A novel wide coplanar waveguide- (CPW-) fed multiband wearable monopole antenna is presented. The multiband operation is achieved by generating slanted monopoles of different lengths from an isosceles triangular patch. The different operating frequencies of the proposed antenna are associated with the lengths of the slanted monopoles, which are determined under quarter wavelength resonance condition. The CPW line is used as a multiband impedance-matching structure. The two grounds are slightly extended for better impedance matching. The proposed antenna is designed to cover the 1800 MHz GSM, 2.4 GHz/5.2 GHz WLAN, and 3.5 GHz WiMAX bands. The measured peak gains and impedance bandwidths are about 4.18/3.83/2.6/2.94 dBi and 410/260/170/520 MHz for the 1550-1960 MHz/2.3-2.56 GHz/3.4-3.57 GHz/5.0-5.52 GHz bands, respectively. The calculated averaged specific absorption rate (SAR) values at all the resonant frequencies are well below the standard limit of 2 W/kg, which ensures its feasibility for wearable applications. The antenna performance under different bending configurations is investigated and the results are presented. The reflection coefficient characteristics of the proposed antenna is also measured for different on-arm conditions and the results are compared. A good agreement between experimental and simulation results validates the proposed design approach.
In today's world, for applications which involve wearable antennas, like health monitoring of patients and firefighting and military personal or body-centric communications, it becomes absolutely necessary that a single antenna operates on multiple bands, with SAR values well below the standard limits and its performance should not degrade under bent conditions. In order to satisfy these specific wireless communication requirements, multiband wearable antennas which can operate at 880–960 MHz/1710–1880 MHz for Global System for Mobile (GSM) communications, 2.4–2.484 GHz/5.15–5.825 GHz for wireless local area network (WLAN), and 3.4–3.69 GHz/5.25–5.85 GHz for Worldwide Interoperability for Microwave Access (WiMAX), are desired.
In the past few years, many single-, dual-, and multiband wearable antennas have been proposed [1–13]. In previous works, different methods have been used for designing single-band wearable antennas, such as antennas with an electromagnetic band gap (EBG) structure [
CPW-fed monopole antennas have been used frequently for the achievement of multiband operations [14–19]. CPW-fed dual-frequency monopole antennas have been presented in [14–16]. In [
The specific absorption rate of a wearable antenna should be calculated to ensure the safety of the user. Either single-layer or multilayer human head and human tissue models are used to calculate the specific absorption rate (SAR) of an antenna [
In this paper, we propose a four-band wearable monopole antenna fed by a wide CPW line. The small values of the substrate's dielectric constant increase the width of the CPW line. This limitation is exploited in this design. The proposed design has used the wide CPW line as a multiband impedance-matching structure. An isosceles triangular patch is slotted to generate four slanted monopoles for multiple wide-band operations. The antenna is tested under bent conditions and the results are presented. Antenna performance is also investigated for different on-arm conditions, with and without a cloth, and the results are compared. The specific absorption rate of the proposed antenna is found to be within standard limits. The geometrical structure and design procedure of the proposed antenna are presented in Section 2. Section 3 describes the simulation and measurement methodology. The results of the simulated and handmade proposed antenna are presented, compared, and analyzed in Section 4. The estimation of SAR values, effect of bending, and on-arm performance of the proposed antenna are also explained in this section. The paper is concluded in Section 5.
The geometry of the proposed multiband wearable antenna is shown in Figure 1. The antenna is formed by slanted monopoles using the approach applied in [
PHOTO (COLOR): A wide CPW-fed multiband wearable monopole antenna with extended grounds.
The width of the CPW strip (W
The size of the proposed antenna considering the substrate size is 45.5×85×1 mm
Initially, a CPW-fed triangular patch with parameters L=85.0 mm, W=45.5 mm, Lg=40.0 mm, Wg=19.4 mm, LT=38.5 mm, WT=3.15 mm, W
The first slanted monopole m
For the selected length LT and width WT of the isosceles triangular patch, the value of θ comes out to be approximately 85.5°, which is estimated as(
Due to the mutual coupling between the slanted monopole m
The second slanted monopole m
The third slot with a width of Ws
In the case of Antenna IV, the reflection coefficient and bandwidth of the four bands at 1800 MHz, 2.4 GHz, 3.5 GHz, and 5.38 GHz are not adequate and need to be improved. To improve the reflection coefficient and bandwidth of the four bands, both ground patches are extended with length Lag and width Wag. The required four-band operation is obtained when Lag=9 mm and Wag=12 mm. These extended grounds provided voltage standing wave ratio (VSWR) values close to 1 at four resonating frequencies, which proves improved impedance matching. Improved impedance matching further decreases the reflection coefficient and increases the bandwidth of the four respective bands. The antenna with four monopoles along with extended grounds is the final required design denoted Antenna V. Antenna V is shown in Figure 2(e). The photograph of the handmade Antenna V is shown in Figure 2(f).
PHOTO (COLOR): Geometries of (a) Antenna I, (b) Antenna II, (c) Antenna III, (d) Antenna IV, and (e) Antenna V and (f) photograph of the proposed handmade Antenna V.
The antenna design is simulated using a high-frequency structure simulator (HFSS). The simulation is performed for the reflection coefficients (S
A three-layer human tissue model is used for the estimation of SAR values at the measured resonant frequencies of the proposed antenna. The permittivity (εr) and conductivity σ (S/m) of the skin (dry and wet), fat, and muscle layers at 1780 MHz, 2.40 GHz, 3.46 GHz, and 5.26 GHz are listed in Tables 1 and 2 [
Permittivity (εr) of skin, fat, and muscle.
Layer Permittivity (εr) 1780 MHz 2.40 GHz 3.46 GHz 5.26 GHz Skin (dry) 38.90 38.06 37.04 35.56 Skin (wet) 43.88 42.92 41.53 39.30 Fat 5.35 5.28 5.17 5.00 Muscle 54.47 53.63 52.25 49.77
Conductivity σ (S/m) of skin, fat, and muscle.
Layer Conductivity σ (S/m) 1780 MHz 2.40 GHz 3.46 GHz 5.26 GHz Skin (dry) 1.17 1.44 1.99 3.25 Skin (wet) 1.22 1.56 2.27 3.80 Fat 0.07 0.10 0.15 0.25 Muscle 1.37 1.77 2.62 4.53
Thickness, density, and loss tangent of skin, fat, and muscle layers.
Layer Thickness (mm) Density (kg/m3) Loss tangent Skin 3 1090 0.418 Fat 10 1100 0.186 Muscle 30 1050 0.342
In order to investigate the effects of slight and severe bending on antenna performance, two diameters have been taken for bending analysis. The 160 mm diameter offers slight bending, while the 110 mm diameter offers severe bending. The reflection coefficients (S
The simulated reflection coefficients (S
PHOTO (COLOR): Simulated reflection coefficient (S11) of Antenna I to Antenna V.
PHOTO (COLOR): Reflection coefficient (S11) of Antenna V at Wag=12 mm and Lag=8, 9, and 10 mm.
PHOTO (COLOR): Reflection coefficient (S11) of Antenna V at Lag=9 mm and Wag=11, 12, and 13 mm.
Antenna II has three bands that cover from 1660 MHz to 1920 MHz (260 MHz, 14.53%), 2.52 GHz to 2.7 GHz (180 MHz, 8.34%), and 5.2 GHz to 5.8 GHz (600 MHz, 10.9%). Antenna III covers three bands from 1660 MHz to 1960 MHz (300 MHz, 16.57%), 2.36 to 2.46 GHz (100 MHz, 4.15%), and 5.3 GHz to 5.84 GHz (540 MHz, 9.69%).
Antenna IV has three bands that cover from 1640 MHz to 1960 MHz (320 MHz, 17.78%), 2.36 GHz to 2.48 GHz (120 MHz, 4.96%), and 5.26 GHz to 5.74 GHz (480 MHz, 8.73%). Antenna V covers four required bands from 1550 MHz to 1980 MHz (430 MHz, 24.36%), 2.36 GHz to 2.56 GHz (200 MHz, 8.13%), 3.43 GHz to 3.59 GHz (160 MHz, 4.56%), and 5.08 GHz to 5.65 GHz (570 MHz, 10.62%). The resonant frequencies of the simulated Antenna V are 1800 MHz, 2.44 GHz, 3.5 GHz, and 5.38 GHz, with reflection coefficient values of −33.7 dB, −22.3 dB, −23.6 dB, and −25.6 dB, respectively.
The first band of Antenna V covers the 1710–1880 MHz GSM band. The second band of Antenna V satisfies the requirement of 2.4–2.484 GHz WLAN applications. The third band of Antenna V operates within the 3.4–3.7 GHz WiMAX range. The fourth band of Antenna V covers the 5.15–5.35 GHz WLAN operational band. Figure 6 shows the measured and simulated reflection coefficients of Antenna V.
PHOTO (COLOR): Measured and simulated reflection coefficient of Antenna V.
The measured operating bands of Antenna V are 1550 MHz to 1960 MHz (410 MHz, 23.36%), 2.3 GHz to 2.56 GHz (260 MHz, 10.69%), 3.4 GHz to 3.57 GHz (170 MHz, 4.87%), and 5.0 GHz to 5.52 GHz (520 MHz, 9.89%). The measured resonant frequencies of Antenna V are 1780 MHz, 2.40 GHz, 3.46 GHz, and 5.26 GHz, with reflection coefficient values of −30.8 dB, −24.15 dB, −24.3 dB, and −30.4 dB, respectively.
From the simulated and measured results, a slight shift in the resonant frequencies accompanied by a change in reflection coefficient values has been found. This can be attributed to the inconsistency in the fabrication process and soldering tolerance.
Refereeing to these results, the proposed antenna can satisfy the 1800 MHz GSM, 2.4/5.2 GHz WLAN, and 3.5 GHz WiMAX bands, resulting in a four-band operation.
Figure 7 shows the surface current distributions of Antenna V at 1800 MHz, 2.44 GHz, 3.5 GHz, and 5.38 GHz. The surface current densities are at the maximum in the corresponding monopoles at the respective simulated resonant frequencies of 1800 MHz, 2.44 GHz, 3.5 GHz and 5.38 GHz. In monopole elements that are sufficiently thin electrically and not too long, the element current distribution is approximately sinusoidal [
PHOTO (COLOR): Surface current density distributions of Antenna V at (a) 1800 MHz, (b) 2.44 GHz, (c) 3.5 GHz, and (d) 5.38 GHz.
Since the measured resonant frequencies of Antenna V are 1780 MHz, 2.40 GHz, 3.46 GHz, and 5.26 GHz, Figure 8 shows the 3-dimensional simulated radiation patterns of Antenna V at measured resonant frequencies. Figures 9–12 illustrate the 2-dimensional simulated and measured radiation patterns in x–z, y–z, and x–y planes of Antenna V at measured resonant frequencies.
PHOTO (COLOR): 3-Dimensional simulated radiation patterns of Antenna V at (a) 1780 MHz, (b) 2.40 GHz, (c) 3.46 GHz, and (d) 5.26 GHz.
PHOTO (COLOR): 2-Dimensional simulated and measured radiation patterns of Antenna V at 1780 MHz.
PHOTO (COLOR): 2-Dimensional simulated and measured radiation patterns of Antenna V at 2.40 GHz.
PHOTO (COLOR): 2-Dimensional simulated and measured radiation patterns of Antenna V at 3.46 GHz.
PHOTO (COLOR): 2-Dimensional simulated and measured radiation patterns of Antenna V at 5.26 GHz.
CPW-fed antennas normally suffer backward radiation. The radiation patterns at 1780 MHz and 2.4 GHz are quite similar and also suffered backward radiation, which is the reason for the shapes of their radiation patterns. However, at 3.46 GHz and 5.26 GHz, the radiation patterns are distinct.
Figure 13 shows the measured and simulated peak gains versus the frequency of Antenna V. The measured and simulated peak gains in the 1550–1960 MHz band vary from 3.3 to 4.18 dBi and 8.51 to 9.33 dB, respectively. The measured and simulated results in the 2.3–2.56 GHz band described that the gain variations are from 3.24 to 3.83 dBi and 5.63 to 7.15 dB. For the third and fourth operating bands of 3.4–3.57 GHz and 5.0–5.52 GHz, the measured peak gains are about 1.86–2.6 dBi and 2.01–2.94 dBi. The simulated peak gains for the 3.4–3.57 GHz and 5.0–5.52 GHz bands lie within 1.74 to 2.41 dB and 1.59 dB to 2.75 dB, respectively. The simulated radiation efficiency of the proposed Antenna V is shown in Figure 14. For the 1550 MHz–1960 MHz band, the radiation efficiency is about 91.92 to 94.94%. In the second band of 2.3–2.56 GHz, the radiation efficiency varies from 82.16 to 95.69%.
PHOTO (COLOR): Measured and simulated peak gain of the proposed Antenna V.
PHOTO (COLOR): Radiation efficiency of the proposed Antenna V.
The results in the 3.4–3.57 GHz and 5.0–5.52 GHz bands described that the radiation efficiency variations are from 80.89 to 94.10% and 80.20 to 87.82%, respectively.
Figure 15 shows the three-layer human tissue model [
PHOTO (COLOR): Modelling of (a) Antenna V on a three-layer human tissue model for SAR calculation and (b) Antenna V over a three-layer human tissue model in HFSS [
Estimated SAR (W/kg) values for the three-layer human tissue model with dry and wet skin.
Frequency Average SAR (W/kg) Dry skin Wet skin 1780 MHz 0.2293 0.2549 2.40 GHz 0.3892 0.3861 3.46 GHz 0.4640 0.4447 5.26 GHz 0.2535 0.2512
Referring to these results, SAR values tend to increase with an increase in the simulating frequency of the human tissue model except at 5.26 GHz. It is due to the pointed radiation pattern in the z direction at 5.26 GHz, which shows that a small amount of radiation is illuminating the human tissue model, and therefore, the reason for the low SAR value at 5.26 GHz.
The estimated SAR value for the human tissue model with wet skin is higher than that of the model with dry skin for 1780 MHz, whereas for 2.4 GHz, 3.46 GHz, and 5.26 GHz, the SAR values for the model with dry skin are higher than that of the model with wet skin.
Bending is investigated in the x and y planes by keeping the handmade Antenna V around the PVC (polyvinyl chloride) pipes [
PHOTO (COLOR): Photographs of Antenna V under four bending conditions on a PVC pipe (a) bent along the x plane on an 80 mm radius, (b) bent along the x plane on a 55 mm radius, (c) bent along the y plane on an 80 mm radius, and (d) bent along the y plane on a 55 mm radius.
PHOTO (COLOR): HFSS simulation photographs of Antenna V under four bending conditions on a PVC pipe (a) bent along the x plane on an 80 mm radius, (b) bent along the x plane on a 55 mm radius, (c) bent along the y plane on an 80 mm radius, and (d) bent along the y plane on a 55 mm radius.
Figures 18–21 compare the measured reflection coefficient of the flat Antenna V in free space with the simulated and measured reflection coefficients of Antenna V, bent along the x and y planes on PVC pipes with radii of 55 mm and 80 mm. Frequency detuning, which is observed when simulated and measured results are compared for all four cases of the bending of Antenna V, is presented in Table 5.
PHOTO (COLOR): Simulated and measured reflection coefficients of Antenna V bent along the x plane on a PVC pipe with an 80 mm radius.
PHOTO (COLOR): Simulated and measured reflection coefficients of Antenna V bent along the x plane on a PVC pipe with a 55 mm radius.
PHOTO (COLOR): Simulated and measured reflection coefficients of Antenna V bent along the y plane on a PVC pipe with an 80 mm radius.
PHOTO (COLOR): Simulated and measured reflection coefficients of Antenna V bent along the y plane on a PVC pipe with a 55 mm radius.
Frequency detuning of Antenna V for bending in the x and y planes.
Resonant frequency Simulated/measured Frequency detuning (%) x plane y plane Case 1 Case 2 Case 3 Case 4 80 mm bending radius 55 mm bending radius 80 mm bending radius 55 mm bending radius 1780 MHz Simulated 6.74 6.74 3.37 2.24 Measured 4.49 6.74 4.49 1.12 2.4 GHz Simulated 0.83 2.50 2.50 2.50 Measured 0.00 0.83 2.50 0.00 3.46 GHz Simulated 1.15 1.73 1.15 0.57 Measured 1.15 0.00 1.15 0.57 5.26 GHz Simulated 6.84 6.84 7.98 7.22 Measured 7.60 7.60 8.36 7.60
For both simulated and measured results, the 1780 MHz band shifts to the right side of the spectrum for all four bending cases of Antenna V. Similarly, in the case of the simulated results, the 2.4 GHz band shifts to the right side of the spectrum for all four cases of bending of Antenna V along the x and y planes. The measured results described that the 2.4 GHz band shifts to the right side of the spectrum for Cases 2 and 3, but it does not shift for Cases 1 and 4.
The simulated results of the four bending cases illustrated that the 3.46 GHz band shifts to the left side for Cases 1, 3, and 4, whereas it shifts to the right side of the spectrum for Case 2. However, the measured results show that the 3.46 GHz band shifts to the left side for Cases 1 and 3, does not shift for Case 2, and shifts to the right side of the spectrum for Case 4.
The simulated as well as measured results show that the last 5.26 GHz band shifts to the left side of the spectrum for all four bending cases of Antenna V. In our proposed antenna, it could be easily seen that the antenna has strong currents on its ground planes, especially around the CPW feed line. The ground planes are actually a part of the proposed CPW-fed antenna. Taking this into account, it could be deduced that the distribution of the currents in the ground plane will change with the size and or/shape of the ground plane. As a result, the impedance and radiation performance of the antenna will change, similar to that in [
For bending along the x plane, the measured and simulated frequency detuning values of Case 2 are either equal to or higher than that of Case 1, except for the 3.46 GHz band, for the measured value. On the contrary, for bending along the y plane, the measured and simulated frequency detuning values of Case 4 are either equal to or less than that of Case 3.
Maximum frequency detuning is observed for Case 3 at the 5.26 GHz band. In the measured results of the 2.4 GHz and 3.46 GHz bands, no frequency shift is observed for Cases 1, 4, and 2, respectively. Minimum frequency detuning has been found at the 3.46 GHz band for Case 4.
Figure 22 illustrates the setup for two on-arm conditions of Antenna V during reflection coefficient measurement. The first condition is Antenna V on arm, without cloth, and the second condition is Antenna V on arm, with cloth [
PHOTO (COLOR): Setup for performance measurement of Antenna V for on-arm (a) without-cloth and (b) with-cloth conditions.
The measured reflection coefficients for the two on-arm conditions are compared with the measured reflection coefficient of the flat Antenna V in free space, and the results are presented in Figure 23. For both on-arm conditions, the 1780 MHz, 2.4 GHz, and 3.46 GHz bands shifted to the left side, whereas the 5.26 GHz band shifted to the right side of the spectrum. The reflection coefficient of all bands of Antenna V for the on-arm, without-cloth condition, decreases.
PHOTO (COLOR): Measured reflection coefficients for on-arm conditions of Antenna V.
For the on-arm, with-cloth condition, the reflection coefficient of the 1780 MHz and 5.26 GHz bands decreases, while that of 2.4 GHz band increases and that of 3.46 GHz band remains unchanged.
The frequency detuning of Antenna V for the two on-arm conditions is given in Table 6. The frequency detuning values of Antenna V for the on-arm, without-cloth condition are higher than that for the on-arm, with-cloth condition.
Frequency detuning of Antenna V for on-arm conditions.
On-arm condition Frequency detuning (%) 1780 MHz 2.4 GHz 3.46 GHz 5.26 GHz Without cloth 3.37 2.50 1.73 2.66 With cloth 2.24 1.66 1.15 1.90
Maximum frequency detuning is calculated at the 1780 MHz band for the without-cloth condition during on-arm performance measurements. Minimum frequency detuning is observed at the 3.46 GHz band for the with-cloth condition.
Finally, we compare the proposed antenna with other wearable antennas having single-band [
Comparison of the proposed antenna with existing literature.
Ref. Size (mm3) Frequency (GHz) BW (MHz/%) Gain/directivity (dB or dBi) Radiation efficiency (%) SAR (W/kg) [ 46×46×2.4 2.4 660 7.8 dBi — 0.0138 [ 100×100×3 2.4 157 2.0 dB 45 0.0500 [ 50×50×1 5.4 52.96% 9.67 dBi — — [ 40×40×0.6 5.7 — 6.49 dB 59 0.0043 [ 34.29×48×0.45 2.45 458 3.9 dB 85 — 5.8 590 4.6 dB 90 — [ 240×240×1 0.9 — 8.1 dBi 20.5 0.0011 1.8 — 7.4 dBi 10.3 0.0034 [ 24×32×0.305 5.2 4.07% 7.9 dB 90 0.0646 5.8 4.16% 8.2 dB 90 0.0258 [ 31×34×0.0508 2.46 648 1.68 dBi 91.6 — 5.48 1378 1.64 dBi 91.6 — [ π ×352×3 2.45 84 4.16 dBi 63.5 0.248 5.8 247 4.34 dBi 53 0.091 [ 105.5×80.8×0.015 0.9 — 2.0 dBi at all bands 41.6 — 1.9 — 75.8 — 2.45 — 65.6 — [ 40×40×0.8 1.55 — >1.6 dB at all bands >50 at all bands 0.78 1.95 — 0.62 2.1 — 0.58 [ 70×70×2 1.8 320 4.91 dBi 92.45 0.019 2.4 60 7.84 dBi 75.22 0.358 3.6 80 2.58 dBi 63.26 0.566 5.5 180 4.12 dBi 88.95 0.798 Proposed antenna 45.5×85×1 1.78 410 4.17 dBi 94.71 0.2293 2.40 260 3.81 dBi 93.42 0.3892 3.46 170 2.60 dBi 90.52 0.4640 5.26 520 2.74 dBi 83.97 0.2535
The gains of the dual-band wearable antennas in [
A novel wide CPW-fed multiband wearable monopole antenna and its design procedure have been proposed and successfully implemented. The antenna has a simple geometry and is easy to design on a polyester substrate. It has been shown that the proposed antenna with a wide CPW line is sufficient to cover the 1800 MHz GSM, 2.4/5.2 GHz WLAN, and 3.5 GHz WiMAX bands, resulting in a four-band operation. The antenna provides good radiation pattern characteristics and appreciable gain over each of the operating bands. The calculated SAR values at all the resonant frequencies of the wearable antenna are well below the acceptable limit of 2 W/kg, ensuring the safety of the user and viability of the proposed antenna for wearable applications. Further, the effect of bending in the x and y planes on antenna performance has been investigated and frequency detuning is discussed. Finally, the reflection coefficient characteristics has been measured for two on-arm conditions, with and without cloth, and the results are presented.
The data used to support the findings of this study are included within the article.
The authors declare that there is no conflict of interests regarding the publication of this paper.
We are thankful to I. K. Gujral Punjab Technical University, Jalandhar for providing the opportunity to do research and publish results.
By Danvir Mandal and S. S. Pattnaik