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In this paper, two different methods were used for investigating the RF characteristics of three types of textile materials. Goch, Jeans and Leather substrates were studied. A microstrip ring resonator method and DAK (Dielectric Assessment Kit) method were used. Bluetooth antennas were designed and fabricated using these substrates. The results were compared for the two methods. The bending effect of these antennas on its impedance characteristics due to human body movements was also studied. Finally, all antennas were simulated by CST simulator version 2016, fabricated using folded cupper and measured by Agilent 8719ES VNA. The measured results agree well with the simulated results.

In recent years, flexible and textile antennas have taken a lot of attention due to their application in wearable systems. Nowadays, research efforts focus on the development of novel textiles to be used as antenna substrates. Wearable antennas need to be integrated within everyday clothing, be low profile, and be hidden as much as possible [

In this paper, two different methods for measuring the dielectric constant and loss tangent were used: the first using a microstrip ring resonator method and the second using DAK (Dielectric Assessment Kit). Therefore, the second has been used to confirm the results determined using the first. The measurement of dielectric constant and loss tangent of three different textile materials were performed: Goch, Jeans and Leather. First the microstrip ring resonator is a very popular technique for measuring the dielectric constant, and loss tangent because of its easy fabrication, planar structure, compact size and high quality factor, so there are few radiation losses of ring resonator and more accurate results [

By fabricating this ring model above using the three different textile materials: Goch, Jeans and Leather and obtaining the S_{21} for each other, the peak in the S_{21} was recorded around each resonance and the n^{th} resonance occurs at [

F n = n c 2 π r ε r (1)

where r is the mean radius; c is the speed of light in a vacuum and ε_{r} is the required dielectric constant.

Also, the insertion loss (S_{21} (dB)) is introduced as [

I L = 20 log ( 1 − Q L Q u ) (2)

where: Q_{L} is the loaded quality factor, and Q_{u} is the unloaded quality factor.

(Hint: if Q_{L} and Q_{U} are similar, this means that this material is very lossy, so it must be Q_{U} ? Q_{L} to avoid excessive losses).

Also, the loaded quality factor can be measured from S_{21} curve as [

Q L = f 0 Δ f (3)

where: f_{0}: is the resonance frequency, and ∆f: is the difference between high freq. and low freq. around f_{0} ( Δ f = f h − f l ).

To calculate the loss tangent of any material firstly must be determined unloaded quality factor (Q_{u}) from Equation (2) and then applied it in Equation (4) [

1 Q u = 1 Q c + 1 Q d (4)

where: Q_{d} is the dielectric quality factor, and Q_{c} is conduction quality factor.

Also the conduction quality factor can be calculated by [

Q c = h f 0 μ 0 Π σ c (5)

where: h is the thickness of substrate, f_{0} is the resonance frequency, µ_{0} is the permeability of free space, and σ_{c} is the conductivity of conduction walls.

From Equation (4), the dielectric quality factor (Q_{d}) can be determined by subtracting the conductor quality factor (Q_{c}) from the unloaded quality factor (Q_{u}), and then the loss tangent can be obtained as [

tan δ = 1 Q d (6)

The second method is The Dielectric Assessment Kit (DAK) equipment which offers high-precision dielectric parameter measurements (permittivity, permeability, conductivity, loss tangent) over the very broad frequency range from 10 MHz to 67 GHz for applications in the electronic, chemical, food, and medical industries [

In Sections 2 and 3, based on the results of dielectric constant and loss tangent of three different textile materials: Goch, Jeans and Leather, we designed and fabricated three wearable Bluetooth antennas to compare between three textile materials in Section 4. However, it’s not possible to keep wearable antenna position in flat at all the time due to human body movement [

In order to design any microstrip antenna on textile substrate the dielectric constant and loss tangent of this substrate must be known. For this purpose, we used the microstrip ring resonator method to measure the dielectric constant and loss tangent of three different textile materials: Goch, Jeans and Leather. The thickness of the different substrate materials are measured by using digital vernier gauge and are mentioned in _{21} for the ring resonator for three different substrates is shown in _{r} and loss tangent tanδ) are tabulated in

Substrate Thickness | Material | ||
---|---|---|---|

Goch | Jeans | Leather | |

Thickness (mm) | 1.2 | 0.6 | 1.3 |

Ring Dimensions | Length | |||
---|---|---|---|---|

Mean Radius (R_{m}) | inset feed Width (W_{f}) | inset feed Length (L_{f}) | Gap (∆) | |

Value (mm) | 8.5 | 3 | 14 | 1 |

Ring Resonator Results | Dielectric Properties Characterization | ||||
---|---|---|---|---|---|

Mode | Resonant Frequency (GHz) | Insertion loss (S_{21} (dB)) | dielectric constant (ε_{r}) | loss tangent (tanδ) | |

Goch Substrate | n = 1 | 5.1 | −11.8 | 1.266 | 0.029 |

n = 2 | 10.21 | −14.6 | 1.25 | 0.04 | |

Jeans Substrate | n = 1 | 4.26 | −35.5 | 1.73 | 0.077 |

n = 2 | 8.89 | −36.9 | 1.69 | 0.073 | |

Leather Substrate | n = 1 | 4.2 | −16.6 | 1.788 | 0.039 |

n = 2 | 8.81 | −13.9 | 1.72 | 0.044 |

DAK Results | Dielectric Properties Characterization | |
---|---|---|

dielectric constant (ε_{r}) | loss tangent (tanδ) | |

Goch Substrate | 1.32 | 0.034 |

Jeans Substrate | 1.78 | 0.085 |

Leather Substrate | 1.79 | 0.042 |

Another method for measuring the dielectric constant and loss tangent of the same three textile materials: Goch, Jeans and Leather, is called the dielectric assessment kit (DAK) equipment. This method is used to confirm the results which obtaining from the ring resonator method. The measured results of the DAK equipment for dielectric extraction (dielectric constant ε and loss tangent tanδ) are tabulated in

From comparing the obtained results using the two methods, it can be observed that both of them estimate the values for Goch, Jeans and Leather, but there are some slight differences between them, this is an inherent thing.

A three Bluetooth microstrip patch antennas using three different types of textile materials as substrates are designed. These substrates are Goch, Jeans and Leather which already have been identified their dielectric characteristics (dielectric constant ε and loss tangent tanδ) in sections 2 and 3.

The simulated and measured S_{11} for this antenna are plotted in

Antenna Dimensions | Length | |||||||
---|---|---|---|---|---|---|---|---|

Width (W) | Length (L) | Ground Width (W_{g}) | Ground Length (L_{g}) | Inset feed Width (W_{f}) | Inset feed Length (L_{f}) | Stub Length (y) | Gap (g) | |

Goch | 30 | 52 | 50 | 72 | 2 | 10 | 10 | 1 |

Jeans | 30 | 45 | 50 | 65 | 3 | 10 | 5 | 1 |

Leather | 30 | 43 | 50 | 63 | 3 | 10 | 10 | 1 |

some problems during manufacturing. The simulation performances of these antennas are also mentioned in

From _{11} curves has shifted up relative to the simulated one but this shift is more in the Goch and Leather substrates than Jeans substrate. In

Antenna Performance | Simulated Results | ||
---|---|---|---|

S_{11} (dB) | Realized Gain (dB) | Efficiency % | |

Ant. with Goch | −15.6 | 4.54 | 80.3 |

Ant. with Jeans | −18.9 | 1.82 | 53 |

Ant. with Leather | −19.87 | 4.901 | 82.1 |

leather substrate, due to the two substrate material with similar loss tangent, dielectric constant and thickness.

To investigate the effect of antenna bending on its impedance characteristics for three Bluetooth patch antennas with three different textile materials: Goch, Jeans and Leather, this study is carried out by bending each textile antenna around curved surfaces with diameters of 70 mm and 150 mm. These dimensions are typical for human body (e.g., arm, leg, and shoulder). The results for wearable patch antennas for flat position and for both bending diameters are shown in

Two methods for fabric characterization were presented in this paper: a microstrip ring resonator method and DAC (Dielectric Assessment Kit) method. They used for measuring the dielectric constant (ε_{r}) and loss tangent (tanδ) of three different textile materials: Goch, Jeans and Leather where the second has been used to confirm the results determined using the first. Based on these data, three Bluetooth rectangular patch wearable antennas were designed and fabricated on these materials. Comparing between the simulated and measured results and also keeping in mind the effect of bending on each textile antenna, we found that the Bluetooth patch wearable antenna with Leather textile material is easier to place within clothing where it has higher water resistance than the other. In addition, the Goch textile material is fluffy and hairy. When the patch is pasted on

it, there is air layer between patch and substrate to make it press and then return to the original thickness, so the results are not more accurate. Also the jeans textile material is very lossy material because the loaded and unloaded quality factors are very close to each other according to insertion loss results. Therefore, the leather textile material is the best choice as a substrate for wearable microstrip antennas.

Ahmed, M.I., Ahmed, M.F. and Shaalan, A.A. (2017) Investigation and Comparison of 2.4 GHz Wearable Antennas on Three Textile Substrates and Its Performance Characteristics. Open Journal of Antennas and Propagation, 5, 110-120. https://doi.org/10.4236/ojapr.2017.53009