3870IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 53, NO. 12, DECEMBER 2005
Antenna Design for UHF RFID Tags:
A Review and a Practical Application
K. V. Seshagiri Rao, Senior Member, IEEE, Pavel V. Nikitin, Member, IEEE, and Sander F. Lam
Abstract—In this paper, an overview of antenna design for pas-
sive radio frequency identification (RFID) tags is presented. We
discuss various requirements of such designs, outline a generic de-
sign process including range measurement techniques and concen-
trate on one practical application: RFID tag for box tracking in
warehouses. A loaded meander antenna design for this application
is described and its various practical aspects such as sensitivity to
fabrication process and box content are analyzed. Modeling and
simulation results are also presented which are in good agreement
with measurement data.
Index Terms—Antennas, passive modulated backscatter, radio
frequency identification (RFID), tags, transponders.
matic identification of objects. Although the first paper on mod-
ulated backscatter (basic principle of passive RFID) was pub-
lished in 1948  it took considerable amount of time before
the technology advanced to current level . Now RFID finds
tion, asset identification, retail item management, access con-
trol, animal tracking, and vehicle security . Several standards
of RFID systems are currently in use (ISO, Class 0, Class 1, and
Globally, each country has its own frequency allocation for
RFID. For example, RFID UHF bands are: 866–869 MHz
in Europe, 902–928 MHz in North and South America, and
950–956 MHz in Japan and some Asian countries. A typical
passive RFID transponder often called “tag” consists of an an-
tenna and an application specific integrated circuit (ASIC) chip.
RFID tags can be active (with batteries) or passive(batteryless).
A passive back-scattered RFID system operates in the fol-
antenna. The RF voltagedevelopedon antenna terminals during
unmodulated period is converted to dc. This voltage powers up
the chip, which sends back the information by varying its front
gles between two different states, between conjugate match and
some other impedance, effectively modulating the back-scat-
tered signal. Fig. 1 illustrates a passive RFID system operation.
ADIO FREQUENCY identification (RFID) is a rapidly
developing technology which uses RF signals for auto-
Manuscript received December 18, 2004; revised June 8, 2005.
The authors are with the RFID Intellitag Engineering Department, Intermec
Technologies Corporation, Everett, WA 98203 USA (e-mail: kvs.rao@in-
termec.com; email@example.com; firstname.lastname@example.org).
Digital Object Identifier 10.1109/TAP.2005.859919
Proper impedance match between the antenna and the chip
is of paramount importance in RFID. Since new IC design and
manufacturing is a big and costly venture, RFID tag antennas
an external matching network with lumped elements is usually
prohibitive in RFID tags due to cost and fabrication issues. To
overcome this situation, antenna can be directly matched to the
and the input power applied to the chip.
Several papers have been published on RFID antennas for
both passive and active tags, including covered slot antenna
design , circular patch antenna analysis , meander antenna
optimization , planar inverted F-antenna , folded dipole
antenna , etc. However, very few papers  provided an
overview of criteria for RFID tag antenna design and an anal-
ysis of practical application aspects. At the same time, there
exist many papers on practical analysis and design of particular
classes of antennas used for other applications –.
In the current article, we attempted to fill the existing gap.
We reviewed design requirements for passive UHF RFID tag
UHF tag design for a RFID tag placed on a cardboard box that
is being tracked in standard supply chain. The design is a versa-
with various content like dry goods or plastics. This example is
supplemented with modeling and simulation results which are
in close agreement with measured data.
II. ANTENNA DESIGN
A. Performance Criteria
The most important tag performance characteristic is read
range—the maximum distance at which RFID reader can de-
tect the backscattered signal from the tag. Because reader sensi-
tivity is typically high in comparison with tag, the read range is
defined by the tag response threshold. Read range is also sensi-
tive to the tag orientation, the material the tag is placed on, and
to the propagation environment.
The read range
can be calculated using Friis free-space for-
gain of the receiving tag antenna,
is the wavelength,
is the gain of the transmitting antenna,
is the power transmitted by the
is the minimum threshold
0018-926X/$20.00 © 2005 IEEE
RAO et al.: ANTENNA DESIGN FOR UHF RFID TAGS: A REVIEW3871
Fig. 1.RFID system operation. The backscattered signal is modulated by changes in chip impedance ? .
Fig. 2. Antenna impedance, chip impedance, and range as functions of frequency for a typical RFID tag.
power necessary to provide enough power to the RFID tag chip,
is the power transmission coefficient given by
is antenna impedance.
Qualitative behavior of antenna impedance, chip impedance,
and read range as functions of frequency for a typical RFID tag
is illustrated in Fig. 2. The frequency of the peak range is re-
ferred as the tag resonance. The tag range bandwidth can be
defined as the frequency band in which the tag offers an accept-
able minimum read range over that band. From (1) one can see
that read range is determined by the product
(transmitter EIRP), tag antenna gain
ficient . Typically
is dominant in frequency dependence and
quency of the best impedance match between chip and antenna.
This frequency is different from the resonant frequency of an-
tenna loaded with 50 Ohm and the antenna self-resonance.
The range in (1) can be normalized with a factor
4 . This factor is the range of the
tag with 0 dBi antenna perfectly matched
chip impedance at a fixed frequency. Contours of constant
range from (1) can be plotted on gain-transmission coefficient
plane as shown in Fig. 3 where they are labeled with their
values normalized to
. The chart in Fig. 3 can be used as a
common reference frame to present the performance of any
RFID tag antenna similar to impedance presentation of a circuit
is chip impedance and
of the reader
, and transmission coef-
range in gain-transmission coefficient plane where the range multiplier is
? ? ???4?? ? ? ??
. Peak free-space performance of RFID tag in
example given in this article is also shown.
Tag antenna performance chart: contours of constant normalized
on a Smith chart. The same range can correspond to several
gain-transmission coefficient combinations.
The RFID tag antenna design process involves inevitable
tradeoffs between antenna gain, impedance, and bandwidth.
The performance chart in Fig. 3 helps the designer to estimate
the range tradeoff between the impedance matching and the
gain. The normalization factor for this performance chart can
be easily calculated for any case of EIRP and threshold power
of the chip for a given frequency.
3872 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 53, NO. 12, DECEMBER 2005
B. Design Requirements
Several general RFID tag design requirements whose rela-
tive importance depends on tag application are discussed in the
following paragraph. These requirements largely determine the
criteria for selecting an RFID tag antenna.
Frequency band. Desired frequency band of operation
depends on the regulations of the country where tag will
Size and form. Tag form and size must be such that
it can be embedded or attached to the required objects
(cardboard boxes, airline baggage strips, identification
cards, etc.) or fit inside a printed label.
Read range. Minimum required read range is usually
specified for different:
EIRP. EIRP is determined by local country regu-
Objects. Tag performance changes when it is
placed on different objects (e.g. cardboard boxes
with various content), or when other objects are
present in the vicinity of the tagged object. Tag an-
tenna can be designed or tuned for optimum perfor-
mance on a particular object or designed to be less
sensitive to the content on which the tag is placed.
Orientation. Read range depends on antenna ori-
entation. Some applications require a tag to have a
specific directivity pattern such as omnidirectional
or hemispherical coverage.
Applications with mobility. RFID tag may be used in
situations where tagged objects like pallets or boxes
travel on a conveyor belt at speeds up to either 600
ft/min or 10 mph. The Doppler shift in this case is less
than 30 Hz at 915 MHz and does not affect RFID oper-
ation. However, the tag spends less time in the read field
of RFID reader, demanding high read rate capability. In
such cases, RFID system must be carefully planned to
ensure reliable tag identification.
Cost. RFID tag must be a low-cost device. This im-
poses restrictions both on antenna structure and on the
choice of materials for its construction including the
ASIC used. Typical conductors used in tags are copper,
aluminum, and silver ink. The dielectrics include a flex-
ible polyester and rigid PCB substrates like FR4.
Reliability. RFID tag must be a reliable device that can
sustain variations due to temperature, humidity, stress,
and survive such processes as label insertion, printing
C. Design Process
RFID tag antenna performance strongly depends on the fre-
quency-dependent complex impedance presented by the chip.
Tag read range must be closely monitored in the design process
in order to satisfy design requirements. Since antenna size
and frequency of operation impose limitations on maximum
attainable gain and bandwidth ,  compromises have to
be made to obtain optimum tag performance to satisfy design
requirements. Often a tunable antenna design is preferable to
provide tolerance for tag fabrication variations and for opti-
mizing antenna performance on different materials in different
Fig. 4.RFID tag antenna design process.
RFID tag antenna design process is illustrated on a flow chart
shown in Fig. 4. Once the RFID application is selected, system
requirements can be translated into tag requirements. These re-
quirements determinethe materials for tag antenna construction
and ASIC packaging. The impedance of the selected ASIC in a
chosen RF package (like flip-chip, etc.) to which antenna will
be matched can be measured with a network analyzer.
Antenna parametric study and optimization is performed
until design requirements are met in simulation. Like most
antennas, RFID tag antennas tend to be too complicated for
analytical solution as they can be used in complex environment.
Tag antennas are usually analyzed with electromagnetic mod-
eling and simulation tools, typically with method of moments
(MoM) for planar designs (e.g. thin flexible tags) and with
finite-element method (FEM) or finite-difference time-domain
method (FDTD) for more complicated three-dimensional de-
signs (e.g. thick metal mounted tags). Fast EM analysis tools
are crucial for efficient tag design. In a typical design process,
modeling and simulation tools can be benchmarked against
measurements. Read range calculation can be implemented di-
rectly in EM software. Tag antenna is first modeled, simulated,
and optimized on a computer by monitoring the tag range,
antenna gain, and impedance which give to a designer a good
understanding of the antenna behavior.
In the last step of the design process, prototypes are built and
D. Range Measurement
Accurate tag range measurement can be conducted in a con-
trolled environment, such as anechoic chamber or transverse
fixed and transmitter output power can be varied by controlled
RAO et al.: ANTENNA DESIGN FOR UHF RFID TAGS: A REVIEW 3873
Fig. 5.RFID tag range measurement using anechoic chamber.
Fig. 6. RFID tag range measurement using TEM cell.
attenuation. This allows one to carry accurate tag range char-
acterization and avoid using large and prohibitively expensive
chambers or cells. Compact TEM cell is a convenient tool for
to measure tag performance on various objects.
the reader antenna as illustrated in Fig. 5.
At each frequency, the minimum power
communicate with the tag is recorded. Since the loss
the connecting cable, the gain
and the distance
to the tag are known, the tag range for any
transmitter EIRP of interest can be determined from (1) as
of the transmitting antenna
The general guidelines for selecting the tag position in ane-
choic chamber are the following: i) the distance must be such
that the tag will respond and will be in far field and ii) the tag
must be placed in a quiet zone of the anechoic chamber where
multipath is minimal.
In TEM cell, the tag is placed inside, and the cell is con-
nected to RFID reader with variable output power as illustrated
in Fig. 6.
At each frequency, the minimum power
range can be calculated from known expressions for the field
inside TEM cell  as
at the input of
and being tracked.
RFID conveyor belt application. Boxes are equipped with smart labels
RFID tag is placed and
In our measurements, we used an anechoic chamber with 3
ft (0.91 m) distance to the tag and 6.2 dBi linearly polarized
reader antenna. The TEM cell had parameters
50 Ohm. Cable loss was
is the height of the TEM cell at the cross-section where
is TEM cell input impedance.
220 mm and
0.5 dB in both cases.
III. RFID EXAMPLE
Consider a specific application: a smart label for cardboard
with a bar code printed on it and an RFID tag embedded under-
neath can be placed on cardboard boxes with various contents.
Boxes may travel on a conveyor belt as shown in Fig. 7 or may
be movedthrough largeportals byforkliftsequipped with RFID
readers. Requirements to the tag design are the following:
• Tag should be easily tunable to any frequency in
860–960 MHz range for various contents in the final
assembly when the tag is inserted into a label and
placed on a cardboard box.
• Tag should have at least 2.5 m range with 4 W EIRP in
915 MHz band (902–928 MHz frequencies, US) and
at least 2.2 m range with 2 W ERP (3.3 W EIRP) in
868 MHz band (866–869 MHz frequencies, Europe)
to meet worst case application requirements.
• Tag should fit into a standard 6
desirable to have as small footprint as possible to fit
• It is preferred to have omni-like read range perfor-
mance for a tag.
4 inch label. It is
B. Antenna Design
The process of antenna design is briefly described below.
Materials. Because of cost and application considera-
tions, it was decided to use for the antenna a thin flex-
iblepolyester substrate witha dielectricpermittivity of
trace was made of copper and had a thickness
mil (0.018 mm).
3874 IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 53, NO. 12, DECEMBER 2005
PARAMETERS OF THE LOADED MEANDER TAG ANTENNA (mm)
Fig. 8. RFID chip affixed to antenna terminals using flip-chip packaging.
Fig. 9. Geometry of the loaded meander tag antenna.
EPC 1.19 G2 RFID ASIC chip  mounted on an-
ular low-cost technique.
sured in the frequency domain with the network ana-
RFID reader that the minimum power needed by the
chip to turn on is
10 dBm, which agreed with manu-
Antenna type. The antenna geometry is shown
in Fig. 9. Because of the size and tunability require-
Meandering allowed the antenna to be compact and
to provide omnidirectional performance in the plane
perpendicular to the axis of the meander. To have a
better control over the antenna resistance, we added
one loading bar with the same width as the meander
trace. To provide a better match for the chip capacitive
impedance, one meandered section was further me-
andered to obtain additional inductance. This antenna
can be easily tuned by trimming. Lengths of meander
trace and loading bar can be varied to obtain optimum
reactance and resistance matching. The trimming is
realized by punching holes through the antenna trace
at defined locations. Such tunable design is desirable
where it is often needed to provide a solution for a
particular application with a minimum lead time.
Parametric study and optimization. The loaded me-
ander antenna has several key parameters: loading bar
, distance , spacing , meander step width ,
and meander step height . Maximum allowed antenna
is dictated by the size requirements. The pa-
of meander trace length trimming ??. Tag resonance is determined by the best
match with the chip impedance (? , ? ).
Impedance of the loaded meander tag antenna (? , ? ) as a function
rameters mentioned above influence antenna gain and
using Ansoft Designer. Tag range was computed from
(1) using simulated values for gain
and measured values for chip impedance
cessing capabilities were not sufficient to create an ad-
equate cost function for our design requirements. The
optimal realizable combinations of tag antenna param-
Antenna reactance and resistance can be controlled
by trimming as it is illustrated in Fig. 10. For example,
trimming the meander trace by
the resonant frequency up by 20 MHz. The gain is
not significantly affected by trimming as it is shown
in Fig. 11. The performance of the untrimmed tag is
also shown on the chart in Fig. 3.
Prototyping and validation. Several tag prototypes
similar to the one shown in Fig. 12 were fabricated and
tested in both anechoic chamber and TEM cell range
Fig. 13 presents a comparison between theoretical
and experimentally measured read range for one of the
tags tuned to 915 MHz band in free space. It can be
seenthattheoreticalcurveand experimentaldataare in
close agreement. Differences between TEM cell data
and anechoic chamber data are due partially to imper-
fections of anechoic chamber and partially to electric
field nonuniformities inside TEM cell.