A modular antenna with switchable in-body/off-body radiation characteristics combines a common antenna base with a removable reconfigurable module. The common base is composed of two substrate layers, which include a proximity coupling feed structure, an off-body radiation patch and an in-body radiator. The removable reconfigurable module employs a conductive cloth to reduce the antenna’s height and weight. It is attached to the common base with insulating screws to realize mode reconfiguration. Simulation and measurement verify the feasibility of the antenna’s characteristics for both in-body and off-body modes of operation.
INTRODUCTION
With improvements in implanted devices for personalized healthcare, wearable wireless body area networks (WBANs), including implanted devices, have attracted much attention recently.1 In such wearable systems, both in-body antennas that receive data from implanted devices and off-body antennas that transmit the received data to devices outside the body are desired. Generally, in-body antennas exhibit unidirectional radiation characteristics with the radiation directed internally to the body, while off-body antennas have unidirectional radiation characteristics with the radiation directed externally.
One method to fulfill both requirements is to provide multiple single-function antennas. Both patch antennas and folded-ring antennas have been proposed for off-body communication,2,3 while in-body antennas have been constructed using ultra-wideband antenna structures4 and patches.5
To reduce the area required for multiple single-function antennas, reconfigurable antennas with changeable radiation characteristics have been employed.6,7 However, the use of electronic switches results in a complex design, increased size and additional maintenance costs.
Chen et al.8 proposed a passive modular reconfigurable antenna for wearable applications with detachable radiating elements. Although manual operation is required, passive reconfiguration promotes ease of antenna design and more versatile wearable antenna integration since no active components and bias circuits are needed. It offers a flexible reconfiguration capability as well as low manufacturing and maintenance costs. However, the switching mechanism is still complex since the antennas share feed structures. The antennas also fail to provide both off-body and in-body radiation characteristics, which limits their application in wearable systems with implanted devices.
In this work, a new modular antenna with switchable in-body/off-body mode radiation characteristics is described. Unlike most reconfigurable antennas, no electric switches are used, resulting in reduced manufacturing and maintenance costs. This design shares most antenna elements including the feed structure and the radiator; only one removable element (module) is required to switch between in-body and off-body modes. As a result, switching complexity is reduced compared with previous approaches.8 A modular reconfigurable antenna prototype in the 2.4 GHz frequency band is built and tested. Measurement and simulation results are in good agreement, proving that the design can provide a low-cost, easy-to-maintain antenna solution for a wearable system that includes implanted devices.
ANTENNA DESIGN
Antenna Structure
The modular antenna comprises a common base and one removable patch module (see Figure 1). The common base consists of two longitudinally stacked substrates (see Figure 1a). Both substrates are 0.8 mm thick FR4 with a relative permittivity εr = 4.4 and loss tangent tanδ = 0.02. To facilitate an SMA connector feed, the lower substrate is extended 10 mm longer than the upper substrate. The top layer (see Figure 1b) is a rectangular patch with a square hole in the middle and slots on the edges. The middle layer (see Figure 1c) is an adjacent coupling feed and the bottom layer (see Figure 1d) is a rectangular patch embedded in a circular slot. Dimensions for the relevant parameters are given in Table I.
Figure 1 Modular antenna configuration: common base structure (a), top layer (b) middle layer (c), bottom layer (d) and removable patch (e).
The removable patch shown in Figure 1e is used for off-body and in-body mode switching. To reduce the antenna’s overall height and weight, it is made from a conductive cloth. The conductive cloth is polyester with five layers of Ni-Cu metal film sequentially deposited using a vacuum method. The cloth is about 80 microns thick and has a resistance of approximately 0.05 Ω/square.
The antenna works in the off-body mode when the removable patch is attached to the bottom layer of the common base with four 2 mm radius insulating nylon screws. Conversely, the antenna works in the in-body mode with the removable patch attached to the top layer of the common base. To avoid affecting the radiating patch, the nylon screws are placed at the top corners of the substrate; the presence of the insulating screws does not influence the current distribution of the antenna.
TABLE I - MODULAR ANTENNA CONFIGURATION DIMENSIONS
Simulation Environment
As a wearable antenna, the human body impact must be considered. Therefore, a four-layer phantom is constructed consisting of 1 mm of air, 1 mm of skin, 2 mm of fat and 2 mm of muscle (see Figure 2). Electromagnetic properties of the different human body tissues in the WLAN frequency band are listed in Table II.9
Figure 2 Simulated human tissue environment.
TABLE II - RELATIVE PERMITTIVITY AND BULK CONDUCTIVITY OF HUMAN BODY TISSUE IN THE WLAN FREQUENCY BAND
In-Body Antenna Configuration
The in-body antenna mode is realized by fixing the removable patch (see Figure 1e) to the top layer of the common base (see Figure 3). Figure 4 shows the antenna layout after adding the removable patch. Compared to Figure 1(b), due to the introduction of the removable patch, the top layer of the antenna is changed from a microstrip patch to a metal surface with a square slot in the middle. As a result, the structure is converted to a slot antenna backed by a shielded parallel plate. The shielded parallel plate is used to prevent the antenna from radiating in the off-body direction. To improve the impedance match, the shielded parallel plate is slotted and the diamond parasitic patch in the bottom layer is added.
Figure 3 Schematic of in-body mode switching.
Figure 4 In-body mode configuration: top layer (a), middle layer (b) and bottom layer (c).
