Fully Textile Dual-Band Logo Antenna for IoT Wearable Devices

Abstract

In recent years, the interest in the Internet of Things (IoT) has been growing because this technology bridges the gap between the physical and virtual world, by connecting different objects and people through communication networks, in order to improve the quality of life. New IoT wearable devices require new types of antennas with unique shapes, made on unconventional substrates, which can be unobtrusively integrated into clothes and accessories. In this paper, we propose a fully textile dual-band logo antenna integrated with a reflector for application in IoT wearable devices. The proposed antenna's radiating elements have been shaped to mimic the logo of South-West University "Neofit Rilski" for an unobtrusive integration in accessories. A reflector has been mounted on the opposite side of the textile substrate to reduce the radiation from the wearable antenna and improve its robustness against the loading effect from nearby objects. Two antenna prototypes were fabricated and tested in free space as well as on three different objects (human body, notebook, and laptop). Moreover, in the two frequency ranges of interest a radiation efficiency of 25-38% and 62-90% was achieved. Moreover, due to the reflector, the maximum local specific-absorption rate, which averaged over 10 g mass in the human-body phantom, was found to be equal to 0.5182 W/kg at 2.4 GHz and 0.16379 W/kg at 5.47 GHz. Additionally, the results from the performed measurement-campaign collecting received the signal-strength indicator and packet loss for an off-body scenario in real-world use, demonstrating that the backpack-integrated antenna prototype can form high-quality off-body communication channels.

Keywords: IoT wearable device; RSSI; SAR; fully textile antenna; logo antenna; wearable antenna.

Figure 1

Configuration of the proposed antenna: (a) 3D view; (b) decomposition view; (c) top view with dimensions; (d) top view of substrate second layer with dimensions. Dimensions indicated in the figure are in millimeters.

Figure 2

Voltage Standing Wave Ratio (VSWR) at a different: (a) reflector size; (b) cotton-layer thickness; (c) logo elements size. The dimensions of the other antenna elements are those shown in Figure 1. The reflector size is in wavelengths, where Lambda is the free-space wavelength at 2.44 GHz (central frequency for the ISM 2.4 GHz).

Figure 3

Antenna prototype fabrication process: (a) block diagram; (b) photograph of the fabricated prototype 1, and (c) photograph of the fabricated prototype 2—integrated into a backpack.

Figure 4

Simulated reflection coefficient |S₁₁| of the proposed antenna: (a) in free space; (b) on a model of a human body, notebook, and laptop; (c) antenna without a reflector on a model of a human body, notebook, and laptop.

Figure 5

Simulated 3D radiation patterns of the antenna at 2.40 GHz: (a) in free space; (b) on the human-body model; (c) on the notebook model; (d) on the laptop model.

Figure 6

Simulated 3D radiation patterns of the antenna at 5.47 GHz: (a) in free space; (b) on the human-body model; (c) on the notebook model; (d) on the laptop model.

Figure 7

Normalized current distribution on the: (a) logo elements, CPW, and reflector at 2.4 GHz; (b) logo elements, CPW, and reflector at 5.47 GHz; (c) logo elements at 2.4 GHz; (d) logo elements at 5.47 GHz.

Figure 8

Simulated: (a) maximum gain; (b) radiation efficiency versus frequency; (c) FBR.

Figure 9

SAR distributions in: (a) different planes; (b) different cross-sections at 2.4 GHz.

Figure 10

SAR distributions in: (a) different planes; (b) different cross-sections at 5.47 GHz.

Figure 11

Measured and simulated reflection coefficient |S₁₁|: (a) in free space; (b) on the copy-printer paper; (c) on the laptop; (d) on the human arm, leg, and flat phantom.

Figure 12

Measurement setup: (a) floor plan of the measurement locations and (b) photographs of the empty backpack, backpack with a package of copy-printer paper, and backpack with a laptop. The dimensions indicated in the figure are in millimeters.

Figure 13

RSSI and packet loss for the backpack-integrated antenna prototype in the case of an empty backpack: (a) distributions with respect to the distance between the antennas; (b) histograms. The RSSI and packet loss for all positions in the LoS scenario are displayed with red triangles. The RSSI and packet loss for all positions in the NLoS scenario are displayed with blue squares.

Figure 14

RSSI and packet loss for the backpack-integrated antenna prototype in the case of a backpack with a package of copy-printer paper: (a) distributions with respect to the distance between the antennas; (b) histograms. The RSSI and packet loss for all positions in the LoS scenario are displayed with red triangles. The RSSI and packet loss for all positions in the NLoS scenario are displayed with blue squares.

Figure 15

RSSI and packet loss for the backpack-integrated antenna prototype in the case of a backpack with a laptop: (a) distributions with respect to the distance between the antennas; (b) histograms. The RSSI and packet loss for all positions in the LoS scenario are displayed with red triangles. The RSSI and packet loss for all positions in the NLoS scenario are displayed with blue squares.