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Design of Antennas for UHF RFID Tags
Proceedings of the IEEE (Volume:100 , Issue: 7 ) RFID Virtual Journal, IEEE2012.07
By Etienne Perret, Member IEEE, Smail Tedjini, Senior Member
IEEE, and Raji Sasidharan Nair, Student Member IEEE
Presenter: Yen-Po Chen
Advisor: Dr. Pei-Jarn Chen
Date: 2014.3.26
Outline
Introduction
Purpose
Material and Methods
Preliminary Results
Conclusions
References
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Introduction
Radio-frequency identification (RFID) technology is
one of the major methods for identification purposes.
 Its operating principle was very clearly explained by
Harry Stockman in an IRE publication of 1948.
One of the early application is the identify friend or
foe (IFF).
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RFID tags are more compact and more complex .
Based on an integrated circuit (IC, commonly known as the RFID
chip), which embeds in the same passive components such as analog,
radiofrequency (RF), and digital circuitry.
Today ISO standards are accepted worldwide and allow a certain
level of interoperability between equipment coming from different
sources.
A real high degree of interoperability is still missing in applications
particularly using ultrahigh-frequency (UHF) bands.
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Two main factors should be underlined
for tag designers
First, RF regulations are not the same worldwide.
Indeed, contrary to high-frequency (HF) RFID for which
there exists a single frequency band adopted worldwide,
there are at least three frequency bands for UHF RFID
depending on specific country regulations.
In addition to that, the emission power and the communication
channel parameters are not the same for different geographic
regions.
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Chip less RFID tags (also named RF barcodes) can be
considered as an emerging area of RFID technology for
ultralow-cost RFID applications.
The first realized chip less tags are based on surface
acoustic wave (SAW)substrate , many other low-cost
approaches are under development .
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The second factor is due to the characteristics of antenna in
general.
The role of the tag antenna and its impact on the global
performance of the tag is very critical.
So, for any antenna there are two operating regions: near-field
and far-field depending on the communication distance
between reader and tags.
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principle of operation of RFID
 The principle of operation of a passive RFID system is illustrated
 It usually contains two parts: RFID reader and an RFID tag.
 The tag consists of an antenna and an application specific in-tegrated circuit (ASIC)
chip.
 Both exhibit complex impedances Za and Zc, respectively.
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The tag can operate in the interrogation zone of the reader.
When it receives the modulated signal with sufficient power
from the reader , the tag switches the chip impedance between
two states and
.
 At each impedance state, the RFID tag presents a specific
complex radar cross section (RCS).
The tag generates the information back by varying its input
impedance and thus modulating the backscattered signal .
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chip impedance
 In practice, to maximize the tag read range, the antenna
impedance Za is matched to the chip impedance at the threshold
power Pth.
 When the distance becomes too short, the incident power on the
tag can be very important and can cause a significant variation in
the chip impedance, which in turn causes an impedance mismatch
and a dead zone.
 Hence, more emphasis should be given to the antenna design
process regarding the impedance mismatch.
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 While considering the frequency regulation, if we want to
make a tag that operates worldwide,it must be able to operate
globally in the band 860–960 MHz.
 The information provided by the chip manufacturer is often
incomplete.
is only available from an equivalent circuit
model, at the threshold power,Where
, for a
given capacitance C, and a series resistance Rc.
 The variation of the chip impedance with power and
frequency can drastically affect the performance of the tag.
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Purposes
 The sensitivity of the antenna is certainly one of the major
weaknesses of UHF tags.
 This feature can be used to improve the efficiency of the label
sensing apparatus.
 In fact it has proved in certain designs, RFID tags can obtain
good sensing capabilities.
 There are two main aspects will be studied: the stability and
sensing .
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Material and Methods
 UHF RFID passive tags is to match the impedance of the antenna
in order to transfer maximum power to the chip.
 An antenna optimization should be required in order to achieve
such an antenna design.
 In practice, we consider that the reader should always be able to
collect the information from the tag as long as the chip receives
sufficient power and then is activated.
 However, perfect matching cannot be achieved in practice.
 The best way is to minimize the power reflection coefficient or to
maximize the read range r.
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 In order to satisfy the design requirements.
 Hence, miniaturization techniques must be implemented.
 The most popular method of miniaturization is simply to fold
the arms of the dipole in order to get the desired template as
well as good EM features.
 Also since the material on which the tag will be attached is
unknown, traditional antenna design approaches cannot be
applied directly.
 One possible way is to test different antenna topologies until
we get the solution for the initial problem.
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Conventional Antenna Design

Conventional antenna design approach,three distinct steps
compensate
EM
coupling
optimization
• The first step is to design a loop around the IC to
compensate for its capacitive part.
• The second step involves adding metal strips such as dipoles
to the loop in order to achieve an EM coupling (inductive or
capacitive coupling).
• The final step involves tuning and optimization of the
antenna to satisfy specific design requirements.
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 The specific design of a working 2. 45 GHz of RFID dipole
antenna
 Half-wave dipole antenna model
 Antenna copper material (conductivity: 5.8e
7s/m,permeability: 1)
 Located in the center of the cube filled with air in the outer
surface of the cube is set radiation absorbing boundary.
 The input signal is fed into the center of the antenna, which is
the location of the RFID chip.
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 For 2. 45 GHz operating frequency of the half-wave length of about
61mm, the dipole antenna provided to the arm width w of 1 mm,
and an infinitely thin, the width of the antenna arms, and called the
actual length of a half-wave dipole antenna was 57mm.
 An soft HFSS tools on the platform, using the finite element
method to simulate the antenna input return loss
distribution
obtained radiation field E plane (ie the plane of maximum radiation
direction and the electric field vector is located).
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 Can be seen from the right in the axial direction of the antenna,
the antenna is almost no radiation if at this time in the direction
of the reader, the transponder will not make any response in order
to obtain a full range of the radiation antenna to overcome this
drawback, You can make the appropriate deformation of the
antenna,
 Such as the extension on the end of a vertical dipole antenna arm
in the direction indicated K / 4 as shown in this way to modify
the total length of the antenna (57. 0 mm + 2 @ 28. 5 mm), the
antenna arm width remains 1 mm. Antenna arms extend after K /
4, the antenna resonating at a wavelength, rather than the original
half-wavelength.
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 This allows the input impedance of the antenna is greatly
increased, the simulation results about 2 k Ω. Input return
loss
in FIG.
 Of E-plane radiation pattern (antenna plane), where the
solid lines show the simulation results, black dot is the
actual sample measurement data, both of which are more
consistent with the results illustrate that the design is correct.
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 In the design of the antenna was found to have a correlation
between the change in the antenna topology changes, the
geometrical dimensions and electromagnetic properties of
the antenna.
 In the antenna geometry, such as research arm length of the
radiating element parameters such as pitch, a good
understanding of the electrical characteristics of the
supplied antenna.
 The choice of the form of embodiment of the antenna is the
most important step, since the final performance is entirely
dependent on the initial geometry.
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Preliminary Results
 AUTOMATED ANTENNA DESIGN TOOL
 A fully automated shape generation (FASG) coupled with the
evolutionary programing technique based on the genetic algorithm
(GA) .
 This antenna design approach guarantees to obtain a large panel of
antenna topologies and generate original shapes.
 Due to FASG and contrary to the conventional antenna design, the
users do not have to suggest any initial antenna shape;
 Definition of [the complex impedance of the chip (Z0c )], the
minimum power activation , and concerning the tag environment.
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The principle is based on the ability of the method:
1) to generate the form of the antenna of the
simulation
2) to determine the performance corresponding to
the specifications
3) to select the best performing antennas.
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A. Fully Automated Shape Generation
The idea is to generate the most varied shapes of antennas
with a limited number of unknown parameters.
Thus, the presented antenna generation approach is based
on the class of folded wire structures.
This antenna generation approach allows obtaining
meanders (which intend to reduce the length of the
antenna) and offers the possibility of getting loops.
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 The chip location in the structure can move, and is thus
optimized. To generate antennas, we just have to define a
vector of parameters to describe the antenna shape:
/ N: the elementary wire number
/ Lw: the elementary wire width
/ Cp: the chip location
/
two parameters defining the position of
each wire to another
 To construct each wire, consider two points A and B
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B. FASG Coupled With GA
 The GA is the core of the automated design process.
 Evaluation of the antenna consists of two steps:
 1) EM characteristics of the antenna are obtained using An soft
Designer
 2) fitness function
 that takes into account both the initial specifications and EM
characteristics of the antenna returns a value that describes the
performance of the antenna.
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 Topology evolution, as a function of GA generation. Only one part
of the antennas is shown.
 For each generation,the shown antenna corresponds to the largest
fitness function.
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Conclusions
 The tag design should meet numerous constraints due to RFID
chip characteristics: low cost and small size requirements.
 In addition to that, there are specific standards and RF emission
regulations .
 The latter regards frequency allocation and emission level that
are different from region to region worldwide.
 So designed UHF RFID tag antenna. It is still a very
challenging problem
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REFERENCES
 [1] H. Stockman, BCommunication by means of reflected
power,[ Proc. IRE,vol. 36, no. 10, pp. 1196–1204, Oct. 1948.
 [2] K. Finkenzeller, RFID Handbook: Fundamentals and
Applications in Contactless Smart Cards, Radio Frequency
Identification and Near-Field Communication. New York:
Wiley, 2010.
 [3] Perret, E. and Tedjini, S. ; Nair, R.S. Design of Antennas
for UHF RFID Tags Proceedings of the IEEE (Volume:100 ,
Issue: 7 ) RFID Virtual Journal, IEEE
 [4] http://tcnwo.blogspot.tw/p/rfid-chips.html
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