A first design set is useful to discuss the miniaturization fea-
ture of the proposed layout respect to the slot size, to the number
of teeth and to the permittivity of the dielectric target. The pro-
posed examples will also suggest a guideline to fix some geo-
metrical parameters. All the reported designs refer to a square
tag of size 5 5 cm (corresponding to in the free-space),
IC packaging’s size 2 2 mm and frequency
(European RFID UHF frequency).
The performance of the GA-optimized multitooth antennas
can be appreciated in comparison with those of the simple
H-shaped slot (single tooth, ) described in [4].
Fig. 3 shows some optimized geometries for the case of an
homogeneous half-space target with permittivity and
some different constraints on the slot size, expressed as frac-
tion of the external tag size. The single-tooth antenna
is not adequate to achieve a reasonable impedance matching
, even if the slot occupies half of the available space,
while a nearly unitary power transfer coefficient can be obtained
by using 5-9 teeth even when the slot size is constrained to 2/5
the overall tag size. In this case, a larger number of teeth are re-
quired and the antenna geometry becomes however more com-
plex and more sensitive to fabrication’s imperfections. The re-
sulting shaped slot profile resembles that of a meander-line an-
tenna which is a typical miniaturized tag geometry [8].
Further designs results are summarized in Table I for other
choices of target’s thickness and permittivity. The best tag
having the simple H-slot profile , optimized for the
free space, is such that the slot extends all over the available tag
size without any miniaturization. In spite of the
resulting large slot, the tag has a modest power transmission
coefficient and realized gain . A
relevant slot miniaturization, moreover preserving a -dipole
like gain, can be instead achieved by multitooth configurations
up to a slot maximum size .
CALABRESE AND MARROCCO: MSAS FOR SENSOR-RFID TAGS 7
Fig. 3. Examples of MSA tags with size L = W = 5 cm, placed over an
" = 3 dielectric half-space. The slot-line profile has been optimized for an IC
withZ = 15  j450 
, for different sizes L of the antenna region and for
different number N of independent slot-line sections. G is the maximum gain
in the air half-space (z < 0).
TABLE I
PERFORMANCES OF MSA TAG OPTIMIZED FOR DIFFERENT TARGET’S
THICKNESS AND PERMITTIVITY
Performances similar to the case have been also ob-
tained for configurations but in these case the gain re-
duces in the air since the maximum radiation occurs toward the
dielectric half-space. The table also reports some design for fi-
nite-thickness targets ( , 20 cm) and even in this case a
nice impedance matching is observed.
Finally, the power transmission factor has been computed
for several optimized antennas over homogeneous half space
with an increasing number of teeth. In all the considered con-
figurations (Fig. 4) a saturation effect is clearly apparent. The
( , , ) diagrams offer some guidelines to choose the
number of the teeth depending on the target’s permittivity and
on the space to allocate sensors and electronics. The improve-
ment of the parameter along with is nearly linear in the
first region of the curves, then after a knee-point, which reduces
for smaller slot occupancy, the further increases of are less
relevant. The knee-points, indicated by a marker in the figure,
could be a reasonable choice for , as tradeoff between a nice
impedance matching and fabrication complexity.
B. Dual-Frequency Tag
The GA optimization has been here applied to the design of
a dual-band MSA tag working at (RFID UHF
Fig. 4. Best achieved power transmission coefficient  with respect to the
number of teeth used in the slot profile optimization. Circles indicate the
approximate transition from the linear improvement of  , along with N , to
saturation. The target is a homogenous half-space.
Fig. 5. Layout of dual frequency tag, optimized for dielectric half-space with
" = 3.
frequency) and (RFID microwave frequency).
The fitness function in (4) is maximized for the choice .
No space requirement has been enforced over the slot, which is
now allowed to occupy the whole tag size.
Results in Fig. 5, referring to a 5 cm 5 cm tag having
teeth and placed over a half-space, shows
nearly identical values of realized gain at the two considered
frequencies.
8 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 7, 2008
Fig. 6. (Top) Free-space tag with size L = W = 5 cm, number of teeth
2N = 40, fabricated onto an FR4 board. (Bottom) Tag connected to a choked
coaxial cable for input impedance measurements.
Fig. 7. Power transmission coefficient of the prototype in Fig. 6, measured and
simulated in the RFID frequency band of 870–960 MHz.
V. EXPERIMENTAL EVALUATION
An 870-MHz MSA tag prototype (Fig. 6) for free-space appli-
cations was fabricated on FR4 substrate ( , ),
wherein the slot occupies half of the tag’s space.
To characterize the tag independently on the microchip and
on the reader’s features, the antenna’s input impedance was di-
rectly measured in chipless mode, and a semirigid coaxial cable
was just connected to the central slot (where the IC should be
placed). The terminal part of the cable was then soldered to the
tag’s metallization and a current choke was however required
to avoid unwanted leakage currents onto the Network Analyzer
port. A short-circuit transmission line stub was hence in-
serted as in Fig. 6 (bottom) to choke the current at the tag’s edge.
It was preliminary evaluated numerically that such a configura-
tion did not perturb the nominal tag impedance and gain.
The prototype was mounted over a dielectric tripod and sus-
pended, by means of the semirigid cable, at 1.8 m vertically
above the floor, in a typical lab environment.
Measured and estimated curves in Fig. 7 for power transmis-
sion coefficient shows a reasonable agreement and values larger
that 0.7 within the European RFID band. The difference in the
values are probably due to the nonideal choking mechanism
and to the not-anechoic measurement environment.
VI. CONCLUSION
The proposed antenna layout offers many degrees of cus-
tomizations and promises to be a good candidate to host sen-
sors and electronics in advanced applications. Of particular rel-
evance is the miniaturization capability which permits to de-
sign matched tags with size of fractions of wavelength. The
optimization is, however, still time-consuming and a more ef-
ficient electromagnetic model of the tag could be developed to
speed-up the antenna design.
ACKNOWLEDGMENT
The authors wish to thank P. Tognolatti for suggestions
and helpful discussions, and L. Scucchia, and S. Ricci and
colleagues for technical support.
REFERENCES
[1] M. Philipose, J. Smith, B. Jiang, A. Mamishev, S. Roy, and K. Sundara-
Rajan, “Battery-free wireless identification and sensing,” IEEE Pervas.
Comput. Mag., vol. 4, no. 1, pp. 10–18, 2005.
[2] S. Nambi, S. Nyalamadugu, S. M. Wentworth, and B. A. Chin, “Radio
frequency identification sensors,” in Proc. 7th World Multiconf. Sys-
temics, Cybern. Informatics (SCI 2003), 2003, pp. 386–390.
[3] C. Alippi and G. Vanini, “An application-level methodology to guide
the design of intelligent-processing power-aware passive RFID,” in Int.
Symp. Circuits Syst., 2005, vol. 6, pp. 5509–5512.
[4] G. Marrocco, “RFID antennas for the UHF remote monitoring of
human subjects,” IEEE Trans. Antennas Propag., vol. 55, no. 6, pt. 2,
pp. 1862–1870, Jun. 2007.
[5] B. G. Porter and S. S. Gearhart, “Impedance and polarization charac-
teristics of H and IHI slot antennas,” IEEE Trans. Antennas Propag.,
vol. 48, no. 8, pp. 1272–1274, Aug. 2000.
[6] P. V. Nikitin, K. V. S. Rao, S. F. Lam, V. Pillai, R. Martinez, and H.
Heinrich, “Power reflection coefficient analysis for complex imped-
ances in RFID tag design,” IEEE Trans. Microw. Theory Tech., vol.
53, pp. 2721–2715, Sep. 2005.
[7] D. S. Weile and E. Michielssen, “Genetic algorithm optimization ap-
plied to electromagnetics: A review,” IEEE Trans. Antennas Propag.,
vol. 45, no. 3, pp. 343–353, Mar. 1997.
[8] G. Marrocco, “Gain-optimized self-resonant meander line antennas for
RFID applications,” IEEE Antennas Wireless Propag. Lett., vol. 2, pp.
302–305, 2003.
[9] “FEKO User’s Manual, Suite 5.1,” EM Software & Systems-S.A. (Pty)
Ltd. [Online]. Available: http://feko.info, Stellenbosch, South Africa,
Dec. 2005.


Calabrese C.

MARROCCO, GAETANO

IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 7, 2008 5Meandered-Slot Antennas for Sensor-RFID TagsClaudio Calabrese and Gaetano Marrocco, Member, IEEEAbstract—This letter introduces a planar antenna layout suitedto Sensor-RFID fabrication. The geometry is based on a mean-dered-slot profile on a suspended patch and permits to host sensorsand electronics in a small space. The available geometrical param-eters are optimized by means of a Genetic Algorithm (GA) proce-dure aimed to maximize the antenna realized gain. The antennaperformances are discussed through examples and prototypes.Index Terms—Genetic algorithm (GA), miniaturization, radiofrequency identification (RFID), sensor, slot-line.I. INTRODUCTIONONE of the most fascinating evolutions of the Radio Fre-quency IDentification (RFID) technology is the SensorRFID family (S-RFID) where the tag is augmented with sensingand signal processing capabilities in addition to more traditionallabelling operations [1]. Some applications have been alreadyenvisaged such as the monitoring of environmental parametersas well as of the human activity (temperature, blood pressure,heartbeat, glucose content, motion) [2], [3]. Such devices re-quire a higher degree of integration when compared with stan-dard RFID tags, since the antenna, the chip (IC), and all the rel-evant sensing electronics need to be efficiently packaged withina small space.In a recent paper [4], a versatile tag antenna configuration,having impedance tuning capability and the possibility tohost electronics packaging, has been proposed for applicationin biomonitoring. The antenna consists of a nested H-slotsuspended patch, wherein the external size determines themaximum gain while the H-aspect ratio affects the compleximpedance matching. This layout is now generalized by in-troducing a larger number of design parameters so that, whenproperly optimized, the antenna could permit to fulfill severalelectrical and geometrical constraints, such as the impedancematching to a particular IC, the integration of a given sizesensor, the space miniaturization, and a stable response over alarge variety of tagged dielectrics and multiband performances.II. ANTENNA LAYOUTThe considered antenna layout (Fig. 1) comprises a planarpatch (with no ground plane) provided with a nonuniformslot-line which occupies just a portion of the metallization.The choice of the slot, as main radiating element, is expectedManuscript received September 22, 2007; revised November 27, 2007.The authors are with the Dipartimento di Informatica, Sistemi e Produzione,Università di Roma Tor Vergata, 00133 Rome, Italy (e-mail: marrocco@disp.uniroma2.it).Digital Object Identifier 10.1109/LAWP.2007.914123Fig. 1. General layout of the proposed tag family and distributed modelof the slot-line sections including radiation from step discontinuity (shuntconductance).to simplify the synthesis of the inductive reactance required tomatch the generally capacitive IC’s impedance.As discussed in [4], the H-slot may radiate, depending on theslot’s aspect ratio, as a broadband patch, a doubly folded dipoleor as a dumbbell slot.By generalization of the circuit model of the dumbbellslot in [5], it can be assumed that each short slot-line sectionmainly provides electric energy storage and radiation, while thediscontinuity (tooth) between two slot-lines having differentsection adds an inductive reactance besides producing radiation.The whole slot profile, hereafter denoted to as Meander-SlotAntenna (MSA) can be, therefore, considered as a slot-lineimpedance transformer, with many degrees of freedom toconvert the terminating short circuit to the complex conjugateof the IC impedance.III. OPTIMIZATION PROBLEMThe antenna’s shape optimization problem can be cast as theselection of the geometrical profile of the slot-line transformer,under sensor space constraints, such to achieve the maximumactivation distance of the RFID tag, when it is placed onto aparticular dielectric object.Having fixed the effective power EIRP transmitted by thereader and the sensitivity of the tag’s IC (i.e., the RF1536-1225/$25.00 © 2008 IEEE6 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 7, 2008Fig. 2. Geometric parameters of the slot-line tag to be optimized, for the par-ticular case of 2N = 6 (three independent teeth). Only half of the antennalayout is shown.power required to the microchip electronics to turn on and toperform backscattering modulation), the maximum activationdistance of the tag is given [6] byEIRP(1)where is the maximum tag’s gain, and the factor(2)is the power transmission coefficient which accounts for theimpedance mismatch between the antennaand the chip . The ultimate antenna’sfeature to be maximized is, therefore, the realized gain .To reduce the design complexity, the shaped slot is assumedto be symmetric with respect to the -plane. The geometricalconstraints are: tag’s external size , slot-over-tag ratio, depending on the space to allocate sensors, and thenumber of slot’s teeth. The parameters to optimize are(see Fig. 2) the position of the chip and the vertical exten-sion of the independent teeth given in terms of their upperand lower coordinates . The length ofeach slot-line section is instead regular and fixed by the teeth’snumber and the size. The teeth’s coordinates aresubjected to some constraints for geometrical consistency, e.g.(3)where is a safeguard distance from the patch edges.The fitness function to maximize is(4)where are the frequencies at which the RFID should be bestmatched to the chip, and are arbitrary weights.The optimization problem is solved using a Genetic Algo-rithm (GA) [7] procedure, already successfully applied to me-ander-line tag design [8]. To simplify the electromagnetic com-putation, required to calculate the gain and the impedance ofeach antenna of the GA population in presence of the taggedbody, a canonical reference geometry is assumed for the target,consisting of a homogeneous dielectric slab, of permittivity ,orthogonal to the axis and bounded within . Thetag is placed on . Configurations with represent thefree space and hence is useful to model a tag sensing some en-vironmental parameters, such as humidity or temperature. Thelimiting case with (half-space) could instead modellarge and deep targets, such as walls or containers. Intermediatethickness could account for the tag response over finite objectslike pallets or blood sacs. The evaluation of the fitness in (4) foreach antenna of the GA population is accomplished by a planarMoment Method solver [9], using Sommerfeld integral to modelthe dielectric slab.IV. DESIGN EXAMPLESSome design examples at single and dual frequency, are herereported for an RFID chip having high-angle-phase impedance(which represents one of the most chal-lenging matching condition). All the presented results have beenobtained by the GA procedure using 4 bit coding of thegeometrical parameters to optimize.A. Tag MiniaturizationA first design set is useful to discuss the miniaturization fea-ture of the proposed layout respect to the slot size, to the numberof teeth and to the permittivity of the dielectric target. The pro-posed examples will also suggest a guideline to fix some geo-metrical parameters. All the reported designs refer to a squaretag of size 5 5 cm (corresponding to in the free-space),IC packaging’s size 2 2 mm and frequency(European RFID UHF frequency).The performance of the GA-optimized multitooth antennascan be appreciated in comparison with those of the simpleH-shaped slot (single tooth, ) described in [4].Fig. 3 shows some optimized geometries for the case of anhomogeneous half-space target with permittivity andsome different constraints on the slot size, expressed as frac-tion of the external tag size. The single-tooth antennais not adequate to achieve a reasonable impedance matching, even if the slot occupies half of the available space,while a nearly unitary power transfer coefficient can be obtainedby using 5-9 teeth even when the slot size is constrained to 2/5the overall tag size. In this case, a larger number of teeth are re-quired and the antenna geometry becomes however more com-plex and more sensitive to fabrication’s imperfections. The re-sulting shaped slot profile resembles that of a meander-line an-tenna which is a typical miniaturized tag geometry [8].Further designs results are summarized in Table I for otherchoices of target’s thickness and permittivity. The best taghaving the simple H-slot profile , optimized for thefree space, is such that the slot extends all over the available tagsize without any miniaturization. In spite of theresulting large slot, the tag has a modest power transmissioncoefficient and realized gain . Arelevant slot miniaturization, moreover preserving a -dipolelike gain, can be instead achieved by multitooth configurationsup to a slot maximum size .CALABRESE AND MARROCCO: MSAS FOR SENSOR-RFID TAGS 7Fig. 3. Examples of MSA tags with size L = W = 5 cm, placed over an" = 3 dielectric half-space. The slot-line profile has been optimized for an ICwithZ = 15  j450 , for different sizes L of the antenna region and fordifferent number N of independent slot-line sections. G is the maximum gainin the air half-space (z < 0).TABLE IPERFORMANCES OF MSA TAG OPTIMIZED FOR DIFFERENT TARGET’STHICKNESS AND PERMITTIVITYPerformances similar to the case have been also ob-tained for configurations but in these case the gain re-duces in the air since the maximum radiation occurs toward thedielectric half-space. The table also reports some design for fi-nite-thickness targets ( , 20 cm) and even in this case anice impedance matching is observed.Finally, the power transmission factor has been computedfor several optimized antennas over homogeneous half spacewith an increasing number of teeth. In all the considered con-figurations (Fig. 4) a saturation effect is clearly apparent. The( , , ) diagrams offer some guidelines to choose thenumber of the teeth depending on the target’s permittivity andon the space to allocate sensors and electronics. The improve-ment of the parameter along with is nearly linear in thefirst region of the curves, then after a knee-point, which reducesfor smaller slot occupancy, the further increases of are lessrelevant. The knee-points, indicated by a marker in the figure,could be a reasonable choice for , as tradeoff between a niceimpedance matching and fabrication complexity.B. Dual-Frequency TagThe GA optimization has been here applied to the design ofa dual-band MSA tag working at (RFID UHFFig. 4. Best achieved power transmission coefficient  with respect to thenumber of teeth used in the slot profile optimization. Circles indicate theapproximate transition from the linear improvement of  , along with N , tosaturation. The target is a homogenous half-space.Fig. 5. Layout of dual frequency tag, optimized for dielectric half-space with" = 3.frequency) and (RFID microwave frequency).The fitness function in (4) is maximized for the choice .No space requirement has been enforced over the slot, which isnow allowed to occupy the whole tag size.Results in Fig. 5, referring to a 5 cm 5 cm tag havingteeth and placed over a half-space, showsnearly identical values of realized gain at the two consideredfrequencies.8 IEEE ANTENNAS AND WIRELESS PROPAGATION LETTERS, VOL. 7, 2008Fig. 6. (Top) Free-space tag with size L = W = 5 cm, number of teeth2N = 40, fabricated onto an FR4 board. (Bottom) Tag connected to a chokedcoaxial cable for input impedance measurements.Fig. 7. Power transmission coefficient of the prototype in Fig. 6, measured andsimulated in the RFID frequency band of 870–960 MHz.V. EXPERIMENTAL EVALUATIONAn 870-MHz MSA tag prototype (Fig. 6) for free-space appli-cations was fabricated on FR4 substrate ( , ),wherein the slot occupies half of the tag’s space.To characterize the tag independently on the microchip andon the reader’s features, the antenna’s input impedance was di-rectly measured in chipless mode, and a semirigid coaxial cablewas just connected to the central slot (where the IC should beplaced). The terminal part of the cable was then soldered to thetag’s metallization and a current choke was however requiredto avoid unwanted leakage currents onto the Network Analyzerport. A short-circuit transmission line stub was hence in-serted as in Fig. 6 (bottom) to choke the current at the tag’s edge.It was preliminary evaluated numerically that such a configura-tion did not perturb the nominal tag impedance and gain.The prototype was mounted over a dielectric tripod and sus-pended, by means of the semirigid cable, at 1.8 m verticallyabove the floor, in a typical lab environment.Measured and estimated curves in Fig. 7 for power transmis-sion coefficient shows a reasonable agreement and values largerthat 0.7 within the European RFID band. The difference in thevalues are probably due to the nonideal choking mechanismand to the not-anechoic measurement environment.VI. CONCLUSIONThe proposed antenna layout offers many degrees of cus-tomizations and promises to be a good candidate to host sen-sors and electronics in advanced applications. Of particular rel-evance is the miniaturization capability which permits to de-sign matched tags with size of fractions of wavelength. Theoptimization is, however, still time-consuming and a more ef-ficient electromagnetic model of the tag could be developed tospeed-up the antenna design.ACKNOWLEDGMENTThe authors wish to thank P. Tognolatti for suggestionsand helpful discussions, and L. Scucchia, and S. Ricci andcolleagues for technical support.REFERENCES[1] M. Philipose, J. Smith, B. Jiang, A. Mamishev, S. Roy, and K. Sundara-Rajan, “Battery-free wireless identification and sensing,” IEEE Pervas.Comput. Mag., vol. 4, no. 1, pp. 10–18, 2005.[2] S. Nambi, S. Nyalamadugu, S. M. Wentworth, and B. A. Chin, “Radiofrequency identification sensors,” in Proc. 7th World Multiconf. Sys-temics, Cybern. Informatics (SCI 2003), 2003, pp. 386–390.[3] C. Alippi and G. Vanini, “An application-level methodology to guidethe design of intelligent-processing power-aware passive RFID,” in Int.Symp. Circuits Syst., 2005, vol. 6, pp. 5509–5512.[4] G. Marrocco, “RFID antennas for the UHF remote monitoring ofhuman subjects,” IEEE Trans. Antennas Propag., vol. 55, no. 6, pt. 2,pp. 1862–1870, Jun. 2007.[5] B. G. Porter and S. S. Gearhart, “Impedance and polarization charac-teristics of H and IHI slot antennas,” IEEE Trans. Antennas Propag.,vol. 48, no. 8, pp. 1272–1274, Aug. 2000.[6] P. V. Nikitin, K. V. S. Rao, S. F. Lam, V. Pillai, R. Martinez, and H.Heinrich, “Power reflection coefficient analysis for complex imped-ances in RFID tag design,” IEEE Trans. Microw. Theory Tech., vol.53, pp. 2721–2715, Sep. 2005.[7] D. S. Weile and E. Michielssen, “Genetic algorithm optimization ap-plied to electromagnetics: A review,” IEEE Trans. Antennas Propag.,vol. 45, no. 3, pp. 343–353, Mar. 1997.[8] G. Marrocco, “Gain-optimized self-resonant meander line antennas forRFID applications,” IEEE Antennas Wireless Propag. Lett., vol. 2, pp.302–305, 2003.[9] “FEKO User’s Manual, Suite 5.1,” EM Software & Systems-S.A. (Pty)Ltd. [Online]. Available: http://feko.info, Stellenbosch, South Africa,Dec. 2005.

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Meandered-slot antennas for sensor-RFID tags

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Archivio della Ricerca - Università di Roma Tor vergata

ART