As could be noticed from Fig.3, the presented antenna is a dual band antenna. It covers a frequency band of 12.25 MHz [397.15-409.4 MHz] for MICS band with a return loss of -23.23 dB at 402 MHz. For the ISM band it covers a wide bandwidth of 630 MHz [2.37-3.00 GHz] with a return loss of -20dB at 2.42 GHz. A very good bandwidth enhancement is obtained, especially at the ISM band, due to the capacitive load effect in addition to 53.2% size reduction compared to the results in . A size reduction of 76.2% is obtained compared to the results in . The bandwidth enhancement at MICS and ISM band is calculated to be 23.3% and 87.3% respectively compared to the achieved bandwidth for the planar inverted F-L implant antenna in .
Human safety is an essential issue, so the average Specific Absorption Rate (SAR) value is very important in the design of biomedical antennas. The SAR value measures the absorbed power by a unit mass of biological tissue. The optimal SAR value is less than 1.6 W/Kg for C95.1-1999 system and less than 2 W/kg for C95.1-2005. For our design, the simulated SAR value based on 1 W input power at 402 MHz is 145.13 W/kg for 1g model and 42.56W/kg for 10g model. Similarly, the simulated SAR value at 2.42 GHz is 135.29 W/kg for 1g model and 41.87W/kg for 10g model. To be within the optimal values for the SAR, the maximum input power has to be reduced to 11mW (1 g) and 46.98 mW (10g) at 402 MHz for the MICS band. For the ISM band, the peak input power has to be reduced to 11.82mW (1 g) and 47.7 mW (10g) at 2.42 GHz.
At 402 MHz, the highest current density is mostly focused at the center, which means that the half-wavelength mode is excited. While at 2.42 GHz, the current peak values are mostly focused at the edges of the structure, which means that the full-wavelength mode is excited, Fig.4. For the electric-field intensity, it has its peak value on the center of the structure at 402 MHz. While the field peak values occur on the edges at 2.42 GHz as Fig.5 shows.
The antenna far-fields shown in Fig.6 are mostly directed away from the three-layer body model for both bands. The radiation pattern for the ISM band shows a null at theta=±π/4. The calculated gain for MICS band at 402 MHz is -27.52dBi and for the ISM band at 2.42 GHz is -1.85dBi. Low gain is a drawback in the design of the implantable antennas and a lot of researches focus on the gain enhancement techniques, , and [20-23]. A summary of the results obtained by this work in comparison to other works is given in Table 2.