WILEY ANTENNAS FOR PORTABLE DEVICES phần 9 potx

224 Antennas for Wearable Devices

Figure 6.25 Azimuth plane radiation pattern of sensor antenna when placed in free space and on the

body.

Sensor

88 cm

Control

Post Processing

Turntable

0 – 360°

Spectrum

Analyser

Rx Antenna

Figure 6.26 Measurement setup for sensor angular pattern performance using a patch antenna as the

receiving antenna.

Figure 6.28 shows the results obtained for measurement in both copolar and cross-polar

positions of the sensor antenna in free space and also when placed on the body with the

antenna parallel to the body. When the body shadows the communication link between Tx

and Rx at 180 the loss due to the shadowing is around 18–20 dB.

The angular patterns (Figure 6.28) present reasonable omnidirectional behaviour of the

sensor antenna with maximum variation of 8–10 dB for free space cases (off-body). Following

the set-up described above, path loss analysis of the radio channel between the Tx sensor

and a receiving antenna for cases where the sensor placed is in free space and on the body

in the anechoic chamber and in the indoor environment is performed. Figure 6.29 shows the

6.4 Case Study 225

Figure 6.27 Philips test module sensor placed on the body for radio channel characterization

measurement.

-30

-20

-10

0 dB

30

210

60

240

90

270

120

300

150

330

180 0

Tx Horizontal Free Space

Tx Vertical Free Space

Tx Onbody

-30

-20

-10

0 dB

30

210

60

240

90

270

120

300

150

330

180 0

Tx Horizontal Free Space

Tx Vertical Free Space

Tx Onbody

Figure 6.28 Received power pattern when Tx (sensor) is placed 88 cm from a receiving patch antenna

for horizontal and vertical sensor placements.

226 Antennas for Wearable Devices

-2 -1 0 1 2 3 4

55

60

65

70

y = 1.3*x + 59

OnBody-Standing

Fitted Line

OnBody-NLOS

OnBody-Sitting

OffBody-Hor

OffBody-Ver

) Bd( ssoL ht aP

10*log(d/d0)

55

y = 1.3*x + 59

OnBody-Standing

Fitted Line

OnBody-NLOS

OnBody-Sitting

OffBody-Hor

OffBody-Ver

Figure 6.29 Indoor measured path loss when sensor is placed off and on body with modelled path

loss using the least fit square technique.

path loss measured in the indoor environment. As predicted, the exponent is lower than that

of free space with a value of 1.3 when the sensor is placed on the body due to multipath

components from the different scatterers. For similar distances the loss is higher for non￾line-of-sight (NLOS) cases. The directivity of the antenna increases when it is placed on the

body, as discussed earlier, due to high losses at 2.4 GHz of the human tissue which leads to

greater received power for the same distances as applied in the standalone sensor case.

6.5 Summary

Wireless body area networks have been made possible by the emergence of small and

lightweight wireless systems such as Bluetooth™ enabled devices and PDAs. Antennas are

an essential part of any WBAN system and, due to varying requirements and constraints,

careful consideration of their design and deployment is needed.

This chapter introduced wireless body area networks and their progression from WLAN

and WPAN to satisfy the demand for more personal systems. The main requirements and

features of wearable antennas were presented with regard to design and implementation

issues. A review of the latest developments in body-worn antennas and devices provided a

clearer picture of the current state of the art and the potential areas for additional investigations

and applications. As an inseparable part of the whole communication system, specifically

in WBAN, the influence of different antenna parameters and types on the radio propagation

channel is of great significance, especially when designing antennas for wearable personal

technologies.