INTRODUCTION

In recent years the growing demand for wireless application has led to the development of new wireless access technologies. As all fields are gaining the researchers attraction, wireless communication is not an exception, Recent years have witnessed a great deal of interest in the field of wearable electronics. Their light weight, low-cost manufacturing, ease of fabrication, and availability of inexpensive substrate make wearable electronics an appealing invention for the next generation of consumer electronics.

Consistently, the wearable application system requires the integration of the fabric along with antenna operating in specific bands to provide wireless connectivity which is highly demanded by today's fast moving society. The performance of these systems primarily depends on the characteristics of the integrated antenna. At the same time, these antenna should be mechanically strong, efficient with a reasonably wide bandwidth and desirable radiation characteristics.

Wearable antenna means a dedicated antenna that could be worn. The wearable antenna serves as element of clothes, whose purpose is performing tasks directly related to telecommunications such as tracking and navigation, remote computing and communication tasks related to public safety. The development of wearable antenna has been rapid as they are light weight and compact. The wearable antenna have gained much moreimportance due to the introduction of fabrics as the substrate.

Conventional antenna designs which include planar dipoles, monopoles, and micro strip patches were used in recent research for wearable antennas design. Microstrip antennas are planar and these can be manufactured on a printed circuit board (PCB). This made them a practical antenna type due to their low cost, and ease for fabrication. This fabric-based antenna is designed to use it for WLAN application.While designing these type of antennas, the losses introduced by the substrate, flexibility and antenna's susceptiblity to bending must be taken into account.

1.1 OBJECTIVE:

The main aim of this project is to design a wearable planar inverted-f antenna which has good Specific Absorption Rate (SAR). The antenna is fed by inset feed mechanism. As the actual SAR value measurement is quite difficult, an alternative method is suggested here. Instead of measuring SAR , front to back ratio of the antenna is measured, PIFA is also known for its narrow bandwidth characterstic. In our project we introduce corrugated slot in the ground plane in order to increase bandwidth. The next important objective is to compare the performance of various substrate and conducting materials. The comparison is carried out based on the parameters like return loss, bandwidth, front to back ratio, directivity and gain.Another main focus of this project is to achieve compactness compared to the wearable patch antenna. This antenna is designed to operate at 2.45GHz. Considering application, this project is designed to work for WLAN application.

1.2 WEARABLE ANTENNA

One of the dominant research topics in antennas for body-centric communications is wearable, fabric-based antennas.In simple terms wearable antennas are a simple cloth whose purpose is performing tasks related to tracking and navigation, remote computing and communication tasks related to public safety. Commonly, wearable antenna requirements for all modem applieatioi; require light weight, low gost, almost maintenance-free and no installation. There are number of specialized occupation segments that apply body centric-communication systems, such as paramedics, fire fighters and military. Besides, wearable antennas also can be applied for youngsters, the aged, and athletes for the purpose of monitoring , In wearable antennas design, the important thing is the choice of substrate. Generally fleece, cotton, jean , polyester are used. The electromagnetic properties that are considered are dielectric constant and loss tangent. There are electro-textilematerials that can be used as conductive material. The commonly used electro-textile materials are zelt, Pure Copper Polyester Taffeta Fabric(PCPTF) and flectron. Here , we haye considered jean, cotton and polyester for substrates and copper, aluminum and PCPTF for conductive materials.

1.3 COMMERCIAL APPLICATIONS OF WEARABLE ANTENNA:

• Security and verification purpose

• Environmental sensing

• Sports science

• Tracking people and animals

• Patient assistive technology and patient monitoring.

While the above wearable antennas were analyzed by variety of factors such as gain, bandwidth and return loss, an additional parameter that is exclusively considered for wearable antenna is Specific Absorption Rate (SAR). Specific Absorption Rate is a measure that estimates the amount of radio frequency power absorbed in a unit mass of body tissue. SAR is measured in units of watts per kilogram,or equivalently in milliwatts per gram. This new factor must be equally considered along with the traditional parameters.

3. INTRODUCTION TO PLANAR INVERTED-FANTENNA (PIFA)

Over three decades PIFA structure has emerged as one of the most promising design for low profile antennas. PIFA is obtained from Inverted F. Antenna (IFA) by replacing the thin wire radiator with a planar radiator.A merit of this type of this antenna is that there is more design freedom than a thin wire one. This antenna consist of a patch with a shorting plane and feed point which resembles the F shape and hence the name.

Basically PIFA can be considered as a modification of monopole or microstrip patch antenna. More directly a PIFA can originate from planar monopole by bending the planar radiator for low profile and introducing a shorting pin for good impedence matching. Alternatively , PIFA could be a variation of a shorted patch antenna, where the radiating patch of the antenna is halved in its midline by a short circuiting wall. Therefore, the PIFA could be regarded as the variation of thin wire transmission line IFA or a short-circuit microstrip patch antenna.

This antenna is resonant at a quarter - wavelength thus reducing the required space (for instance in mobile phones).Other, advantages of PIFA includes easy fabrication, light weight, simple structure, small volume, low manufacturing cost. PIFA structure is easy to hide iri the casing of mobile handsets as compared to monopole, rod and helix antennas. PIFA has good SAR properties due to reduced back lobe radiation towards user's head and body.

3.2. VARIOUS SHAPES OF PIFA:

PIFA can be made in different shapes and it is used with different substrates for various applications. The radiating patch is designed in various shapes and the shapes that have been designed so far are :

• Swastik PIFA

• R-Shape PIFA

• Rectangular PIFA

• Circular PIFA

Each shape is having its own merits and demerits. In our project we design an rectangular PIFA with inset feed mechanism. The equation used to design this shape is discussed in the upcoming chapters.The design variables of the antenna are length, width and height of the patch, the width and location of the shorting plate and feed point location. The shorting plate of usual PIFA is a useful method for reducing the dimensions , but results in narrow impedence bandwidth.

3.3 EXISTING WEARABLE ANTENNAS:

• E- shaped wearable PIFA:

This antenna aimed at solving the problem of the close presence of the human body and of guranteeing the functionality in the presence of lossy conductive material.

• S-Shaped wearable PIFA:

This is proposed antenna is for implantable biotelemetry in the Medical Device Radio Communications Service, Wireless Medical Telemetry Services and Industrial, Scientific, and Medical band.

4.2. FABRICATION METHODS:

There are four methods that are used to fabricate wearable antenna. They are :

1. Liquid textile adhesive
2. Pointwise deposition of conductive adhesive

3. Sewing

4. Adhesive sheets

4.2.1 Liquid textile adhesive:

In this method , liquid textile adhesive is applied on the conductive fabric but it showed soaking effects so distribution of an evenly thin film of adhesives was impractical.As a result, adhesive acts as a insulator among the conductive yarns. Because of the uneven distribution, electrical resistance showed ihhomogeneity and it could by a factor of 10 at certain spots.

4.2.2Pointwise deposition of conductive adhesive:

In this method fabric a 1-cm spacing did not increase the sheet resistance; however, mechanical stability was significantly worse compared to the textile adhesive. Second, accurate attachment of the antenna patch was not ensured, in terms of preserving geometry. Conductive adhesives cannot be applied in an area-wide manner on textiles since these types of adhesives usually are stiff and brittle.

4.2.3 Sewing Method:

The seam spacing must be smaller than two cm in order to minimize wrinkling.. A wrinkling corresponds to uneven distances between the antenna p-afcfe a™d the gnound plane ^esultkig m distortion of the antenna characteristic.

A stitch passes through both the patch and the ground plane of the antenna. Electrical measurements revealed shorts between antennas patch and ground plane because the sewing needle pulled small conductive fibers from the patch through the substrate and shorted them with the ground plane. Besides, sewing could not be used with the spacer fabric substrate since the high pressure of the sewing seam compressed the substrate permanently.

4.2.4 Adhesive Sheets:

It show the best results. It evenly deposits as a thin layer on the conductive textile by ironing. Moreover, the adhesive only penetrates the surface of the conductive textile such that patch sheet resistance and substrate permittivity are not changed designed antenna is fabricated using sewing method.

In our project,the thickness of the jean is 0.625mm (measured value). In order to make it to the height of 5mm, 8 folds are made and stitched together. This 11 folds improves the mechanical strength of the antenna. Then the conductive material is stuck on the substrate using an adhesive .

4.3. PIFA FEEDING METHODS

The antennacan be fed by various methods The type of feed is going to have its impact on the radiation characterstics. Generally the feeding methods are classified as contacting and non-contacting. In contacting method, the RF power is directly fed to the radiating patch using a connecting element such as microstrip line. On the other hand, in the non-contacting method electromagnetic field coupling is done to transfer the power between the microstrip line and radiating patch.

4.4. ROLE OF DIELECTRIC MATERIALS

It has been concluded that wearable antennas serves as an element of cloth, that is nothing but the dielectric materials used are of clothes. There are various wearable substrates used in industry, some of them are felt, fleece, jean, cotton, silk, polyester, etc. Among which the materials used in our design are jean, cotton and polyester. All these materials have their own electromagnetic properties. Mainly we concentrate on permittivity, thickness and loss tangent values in our simulation process. In general there are two types of wearable antennas; partial wearable antennas and fully wearable antennas. Partial wearable antenna substrate has dielectric values within the range of 2 to lO.The aim of this project is to develop a fully wearable antenna and so the the substrates has its dielectric constant value less than 2. Initially, substratessuch as jean, panama fabric, fleece, felt and dacron were selected for analysis. The substrates are finalized for fabrication based on their availability. In our project we have chosen three fabrics as substrate and implemented PIFA on it. The three substrates are jean, cotton and polyester. The reason behind choosing these fabrics is explained in later chapters. The pictures of these fabrics is shown below.

4.4.1. PROPERTIES OF TEXTILE FABRICS

The selection criteria for the textile fabrics are dielectric constant, loss tangent and thickness.

4.4.1.a) DIELECRIC CONSTANT:

The dielectric constant is the extent to which a substance concentrates the electrostatic lines of flux. The lower the dielectric constant lower are the surface wave losses. Therefore, lowering the dielectric constant also increases spatial waves and hence increases the bandwidth of the antenna, allowing the antenna to obtain acceptable efficiency and gain. It has to be noted that the dielectric constant depends on frequency and they are inversely proportional.

4.4.1.b) LOSS TANGENT:

Loss tangent (tan§also known as dissipation factor) is the amount of power turned into heat in the material. It is given by the ratio of imaginary s " to the real part 8 ' of the permittivity: tan 8 = 8 "/ 8 '. The lower the loss tangent values the lower loss the dielectric substrate will have. Thus the radiation efficiency will be good. The textile materials here have less loss tangent values.

4.4.1. c) THICKNESS:

The bandwidth and efficiency of an antenna is also mainly decided by its thickness. Increased thickness leads to low value of quality factor which inturn increases the bandwidth as they are inversely proportional. As the substrate thickness increases, it also helps in achieving good mechanical strength of the antenna.

These factors for the three substrates are discussed below along with their properties which clearly indicates the reason for choosing these substrates.

4.4.2.JEAN
Jeans belong to the family of greaser subculture and are made from denim.Theyare dyed with indigo dye, giving them their distinctive blue color. This is of great tensile strength and hence greatly used. The jean material we haveused is of thickness 0.645mm and dielectric constant of 1.7.

Properties of jean:

• Fabric available readily

• Texture and durability

• Absorbency

• Resilience and flammability.

4.4.3.POLYESTER

Polyester is the general name of widely used synthetic fabric. Polyester is a long-chain polymer chemically composed of at least 85% by weight of an ester and a dihydric alcohol. Polyesters are strong, tough materials that are manufactured in a variety of colors, shapes and sizes.

Properties of the polyester;

• It is resists wrinkling.

• It is easy to launder and it retains its shape

• It dries quickly.

• It is resistant to stretching and shrinking.
4A4.COTTON

Cotton fiber is made from other cellulose fibers in morphological traits. The following advantages make us to use cotton as one of our substrate.In our project, cotton fabric used is of thickness 0.0625mm and dielectric constant 1.6.

Properties of cotton:

• Cotton shows good durability and utility; it stays undamaged even under the exposure of any acids or alkalis.

• It has good water-absorbing capacity and they are easy to dye.

• As we are using it for wearable applications, it is noted that it suits perfectly for people and preserves the warmth of human body.

4.5. ROLE OF CONDUCTIVE MATERIALS

Conductive materials are used as ground plane and patch in our wearable PIFA design. They are constructed by interpolating conductive metal with normal fabric. Characteristics of these fabrics like durablity and flexiblityhas made it suitable to be integrated into clothing. The conductive materials that we have chosen is desired to have a low and stable losses. By incorporating the above standards, the materials used here are copper, aluminium and pure copper polyester taffeta fabric(PCPTF).

4.5.1.COPPER

• Copper offers fairly good transparency and excellent corrosion resistance. It offers better attenuation makes this as perfect choice. Flexible and light weight, cuts easily for custom sizing. Can be used indoors or out. It is tested to be used over 10MHz-3GHz. The thickness is of 0.035mm.

4.5.2.PCPTF

• It is shiny, smooth fabric with pure copper, light weight and flexible. Easy to cut and sew like ordinary fabric. Better color stability due to tarnish resistant finish. High conductivity and shielding performance.

• Thickness is of 0.08mm.

5. RESULTS AND DISCUSSION

The Wearable Planar Inverted - F Antenna has been designed using the above explained design considerations and equations. The goal of this project as mentioned in the abstract is to achieve compactness and to improve the bandwidth . The antenna is fed using inset feed method and the bandwidth is enhanced by introducing corrugated slot. The slot dimensions can be adjusted to get good improvement in the bandwidth

The substrates play a vital role in the antenna performance. The textile materials are chosen as the substrate. The factors that are considered while choosing the material are loss tangent , availability and dielectric constant. Similarly three conductive materials are chosen and one among them is an electro-textile material. So nine antennas are designed using this combination and the best is identified from the parametric study. The return loss decreases at the operating frequency and the mechanical strength of the antenna is improved having the substrate in layers.

Absorption of radiation by the human body is characterised through front to back ratio. VSWR obtained at the primary resonant frequencies (2.45GHz) is the least and tend towards the ideal value of VSWR.The presence of corrugated slot in the ground plane increases the capacitive load of the antenna .

5.1. PARAMETERIC STUDY:

GAIN:

Antenna gain is usually defined as the ratio of the power produced by the antenna from a far-field source on the antenna's beam axis to the power produced by a hypothetical lossless isotropic antenna, which is equally sensitive to signals from all directions. Usually this ratio is expressed in decibels, and these units are referred to as "decibels-isotropic" (dBi).
Gain is the measure that combines antenna efficiency2sfl/J/ew/lfland directivity!)

EFFICIENCY:

Efficiency of an antenna is the ratio of the total power radiated by an antenna to the net power accepted by the antenna from the connected transmitter. It is expressed as a percentage (less than 100), and is frequency dependent.

RETURN LOSS:

Return loss is the loss of signal power resulting from the reflection caused at a discontinuity in a transmission line. It is usually expressed as a ratio in decibels (dB);

RL(dB)= 101og10

where RL(dB) is the return loss in dB, Pj is the incident power and Pr is the reflected power.

DIRECTIVITY:

Directivity is " a figure of merit for an antenna. It measures the power density the antenna radiates in the direction of its strongest emission, versus the power density radiated by an ideal isotropic radiator (which emits uniformly in al directions)radiating the same total power.

BANDWIDTH:

Bandwidth is a fundamental parameter which describes the range of frequencies over which the antenna can properly radiate or receive enegy.
FRONT TO BACK RATIO:

The term front-to-back ratio (also known as front-to-rear ratio) can mean:

1. The ratio of power gain between the front and rear of a directional antenna.

2, Ratio of signal strength transmitted in a forward direction to that transmitted in a backward direction. The use of different substrates and conductive materials gives different performance which is formulated in the table below.

5.2.BANDWIDTH ENHANCEMENT

For any antenna, bandwidth is an important factor. Particularly in the field of electronic warfare, wideband radar and measuring system are required. PIFA has narrow bandwidth due to the presence of shorting pin. Commonly used techniques to improve the bandwidth are:

• Introducing slots

• Increasing substrate height and dielectric constant

• Using stacked layers and impedance matching networks.

In our design, we introduce slots in the ground plane to enhance the bandwidth. The slot acts as a capacitive load thereby helping in improving the bandwidth.By proper selection of the dimensions and position of the slot two radiation modes of the antenna are perturbed such that their resonant frequencies get close to each other and form wide band. Slots can be different shapes Rectangular shaped slot, circular shaped slot are commonly used for bandwidth enhancement. In our project, corrugated slot is used for bandwidth enhancement. Figure 5.1 shows the geometry of corrugated slot. The dimensions are varied in order to get better performance. Thus a comparison is made between the antenna with slot and the antenna without slot. Table 5.1 shows the difference in parameters obtained with slot and without slot. Better return loss and bandwidth is achieved in antenna with slot.

From the above tabulation, by analyzing the parameters of all nine combinations, jean substrate with copper as conductive material shows better results. Hence we chose this for our design.The fabricated PIFA is shown below.


CLAIMS are

1. Wearable design

2. Compactness

3. High directivity, improved return loss and bandwidth

4. Suitable for most secure communication like military and defence

5. A new approach of Planar Inverted F Antenna(PIFA) is designed in jean cloth.

6. More stable system

7. Most suitable for practical applications in the frequency range of 2.45 GHz and ISM band of applications