Metamaterial Based Wearable Antenna with Low SAR

In this paper, Wearable antenna based on metamaterial (MTM) with a low specific absorption rate (SAR) has been developed. The suggested antenna design has four metamaterial unit cells underneath the radiating element in the same plane as the ground, together with partial ground. Rectangular patches with I-shaped slots are used to create the MTM unit cells. The jeans fabric is positioned on top of the metamaterials. The MTM unit cells were significant in reducing SAR. Through comparative simulated analysis, such as reflection coefficient, VSWR, radiation patterns, and other antenna parameters, HFSS software presents the performance of the suggested antenna


Introduction
Artificial structures known as metamaterials are made of regular or irregular subwavelength macrocells.The ability to regulate the effective medium properties of metamaterials is one of their advantages.This is achieved by creating the macro units to create unique materials that do not exist in nature.The dimensions and bandwidth of the metamaterial antenna design are five times smaller.In contrast to conventional phased array antenna, it does not require active phase shifters or amplifiers.Wide angle scanning and superior beam performance are both offered.It features pointing and polarization that may be manipulated electronically.It uses incredibly little power.Metamaterial Antennas are compact, flat, and light in weight.Instead of using mechanical components, it steers using software [10].Planar integration with other components is possible.Properties of metamaterials are obtained from their physical structures rather than their chemistry.They display characteristics not found in nature.They display qualities not seen in the components that make up their composition.A composite material that has been specifically designed to perform better.They display qualities that are not possible to achieve with regular material.The ratio between the capacitance of a capacitor utilizing that substance as its dielectric and a capacitor with a vacuum-based dielectric is known as relative permittivity.A composite medium with simultaneously negative permittivity and permeability had been postulated by D.R. Smith [4].They present a composite medium that exhibits a frequency region in the microwave regime with simultaneously negative values of effective permeability and permittivity.This composite medium is based on a periodic array of interspaced conducting non-magnetic split ring resonators and continuous wires.Wearable refers to anything that can be worn on the body.The concept "wearable electronics" is relatively new, but it has started to gain traction among techies over the past ten years.A wearable electronics device is one that has intelligence, can process information, and can produce useful output.Products with wearable electronics that incorporate computing and electronic technologies into their functionality.The wearable antenna for the body area networks are used as off 1291 (2023) 012049 IOP Publishing doi:10.1088/1757-899X/1291/1/012049 2 body/ on body communication, which find wide applications like Health care monitoring, military applications, personal Communications RFID tags etc.For the wearable technology the antenna should be flexible to mount on any part of the body.It is very important for the designer to design the wearable appropriately [5].The main procedural steps to obtain a better performance wearable antenna include, Identification of the best design, checking for grip of the wearable, Ruggedness, developing wearables prototypes, Testing and verification, Selection of appropriate antennas and Flexibility of antenna.A wearable antenna uses two types of materials, namely conductive and substrate.The conductive materials for antenna depends on the thickness and conductivity parameters.Whereas, Substrate materials for antenna depends on dielectric constant and loss tangent parameters.Wearable material should ideally have Light weight [9].The interactions between the lossy human body tissues and wearable antennas cause changes in the reflectivity and radiation properties of the antennas.Second, the type (skin, fat, muscle, etc.) and working frequency of human tissues affect their permittivity [6].Finally, varying permittivity and conductivity levels primarily alter reflection coefficients, which in turn affect how much power is absorbed by the body.This results in a decrease in the antenna's radiated efficiency.

Literature survey
In this paper [1] the antenna is designed using polyester substrate.The use of copper material on the substrate is minimized to a great extent.The reduction in the size of the antenna results in the increase in resonant frequency.They have reduced the use of copper material and hence size of antenna is reduced.This antenna is designed for IEEE 802.11a wireless local area network applications.The substrate thickness is 1mm and dielectric constant is 1.39.copper tape with thickness of 0.1mm is used.The feedpoint location is 11mm in the x direction and 7mm in the y direction.Under unloaded condition, this antenna is resonating at 8.97 GHz and reflection coefficient is -19.5 dB.Under loaded condition, this antenna is resonating at 5.10 GHz and reflection coefficient is -24 dB.The suggested antenna's measured reflection coefficient is -30 dB.The suggested antenna has a 4 dBi gain.Using PVC pipe, the impact of bending on the performance of the antenna was also examined.The antenna's return loss was measured to be -18 dB when bent in a 54.5mm radius.The antenna's return loss was measured to be -21 dB when bent in a 44.5mm radius.Wearable microstrip patch antenna with interdigital capacitor and rectangular stub is presented in this research [2].Inductance is produced by a rectangular stub, and capacitance by an interdigital capacitor.The ground plane is soldered properly to the polyester substrate's opposite side, which is where the antenna is constructed.The polyester substrate's utilised thickness is 3.14 mm, and its dielectric constant is 1.39.The copper tape has a 0.1 mm thickness.The feedpoint is situated at a distance of 7mm in the Y direction and 13mm in the X direction.At 2.45 GHz, the antenna's simulated reflection coefficient is -25 dB under no load.At 2.54 GHz, the antenna's observed reflection coefficient is -32 dB.The filters have been used to remove the unwanted bands.The proposed antenna offers a good gain.In this paper [3], they have designed aesthetic pattern antenna.They have used 3 petals and in the middle circular patch is placed, circular patch is used to connect 3 petals [8].Spline curve method available in the CST software is used to design the petals.Depending on the requirements the ground structure can be changed.Partial ground has been used in order to increase the bandwidth.The substrate thickness has been increased by stacking one more substrate upon the another substrate.They have used two substrates since the available substrate is only 1mm thickness.By increasing the substrate thickness, the antenna bandwidth and gain has been improved.The SAR calculation performed with antenna placements at 0mm (touching the skin), 1mm and 3mm away from the body part or using phantom model.

Antenna design
Steps for Designing the Antenna in HFSS Software: In step 1, create ground plane by using the rectangle which is the 2D and name it as ground.In step 2, After creating ground plane place a dielectric substrate, draw a box of same size and extend in the zdirection.In step 3, Click on the box substrate, jeans substrate will be used in our design.over this click on create substrate dimensions are ok.But Z dimension has to be 1.6, click on ok, can be rotated to see the plane.In step 4, Over the substrate a patch has been placed with specified length and width, patch has to be placed on the top.field line impedance of 50 ohm is provided.In step 5, Once everything has been done.Then the design need to be validated.First save the design go to HFSS and click on validation check.In step 6, Go to the simulation and click on analysis and after that progress window can be seen, so once the analysis is completed, results can be seen and possible to find maximum gain.where it is occurring and at what frequency [7].The proposed antenna design using HFSS software has been shown in figure 1 and 2. Figure 3 and 4 shows the fabricated proposed antenna design.The dimensions of the proposed antenna are listed in table.1 below.

Results
As can be seen in fig.5, the reflection coefficient of the antenna for free space is -27.2 dB and the reflection coefficient of the antenna for on body is -26 dB. Figure 6, shows the VSWR characteristics of the proposed antenna.The VSWR of the antenna for free space is 1.15 and the VSWR of the antenna for on body is 1.09.The SAR value of the proposed antenna when placed on Phantom arm is 0.97 W/Kg.The suggested antenna design has four metamaterial unit cells underneath the radiating element in the same plane as the ground, together with partial ground.Rectangular patches with I-shaped slots are used to create the MTM unit cells.The jeans fabric is positioned on top of the metamaterials.The MTM unit cells were significant in reducing SAR.Through comparative simulated analysis, such as reflection coefficient, VSWR, radiation patterns.

Conclusion
The simulation research demonstrated that the presence of the metamaterial unit cell improves the antenna performance and the reflection coefficient.According to FCC regulations, the SAR value has been decreased to below 1.6 W/Kg, making the developed antenna an excellent contender for wearable applications.The suggested antenna design has four metamaterial unit cells underneath the radiating element in the same plane as the ground, together with partial ground.Rectangular patches with I-shaped slots are used to create the MTM unit cells.The jeans fabric is positioned on top of the metamaterials.
The MTM unit cells were significant in reducing SAR.Through comparative simulated analysis, such as reflection coefficient, VSWR, radiation patterns, and other antenna parameters.

Fig. 4 :
Fig.4: Bottom view of the fabricated proposed antenna

Table . 1
: Dimensions of the proposed antenna