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.
IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 54, NO. 6, JUNE 2006
reader antennaand the tagantenna. Theabsolute value ? ofthe antenna
polarization ratio is related to the antenna axial ratio ? (measured in
are the complex polarization ratios of the
? ? ?????? ? ?
? ? ?
There are several special cases of interest for RFID applications. When
both antennas are linearly polarized and oriented with a misalignment
angle ?, the polarization efficiency is ? ? ?????. When the reader
antenna iscircularlypolarized andthetagantenna islinearlypolarized,
as correctly pointed out by Dr. Tikhov.
Dr. Tikhov also mentions that (1) is invalid at near-field distances.
We agree with that but would like to emphasize again that (1) gives
the maximum distance at which RFID reader can read the tag in free
space. In most UHF RFID systems, this distance extends well into the
far-field zone. This is reflected in the very form of (1) which uses Friis
free-space transmission formula valid only in the far-field region .
While some tag application scenarios involve near-field tag scanning,
closer to RFID reader antenna into the near field. Hence, the readrange
of most UHF RFID tags is determined by the tag performance in far
We would like to thank Dr. Tikhov for his valuable Comments 
which helped to clarify some aspects of our work  and make it more
useful for tag antenna designers and other RFID community.
 Y. Tikhov, “Comments on ‘Antenna design for UHF RFID tags: A re-
view and a practical application’,” IEEE Trans. Antennas Propag., vol.
54, p. 1906, Jun. 2006.
 K. V. S. Rao, P. V. Nikitin, and S. Lam, “Antenna design for UHF
RFID tags: A review and a practical application,” IEEE Trans. An-
tennas Propag., vol. 53, no. 12, pp. 3870–3876, Dec. 2005.
 P. Talaga, “The measurement of a large antenna using a spacecraft as a
receiver,” IEEE Trans. Antennas Propag., vol. 38, no. 6, pp. 883–888,
 IEEE Standard Test Procedures for Antennas, , Dec. 1979, IEEE Stan-
dard 149-1979, pp. 61–70.
Antennas Propag. Mag., vol. 35, no. 4, pp. 33–35, Aug. 1993.