Shorted Microstrip Patch Antenna Using

Inductively Coupled Feed for UHF RFID Tag

Jeong-Seok Kim, Wonkyu Choi, Gil-Young Choi, Cheol-Sig Pyo, and Jong-Suk Chae

ABSTRACTA very small patch-type RFID tag antenna

(UHF band) using ceramic material mountable on
metallic surfaces is presented. The size of the proposed tag is
25 mm25 mm3 mm. The impedance of the antenna can be
easily matched to the tag chip impedance by adjusting the size
of the shorting plate of the patch and the size of the feeding
loop. The measured maximum reading distance of the tag at
910 MHz was 5 m when it was mounted on a 400 mm
400 mm metallic surface. The proposed design is verified by
simulation and measurements which show good agreement.
KeywordsRFID, microstrip patch antenna, metal tag,
ceramic, shorting plate, inductively coupled feed.

I. Introduction
Radio frequency identification (RFID) has been widely used
in supply chain, service industries, distribution logistics, and
manufacturing companies to identify goods. In some RFID
applications such as metallic components, label tags generally
cannot operate on the surface of conducting materials because
of the degradation of tag antennas. Proper antenna design for
RFID tag applications is becoming essential for the
maximization of RFID system performance. Recently, there
have been many studies on RFID tag antennas in the UHF
band, especially at 900 MHz. In many applications, RFID tags
need to be very small and mountable on metallic surfaces. To
meet this application requirement, the planar inverted-F
antenna which can be used on metallic surface has been
Manuscript received Mar. 5, 2008; revised May 26, 2008; accepted June 24, 2008.
This work was supported by the IT R&D program of MKE/IITA[2008-S-023-1,
Development of Next Generation RFID Technology for Item Level Applications], Rep. of
Korea.
Jeong-Seok Kim (phone: + 82 42 860 1365, email: jskim0113@etri.re.kr), Wonkyu Choi
(email: wkchoi@etri.re.kr), Gil-Young Choi (email: kychoi@etri.re.kr), Cheol-Sig Pyo (email:
cspyo@etri.re.kr), and Jong-Suk Chae (email: jschae@etri.re.kr) are with the IT Convergence
Technology Research Laboratory, ETRI, Daejeon, Rep. of Korea.

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Jeong-Seok Kim et al.

proposed as a tag antenna [1], [2]. To reduce the size of the

patch antenna, a microstrip antenna with two-shorted-patches
and a feed loop has been proposed [3]. To expand the
bandwidth of metal tag antennas, the use of orthogonal
proximity-coupled patch antennas in RFID tags has been
studied [4]. Also, a tag antenna must be directly matched to the
tag chip, which in most cases has a complex impedance rather
than 50 . A impedance matching technique using inductive
coupling has been studied in relation to RFID tag antennas [5].
In this paper, we present a very small tag antenna which is
mountable on metallic surfaces and suitable for a UHF-band
RFID tag. It is a patch antenna with a shorting plate and uses a
ceramic material and an inductively coupled feed. It can be
placed on conducting materials and measures 25 mm25 mm
3 mm. It can be used in applications which require small size.

II. Tag Antenna Design

The structure of the proposed antenna is presented in Fig. 1.
The proposed tag consists of a tag chip, an inductively-coupled
feed line, a radiating patch with shorting plates, the substrate
filled with ceramic material (r = 48), and the ground plate. The
radiating patch is a metal plate with horizontal and vertical slits
to adjust the radiation frequency. The vertical slit has a length
of Lf. The tag chip is electrically connected to the feed line,
which is located in the same plane as the radiating patch. The
horizontal length of the feed line is Li, and the width of the
shorting plate is Lr. The proposed antenna is designed for a tag
chip (commercial RFID tag chip: Alien Higgs) with an
impedance of Zc=(12-j140) at a resonant frequency of
910 MHz. The conjugate match between the impedance of the
proposed antenna and that of the tag chip is achieved by
adjusting the width of the shorting plate and the feed line length.

ETRI Journal, Volume 30, Number 4, August 2008

unit : mm

25

+j

+j2
Lf

Li=14.2 mm(xr=113 at 915 MHz)

Li=15.8 mm(xr=148 at 915 MHz)
Li=16.6 mm(xr=220 at 915 MHz)
Measurement
ZC*

+j4

25
1

860 MHz
960 MHz

Li

1
3
X

+j
1

Shorting plate

2
4
(a) Variation in Li

Microchip
Radiating plate

+j2

Shorting plate

Lr=0.2 mm
Lr=1.0 mm

Z
Ceramic
substrate
X

r=48

ZC*

Lr

Lr

916 MHz
902 MHz

Ground plate
(a) Structure of the antenna

Lr=1.5 mm
Measurement

+j4

897 MHz
860 MHz
960 MHz
1
+j

(b) Variation in Lr

+j2
843 MHz
877 MHz

+j4

(b) Photograph of the implemented tag

916 MHz

Fig. 1. Structure of the proposed antenna.

The operating frequency is adjusted by varying the slit length

of the radiating patch, while the impedance of the antenna is
almost unaffected.

III. Simulation and Measurement

A prototype antenna was designed and implemented for a
tag chip with a complex conjugate impedance of
Zc*=(12+j140) . The operating frequency is 910 MHz.
Figure 2 shows the simulated and measured data for the
input impedance of the antenna when it is attached to an
infinite metallic surface. The simulation was performed using
CST Microwave Studio. The locus of the input impedance of
the antenna has an shaped feature in the Smith chart. This is
because the reactance component of the coupled impedance of
the radiating patch and the self-reactance of the feed line cancel
each other around the resonance frequency of the radiating

ETRI Journal, Volume 30, Number 4, August 2008

ZC*

Lf=6 mm
Lf=7 mm
Lf=8 mm
Lf=9 mm
Measurement

935 MHz

860 MHz
960 MHz
1

2
4
(c) Variation in Lf

Fig. 2. Input impedance characteristics.

patch in a manner similar to that of the inductively coupled

feed [5]. The reactive component of the input impedance
depends on the self-reactance of the feed line Xf, and we can
easily match it to the conjugate impedance of the tag chip by
varying the length of the feed line Li. The resistive component
of the input impedance has simple dependence on the width of
the shorting plate Lr. The reactive and resistive components of
the input impedance characteristics of the antenna for various
values of Li and Lr are shown in Figs. 2(a) and (b). As the value
of Li increases, the reactance of the input impedance increases.
As the value of Lr increases, the resistance of the input

Jeong-Seok Kim et al.

601

maximum reading distance was 5 m when a 400 mm400 mm

metallic surface was used, while the operating frequency of the
reader hopped to the frequency band in the range from
908.5 MHz to 914 MHz. The simulated results were in good
agreement with the measured results. The maximum reading
distance rapidly decreased as the size of metallic surface was
reduced to smaller than 200 mm200 mm.

Return loss (dB)

-5

3 dB bandwidth = 21 MHz

-10
-15
-20

IV. Conclusion
-25

Simulation
Measurement

-30
900

905

910
915
920
Frequency (MHz)

925

930

Fig. 3. Calculated and measured return loss of the antenna.

0
-10

30

(dB)

-10
-20

xz-plane co-pol
xz-plane cross-pol
yz-plane co-pol
yz-plane cross-pol

60

300

References

-30
-40
-50

90

270

Fig. 4. Simulated radiation pattern.

impedance decreases, and the resonant frequency does not

change. Figure 2(c) shows that the vertical length of the slit Lf
in the radiating patch has the effect of moving the -shaped
locus with variation of the resonant frequency. Figure 2 shows
that the measured impedance locus agrees well with the
simulated result. The measurement was carried out with the tag
placed at the center of a 400 mm400 mm metallic surface.
Figure 3 shows the return loss of the proposed antenna with the
simulation and measurement results with respect to Zc*. The
bandwidth at 3 dB return loss is 21 MHz, which covers the
bandwidth of the Korean RFID frequency band (908.5 MHz to
914 MHz).
Figure 4 shows the simulated radiation pattern of the
proposed antenna mounted on a metal surface at 910 MHz.
The radiation pattern in the yz plane and xz plane has fairly
good omni-directional performance. The simulated directivity
of the proposed antenna with an infinite metallic surface is
about 5.21 dBi, and the radiation efficiency is about 35%.
These characteristics can be verified by measuring the reading
distance of the proposed tag.
We measured the maximum reading distances of the tag on a
metallic surface using the commercial RFID reader made by
Alien Technologies (ALR-9800) in an anechoic chamber. The

602

Jeong-Seok Kim et al.

A design for very small ceramic tag antenna (25 mm

25 mm3 mm) for the UHF band mountable on metallic
surfaces was implemented. The antenna can be directly
matched to the arbitrary complex impedance of a tag chip. We
verified that the proposed tag achieves good performance by
measuring the bandwidth of 21 MHz at 3 dB return loss and a
maximum reading distance of 5 m on metallic surfaces. The
proposed tag may be used with conducting plates such as
automobile components if necessary.

[1] M. Hirvonen et al., Planar Inverted-F Antenna for Radio

Frequency Identification, Electron. Lett., vol. 40, no. 14, July
2004, pp. 848-850.
[2] W. Choi et al., An RFID Tag Using a Planar Inverted-F Antenna
Capable of Being Stuck to Metallic Objects, ETRI Journal, vol.
28, no. 2, Apr. 2006, pp. 216-218.
[3] H. Son and G.. Choi, Orthogonally Proximity-Coupled Patch
Antenna for a Passive RFID Tag on Metallic Surfaces,
Microwave and Optical Technology Letters, vol. 49, no. 3, Mar.
2007, pp. 715-717.
[4] B. Yu et al., RFID Tag Antenna Using Two-Shorted Microstrip
Patches Mountable on Metallic Objects, Microwave and Optical
Technology Letter, vol. 49, no. 2, Feb. 2007, pp. 414-416.
[5] H. Son and C. Pyo, Design of RFID Tag Antennas Using an
Inductively Coupled Feed, Electron. Lett., vol. 41, Sept. 2005, pp.
994-996.
[6] H. Son, G Choi, and C. Pyo, Open-Ended Two-Strip MeanderLine Antenna for RFID Tags, ETRI Journal, vol. 28, no. 3, June
2006, pp. 383-385.

ETRI Journal, Volume 30, Number 4, August 2008