Reading of UHF band passive RFID tags

using a reflector #Yuki Oowashi

1, Shigeki Takeda

1 Kenichi Kagoshima

1

1 Ibaraki University

4-12-1 Nakanarusawa, Hitachi, Ibaraki, 316-8511 Japan, Email:obote@mx.ibaraki.ac.jp

Abstract Due to the high cost of tags which are designed for metallic objects, they have not been

commonly used compared to the conventional ones. On the other hand, although reading distance

becomes short, a recognition technique using a dipole tag is very effective to reduce introduction

cost of RFID. In this paper, we have proposed the technique for improving the recognition

performance of UHF RFID which is used in close proximity of metallic objects. We have employed

a waveguide like square shaped reflector in order to be able to read tags attached any position

including the reverse side of a metallic objects which is assumed to be a cylinder. It is clarified that

optimum electromagnetic distribution is similar to TM01 mode of a coaxial waveguide and that the

validity of this property is evaluated by method of moment based electromagnetic simulation and

experiment.

Keywords : UHF RFID tag, Reflector, Reverse side

1. Introduction Radio frequency identification (RFID) is noncontact recognition technology to send and

receive information by wireless. Recently, the RFID technology becomes to be widely used.

However, RFID tags have weakness that their performance is easily affected by metallic objects.

We will present a performance of RFID tags which are used in the close proximity of a

metallic objects. Fig.1 shows the introduction example. In general, when the tag is got close to

metal, the recognizing performance is greatly deteriorated because the impedance of tag is changed.

Especially, receiving power of tags attached to reverse side of metallic object is lower than those

which are attached to front or lateral side. Thus, it is difficult to recognize tags.

In this paper, we have proposed a technique for improving the recognition performance of

UHF RFID which is used in close proximity of metallic objects. We have employed a waveguide

like square shaped reflector in order to be able to read tags attached to reverse side of metallic

objects and assumed a shape of a metallic object to be cylinder. It will be clarified that optimal

electromagnetic distribution is similar to TM01 mode of coaxial waveguide and validity of this

property is evaluated by method of moment based electromagnetic simulation and experiment.

Fig.1 Example of practical use.

2. Problem of recognizing tags attached to reverse side of metallic objects

at free space Reading characteristics of RFID tags which is attached to front, lateral and reverse side of a

metallic cylinder are evaluated in free space. Geometry of a experimental set up is shown in Fig.2.

Since radius of a metallic cylinder is 34.45 mm, distance between the reader and tag is started from

50 mm, and it is varied up to 200 mm with intervals of 50 mm in each three tags position of front,

lateral, and reverse side. Furthermore, measurement has been carried out for different z of 0 mm and

100 mm. z of 100 mm corresponds to the edge of the reader antenna. The reader is RF-ATLP002 by

Mitsubishi Electric Corporation [1]. Width, height and thickness are 200 mm, 200 mm and 25 mm,

respectively. Transmitting power is 30 dBm. A UHF band RFID tag is RF-TGP005-W (Fig.3) [2].

metallic object

direction of movement metallic plate

reader

UHF RFID tagbelt conveyer

waveguide like reflector

metallic object

direction of movement metallic plate

reader

UHF RFID tagbelt conveyer

waveguide like reflector

Experimental results are shown in Fig.4. From Fig.4, the edge located tags have better

characteristic than the center located one in the front side in x=50 mm, because the distance of the

reader and the metallic cylinder is narrow. Moreover, electric field Z-component Ez is attenuated in

reverse side of metallic object from Fig.5. So, the tag in reverse side cannot be recognized.

dUHF RFID tag

metallic

cylinder

Z=0Z

x

reader

68.9

reverse side

lateral side

front side

dUHF RFID tag

metallic

cylinder

Z=0Z

x

reader

68.9

reverse side

lateral side

front side

Fig.2 Experimental environment in free space. Fig.3 UHF band RFID tag (RF-TGP005-W).

Fig.4 Recognition performance as a function of distance between the metallic cylinder and reader.

(a) Ez in x-y plane (z=0)

（i）x=50 [mm] （ii）x=100 [mm] （iii）x=150 [mm] （iv）x=200 [mm]

(b) Ez in x-z plane (y=0)

Fig.5 Electric field distribution (Ez component) in free space.

3. Reading technique of RFID tags attached to metallic objects 3.1. Method of moment analysis As shown in Fig.4 and Fig.5, it is very hard to recognize a tag in the reverse side. Therefore,

we propose a square shaped reflector to improve reading characteristics of tags attached to a reverse

side of a metallic cylinder. Fig.6 shows the analytical model. In this way, the technique for

improving the recognition rate of tag on reverse side was examined. The input impedance of the

patch antenna is 47.0-j8.02 ohm in free space at 953MHz and source impedance is set to be

complex conjugate of this input impedance. Transmitting power of the reader is set to be 30 dBm.

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7.3 mm

2.2 mm

Unit:mmx

zy

x

zy

（a）d=4 [mm] （b）d=6 [mm]

metallic

cylinder patch

antenna

Simulation results are obtained by EEM-MOM ver.1.5 [3] that is based on method of

moment.

3.2. Simulation results Electric field Ez that is parallel to a tag is shown in Fig.7. Fig.7 (a) and (b) show z

component of electric field in x-y plane (z=0) and x-z plane (y=0), respectively. The width of the

square shaped waveguide is varied from 340 mm to 400 mm every of 20 mm. From these results,

we can see uniform electric field distribution around the metallic cylinder is obtained at width of

360 mm. This kind of electric field distribution is desirable, because tags attached to front, lateral

and reverse sides must be recognised. This electric field distribution is similar to TM01 mode of a

coaxial waveguide [4]. On the other hand, complex electric field distribution appears for larger

width of the square shaped reflector than 360 mm. The complex electric field distributions results in

dead spots. The dead spot means the position where recognition rate is lower than 100 %. Therefore,

we can conclude that TM01 mode electric field distribution is optimum for RFID systems. In the

experiment presented in the next section, the size of copper reflector will be chosen to be 360×360

×400 mm.

patch antenna

metallic cylinder

a

a

400

square shaped

waveguide

68.9z

x

y

0

200

200

10

140

140

patch antenna

35

patch antenna

metallic cylinder

a

a

400

square shaped

waveguide

68.9z

x

y

0z

x

y

0

200

200

10

140

140

patch antenna

35

Fig.6 Analysis model.

(a) Electric field in x-y plane (z=0)

（i）a=340 [mm] （ii）a=360 [mm] （iii）a=380 [mm]

(b) Electric field x-z plane (y=0)

Fig.7 Electric field distribution (Ez component).

4. Experimental results 4.1 Experimental set up The width and the height of the reflector are 360 mm and 400mm, respectively. These are

determined based on the simulation in the preview section. Fig.8 shows the photos of the

experimental set up.

Unit:mm

metallic

cylinder

square shaped

waveguide patch

antenna

4.2 Experimental results Fig.9 shows experimental results in terms of recognition rate with or without the reflector

when distance of d is 4 mm and 5 mm. The recognition performance of the tag on lateral and

reverse side has been improved by the reflector. Especially, the recognition performance on the

reverse side is considerably improved.

When d is grater than or equal to 3 mm, recognition rate on reverse side at z of 0 becomes

to be 100%.

（a）Perspective view （b）Internal structure

Fig.8 An experimental environment.

Fig.9 Recognition performance with or without the reflector.

5. Conclusion We have proposed the recognition technique of UHF band RFID tags attached to a

cylindrical metallic object. In order to recognize tags attached to the reverse side of the metallic

object, we have introduced the square shaped reflector. The square shaped reflector and cylindrical

metallic object generate electric field distribution similar to a coaxial waveguide. It is clarified that

TM01 mode electromagnetic distribution is optimum to recognize tags attached on front, lateral and

reverse side of the cylindrical metallic objects.

Derivation of optimum size of a square shaped reflector for various kind of metallic objects

will be a further study.

References [1] http://www.mitsubishielectric.co.jp/device/rfid/products/index.html#02

[2] http://www.mitsubishielectric.co.jp/device/rfid/products/index.html#03

[3] http://www.imsab.co.jp/eem/

[4] N. Marcnvitz, “Waveguide Hand Book,” Mc Graw-Hill Co., Inc., pp.39-81, 1951.

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UHF RFID reader UHF RFID

tag

z

xy

z

xy

x

y

zx

y

z

（a）d=4 mm (b) d=5 mm