As shown in Figure 5a, when the antenna is not loaded with a removable patch, it is a slot antenna fed by microstrip line. When capacitance is introduced through the diamond-shaped parasitic patch in the middle of the slot, the antenna resistance is reduced and two resonant modes are excited. As shown in Figure 5b, after the antenna is loaded with the removable patch, its inductance and resistance both increase. The higher-order mode of the antenna is moved higher in frequency and impedance matching to 50 Ω is improved in the WLAN frequency band. An impedance bandwidth of 35.6 percent is achieved. As shown in Figure 5c, after loading with the reflective removable patch, antenna gain is increased by 3.3 dB and the front-to-back ratio is 14.3 dB.

Figure 5

Figure 5 Simulated performance of the in-body mode antenna configuration with and without the removable patch: |S11| (a), impedance (b) and gain (c).

Off-Body Antenna Configuration

The removable patch is fixed to the bottom layer of the common base with nylon insulating screws to function in the off-body mode (see Figures 6 and 7). Note that the patch antenna with a slot in the middle is used for compatibility with in in-body mode configuration. Due to the introduction of the removable patch (see Figure 6), the bottom layer of the antenna is now a complete ground, which prevents leakage of electromagnetic energy into the human body.

Figure 6

Figure 6 Schematic of off-body mode switching.

Figure 7

Figure 7 Off-body mode configuration: top layer (a), middle layer (b) and bottom layer (c).

The antenna after loading with the removable patch becomes a microstrip patch antenna with a complete ground plane. The antenna is slotted at the edge, which excites a resonance at 2.51 GHz, improving impedance matching and bandwidth.

Figure 8

Figure 8 Surface current distribution of the off-body mode antenna at 2.40 and 2.51 GHz.

EXPERIMENTAL RESULTS

To validate the modular antenna concept, a prototype 2.4 GHz antenna is characterized using fresh pork to simulate the human body. S11 is measured with a Keysight AV3656A vector analyzer and antenna radiation patterns are measured in an anechoic chamber.

In-Body Mode Antenna Simulation and Measurements

Figure 9 compares the simulated and measured |S11| of the in-body mode antenna with loading from the human body. The simulated |S11| is below – 10 dB between 1.80 and 2.58 GHz while the measured |S11| is below – 10 dB between 1.65 and 2.60 GHz. The measurements are in good agreement with the simulations. The slight frequency shift between simulation and measurement is attributed to fabrication and measurement error.

Figure 9

Figure 9 Measured and simulated |S11| of the in-body mode antenna.

The simulated radiation pattern of the in-body mode antenna is shown in Figure 10. It achieves a maximum gain of –0.15 dBi at θ = 180 degrees and a minimum gain of –23.2 dBi at θ = 40 degrees in the XoZ plane. Radiation in the YoZ plane is concentrated between 40 and 320 degrees.

When an implanted antenna is placed in muscle tissue below the in-body antenna at a depth of 30 mm, the simulated |S21| between the in-body antenna and the implanted one is −31.4 dB and the received power is –26.8 dBm with a transmit power of 2.9 mW. Assuming the transmit power degrades further by the distance between two antennas at a 26 dB/m rate,10 the receive power is reduced to −65.8 dBm at 1.5 m, which is still larger than the minimum required receive power level of −75 dBm.10

Figure 10

Figure 10 Simulated radiation patterns of the in-body mode antenna.

With the antenna spaced 1 mm from the human body model, the specific absorption rate (SAR) of the in-body mode antenna is simulated on the surface of the numerical model at the frequency where |S11| is a minimum. With an incident power of 24 mW, the 10 g average SAR for the in-body mode is 1.95 W/Kg, within the 2 W/kg limit specified by European standards.

Off-Body Mode Antenna Simulation and Measurements

The simulated and measured |S11| of the off-body mode antenna after loading from the human body are compared (see Figure 11). The measured |S11| – 10 dB band is from 2.4 to 2.52 GHz, while the simulated band is from 2.4 to 2.51 GHz. Both simulated and measured results meet the operating bandwidth requirements for WLAN and are in good agreement.

Figure 11

Figure 11 Measured and simulated |S11| of the off-body mode antenna.

The normalized radiation patterns in the XoZ and YoZ planes of the antenna are obtained in an anechoic chamber (see Figure 12). The radiation is concentrated on the upper half plane of the antenna, which is in good agreement with the simulation. The measured peak off-body gain reaches 1.46 dBi.

Figure 12

Figure 12 Measured and simulated radiation patterns of the off-body mode antenna: XoZ plane (a) and YoZ plane (b).  

CONCLUSION

A modular wearable antenna design for WBAN applications uses a relocatable patch to change the antenna’s radiation properties between in-body and off-body radiation modes. Mode configurations are mechanically secured by insulating screws. The mechanical reconfiguration eliminates the additional volume occupied by active components and bias circuits, giving this antenna the advantages of compactness, ease of manufacture and low maintenance costs.

References

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  2. H. Lee, J. Tak and J. Choi, “Wearable Antenna Integrated Into Military Berets for Indoor/Outdoor Positioning System,” IEEE Antennas and Wireless Propagation Letters, Vol. 16, March 2017, pp. 1919-1922.
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  7. S. Yan and G. A. E. Vandenbosch, “Radiation Pattern-Reconfigurable Wearable Antenna Based on Metamaterial Structure,” IEEE Antennas and Wireless Propagation Letters, Vol. 15, February 2016, pp. 1715-1718.
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  9. “Tissue Properties,” IT’IS Foundation. Web. https://itis.swiss/virtual-population/tissue-properties/downloads/.
  10. N. Cho, T. Roh, J. Bae and H. Yoo, “A planar MICS Band Antenna Combined witha Body Channel Communication Electrode for Body Sensor Network,” IEEE Transactions on Microwave Theory and Techniques, Vol. 57, No. 10, October 2009, pp. 2515-2522.