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Wireless Personal Communications

, Volume 100, Issue 4, pp 1753–1764 | Cite as

Multiband Flexible Antenna for Wearable Personal Communications

  • Miguel Ángel Bolaños-Torres
  • Richard Torrealba-Meléndez
  • Jesús Manuel Muñoz-Pacheco
  • Luz del Carmen Goméz-Pavón
  • Edna Iliana Tamariz-Flores
Article
  • 189 Downloads

Abstract

In this paper the design and experimental characterization of a coplanar waveguide-feed planar inverted F antenna for wearable personal communications are presented. The proposed antenna is designed to operate at GSM 1800 MHz and ISM 2450 MHz bands. Experimental measurements of the reflection coefficient are achieved by considering two scenarios: the antenna under bending conditions and when it is placed on different textile materials for weareable applications. Moreover the measured radiation patterns and peak gains are obtained in bending and no bending conditions. The results demonstrate that the reflection coefficient is maintained below − 10 dB, the bandwidth is approximately to 10%, and the radiation properties are suitable when the bend is less than or equal to 80\(^{\circ }\).

Keywords

Antennas Flexible substrate Weareable Personal communications 

1 Introduction

In recent years, due to the human need to stay connected all the time through portable devices, send high speed data and optimize the use of the limited bandwidth, it has been generated an exponential growth in technologies and standards of wireless personal communications such as Zigbee, Ultra-Wide Band, Bluetooth, LTE, LTE-A, and so on [1, 2, 3, 4, 5, 6, 7, 8]. The growth in demand for these technologies and standards has generated a challenge for designing devices that operate with different standards and frequencies ranges. On the other hand, wearable electronics is an emerging technology widely used to wireless personal communications [9, 10, 11]. Therefore, the increased use of wearable electronics, seem to be a great research challenge focused on flexible substrates, electronic devices and communications systems operating under bending and twisting conditions.

Besides, in wireless communications systems an important part to obtain an optimum performance is to design antennas that maintain their radiation properties and operating frequency in nonideal conditions, i.e., under bend, twisting and stretching. In this context, the flexible antennas have emerged as a solution. Additionally, in real-world applications, those antennas are being placed into diverse materials such as textile materials, clothes and stickers. Applications of flexible antennas are vast, ranging from wireless communications gadgets, radio frequency identification devices, wearable electronics and smart clothing to new approaches of wireless sensing, e.g., microwave imaging for health care, monitoring of civil construction, and material characterization [12, 13, 14]. In literature, several flexible substrate antennas have been proposed [15, 16, 17, 18, 19, 20]. Among them, the planar inverted F antenna (PIFA) is widely used in industry and research because of its compact size, light weight and, straightforward design and manufacturing [21]. In addition, a multiband operation, wide bandwidth and high gain can be obtained [22, 23]. PIFA is feed by either microstrip or coplanar waveguide (CPW) [24, 25, 26].

In this work, the design and experimental characterization of a coplanar waveguide-feed PIFA with multiband operation, GSM 1800 MHz and ISM 2450 MHz, for wearable personal communications are presented. The antenna characterization is performed by considering the measured reflection coefficient, radiation pattern and peak gain as performance indexes. Two cases are analyzed. First, different bending grades are considered. Then, the antenna is placed on four textile materials, such as cotton-polyester, polyester, polyester-acrylic, polyamide-polyester to study the wereable applications of a t-shirt, jacket, backpack and bag, respectively. The experimental results fulfill the communication standards for required bands.

2 CPW-PIFA Design

The planar inverted F antenna is designed by selecting a coplanar waveguide to feed the antenna resonators because a coplanar waveguide has a single-sided nature [27], as shown in Fig. 1. Since the via holes and wraparound are not required for grounding components, the via hole to ground in a microstrip PIFA can be suppressed. In Fig. 1, a is the signal plane width and b is the distance between ground planes. The characteristic impedance is determined by the ratio a / b, therefore a size reduction is possible without limitations, theoretically [27]. The resulting CPW-PIFA is composed of three coplanar elements; a feeding strip, transverse radiating patch with two stubs to have a dual band operation, and shortening strip. To get an input impedance \(Z_0\)=50 Ohms at GSM 1800 MHz and ISM 2450 MHz bands, the dimension of CPW feed is calculated by [28]:
$$\begin{aligned}&Z_0=\frac{30\pi K(k_{t}')}{\sqrt{\varepsilon _{eff,t}}K(k_t)}, \end{aligned}$$
(1)
$$\begin{aligned}&\varepsilon _{eff,t}=\varepsilon _{eff}-\frac{\varepsilon _{eff}-1}{\frac{(b-a)/2}{0.7t}\frac{K(k)}{K(k')}+1}, \end{aligned}$$
(2)
$$\begin{aligned}&k_{t}=a_{t}/b_{t}, k=a/b, \end{aligned}$$
(3)
where h is substrate thickness, t is metal thickness, \(\varepsilon _{eff}\) is effective permittivity, \(K(\cdot )\) are elliptic integrals, \(k'\) and \(k_{t}'\) are the complementary modules of k and \(k_{t}\), and the subscript t is related to the effect of metal thickness. As a result, CPW-PIFA is designed and fabricated on a 0.1mm thick substrate (Ultralam 3850, Rogers Corporation, USA), with relative permittivity \(\varepsilon _r\)=2.9, loss tangent \(\tan \delta\)= 0.002 and a copper cladding t= 18\(\mu\)m. The length, width, and separation distances of the stubs for the radiation patch and shorting pin are determined by a parametric analysis to obtain a dual band operation. The designed antenna, given in Fig. 2a, has the following dimensions: L1 \(=\) 35, A1 \(=\) 2, L2 \(=\) 40, A2 \(=\) 1.5, F1 \(=\) 3.8, F2 \(=\) 2, R \(=\) 0.2, D1 \(=\) 4, D2 \(=\) 8, G1 \(=\) 10, G2 \(=\) 5, T1 \(=\) 18, T2 \(=\)  5, T3 \(=\) 2 and C \(=\) 1.5 (dimensions in millimeters). The proposed antenna is shown in Fig. 2b. Besides, the CPW-PIFA can be connected using a SMA connector, which is considered in the simulations.
Fig. 1

Transversal view of a coplanar waveguide

Fig. 2

a CPW-PIFA geometry, and b the fabricated antenna

3 Experimental Characterization of CPW-PIFA

In this section, the experimental characterization of CPW-PIFA is performed and the results are compared with the simulation results. For all reflection coefficient (S11) results , the Vector Network Analyzer (Vector Start, Anritsu, USA) and HFSS software (version 13, ANSYS, USA) were used for the experimental measurements and simulations, respectively. The first scenario focuses on the CPW-PIFA in a straight configuration and without interaction with textile materials. The obtained results are shown in Fig. 3. In both frequencies, the reflection coefficient (S11) is below − 10 dB. It means a good impedance matching. In Fig 3, we observed a slight shift in the operating frequencies. The 1800 MHz band has shifts of 40 and 20 MHz for simulations and measurements, respectively. Similarly, shifts of 20 and 30 MHz are perceived for 2450 MHz band. Besides, the fractional bandwidth between simulations and experimental measurements are in accordance as demonstrated in Table  1. It is important to remark that LTE 1700 MHz band can also be covered by the antenna.
Fig. 3

Experimental reflection coefficient of CPW-PIFA at GSM 1800 MHz and ISM 2450 MHz bands

Table 1

Fractional bandwidth

Frequency

1800 MHz (%)

2450 MHz (%)

Simulated

13

8.13

Measured

11

10.7

In addition, the current distribution of CPW-PIFA is simulated for both bands. The normalized current distribution for 1800 MHz and 2450 MHz are given in Fig. 4a, b respectively. These simulations show that the current blue distribution is associated with the proper resonator.
Fig. 4

Current distribution for a 1800 MHz, b 2450 MHz

To complete the characterization of the antenna in straight configuration, the radiation patterns and peak gains are simulated and measured. The radiation patterns were simulated in the xz (E) and yz (H) planes. For the measured radiation patterns, a standard log periodic directional antenna (HyperLOG 7060, AARONIA) was used as reference. In both operating frequencies, the simulated and measured patterns are in concordance as shown in Fig. 5. The xz plane radiation patterns, for 1800 MHz, seem to be omni-directional while in the yz plane present a bidirectional behavior. For 2450 MHz, a bidirectional radiation is observed in both planes. Moreover, the peak gains are measured employing the three-antenna method, i.e., we use two CPW-PIFA and the aforementioned standard antenna. The peak gains are in agreement in all cases as given in Table 2. As a result, the correct operation of CPW-PIFA in straight configuration is demonstrated.
Fig. 5

Measured and simulated radiation patterns for the CPW-PIFA without bent for a 1800 MHz and b 2450 MHz

Table 2

Peak gains of CPW-PIFA without bending

Frequency

1800 MHz (dB)

2450 MHz (dB)

Simulated

2.5

3

Measured

2.4

2.6

3.1 Antenna Performance Under Bending Conditions

One of the most important requirements of personal communications systems for wearable applications relies on antennas which be bended in some degree. Nevertheless, the antenna must preserve a suitable performance. In this section, the performance of CPW-PIFA under bending conditions is analyzed. By considering three bending conditions, 40\(^{\circ }\), 80\(^{\circ }\), and 100\(^{\circ }\), the reflection coefficient is experimentally determined. The set-up for this analysis is sketched in Fig. 6. The measured reflection coefficients under bending conditions and without bend are compared in Fig. 7. From bending conditions scenario, it can be noted that the reflection coefficient, S11, is under − 10 dB at operating frequencies, altought there is a small shift in central frequency. In 1800 MHz band, we have a central frequency of 1810 MHz and Fractional Bandwidth (FBW) of 12.51% for 40\(^{\circ }\), a central frequency of 1730 MHz and FBW of 13.29% for 80\(^{\circ }\), and a central frequency of 1750 MHz and FBW of 10.85% for 100\(^{\circ }\). Similarly, in 2450 MHz band we have the following results: a central frequency of 2400 MHz and FBW of 7.11% for 40\(^{\circ }\), a central frequency of 2510 MHz and FBW of 12.44% for 80\(^{\circ }\), and a central frequency of 2460 MHz and FBW of 8.47% for 100\(^{\circ }\). With these results, we show experimentally a proper impedance matching under different bending conditions of the proposed CPW-PIFA.

Furthermore, we measure the radiation patterns for the three bending conditions and compared them with the radiation patterns without bend. Figures 8 and 9 show the comparison of the radiation patterns for 1800 and 2450 MHz, respectively. From the figures, the radiation patterns for 40\(^{\circ }\) and 80\(^{\circ }\) change slightly with respect to the radiation pattern without bend. In these cases, the CPW-PIFA preserves its radiation properties in the operating frequencies, therefore is suitable for applications in wireless personal communications. On the other hand, the 100\(^{\circ }\) degrades significantly the radiation patterns in both frequencies. Previous results are confirmed by peak gains given in Table 3. The gain decreases slightly for bending condition of 40\(^{\circ }\) compared to the gain in straight configuration; and the gain is quite close to the ideal gain for 80\(^{\circ }\). As a conclusion the performance of CPW-PIFA is adequate up to 80\(^{\circ }\). The peak gain for 100\(^{\circ }\) bending condition is negative.
Fig. 6

Set-up to analyze different bending conditions on CPW-PIFA

Fig. 7

Experimental reflection coefficient of CPW-PIFA at 1800 and 2450 MHz bands by considering, \(0^{\circ }\), \(40^{\circ }\), \(80^{\circ }\), and \(100^{\circ }\) of bending (all cases present S11 \(=-\) 10 dB)

Fig. 8

Measured radiation patterns for CPW-PIFA under bending conditions for 1800 MHz: a plane xz and b plane yz

Fig. 9

Measured radiation patterns for CPW-PIFA under bending conditions for 2450 MHz: a plane xz and b plane yz

Table 3

Peak gains under bending conditions

Bending condition

1800 MHz (dB)

2450 MHz (dB)

0\(^{\circ }\)

2.4

2.6

40\(^{\circ }\)

1.5

1.8

80\(^{\circ }\)

2

2.6

100\(^{\circ }\)

− 1.8

− 1

3.2 Wearable Test for CPW-PIFA

From real engineering applications point of view, flexible antennas can be applied in technological trends such as weareables and internet of things (IoT). A wereable is a thing or device, like clothing, that includes electronic technology to be connected to internet or other things or devices. In that framework, the proposed CPW-PIFA was placed on diverse textile materials to analyze its performance. The reflection coefficient is measured when antenna is used as a wereable. Four textile materials, such as, cotton-polyester, polyester, polyester-acrylic, and polyamide-polyester associated with wereable applications of a t-shirt, jacket, backpack and bag, respectively, are herein considered. The experimental data were collected with the wereable on a human body. The reflection coefficient for the wearable applications is shown in Fig. 10. These results show a proper impedance matching and a S11 paremeter below − 10 dB. The results are summarized in Table 4 for GSM 1800 MHz and ISM 2450 MHz bands, respectively.
Fig. 10

Experimental reflection coeficient measured on t-shirt (cotton-polyester), jacket (polyester), backpack (polyester-acrylic), and bag (polyamide-polyester), respectively

Table 4

Fractional bandwidth and central frequency for wearable test

 

1800 MHz

2450 MHz

Material

FC (MHz)

FBW (%)

FC (MHz)

FBW (%)

Cotton-polyester

1820

14.83

2380

13.86

Polyester

1800

16.11

2350

11.96

Polyester-acrylic

1820

7.69

2350

5.39

Poliyamide-polyester

1820

12.63

2300

11.76

Finally, the measured peak gains when the CPW-PIFA is placed over the four textile materials are shown in Table 5. For all materials the gain is still suitable for the operation frequencies. There is some attenuation respect to the straight configuration which could be caused by the interaction of the radiation field with the material.
Table 5

Peak gains for CPW-PIFA over textile materials

Material

1800 MHz (dB)

2450 MHz (dB)

Cotton-polyester

1.2

1

Polyester

2

1.2

Polyester-Acrylic

1.7

1.5

Poliyamide-polyester

2

1.25

4 Conclusions

A flexible CPW-PIFA has been presented for wearable personal communications at GSM 1800 MHz and ISM 2450 MHz bands. The performance of the antenna has been experimentally tested by measuring its reflection coefficient and peak gains in two scenarios. First, different bending grades were considered. Latter, the antenna has been placed on four textile materials. In both cases the antenna showed an optimum behavior. In addition, the radiation patterns were measured for bending conditions. These radiation patterns showed that the antenna mantains its radiation properties for bendings less than or equal to 80\(^{\circ }\). Also, the antenna presented a broad bandwidth in the required bands, and therefore, the antenna can also cover LTE band (1700 MHz). Experimental results agree with simulations and fulfill communication standards for the required bands. Further, the proposed flexible antenna can be useful to wearable personal applications.

Notes

Acknowledgements

This work was partially supported by projects Cuerpo Académico BUAP-CA-276, and VIEP-BUAP 2017. We also thanks to the Laboratory of Characterization of Systems Based on Microwaves at FCE-BUAP where all experimental characterizations were carried out. J. M. Munoz-Pacheco acknowledges CONACYT for the financial support (no. 258880: Proyecto Apoyado por el Fondo Sectorial de Investigación para la Educación).

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Copyright information

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Authors and Affiliations

  1. 1.Faculty of Electronics SciencesBenemérita Universidad Autónoma de PueblaPueblaMexico
  2. 2.Faculty of Computational SciencesBenemérita Universidad Autónoma de PueblaPueblaMexico

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