With the development of radio frequency (RF) technology and body sensor network (BSN), the interaction of RF electromagnetic field with the human body has been an active research topic for many years [
1,
2]. Parts of these studies are focused on the electromagnetic power absorption by the whole body or parts of the body [
3,
4,
5,
6,
7]. These studies have provided a comprehensive analysis between the power absorption and human body, which are helpful to understand the effects of RF radiation on the human body.
On the other hand, considering the fact that the human tissues are lossy media [
8,
9,
10], the electric current and electric field are induced in the human body when the human body is exposed to the RF electromagnetic field [
11,
12]. Therefore, besides the electromagnetic power absorption in human body, the human body may be used itself as an antenna or medium for signal transmission. In recent years, some researchers have demonstrated the propagation characteristics using the human body itself as the transmission medium, which is referred to as human body communication (HBC) or intra-body communication (IBC) [
13,
14,
15,
16,
17]. However, in the above studies, the transmitter and receiver of HBC are acquired to be located on the same human body due to the fact that the electromagnetic field the human body in HBC technology. In other words, it is indicated that the above studies are mainly suitable for the signal transmission in the same individual rather than among different individuals. Thus far, few researchers have tried to characterize the human body as an antenna to achieve the signal transmission among different individuals. In [
18], the authors demonstrated the feasibility of energy harvesting from ambient electromagnetic wave using the human body as a receiving antenna. However, only the power spectral density was acquired in the paper, and therefore the propagation characteristics were still not clear when the human body was used as an antenna. In [
19], the human-body equivalent monopole antenna, which was made by different combinations of the rectangular blocks, was proposed as a human surrogate. Furthermore, the induced ankle current of model was measured in the frequency range 30 MHz to 100 MHz when the model was exposed to the electromagnetic field. In [
20], the human body was demonstrated as an equivalent cylindrical monopole antenna which was grounded on a highly conductive ground plane. The reflection coefficients of antenna were investigated through the finite-difference time-domain (FDTD) algorithm when the frequency was from 10 MHz to 110 MHz. In addition, the human body was used as an antenna to achieve the wireless implant communication among different individuals in [
21]. Specifically, a toroidal inductor at the cross section of the ankle was used to excite electromotive force inside the human body when the frequency was from 1 MHz to 70 MHz. In addition, the receiving toroidal inductor was at the ankle of the other individual. As previously mentioned [
19,
20,
21], it is indicated that the human body can be represented by a monopole antenna to study the propagation characteristics. However, it requires that the human body stands on a metallic plane or ground plane. Meanwhile, the transmitter and receiver should be fixed on the bottom of the human body (i.e., ankle or sole) which corresponds to the feed point of monopole antenna when the human body is modeled as a monopole antenna. Once the position of transmitter or receiver is changed, the human-body equivalent model of monopole antenna may be unavailable. Furthermore, thus far, the factors affecting propagation characteristics are still ambiguous when the human body is used as an antenna. Therefore, in order to better understand the characteristics of human body when it is utilized as an antenna and provides a prospective communication solution for BSN, a comprehensive investigation is necessary and important.
In this paper, we mainly focused on the propagation characteristics when the human body was employed as an antenna to achieve the signal transmission among different individuals. Specifically, different from the previous investigations which the human body is regarded as a monopole antenna and the power is injected into the human ankle or sole by using current directly, the power is coupled into the human body through electrode in this paper, and then the power is radiated out of the body when the human body is utilized as a transmitting antenna. Similarly, the power is coupled into the receiver when the human body is used as a receiving antenna. The advantage of this method is that the positions of the transmitter and the receiver are alterable, so they are no longer limited to the human ankle or sole. The contribution and originality of this paper was summarized as follows. Firstly, the propagation characteristics were investigated when the human body was utilized as an antenna in the frequency range from 1 MHz to 90 MHz. Secondly, the factors affecting propagation characteristics, including the different human tissues, the electrode position on the human body, and the distance between electrode and human body were studied in this paper.
The remainder of this paper is organized as follows. In
Section 2, we will present the inhomogeneous human body model and the simulation setup under different conditions. In
Section 3, the experimental measurement will be reported.
Section 4 discusses and gives a detailed analysis of both simulation and measurement results. Finally, the conclusions are drawn in
Section 5.