Design and Integration of Dual Band Textile Antenna with High Impedance Surface Muhammad Salman Khan, Shahid Bashir, Muhammad Asif K., Asaf K Dept. of Electrical Engineering University of Engineering and Technology Peshawar, Pakistan
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[email protected] Abstract— This paper describes self conceived design of new dual band wearable FIFA operating at Wifi bands (2.40 & 5.80GHz). Different radiating elements used in this indigenous design, outcomes dual mode resonance. The considered design approach has quasi omni-directional pattern that leads to significant back radiations resulting high value of Specific Absorption Rate (SAR), usually not desirable in wearable application. To overcome this issue, the antenna is integrated with dual band high impedance surface (HIS). This modular integration results in reduction of back radiations by 4-15dB at operating frequencies. Sequel to reduction in back radiations, SAR has also been reduced at the operating frequencies, while evaluating its design response for on-body environment. In addition to these improvements, antenna gain has been increased by 8-9dBi along with improvement in return loss by 23-27dB at the operating frequencies. CST Microwave Studio has been used for the design and simulations.
polyester taffeta textile material, that are used for radiations whereas non conductive fabrics like felt, fleece, jeans, silk, cotton etc are used as the substrate [1], Articles in [l]-[7] has demonstrated single band wearable antennas with acceptable performance. Some of these antennas have ground plane having and inherent advantage of having less interaction with human body when worn. Later some dual band wearable designs were presented in [8]-[ll] for mobile network applications. Recently integration of these antennas with EBG has been performed [11] in order to improve antenna performance in on-body environment.
We present a new dual band wearable PIFA in this paper that is incorporated with dual band high impedance surface (HIS) ground plane. Common clothing fabric felt is used for the design of operating frequencies at 2.40GHz and 5.80GHz. Index Terms—Component Wearable antenna, High impedance Because of lossy nature of human body, the performance of the textile antenna degrades when operated over it. Also it is Surface, Body area networks (BANs) desirable to reduce the back radiation toward the body on which it is worn. For this purpose incorporation of HIS to the antenna has been investigated. All the designs and simulations I. INTRODUCTION are performed in CST Microwave Studio. One of the significant and important branches of 4G Mobile / Portable communication is Body centric wireless This paper is categorized as follow. In section II the design communication. It has gained its popularity and importance of dual band wearable antenna has been explained along with confidently within the globe of personal area networks (PANs) its necessary parameters. Section m presents the incorporation and body area networks (BANs). Due to introduction of of a dual band EBG to the designed antenna along with its personal communication technology, wearable antennas have results after integration. Section IV deals with on-body received its importance and interest. It provides the impulse to performance of the antenna with and without HIS/EBG. In make cell phones smaller and simpler. There are a variety of section V a conclusion has been presented based on the consumer electronics integrated to clothing. results. A wearable or textile antenna can be defined as an antenna that is essentially part of our clothing. These are made of flexible material [l]-[5] in order to make it easy to be worn on human bodies. The general requirements of wearable antennas are that they should be light weight, inexpensive, have almost zero maintenance cost and easy to install. Fabric material is one of the important design elements that need special care in case of wearable antennas. The knowledge of electromagnetic properties like permittivity, permeability and loss tangent, of the fabric/textile material are very much important and should be known for the design purpose. There are conductive fabrics like Flectron, Zelt and pure copper
n. DESIGN OF DUAL BAND WEARABLE ANTENNA In case of body worn antennas, planar configurations are intended because of their ease in fabrication and installation on the human body. Because of this reason, a new dual band fabric based PIFA has been designed from a single band configuration presented in [11], Geometrical modification in the single band wearable PIFA resulted in dual resonance simultaneously at 2.40GHz and 5.80GHz thus covering WIFI bands. The structural design the antenna is given in Fig. 1
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Return loss versus frequency plot for the antenna is shown in Fig.2. Two resonances at 2.40GHz and 5.80GHz can be observed in the plot with good impedance match at these frequencies. At 2.40GHz the value of return loss is found to be -17.2dB and at 5.80GHz a 21.9dB is observed. These values indicate very good impedance match at both the operating frequencies.
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(b) Fig. 1 Geometry of Dual Band PIFA (a) Top View (b) Side View
There are two radiating elements that are responsible for the dual mode operation. One of the radiating elements is the longer one having length of 27.2 mm, and the second is the shorter with length of 9.8 mm. At the lower resonant mode (2.40GHz), the longer radiating element resonates and maximum current will flow through it. Similarly at the higher resonant mode (5.80GHz), the shorter radiating element resonates with maximum current flowing through it . A microstrip transmission line with characteristics impedance of 50 ohm is used for the feeding of the antenna that has length of 25 mm and width of 5mm. The left end of the antenna is connected to the ground with the help of a shorting pin which is for the design purpose. On the back side, a ground plane is used having a length of 25 mm and a width of 40 mm. This is necessary for the microstrip transmission line. The overall dimension of the design was calculated to be 35 x 40 x 1.1 mm which is smaller enough to be easily worn on bicep. Furthermore felt fabric is modeled as the substrate of the antenna with a thickness of 1.1 mm. It is one of the common fabrics available in the industry of textile having dielectric constant of 1.38 and loss tangent 0.02 [11], To model the metallic part of the antenna, conductive fabric Zelt is selected that has conductivity of 1 e + 006 S/m and thickness of 0.06 mm. CST Microwave studio is used for all the simulations and designs.
3 4 Frequency/GHz Fig. 2 Simulated Return Loss of the antenna
Bandwidth at 2.40GHz is 212.3MHz and at 5.80GHz it is 245.9 MHz which are large enough to be used for WLAN applications. Fig. 3 demonstrates the surface currents at both the operating frequencies. It can be seen that the smaller radiating element resonates at 5.80GHz whereas the larger one resonates at 2.40GHz because of maximum flow of currents. 2.40 GHz
5.80 GHz
Fig. 3 Distribution of Surface current at the operating frequencies
Parametric sturdy of the design has shown that the operating frequencies of the antenna can be adjusted by varying lengths of the radiating elements. The operating frequencies are inversely related to the lengths of the corresponding radiating element. Results for the higher resonant mode (5.80GHz) are presented to demonstrate this. By decreasing the length of the shorter radiating element, the higher resonant frequency will move toward the higher frequencies as can be seen in Fig. 4
3 4 Frequency/GHz
Fig. 4 Return Loss comparison for decreasing length of the shorter radiating element
Similar parametric analysis has been performed based on the separation in between the shorter and longer radiating arm. It has been observed that value of return loss, at 5.80GHz, can be improved by decreasing the separation between these radiating elements as can be observed in Fig. 5
(b)
Fig. 6 Simulated Radiation Pattern of the textile antenna (a) 2.40GHz (b) 5.80GHz
It can be seen that the pattern is quasi omni-directional in nature with significant back radiation. For the case of body worn antennas, these back radiations are not desirous as they can cause severe health problems to humans when worn. The gain of the antenna is found to be 1.84dB and 4.45dB at 2.40GHz and 5.80GHz respectively. All these parameter are summarized in Table I. Table I Summary of the Parameters Frequency
Parameter
3 4 Frequency/GHz
2.40GHz
5.80GHz
Return Loss
-17.2Db
-21.9Db
Bandwidth
212.6 MHz
245.9 MHz
Gain
1.84dB
4.45Db
Fig.5 Return loss comparison for decreasing separation between radiating elements
Finally the antenna radiation pattern is evaluated for both the operating frequencies in XZ-plane and YZ-plane. The results are shown in Fig.6.
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m . 1NTEDRATTON OF DUAL BAND WEARABLE ANTENNA AND HIS/EBG As previously mentioned that in case of on-body applications, there is a requirement of minimizing the backward flow of energy toward the human body on which it is worn. Consequently these high radiations toward the human body will result in high SAR, which is undesirable. These backward radiations need to be minimized for optimal operation of the design. To this purpose, a dual band HIS/EBG has been designed over felt fabric. The geometrical configuration of the EBG has been demonstrated in Fig. 7. The design is quite simple and consists of square patches (mushroom type EBG) on which circular slots are etched in order to have two resonances near 2.40GHz and 5.80GHz. Here the outer square patch was 16x16 mm and the inner circular slot was etched at a radius of 9.2 mm. A 3x3 array of the dual band EBG was integrated with the antenna as shown in Fig. 7.
Here we can observe that return loss values have been improved to -23dB and -27dB at 2.40GHz and 5.80GHz respectively. After that, the radiation pattern of the antenna/EBG combination has been evaluated at both the frequencies and is shown in Fig. 10. The antenna alone has a dipole like pattern i.e. Omni-directional. Due to that it will significantly radiate into the body on which it is worn. Placing EBG blow the antenna lowers the back radiation by 4dB at 2.40GHz. Similarly the back radiations are reduced by 15dB at 5.80GHz by making it feasible for on-body operation. It is also found that the gain of the antenna has been increased to 8.3dB and 9dB at 2.40GHz and 5.80GHz. This is because the back radiations are now efficiently radiating in the forward direction because of inclusion of HIS/EBG ground plane. Fig.7 Integration of Dual Band PIFA with EBG
Dual Band W-PIFA Dual bard W-PIFA on EBG
The In phase reflection response for the unit cell of the designed HIS/EBG is presented in Fig. 8. This can be observed that the zero phase points for the designed HIS lies at 2.19GHz and 6GHz which are near to the operating frequencies of the dual band antenna.
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3 4 Frequency/GHz Fig.8 Reflection phase response for Unit Cell of HIS/EBG
The simulated return loss for antenna in combination with HIS/EBG is presented in Fig.9 and show good agreement with that of the antenna without EBG.
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Fig 11 Return Loss dependency with antenna-body separation
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Fig. 10 Simulated Radiation Pattern of Dual Band Wearable Antenna with EBG (a) 2.40GHz (b) 2.40GHz (c) 5.80GHz (d) 5.80GHz
IV. ON-BODY PERFORMANCE Because of lossy nature of human body, the performance of the textile antenna degrades when operated over it. To check the performance of the textile antenna near human body, a four layer human body phantom has been modeled in CST Microwave Studio. These layers consist of skin, fat, muscle and bone. The textile antenna is then placed over phantom and was excited for evaluation of its performance. Observations have shown that the return loss was strongly dependent on the separation between antenna and the phantom. Return loss for different values of separation is presented in Fig 11 which shows that there is a severe variation in resonant frequency of the antenna because of interaction with body. Afterwards the antenna along with HIS/EBG is placed over body phantom and excited for evaluation of performance. It was observed that the return loss showed stability and now
From Fig 12 it can be observed that the variation of resonant frequency due to the interaction/coupling of human body is negligible for the case of antenna along with HIS/EBG.
Fig 12 Comparison of return loss for antenna over body phantom with and without HIS/EBG
Consequently specific absorption rate was evaluated for the antenna alone and in combination with HIS. It was observed that SAR was 11 W/Kg at 2.40GHz and 2.3W/Kg at 5.80GHz when the antenna was operated without HIS/EBG. When the antenna along with HIS is operated near human body phantom, reduced values of SAR were observed being 0.68W/Kg at 2.40GHz and 0.069W/Kg at 5.80GHz hence demonstrates the benefit of EBG/HIS based antenna. Table I summarizes SAR values for both the cases. Table I Simulated SAR Comparison SAR value for 10 g Tissue (W/Kg) Frequency
2.40GHz
5.80GHz
Antenna Alone
11
2.3
Antenna+ HIS
0.68
0.069
V . CONCLUSION
This paper demonstrates the design of a new dual band wearable PIFA. Commonly available textile material felt is utilized for the design of the antenna to operate at 2.40GHz and 5.80GHz Wifi bands. The antenna demonstrates good impedance match at the operating frequencies having return loss value of -17.2dB and -21.9dB at 2.40GHz & 5.80GHz respectively. The gain of the antenna is found to be 1.84dB and 4.45dB at these frequencies. The operating bandwidths of the antenna are enough to be used in applications like Wifi. It is further integrated with a dual band high impedance surface in order to reduce the backward radiations. This makes it safe to be worn on human body. It has been observed that the return loss of the HIS based antenna has improved to 23dB and -27dB at 2.40GHz and 5.80GHz respectively. Similarly backward radiation has reduced by 4 to 15 dB resulting in the improvement of gain in forward direction. Improved values of gain are 8.3dB and 9dB at 2.40GHz and 5.80GHz respectively. After that the antenna was operated near human body alone and in combination with HIS/EBG. For the antenna alone case, resonant frequency was strongly dependent on the separation between antenna and the body phantom. Large values of SAR were also observed at both the operating frequencies being 11 W/Kg at 2.40GHz and 2.3W/Kg at 5.80GHz. When the antenna along with HIS was operated near body phantom, resonant frequencies showed stability and there was almost no more variation in it. SAR values were also reduced being 0.68W/Kg at 2.40GHz and 0.069W/Kg at 5.80GHz. By summarizing this all, incorporation of HIS/EBG structures to wearable antennas brings improvements in its performance and makes it feasible for on-body operation. REFERENCES
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