DESIGN AND IMPLEMENTATION OF TEXTILE ANTENNA FOR WLAN AND WBAN APPLICATION
Dr.Mrs.S. Raju1 Roshni S Nair2
1 Professor and Head, Department of ECE, Thiagarajar College of Engineering Madurai, India, [email protected]
2 PG Scholar, Department of ECE, Thiagarajar College of Engineering Madurai, India, [email protected]
Abstract'In this paper an antenna with circular polarization for WBAN in ISM band applications is designed and made on textile substrate at 2.45 GHz. The circular polarization provides better power reception in whatever the direction body worn antenna turns. For achieving the circular polarization a rectangular slot of 450 at the center of the circular patch is used. In ground plane a rectangular cross slot is introduced for improving the bandwidth. The simulation result shows that the antenna has an impedance bandwidth of 110 MHz and the gain of the antenna is 3.6 dB. The designed antenna is fabricated on the textile substrate where copper tape is used as the conductive material for the patch and the ground plane. The measured operating frequency range of antenna spans from 2.40 GHz to 2.51 GHz having gain of 3.6 dB at 2.45 GHz. The antenna is tested by bending it above a foam sheet and then analyzed the effects on return loss, axial ratio, bandwidth, and radiation characteristics in order to account for its usefulness in the WBAN applications
Index Terms'Axial ratio, Polarization, Microstrip, WBAN
Miniaturization of electronics devices paved the way for creation of a wide range of devices that can be carried by users in their pockets or in some cases attached to their bodies. In recent years the development of wearable computer systems has been rapid. They are becoming more and more light weight and quite soon this computing systems will be integrated into daily clothing to create a so called Body Area Network (BAN) . To reach this featherweight level requires optimization of every single part and subpart of the wearable system. A body worn wearable system consists of electronic devices normally situated on or in close proximity to the human body.
Body Area Network (BAN) which is an expansion of Personal Area Networks (PANs) enables different devices to communicate with each other by placing them on or near to human body. The applications of BAN include tracking, mobile computing, measuring run time physiological changes and navigation . One can also continuously monitoring the changes in human body it is possible to keep the record of patient's health. The frequency bands used in such purposes are Wireless Body Area Network application band and Industrial, Scientific and Medical (ISM) band (2.40 GHz to 2.50 GHz). Wearable antennas are used as a transceiver in WBAN systems to send and receive the data or information. These antennas have to be flexible so that they do not hinder the movements of human body. The BAN antenna has to be a planar structure on a flexible materials . The permittivity and the thickness of the substrate determine the bandwidth and the efficiency of a planar antenna. Ordinary textile fabrics have been used as substrates for these wearable antennas. Textiles present a very low dielectric constant which increases the impedance bandwidth of the antenna and reduces the surface wave losses. However, their electromagnetic properties are varying because of constant exchange of water molecules with the surroundings. In addition, textile fabrics are porous, compressible materials and anisotropic whose density and thickness might change with low pressures. So it is important to know how these characteristics that influence the behavior of the antenna in order to minimize unwanted effects . Due to the constant motion of human body, it is difficult to align the polarization of the transceiver nodes for better power reception. The need for continuously aligning two nodes can be eliminated by circular polarization (CP)  . This paper presents the design and development of textile antennas for WBAN applications.
II. DESIGN METHODOLOGY OF TEXTILE ANTENNA
To design the wearable antenna selection of the substrate fabric is the first step. Jeans material has been chosen as substrate and copper tape for antenna and feed part. The proposed antenna structure is shown in Figure 1 a and b. The circular patch resonates at 2.45 GHz, while the rectangular slot 's' in centre of antenna patch is used to change the path of current and generate two modes with equal amplitude and phase difference of 90'' for achieving circular polarization . The angle between the slot and the feed line is set to 45''. It also have cross slot 'gs' in the ground plane for increasing bandwidth.
Fig.1Antenna (a). Front view (b) back view of patch antenna
The antenna is fed by the transmission line having impedance of 50 '' operating at 2.45 GHz. Parameters of antenna is calculated according to design frequency and permittivity and thickness of the selected materials .
The actual radius of the patch is given by
By considering the fringing effect the effective radius of patch is given by
The resonant frequency for the dominant TMz110is given by
where v0 is the free space speed of light.
Partial and slotted ground plane techniques are used to increase bandwidth of micro-strip patch antennas. The slotted ground will act as capacitor, which in turn lowers the Q factor and increases the bandwidth. By using cross slot 'gs" in the ground plane will further lowers the Q factor of the antenna and there by increases the bandwidth without changing the size of the radiating patch. It has very small effect on the resonant frequency due to the small size of the slot 'gs" because of the increase of the backward radiation, but leakage radiation from slot is less.
III SIMULATION OF WEARABLE ANTENNA
The antenna is designed and simulated in CST microwave studio software with jeans substrate having a thickness of 0.6 mm with dielectric constant (''r) 1.7 and loss tangent ('') 0.03.
Fig.2 Antenna design in CST
The real time applications of the wearable antenna operate near human body vicinity. Water constitutes two-thirds of human body and it is attributed as polar in nature, the antenna property changes in the vicinity of human body due to the polarization of water molecules in the presence of electromagnetic radiations. This phenomenon is known as dielectric loading. The electrical properties of human body change with frequency. Table 1 tabulates the properties of different human body layers at 2.45 GHz . All the commonly used antenna parameters, such as resonant frequency, bandwidth, radiation pattern, and particularly efficiency are likely to change radically as an antenna moves closer to the body and therefore a free space design may only be a rough approximation of antenna suitability.
The Properties of human body at 2.45 Ghz is shown in table.1.
Phantom layers Parameters
Relative permittivity (''r) Conductivity ('' in S/m) Loss tangent (tan'')
skin 35.114 3.717 0.328
fat 4.955 0.293 0.183
muscle 48.485 4.962 0.317
bone 20.76 1.07 0.241
In order to account for the above the designed antenna is also analyzed with human body model in the simulation with layers skin, fat, muscle, bone. Antenna is separated from skin by a gap of 2 mm.
Fig.3 Antenna Simulation with human phantom model
IV RESULTS AND DISCUSSIONS
Effect of Slot 's' on Return Loss
The Figure 4 shows the variation in return loss with parameter 's', where 's' is the slot in patch. It can be seen that increasing the length of rectangular slot 's' beyond certain point (12 mm), increases the return loss but shifts the frequency to lower bands. Further increasing 's' frequency band splits into two bands with decreased return loss. So we take 12 mm as optimal length of slot 's'. The impedance bandwidth of the textile antenna at optimal length is 110 MHz, spans from 2.40 GHz to 2.51 GHz, with S11< -10 dB.
Fig.4 Effect of different values on slot 's' on s-parameter
The antenna parameters are analyzed in CST microwave studio with cutoff frequency as 2.45 GHz and also measured the result of fabricated antenna.
Fig.5 Fabricated antenna front view
The simulated return loss result of textile antenna with human body model and without human body model is given in Figure 6.
Fig.6 Plot of frequency vs. return loss of the antenna
While simulating the antenna on the human body there is some leakage of radiation into the human body. The return loss plot with human body model there occur splits in single frequency band into two bands. The impedance bandwidth is 110 MHz for 10dB return loss. The calculated specific absorption rate is 0.0126 w/kg in 10 gm of tissue, which is the allowed SAR value for wearable antennas .
Measurements are carried out using vector network analyzer (VNA) Agilent PNA E8358A. Figure 7 shows the measured return loss of the antenna and it decreases to
-32.9 dB at the centre frequency of 2.45 GHz. The measured impedance bandwidth of the antenna is 170MHz (2.40 GHz ' 2.70 GHz) as compared to the simulated result of 110 MHz.
Fig.7 Measured return loss of the antenna
Simulated Radiation Pattern
The Figure 8 shows the polar plot of radiation pattern with ''=900& '' =00. Getting gain of 3.6dB. From the plot it is clear that main lobe magnitude of ''=900& ''=00 is 3.6 dB.
Fig.8 Simulated Radiation pattern with (a) ''=900 (b) ''=00
Fig.9 Measured Radiation pattern with ''=900
Fig.10 Measured Radiation pattern with ''=00
The radiation pattern of the antenna is measured in an anechoic chamber. From the Figure 9 it is seen that at ''=900 cut the beam peak magnitude is -67.51 dB and in Figure 10 at ''=00 cut the beam peak magnitude is -67.72 dB. So the difference between the main lobe magnitude of the both is nearly equal to 1 implied that antenna is circularly polarized .
Current Distribution on the Surface of Patch
The Figure 11 shows the current distribution on the surface of the patch at 2.45 GHz with different phases. It seen that the current rotates in a circle with different phases, which is the reason for achieving circular polarization in the antenna.
Fig.11 Current on the Surface of Patch at 2.45 GHz (a) 0'' (b) 90'' (c) 180''
Bending Analysis of Antenna
To analyze bending effect, two cylindrical shaped plastic bottles with radii 100 mm and 75mm are used. It is observed that when the radius of the cylinder increases, the bending angle decreases, and vice versa. The selected radii are for typical human body parts like legs and arms etc. The material of the cylinder will affect the surface currents of the antenna. So the antenna is bent on the foam cylinder with transparent paper tape is used to hold the antenna in the proper place during measurements .
Fig.12 Antenna under bend
Fig.13 Simulated return loss of the antenna under bend
Fig.14 Measured Return loss of antenna under bend
By comparing the measured return loss of the antenna in flat and bend states it is clear that bending an antenna changes the effective length which changes the resonant frequency. Increasing the bend decreases the effective length, there by shifting resonant frequency to higher bands . Return loss after bending on the foam cylinder is almost similar to the measured return loss of the antenna without bend because of the small bending angle. Bending antenna on large cylinder impedance band breaks into two in xz-plane. Effect of bending in return loss, shifts the resonant frequency to higher bands. One reason for this can be that the effective dimensions of transmission line and slot 's' are slightly modified by bending, which detunes the input matching of the antenna. So it is preferable to use antenna in flat portions of human body , when positioning antenna in curved portions like arms, legs antenna should be placed in a way such that bend should be along xz direction.
Fig.15 Measured VSWR of antenna under bend
The comparison of result is shown in table.2.
Parameters Simulated without phantom Simulated with phantom Measured result Under bent
Return loss -23.167 -28.532 -33.911 -32.932
VSWR 1.26 1.184 1.0661 1.1858
Bandwidth 110MHz 110MHz 170MHz 160MHz
In this paper, the design, simulation and prototype measurement of a wearable antennas at 2.45 GHz has been done. To achieve circular polarization a rectangular slot is inserted at the center of the circular patch along the diagonal axis. The antenna analysis showed good agreement between measured and simulated free space results. Free space bending with two different radii (100 mm and 75 mm) was analyzed in which 100 mm bent is the optimum with more agreement with flat antenna measurement. Analysis showed that when bend along the direction which determines its resonance length the performance of the antenna is more affected.
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