WILEY ANTENNAS FOR PORTABLE DEVICES phần 5 ppsx

104 RFID Tag Antennas Transmission coefficient, τ(dB)

–30

–25

–20

–15

–10

–5

0

Free space

d = 1 mm

d = 5 mm

d = 10 mm

d = 15 mm

Frequency, MHz

800 850 900 950 1000

Antenna

Metal plate

d

Figure 3.32 Transmission coefficient of the antenna as a function of distance from metal plate

(calculated by IE3D).

Table 3.6 Effect of metal on the tag at 915 MHz.

Directivity (dBi) Radiation

efficiency

(%)

Gain

(dBi)

Input

impedance

(

)

Transmission

coefficient

(dB)

Reading*

range (m)

Rpower−link

Free space 181 8184 094 33.65 + j427.00 −132 500

d = 1 mm 779 067 −1397 3.20 + j336.00 −1717 015

d = 5 mm 808 647 −380 3.45 + j372.50 −1227 082

d = 10 mm 811 1748 −053 4.49 + j404.30 −440 296

d = 15 mm 810 3001 288 7.08 + j423.80 −086 660

a The system parameters were described in Section 3.3.1.6.

the reading distance of the tag may be enhanced because the metal object functions as a

reflector.

3.4.2.2 Effects of Water on Tag Antenna

Figures 3.33–3.36 show the characteristics of an RFID tag antenna which is placed close

to a water cuboid. As in the case of the metal plate, the antenna used is a folded dipole

antenna and is positioned parallel to and above a water cuboid measuring 250 mm × 80 mm

× 80 mm; r = 77.3 and tan = 0.048. The directivity, radiation efficiency, gain, and input

impedance are investigated for the different distances away from the water cuboid. When the

antenna is placed close to water (d = 1 mm), the directivity of the antenna increases while

the radiation efficiency decreases significantly, which results in a reduction in the antenna

3.4 Effect of Environment on RFID Tag Antennas 105 Gain, dBi

–40

–30

–20

–10

0

10

Free space

d = 1 mm

d = 5 mm

d = 10 mm

d = 20 mm

Frequency, MHz

800 850 900 950 1000

Antenna

d

water

Figure 3.33 Gain of the tag antenna as a function of distance from water (calculated by IE3D).

R, ohms

0

200

400

600

800

1000

1200

1400

Free space

d = 1 mm

d = 5 mm

d = 10 mm

d = 20 mm

Frequency, MHz

800 850 900 950 1000

Antenna

d

water

Figure 3.34 Real part of the input impedance of the antenna as a function of distance from water

(calculated by IE3D).

X, ohms

200

400

600

800

1000

1200

Free space

d = 1 mm

d = 5 mm

d =10 mm

d = 20 mm

Frequency, MHz

800 850 900 950 1000

Antenna

water

d

Figure 3.35 Imaginary part of the input impedance of the antenna as a function of distance from

water (calculated by IE3D).

106 RFID Tag Antennas

Frequency, MHz

Transmission coefficient, τ(dB)

–25

–20

–15

–10

–5

0

Free space

d = 1 mm

d = 5 mm

d =10 mm

water d =20 mm

Antenna

d

800 850 900 950 1000

Figure 3.36 Transmission coefficient of the antenna as a function of distance from water (calculated

by IE3D).

Table 3.7 Effect of water on the tag at 915 MHz.

Free

Space

Directivity

(dBi)

Radiation

efficiency

(%)

Gain(dBi) Input impedance

(ohms)

Transmission

coefficient

(dB)

Reading*

distance(m)

(power-link)

Free Space 181 8184 094 3365 + j42700 −132 5.00

d = 1 mm 399 578 −839 18130+j77970 −1296 0.45

d = 5 mm 244 303 −1274 3256 + j44150 −190 0.97

d = 10 mm 264 1417 −585 1636 + j41410 −047 2.52

d = 20 mm 461 3028 −058 1213 + j41790 −014 4.81

a The system parameters were described in Section 3.3.2.7.

gain. In contrast with the metal plate, the water will always cause a reduction in the gain

regardless of the distance between the water and the antenna. As antenna is moved further

away, the antenna gain approaches the value obtained in free space. The input impedance

shows a smooth variation except when the antenna is very close to the water (d = 1 mm).

The effect of the water on the tag antenna and reading distance at 915 MHz are summarized

in Table 3.7. When the tag is very close to water, the reading distance drops significantly

to 0.45 m. As the tag is moved further away, the effect of the water is decreased and the

reading distance is enhanced.

3.4.3 Case Study

The results of measurements of the effect of various objects on a tag antenna are reported in

this section. The measurement set-up is shown in Figure 3.37. The effect of the objects on