Inexpensive Fabric Antenna for Off-Body Wireless Sensor Communication

Jason Carter, Jason Saberin*, Tejal Shah, Sai Ananthanarayanan P.R., and Cynthia

Furse

Department of Electrical and Computer Engineering University of Utah, Salt Lake City, UT,84112

E-mail: jasonsaberin@gmail.com,saianantha21@gmail.com

Introduction

Emerging trends in monitoring people (patients, soldiers, athletes, etc.) have led to numerous recent advances in body area communication networks (BAN). Wireless sensor communication opens up tremendous potential for wireless patient monitoring. Body centric wireless networks use RF sensor nodes in close proximity to the human body. Body networks include on body, body to body and off body communication. Antenna design and analysis plays an important part in the development of sensors for BAN. Antennas for on-body communication include the inverted F antenna, (IFA) and the planar inverted F antenna (PIFA) [3], low profile microstrip patch antenna [2], sleeve dipole[1], printed-F antenna [1], wearable (fabric) antennas [3]-[6], broadband discone antennas [7] etc.. In this paper we discuss a low cost, nearly circularly polarized truncated patch antenna design on Zelt [11] fabric and Felt substrate for performing off-body communication centered at 915 MHz for monitoring patients after operation. This low cost antenna provides good return loss and high directivity and efficiency comparable to the fabric antennas designed in the literature. The antenna also has a 10-dB bandwidth of 75 MHz which accounts for the frequency shifting due to the bending of the antenna.

Textile Material and Antenna Design One of the main criteria for choosing material for fabric antenna design is the ease with which it can be incorporated. The second criterion is that the fabric for the antenna and the ground plane must have good conductivity. The third criterion is that the fabric substrate must have constant thickness and stable permittivity. Based on the basic properties required for a textile antenna, Zelt and Felt were chosen for the antenna and the substrate, respectively. The material properties of the fabrics are given in Table 1. Table 1: Properties of Zelt and Felt materials for 915-925 MHz

Zelt [11] FeltConductivity (S/m) 1x106 Resistivity (ohm/sq) 0.01 Permittivity 1.38Loss Tangent 0.023 Cost (per sq meter) $15 $5 Substrate thickness (mm) 2.2

Figure 1 shows the fabric antenna built using the Zelt and Felt. This is based on a simple microstrip patch design (Zelt is also used as a ground plane and backing behind the blue Felt in Figure 1. The corners were truncated to provide circular polarization. The antennas were simulated using the transient solver in CST Microwave studio [9].

Truncation and feed optimization were performed for obtaining a well matched, left hand circular polarized (LHCP) truncated patch design. The antenna was fabricated by stitching together the fabric material and the final product is as shown in figure 1. The manufacturing process was less complex as compared to methods used in previous designs [10].

Fig1: Truncated patch antenna using Zelt fabrioc for antenna and a Felt substrate

Measurement and Simulation Results

0.4 0.6 0.8 1 1.2 1.4-40

-30

-20

-10

0

Mag

nitu

de (

dB)

Frequency (GHz)

Measured

Simulated using CST

Fig 2: Measured and simulated return loss

In this section we compare the simulated antenna characteristics with measurements. Measured and simulated return loss (S11) is compared in Figure 2 and show excellent agreement. The simulated antenna has a return loss of about -28 dB, and the measured antenna has a return loss of about -32 dB at 915 MHz. The simulated antenna has a 10 dB-bandwidth of 40 MHz while the measured bandwidth is about 75 MHz. These S11 measurements were also performed by flexing the patch antenna, about 20-30 degrees which lowered the resonance frequency by 10-15 MHz. The radiation pattern in the X-Y plane is shown in Figure 3. The effects of bending and the presence of the body on antenna radiation will be presented at the conference. The measured and simulated antenna gain is 2.346 (3.7033 dB) at 915 MHz. The half power beam width of the truncated patch antenna was about 75o. Because of this directionality we expect the antenna pattern to be less affected by the presence of the human body. The fabric antenna has a total efficiency of 35 % which is similar to the efficiency obtained in [10]. Thus we obtain a low cost antenna giving characteristics similar to those obtained in previous designs [6][10]. The methods for improving the total efficiency will be presented at the conference.

Fig 3: Measured and simulated radiation pattern at 915 MHz. Another important parameter of the patch antenna is the axial ratio which is a parameter to describe either elliptical or circular polarization. The axial ratio was computed using the expression in [10]. For the truncated patch antenna we observe that the axial ratio is about 0.72 dB which shows that the antenna is nearly LHCP (which would have an axial ratio of 0 dB). It is observed that the fabric antenna has an axial ratio comparable with those obtained in [10] as well as using copper conductor.

Conclusion

This paper simulates and fabricates an inexpensive fabric antenna for performing communication from sensors on the human body to a nearby receiver. The antenna is well matched at 915-925 MHz and has a 10 dB-bandwidth of 75 MHz. The antenna has a gain of 3.7 dB and a directivity of 8. It is nearly LHCP in the frequency band of interest. The gain and efficiency of the designed antenna matches well with the antennas presented in the literature. The effects of antenna flexing as well as the presence of the human body along with designs with improved antenna efficiency will be provided at the conference.

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