Hơn nữa, bộ tản nhiệt có thể được xẻ rãnh cho phù hợp với trở kháng tốt và giảm kích thước, như thể hiện trong hình 7,22 (c) [45]. Phù hợp với trở kháng cũng có thể được tăng cường bằng cách notching tản nhiệt như thể hiện trong | 224 Antennas for Wearable Devices Figure Azimuth plane radiation pattern of sensor antenna when placed in free space and on the body. Turntable 0-360 Rx Antenna V 88 cm Figure Measurement setup for sensor angular pattern performance using a patch antenna as the receiving antenna. Figure 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 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 shows the Case Study 225 Figure Philips test module sensor placed on the body for radio channel characterization measurement. 90 0 dB Tx Horizontal Free Space 270 Tx Vertical Free Space Tx Onbody Figure 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 Figure 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 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 GHz of the
