Wearable Antenna Measurements in Vicinity of Human Body

This paper presents measurements techniques of wearable antennas and RF medical systems in vicinity of human body. The antennas radiation characteristics on human body have been measured by using a phantom. The phantom electrical characteristics represent the human body electrical characteristics. The phantom has a cylindrical shape with a 40 cm diameter and a length of 1.5 m. The phantom electrical characteristics are similar to the human body electrical characteristics. The antenna under test was placed on the phantom during the measurements of the antennas radiation characteristics. The phantom was employed to compare the electrical performance of several new wearable antennas. The phantom was also employed to measure the electrical performance of several antenna belts in vicinity of human body. The results of antenna with thinner belt are better than the results of the same antenna array with thicker belt.


Introduction
Wearable antennas and wearable systems were presented in books and journals; see [1]- [15].However, several of the presented wearable antennas were not measured in the vicinity of human body.Moreover, details of measurements in the vicinity of human body are not presented in several of these publications.The antennas radiation characteristics on human body may be measured by using a phantom.The phantom electrical characteristics are similar to the human body electrical characteristics.This paper presents measurements techniques of antennas and RF medical systems in vicinity of human body.Also an automatic RF medical system tester will be presented in this paper.By employing this antenna tester we do not have to connect or disconnect the sensor ca-ble from the medical system board.The automatic RF medical system tester may be applied to measurements of wearable wireless communication systems.RF transmission properties of human tissues have been investigated in several articles [8] [9].Several wearable antennas have been presented in the last decade [10]- [14].A review of wearable and body mounted antennas designed and developed for various applications at different frequency bands over the last decade can be found in [10].In [11] meander wearable antennas in close proximity of a human body are presented in the frequency range between 800 MHz and 2700 MHz.

Phantom Configuration
The phantom is fiberglass cylinder with 1.5 m height and 0.4 m diameter as shown in Figure 1.The thickness of the cylinder surface is around 2.5 mm.The phantom contains a mix of water, sugar and salt.The relative concentration of water, sugar and salt determines the electrical characteristics of the phantom environment.A mixture of 55% water 44% sugar and 1% salt presents the electrical characteristics of stomach tissues.The phantom may be used to measure electromagnetic radiation from inside or outside the phantom.The phantom contain a plastic rod with 5 mm thickness.The position of the plastic rod inside the phantom may be adjusted.The plastic rode may be rotated as shown in Figure 2. A small transmitting antenna may be attached to the plastic rod at different height positions.The antenna may be rotated in the x-y plane.

Measurements of Wearable Antennas
The electrical characteristics of several wearable antennas were measured by using the phantom.The position of the transmitting antennas along the z axis and x axis was varied as listed in tables, (Z = 0, Z = −15 cm and Z = 15 cm).The angular angle of the transmitting antenna was varied from 0˚ to 270˚.Signal reception levels and immunity to noise was measured for several types of wearable antenna.The antennas electrical performance was

Plastic rod
compared and the best antenna was chosen according to the system electrical requirements.

Test Procedure and Process
Test procedure and process is described below.
1) Test procedure The test checks two parameters of the antennas array:  Signal reception levels. Immunity to noise.
3) Test process  Place the antenna array on the phantom as shown in the figure and connect to the recorder. Signal reception levels. Place the transmitter in the phantom in the following coordinates, 5 minutes per each location. Z values: from −15 cm to +15 cm in 15 cm increments, 0 being the level of the antennas center. X values: from −5 cm to −20 cm in 15 cm increments, 0 being the container's wall where the antennas are attached.
 θ values: from 0˚ to 270˚ in 90˚ increments, 0˚ being perpendicular to the middle of the antennas set.

4) Immunity to noise
Place the transmitter outside the phantom in the following coordinates, 5 minutes per each location:  Z values: from −40 cm to +40 cm in 10 cm increments, 0 being the level of the antennas center. X values: 100 cm from container's wall θ values: from 0˚ to 270˚ in 90˚ increments, 0˚ being perpendicular to the middle of the antennas set.

Measurements Results of Wearable Antennas
Measurements of five antenna array configuration was measured.For all the configurations the lower measured signal level was when the transmitting antenna is located at z = 15 cm and x = −5 cm.For all the configurations the higher measured signal level was when the transmitting antenna is located at z = 0 cm and x = −5 cm.

Measurements of Antenna Array 1
The measured antenna consists of four loop antennas with a tuning capacitor as shown in Figure 3.The loop antenna are printed on FR4 substrate with dielectric constant of 4.8 mm and 0.25 mm thickness.The loop radiators orientations are, +45˚, −45˚, −45˚, −45˚.The antennas was inserted in a thin belt.Measurements results of antenna number 1 are listed in Table 1.

Measurements of Antenna Array 2
The measured antenna consists of four loop antennas without a tuning capacitor as shown in Figure 4.The loop radiators orientations are, +45˚, −45˚, −45˚, −45˚.The antennas was inserted in a thin belt.Measurements results of antenna number 1 are listed in Table 2.

Measurements of Antenna Array 3
The measured antenna consists of four tuned loop antennas with a tuning capacitor as shown in Figure 5.The loop radiators orientations are, +45˚, −45˚, +45˚, −45˚.The antennas was inserted in a belt.Measurements results of antenna number 3 are listed in Table 3.

Measurements of Antenna Array 4 in a Thinner Belt
The measured antenna consists of four loop antennas with a tuning capacitor as shown in Figure 6.The loop radiators orientations are, +45˚, −45˚, +45˚, −45˚.The antennas was inserted in a thinner belt.Measurements results of antenna number 4 are listed in Table 4.

Measurements of Antenna Array 5
The measured antenna consists of four loop antennas without a tuning capacitor inserted in a sleeve, as shown in Figure 7.The sleeve improves the antenna VSWR from 4:1 to 2:1.The loop radiators orientations are, (90˚, 90˚, 90˚, 90˚).The antennas was inserted in a thin sleeve.Measurements results of antenna number 5 are listed in Table 5.

Loop antenna Tuning capacitor
Loop antenna Sleeve

Conclusion
Best results are achieved with the first antenna with tuning capacitor antennas in orientations of +45˚, +45˚, +45˚, −45˚.The results of antenna number 4 with thinner belt are better than the results of the same antenna array with thicker belt.For all the configurations the higher measured signal level was when the transmitting antenna is located at z = 0 cm and x = −5 cm.For all the configurations the lower measured signal level was when the transmitting antenna is located at z = 15 cm and x = −5 cm.The effect of the antenna location on the human body should be considered in the antenna design process.