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Antennas for RFID Tags
Introduction :
Radio-Frequency Identification (RFID) is a wireless data capturing technique from a tagged item.
RFID consideration had started during the World war II for tracking system. Since then it has
revolutionized in all aspects like antenna performance chip performance and applications. It is
used for Automatic detection and application such as product IDs. It has emerged as effective
replacement for barcode system. It has memory and it can be printed on products surfaces.
RFID turned out to be superior due to its read range as compared to barcode system which
works only in 'line-of-sight'. (Roy & Karmakar, 2010) With more number of bits, it has capability
of unique ID number for all tags. RFID tag antenna is important factor of RFID characteristics
and it has various designs depending on their application. Quality and effectiveness of RFID
system is generally categorized on the basis of size, reading range, bandwidth. But it is been
found out that RCS is better criteria to define the tag antenna performance. (Kang, Kim, Lee, &
Chung, 2012) In this report, various RFID tag antenna designs are discussed and their
advantages over other systems are considered.
RFID system :
Figure 1 shows typical RFID system. (Roy & Karmakar, 2010) An RFID system comprises an
interrogator (reader) and a tag or transponder. A middleware is a buffer stage that encodes the
data captured from the tag in meaningful identification codes. RFID tags or radio transponders
are high frequency electronic circuits that broadcast the position or attributes of items to which
they are attached. This allows these items to be remotely detected, identified, and tracked. The
basic RFID system consists of three components: (i) a small and mobile data carrying tag unit
that is attached to items of interest, as well as (ii) a reader (or transceiver) whose location is
generally fixed and which contains (iii) an attached directive antenna. (Roy & Karmakar, 2010)
Figure 1 RFID System
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In RFID systems, Modulated Signals are broadcast by the reader via its attached antenna. The
tag receives these signals and responds either by reading or writing the data or by replying with
another signal containing some data, such as an identity code or a measurement value. The tag
may also rebroadcast the original signal received from the reader, sometimes with a
predetermined time delay. In passive tags power for tag is also provided by signal from reader.
RFID system is divided into two layers: physical layer and IT layer. As can be seen in the figure 1,
the physical layer comprises tag, reader/interrogator, and interrogation zone (IZ). IT layer is
mainly made up for data interface between enterprise application and middleware of system.
Following are the detailed discussion (Roy & Karmakar, 2010) of each component of the
physical layer and IT layer of the RFID system.
Tag : As RFID is most of the time compared with barcode system, one can say, tags are similar
to the optical barcodes, which are attached to the product and which store the unique
identification of the product. Tags are also called transponders. The tags primarily consist of
two components: the antenna and the IC chip. In some cases, depending on the business
process involved, they have environmental sensors for measurement values such as
temperature, humidity, and so on. The tag antenna communicates with the reader/interrogator
by means of electromagnetic waves. Also in semi-active and passive tags, antennas scavenge
power from the interrogator to operate the on-board IC chip of the tag. The IC stores the
unique identification of the product in the form of some numbers. Also, depending on the
business process involved, they have provision for subsequent read and write of data and their
retrieval. If there is any environmental sensor included in the tag, they communicate directly
with the IC chip.
Reader/Interrogator : The reader of the RFID system is compared to the scanner used for
optical barcodes. They come in different forms such as handheld, mobile, or stationary. Readers
are made up of primarily two components: the antenna and the interrogator circuitry. The
antenna is used for communication with the tag using electromagnetic waves. For semi-active
and passive tags, the reader antenna is used to supply power to the tags for the operation of
their IC. The interrogator circuitry is a pathway or intermediary between the reader antenna
and the IT layer. Interrogator circuitry performs the task of sending data through the reader
antenna and also receiving data and then sending it to the back end for processing. Interrogator
circuitry also performs the task of coordination between different reader antennas for the
efficient and successful reading of tags.
Interrogation Zone (IZ) : The interrogation zone consists of the area in which the reader and tag
communicate with each other. It includes shake hand signals between the two. This interface
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includes power signal in case of semi active signals and clock signals two keep synchronization
in communication.
Middleware : This is the intermediate between the interrogator and the enterprise layer.
Middleware sends and collects data directly from the interrogator, performs a business-related
process regarding the data, stores the data, and, as per the requirement, sends data to theenterprise applications.
Enterprise Application. Data gathered from middleware are used in here, and relevant business
processes such as the creation of an invoice are carried out using those data in the required
formats.
Now RFID is application specific designs all the above mentioned components are selected
according to the requirements of the application. Criteria such as read range, electromagnetic
power involved, frequency, protocol, shape and size of the tags have to be carefully selected,
depending on the application. (Roy & Karmakar, 2010) Here in next section, we will focus on theRFID tag antennas.
Antennas for RFID tag :
Antennas are the spatial filters that couple guided electromagnetic energy to free space
electromagnetic energy (vice versa) to enable communication in an RFID system. In any RFID
installation, the readers and interrogators are one-time investment only, whereas the tags are
the consumables that are required in thousands, millions depending on the application
process. There is no perfect antenna for all applications. It is the application that defines the
antenna specifications. There is a high probability that many types of transponders will share
the same IC but will connect to different antenna types. (Ahmed M. A. Salama, 2010) Tags areplaced on the items to be monitored. Hence, for proper designing of any RFID tag antenna,
detailed knowledge of the tag and following components is essential.
Integrated Chip (IC) : This is a semiconductor circuitry normally designed by a chip
manufacturer by silicon . The IC is roughly divided into the following parts:
A part of the IC is dedicated for controlling power. This power may come from a battery(semi-passive/active) or radiated energy from the reader/interrogator (passive).
Modulation/demodulation of the signals, encoding/decoding of the digital bits, andimplementation of the communication protocol take place in the IC.
The memory is divided into blocks called banks, which may be read only or read writeenabled depending on the usage. The unique identification number, error checking
codes, public and private passwords, and so on, are stored in the IC memory.
Tag antenna and IC must have conjugate matched impedance for the maximum power transfer
between the two.
Antenna : This is the largest part of the tag which is directly connected to the IC.
Communication and also flow of power between the reader and the tag occur through the
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antenna. Different designs of antennas are available in the market. They are manufactured by
screen-printing, foil-stamping, or copper-etching methods; screen-printing is the fastest and
most common one. One important thing to be kept in mind is that the interconnection between
the antenna and the IC of the tag is the weakest link in any tag.
Operating frequency : All around the world RFID tags are operated at following frequencies.According to frequency selection of tag antenna is performed. Antenna coupling (Inductive or
Backscatter) depends on frequency range. (Dressen)
Low Frequency - 125 to 135 kHz High Frequency - 13.56 MHz Ultra High Frequency - 868 to 928 MHz Microwave Frequency - 2.45 GHz and 5.8 GHz
Dipole antennas, Inverted F antennas, Fractal antennas, Patch antennas are used as RFID tag
antenna. However, some properties of the tag antenna are being presented which must be
given special care : Be small enough to be attached to the required object Have Omni-directional or hemispherical coverage to ensure non-line-of-sight operation
of the tag
Must provide maximum possible signal to the tag IC. The far-field tag antenna isrequired to be conjugate matched to the microchip. For near-field tag antenna (coil),
proper inductance is required to configure a resonant circuit with chip capacitance at
the operating frequency.
Have a polarization such as to match the enquiry signal regardless of the physicalorientation of the tagged object
Be robust and very cheap
Work Satisfactorily in presence of metals (Ning & Qing, 2010)We will have a look at few of these designs.
1] Text shaped meander line antenna :
One of the most widely used tag antenna is meander line resonant antenna. These are
designed by properly shaping the conductor such that to obtain maximum utilization from wire
current while keeping smaller size. (Marrocco, Fonte, & Bardati, 2002)
In this antenna wire is continuouly folded intended to reduce the resonant length. Increasingthe total wire length in antenna of fixed axial length lowers its resonant frequency as shown in
figure 2. It can be seen that current on adjacent horizontal segments are opposite phase and
they cancel each other, therefore vertical segments are effectively the radiation contributors. It
is observed that central segment h00 has the maximum contribution in the radiation resistance
of this antenna. When wni = w00 and hni = h00 then configuration is called uniform meander line
antenna (U-MLA).
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Figure 2 Partial view of Meander line Antenna
As most of the tags are printable on surfaces, Using text as an antenna element in RFID tags can
be effective in commercial aspect such that brand names or manufacturer logos can be used to
form a radiating element for the tag. If the meander lines are used for labeling then depending
on the font used the structure of the text meander line antenna can be close to either uniform
of non-uniform geometry. Thus giving additional value to the transponder itself as a hi-tech
advertisement. Figure 3 shows antenna configurations where the letters 'Institute of
Electronics' is made up from meander wire antenna. These can be classified as nearly uniform
meander line structures where the lengths of the vertical and horizontal sections are nearly
constant throughout the structure. (Keskilammi & Kivikoski, 2004)
Figure 3 Meander Line Antenna used as Text
The degree of reduction in size as compared to half wave dipole is given by shortening ratio as :
g is the wavelength of the operational frequency on the substrate where the antenna is
manufactured and Lax is the axial length of the meander line dipole antenna. Notable point is
that gain of antenna is reduced when meander line antenna is used. It was also noted that
handwriting fonts are suitable for text meander line antennas, as the connection of letters can
be made without discontinues. If the height of the text used for meander line compared to the
length of the text or the text has only a couple of letters like RFID the shortening ratio could
be larger.
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2] Fractal antenna :
The fractal antennas are made up of fractal, self similar shapes which are obtained by infinite
number of iterations. The figure 4 shows iteration of fractal curve. Algorithms such as Iterated
function systems(IFS) can be used to perform iterations to get fractal curve.
Figure 4 Fractal curve iterations
Shape of the fractal antenna depends on number of iterations performed. Two fractal loop
antennas are shown in figure 5 which are obtained by 2nd iteration of 2 different curves Koch
fractal curve and the curve proposed by (Salama & Quboa)
Figure 5 Fractal loop antennas
A loop antenna responds mostly to the time varying magnetic flux density B of the incident EM
wave. The induced voltage can be increased by increasing the area (S) enclosed by the loop, and
thus increase the read range of the tag. By deforming the geometry close match with IC
impedance can be obtained.
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3] Bent coil-antenna for RFID Tag :
For near field cases, coil is used in inductively coupled tags this inductance of the coil then
decides the read range of the tag. But these antennas face problem that effective inductance
values get differed from deigned values after being bent to mount on the tag surface as shown
in figure 6 (Ohnimus, Ndip, Guttowski, & Reichl, 2008). The coil of the transponder is used not
only for the data transfer, but must also generate enough current from the oscillating magneticfield, generated by the reader, to supply the IC with energy.
Figure 6 Planer Antenna Coil and bent version
Read range of the tag antenna can be maximized by reducing the required interrogation field
strength. This can be achieved by increasing the coil area and using the resonance circuit
formed by coil and IC chip. To have optimum use of volume inside the volume on which
antenna coil is mounted, bent tag is designed which can be manufactured on flexible substrate.
Self Inductance of coil L is function of geometry of the coil. Following expression gives loose
approximation of the value
where CIC stands for capacitance of IC and C is parasitic capacitance.
When coil is bent around x-axis two effects take place. Firstly, self inductance L decreases and
minimum required interrogation field increases. Bent radius is important factor of this antenna-
coil as inductance is primarily dependent on bent radius . (Ohnimus, Ndip, Guttowski, & Reichl,
2008)
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4] Bowtie antenna :
Evolution of frequency hopping technology gave birth to consideration of wide impedance
bandwidth tag antennas. The planar bowtie antenna shown in figure7, which is derived from
double conical antenna, has the reputation of wide bandwidth and low profile. (Zhoul & Lai,
2006)
Figure 7 Bowtie Antenna
Empirical formula given below has pointed out that for big flare angle of bowtie, its arm length
determines the important parameters of low frequency of the antenna and the longer the arm
length, the better the covering range of the low frequency.
where
It shows impedance of the bowtie antenna is completely real and tag antennas are connected
to chips which has complex impedance due to negative reactance. This imaginary part is
introduced in the antenna impedance by introducing short circuit stub at the feeding point.
Using the stub is really helpful as it easy fabricate on PCB. Stub has reactance having opposite
sign of that of chip reactance. The best advantages of this match are that there may be no
lumped elements introduced around the periphery, it is very convenient for printed antenna,
and it increases the reliability of the antenna and efficiently reduces the cost of the tag.
Radiation efficiency of this antenna is about 65.8% and its pattern type is Omni-directional.
Another variation of bowtie antenna is fan bowtie antenna as shown in figure 8 which offers
similar advantages.
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Figure 8 Fan bowtie Antenna
5] DRA Tag Antenna :
Compact size, low metal loss high gain and read range can be achieved by DRA as a tag as
shown in figure 9. (Zauind-Deen, Malhat, & Awadalla, 2011) This is dual band antenna which
works efficiently on curved surface. High dielectric constant cylindrical ring is mounted on
substrate. Resonator is fed by proximity feeding microstrip monopole. This antenna is not
susceptible to metallic surface and curvature of surface on which it is mounted.
Figure 9 DRA used as tag antenna
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6] L shaped tag:
Another variation of application requirement is presented here. Sometimes placing the tag on
plane surface is difficult as space is limited tags must be placed on corner. figure 10 shows
novel L shaped antenna designed to be placed at the corners (Rawal & Karmakar, 2007). It is
designed for special application where tag is to be placed on bigger pallet but in smaller areaand in corner.
Figure 10 L shaped tag antenna for corner
It gives radiation in 2 planes and it is less directive having gain of around 4 to 6 dB. Simple to
implement and can be adjusted in small area. Inset fed microstrip line used for feeding.
Full Wave Analysis :
Figure 11 Design Model of L shaped tag antenna
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L shaped tag antenna discussed above is simulated with CST Microwave Studio, 2011. It uses
with Finite Integration Technique for full wave analysis. Antenna model is shown in figure 11.
This carton is made up of paper substrate having dielectric constant 2.5. Substrate used for
patch is of dielectric constant 2.45 having thickness of 0.787mm. It has radiated field mostly in
two planes where patch is pointing as shown in figure 12. Return loss is shown is satisfactoryand shown in figure 13. Return loss can be further improved as per requirement by adjusting
the inset feed depth.
Figure 12 E and H plane radiation pattern
Figure 13 Return loss in dB
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Discussion :
There are many other antennas such as few designed for mounting on clothing, antennas
having strong RCS, slot ring antenna for tag, printing friendly antenna made up conductive
antennas. (Jihui, Fuhai, & Bing, 2011) (Zhou, 2010) (XI, ZHU, & YE, 2011) It can be seen all of
them are designed specifically for particular application requirements. With new designs not sosusceptible to metallic surface of tag on which they are printed or attached, RFID system are
more reliable.
References :
Ahmed M. A. Salama. (2010). Antennas of RFID Tags. In C. Turcu, Radio Frequency Identification
Fundamentals and Applications Design Methods and Solutions.
Dressen, D. (n.d.). Consideration for RFID selection. Retrieved from www.atmel.com.
Jihui, G., Fuhai, L., & Bing, D. (2011). A Design of RFID Tag Antenna for Clothing. IEEE.
Kang, W., Kim, J., Lee, K., & Chung, Y. (2012). Analysis of the RFID antenna with the nonlinear
component. IEEE.
Keskilammi, M., & Kivikoski, M. (2004). Using Text as a Meander Line for RFID Transponder Antennas.
IEEE.
Marrocco, G., Fonte, A., & Bardati, F. (2002). Evolutionary Design of Miniaturized Meander-LineAntennas for RFID. IEEE.
Ning, C. Z., & Qing, X. (2010). Antennas for RFID Applications. IEEE.
Ohnimus, F., Ndip, I., Guttowski, S., & Reichl, H. (2008). Design and Analysis of a Bent Antenna-coil for a
HF RFID Transponder. EuMA.
Rawal, A., & Karmakar, N. C. (2007). A Novel L-Shaped RFID Tag Antenna. EuMA.
Roy, S. M., & Karmakar, N. (2010). Handbook of Smart Antennas for RFID Systems. John Wiley & Sons,
Inc.
Salama, A. M., & Quboa, K. M. (n.d.). A New Fractal Loop Antenna For Passive UHF RFID Tag
Applications.
XI, ZHU, & YE. (2011). Exploration of Printing-friendly RFID Antenna Designs on Paper Substrates. IEEE.
Zauind-Deen, S. H., Malhat, H. A., & Awadalla, K. H. (2011). Curved Dual- Band Dielectric Resonator Tag
Antenna for RFID Applications. NRSC.
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Zhou, Y. (2010). A novel slot antenna for UHF RFID tag. IET.
Zhoul, Y., & Lai, S. (2006). A Design of RFID Tag Antenna Based on Bowtie. ICWMMN.
Table of figures
Figure 1 RFID System ..................................................................................................................................................... 1
Figure 2 Partial view of Meander line Antenna ............................................................................................................. 5
Figure 3 Meander Line Antenna used as Text ................................................................................................................ 5
Figure 4 Fractal curve iterations .................................................................................................................................... 6
Figure 5 Fractal loop antennas ...................................................................................................................................... 6
Figure 6 Planer Antenna Coil and bent version .............................................................................................................. 7
Figure 7 Bowtie Antenna ............................................................................................................................................... 8
Figure 8 Fan bowtie Antenna ......................................................................................................................................... 9
Figure 9 DRA used as tag antenna ................................................................................................................................. 9
Figure 10 L shaped tag antenna for corner ................................................................................................................. 10
Figure 11 Design Model of L shaped tag antenna ....................................................................................................... 10
Figure 12 E and H plane radiation pattern .................................................................................................................. 11
Figure 13 Return loss in dB .......................................................................................................................................... 11