Figure 1 shows the geometry of the proposed double-sided dual band antenna fed with a microstrip feed line. The antenna was printed on a flexible Duroid 5880 RT substrate of thickness T = 1 mm, loss tangent = 0.009 and dielectric constant εr = 2.2. The design process was first carried out using the Ansys High Frequency Structure Simulator (HFSS) tool and the final antenna design was then fabricated and experimentally validated.
Figures 2 and 3 show the evolution of the antenna designs and their corresponding return losses. As can be seen from Fig. 2(a), the fundamental structure of the design originates from the conventional microstrip patch antenna. We first arbitrarily set the resonance frequency fr1 of the design to lie in between the ISM and IEEE 802.11 WLAN band, i.e. at 3.55 GHz. The width W and length L of the antenna can be approximated by substituting fr1 into the following equations developed from the transmission line model [13].
$$W= \frac{c}{2{f}_{r1}\sqrt{\frac{{\epsilon }_{r}+1}{2}}}$$
1
$$L= \frac{c}{2{f}_{r1}\sqrt{{\epsilon }_{eff }}}-2{L}_{1}$$
2
where c is the velocity of electromagnetic waves in free space. The effective dielectric constant εeff and length of the feed L1 can be obtained from [14, 15]
$${\epsilon }_{eff}= \frac{{\epsilon }_{r}+1}{2}+ \frac{{\epsilon }_{r}-1}{2}{\left[1+12\left(\frac{{H}_{S}}{W}\right)\right]}^{-1/2}$$
3
$${L}_{1}=0.412{H}_{S}\left[\frac{\left({\epsilon }_{eff }+0.3\right)\left(\frac{W}{{H}_{S}}+0.264\right)}{\left({\epsilon }_{eff }-0.258\right)\left(\frac{W}{{H}_{S}}+0.8\right)}\right]$$
4
where HS is the height of the dielectric substrate.
The antenna structure was first simulated in a free space environment to verify its resonance frequency and the magnitude of its return loss; it was then simulated again by depositing it onto a phantom model which emulates human body tissues. Figure 4 depicts how the antenna is deposited onto the phantom model during simulation, whereas Table 1 summarizes the thicknesses and electrical properties for each layer of the body tissue [12, 16]. As can be seen from Fig. 3(a), the antenna at free space resonates at 3.5 GHz (with a return loss of 26.14 dB) which agrees reasonably well with the theoretical result. It is also apparent from Fig. 3(a) that fr1 shifts towards 6.1 GHz when the antenna was adhered to the body tissue, indicating that frequency detuning is inevitable because of the change in its effective dielectric constant. The shift, however, is considered acceptable, since fr1 remains at the vicinity of the IEEE 802.11 WLAN band. It is worthwhile noting that, a significant drop can also be observed at the magnitude of the return loss with the introduction of the phantom model. This phenomenon can be attributed to the lossy nature of the body tissue. To generate a second resonance frequency fr2 at the ISM band, two C-shaped slots were etched from both sides of the E-shaped patch, as shown in Fig. 2(b). At this stage, however, the return losses of both resonance frequencies are still rather low, with their magnitudes below 10 dB (i.e., the return losses of fr1 and fr2 are, respectively, 9.81 dB and 5.74 dB, as can be seen in Fig. 3(b)). To ameliorate the performance of the resonance bands, two horizontal strips were added to the top and bottom of the antenna structure and an additional n-shaped slot was etched right at the centre of the radiating patch, as shown in Fig. 2(c). This results in an enhancement of the return losses of fr1 and fr2 to 13.11 dB and 6.51 dB, respectively. Although this modification evinces a distinctive improvement, the antenna performance – particularly that at the second band, is still clearly unsatisfactory.
Like the case in [17], a back patch with an n-shaped slot was introduced to further improve the return losses of these two bands. As can be seen in Fig. 2(d), the patch is appended to the back of the substrate, gap-coupled with the microstrip ground plane. The conductor-backed patch serves as a parasitic structure which is commonly used to improve impedance matching around the resonance bands [17, 18]. It can also be observed from Fig. 2(d) that an additional rectangular slot is etched at the centre of the back patch to optimize the performance of the bands. By carefully modifying the geometrical dimensions of the radiating and back patches, the resonance frequencies were then parametrically shifted to the targeted operating bands. The return loss of the final design is represented by the solid line curve in Fig. 3(b). The final resonance frequencies are 2.13 GHz and 5.87 GHz, with corresponding return losses of 13.22 dB and 25.43 dB. The final geometrical dimensions of the proposed antenna are summarized in Fig. 1 and Table 2.
Table 1
The properties of the human body tissue at the ISM band [12, 16].
Tissues | Thickness (mm) | Relative Permittivity (∈r) | Electrical Conductivity (σ) |
Skin | 4 | 38 | 1.46 |
Fat | 4 | 5.28 | 0.10 |
Muscle | 8 | 52.7 | 1.73 |
Table 2
Parameters of the proposed adhesive antenna.
Parameter | Value (mm) | Parameter | Value (mm) |
H | 18.0 | V5 | 1.0 |
H1 | 10.0 | V6 | 1.0 |
H2 | 6.0 | V7 | 1.0 |
H3 | 5.0 | V8 | 2.5 |
H4 | 1.75 | G1 | 1.0 |
H5 | 3.4 | G2 | 0.5 |
H6 | 1.5 | G3 | 0.475 |
H7 | 1.2 | G4 | 0.5 |
H8 | 0.5 | G5 | 0.5 |
V | 18.0 | G6 | 0.5 |
V1 | 5.0 | G7 | 0.5 |
V2 | 7.5 | B1 | 5.0 |
V3 | 6.0 | B2 | 6.0 |
V4 | 5.5 | B3 | 10.0 |
B4 | 8.0 | B7 | 1.625 |
B5 | 1.5 | B8 | 7.0 |
B6 | 2.0 | T | 1.0 |