**4. Observations**

#### **4.1. Impact on antenna performance with radiating element having different surface resistivity.**

To know the relation between these parameters, a measurement is done for 20 different microstrip antennas keeping other parameters same and only changing the surface resistivity. The result is then plotted in matlab. These measurement results obtained when analyzing different antennas provide valuable information when a conductive textile material is to be

RFID Textile Antenna and Its Development http://dx.doi.org/10.5772/53521 177

Figure 5 depicts that for very low surface resistivity, the gain is maximum. When the radiating element (antenna) surface resistivity is increased, the gain of the antenna starts to decrease.

Figure 6 illustrates the relation between the conductivity and the radiation efficiency for above mentioned conductive textile material when used as an antenna. Radiation efficiency is a ratio of power radiated by antenna and power input to antenna. If most of the power input to the antenna is radiated, then the antenna is said to have high radiation efficiency. It can be seen from Figure 7 that conductivity is related to radiation efficiency in logarithmic manner. When the conductivity of radiating element is lower, the radiation efficiency is also very small, and increases as the conductivity increases. However radiation efficiency does not increase in linear way, and to achieve the radiation efficiency in higher percentage, the conductivity of the

The entire simulated antenna has reflection (*S11*) less than -35 dB. However the entire antenna does not have same *S11*. This affects the smoothness of the curve obtained in Figure 5 and 6.

**4.2. The impact on antenna performance when the thickness of dielectric material is changed**

When the dielectric material thickness is changed, this affects the radiation efficiency. To

analyze this effect, three microstrip patch antenna is designed.

used to design an antenna.

**Figure 6.** Radiation Efficiency vs. Conductivity Plot

radiating material should be very high.

**to different values.**

While working with textile antenna, it is found that the antenna is not working properly as it should work. This is because the antenna which is generally made of very high conductive material has very good radiation efficiency and gain. However the antenna made of conductive textile material has very high surface resistivity and hence lower conductivity. Because of this property of textile material it is difficult to choose the appropriate conductive textile material for desired gain. Also the height of dielectric constant plays a major role in determining the radiation efficiency.

Because of the problem of higher surface resistivity associated with the conductive textile material, a relation between surface resistivity, gain and radiation efficiency is analyzed. For this purpose the microstrip patch antenna is simulated in simulation software IE3D. First the measurement is performed by simulating two different microstrip patch antenna. Both of these antennas are simulated to work at a frequency of 2.45 GHz but with the radiating element having different surface resistivity.

The surface resistivity for two different radiating elements was chosen to be 0.02 Ω/sq and 1.19 Ω/sq. Fleece fabric is used as dielectric material which has a dielectric constant of 1.25. The antennas are simulated for reflection loss less than -40 dB and the result is noted.

It is observed from the simulation result that, though all the other antenna parameter are same, the difference in surface resistivity of the two radiating element affect a lot in their radiation efficiency and gain.

**Figure 5.** Gain Vs Surface Resistivity Plot

To know the relation between these parameters, a measurement is done for 20 different microstrip antennas keeping other parameters same and only changing the surface resistivity. The result is then plotted in matlab. These measurement results obtained when analyzing different antennas provide valuable information when a conductive textile material is to be used to design an antenna.

Figure 5 depicts that for very low surface resistivity, the gain is maximum. When the radiating element (antenna) surface resistivity is increased, the gain of the antenna starts to decrease.

**Figure 6.** Radiation Efficiency vs. Conductivity Plot

**4. Observations**

176 Radio Frequency Identification from System to Applications

radiation efficiency.

efficiency and gain.

**Figure 5.** Gain Vs Surface Resistivity Plot

having different surface resistivity.

**resistivity.**

**4.1. Impact on antenna performance with radiating element having different surface**

While working with textile antenna, it is found that the antenna is not working properly as it should work. This is because the antenna which is generally made of very high conductive material has very good radiation efficiency and gain. However the antenna made of conductive textile material has very high surface resistivity and hence lower conductivity. Because of this property of textile material it is difficult to choose the appropriate conductive textile material for desired gain. Also the height of dielectric constant plays a major role in determining the

Because of the problem of higher surface resistivity associated with the conductive textile material, a relation between surface resistivity, gain and radiation efficiency is analyzed. For this purpose the microstrip patch antenna is simulated in simulation software IE3D. First the measurement is performed by simulating two different microstrip patch antenna. Both of these antennas are simulated to work at a frequency of 2.45 GHz but with the radiating element

The surface resistivity for two different radiating elements was chosen to be 0.02 Ω/sq and 1.19 Ω/sq. Fleece fabric is used as dielectric material which has a dielectric constant of 1.25. The

It is observed from the simulation result that, though all the other antenna parameter are same, the difference in surface resistivity of the two radiating element affect a lot in their radiation

antennas are simulated for reflection loss less than -40 dB and the result is noted.

Figure 6 illustrates the relation between the conductivity and the radiation efficiency for above mentioned conductive textile material when used as an antenna. Radiation efficiency is a ratio of power radiated by antenna and power input to antenna. If most of the power input to the antenna is radiated, then the antenna is said to have high radiation efficiency. It can be seen from Figure 7 that conductivity is related to radiation efficiency in logarithmic manner. When the conductivity of radiating element is lower, the radiation efficiency is also very small, and increases as the conductivity increases. However radiation efficiency does not increase in linear way, and to achieve the radiation efficiency in higher percentage, the conductivity of the radiating material should be very high.

The entire simulated antenna has reflection (*S11*) less than -35 dB. However the entire antenna does not have same *S11*. This affects the smoothness of the curve obtained in Figure 5 and 6.

#### **4.2. The impact on antenna performance when the thickness of dielectric material is changed to different values.**

When the dielectric material thickness is changed, this affects the radiation efficiency. To analyze this effect, three microstrip patch antenna is designed.


**Table 2.** Technical specification for the antenna

Three rectangular patch antennas are designed for different height of dielectric material. On doing simulation, various parameters like reflection, gain, radiation efficiency and antenna efficiency for different patch antenna were observed. The obtained results are shown in Figure 7 and 8.

**5. Conductive textile materials**

of easily wearable conductive textile [6].

conditions as given below.

the ohmic losses in the fabric.

the variation of resistance should be minimum.

The fabric that can conduct electricity is called conductive fabric. The conductivity of the fabric depends on how it is manufactured. Conductive fabric can be made in various ways. They can either be produced by metal inter woven fabric during manufacturing or by metal coated fabric [5] also called electro thread. These conductive textiles have wider application in various fields as they are used for shielding human body and some special equipment from external electromagnetic radiation, and also as pressure sensor or flexible heaters, which are made out

RFID Textile Antenna and Its Development http://dx.doi.org/10.5772/53521 179

**Figure 8.** Radiation Efficiency for antennas having thickness 1 mm, 2 mm, 3 mm respectively

For a good design of a textile antenna, the conductive fabric should satisfy some of the

**•** The electrical resistance of the conductive textile fabric should be small in order to reduce

**•** The surface resistivity should be homogeneous over the entire conductive textile fabric i.e.

The antenna performs better if the conductive textile fulfills the above given characteristics.

**•** The fabric should be flexible enough to be able to use as a wearable antenna.

**Figure 7.** S11 for antenna with dielectric thickness 1 mm, 2 mm and 3 mm correspondingly

From above *S11* plot it can be seen that the reflection is less than -30 dB for all three antennas with dielectric thickness 1 mm, 2 mm and 3 mm.

For the same specification of antennas, the radiation efficiency is measured with different height of dielectric material.

The above plot gives the measure of radiation efficiency of the antenna with three different thicknesses. It can be seen that for an antenna working at 2.45 GHz and dielectric thickness of 1 mm, the radiation efficiency is 61.2 %, for dielectric thickness of 2 mm, the radiation efficiency is 83.4 % and for dielectric thickness of 3 mm, the radiation efficiency is 90.2 %.

**Figure 8.** Radiation Efficiency for antennas having thickness 1 mm, 2 mm, 3 mm respectively

### **5. Conductive textile materials**

**Frequency** 2.45 GHz

**Surface resistivity of radiating element** 0.02 Ω/sq

**Thickness of radiating element** 0.14 mm

**Dielectric material** Fleece fabric -

**Dielectric constant** 1.25 -

**Table 2.** Technical specification for the antenna

178 Radio Frequency Identification from System to Applications

7 and 8.

**Height of Dielectric material** 1, 2, 3 mm

Three rectangular patch antennas are designed for different height of dielectric material. On doing simulation, various parameters like reflection, gain, radiation efficiency and antenna efficiency for different patch antenna were observed. The obtained results are shown in Figure

**Figure 7.** S11 for antenna with dielectric thickness 1 mm, 2 mm and 3 mm correspondingly

with dielectric thickness 1 mm, 2 mm and 3 mm.

height of dielectric material.

From above *S11* plot it can be seen that the reflection is less than -30 dB for all three antennas

For the same specification of antennas, the radiation efficiency is measured with different

The above plot gives the measure of radiation efficiency of the antenna with three different thicknesses. It can be seen that for an antenna working at 2.45 GHz and dielectric thickness of 1 mm, the radiation efficiency is 61.2 %, for dielectric thickness of 2 mm, the radiation efficiency

is 83.4 % and for dielectric thickness of 3 mm, the radiation efficiency is 90.2 %.

The fabric that can conduct electricity is called conductive fabric. The conductivity of the fabric depends on how it is manufactured. Conductive fabric can be made in various ways. They can either be produced by metal inter woven fabric during manufacturing or by metal coated fabric [5] also called electro thread. These conductive textiles have wider application in various fields as they are used for shielding human body and some special equipment from external electromagnetic radiation, and also as pressure sensor or flexible heaters, which are made out of easily wearable conductive textile [6].

For a good design of a textile antenna, the conductive fabric should satisfy some of the conditions as given below.


The antenna performs better if the conductive textile fulfills the above given characteristics.

#### Non woven fabric

By the name it can be concluded that non woven fabric is prepared by neither knitting process, nor are woven fabric. Thus the non woven fabric does not go through the initial stage of yarn spinning and also a definite web pattern as that of a woven fabric is not obtained. Non woven fabric manufacturing process is similar to that of paper manufacturing process.

The material used is Cu-Ni with the thickness measured in the lab is 0.14 mm.


**Table 3.** Technical specification of Cu-Ni textile material [7]

These kind of fabric are generally manufactured in three ways namely, drylaid syste, wetlaid system and polymer based system. After the fabric is manufactured, it is then strengthened. There are various ways for strengthening fiber web as by using chemical means by spraying, coating. This can also be achieved by thermal means by blowing air or by ultrasonic impact which partially fuses (connects) the fiber thread. Thus finally the metal layer is coated.

#### Woven Fabric

Woven fabric is a construction design for lab use at CTU in Prague. The woven fabric consists of silver nano particles attached to the thread of fiber when being constructed and then woven to form a conductive textile, Figure 9.


**Table 4.** Technical specification of Betex textile material

#### **5.1. Electrical resistance and resistivity of textile materials**

Electrical conductivity of textile materials is calculated from electrical resistivity as:

$$
\sigma = 1/\varrho \tag{2}
$$

Electrical resistivity can be obtained via resistance mesurement [8-10]. We differentiate surface and bulk electrical resistance. Surface resistance is defined as the ratio of a DC voltage *U* to the current *IS* which flows beteen two specific electrodes. The electrodes are placed on the same side of measured material and it is assumed all currents flows only between electrodes and do

RFID Textile Antenna and Its Development http://dx.doi.org/10.5772/53521 181

Bulk resistance or electrical resistance takes into account all currents flowing in the material, not only on the surface. It can be measured by RLCG bridge or DC power source (showing

The textile material can be modelled as finite grid of resistors. However, it assumes only woven

not penetrate into the bulk of material [8].

**Figure 9.** Photo of woven Betex textile material

fabric [11]. The example is depicted in Figure 10.

**Figure 10.** Equivalent circuit diagram of Betex textile material

voltage and current values).

**5.2. Resistance modelling**

whereσ is electrical conductivity [S/m], ρ is electrical resistivity [Ω m].

**Figure 9.** Photo of woven Betex textile material

Non woven fabric

Woven Fabric

By the name it can be concluded that non woven fabric is prepared by neither knitting process, nor are woven fabric. Thus the non woven fabric does not go through the initial stage of yarn spinning and also a definite web pattern as that of a woven fabric is not obtained. Non woven

These kind of fabric are generally manufactured in three ways namely, drylaid syste, wetlaid system and polymer based system. After the fabric is manufactured, it is then strengthened. There are various ways for strengthening fiber web as by using chemical means by spraying, coating. This can also be achieved by thermal means by blowing air or by ultrasonic impact which partially fuses (connects) the fiber thread. Thus finally the metal layer is coated.

Woven fabric is a construction design for lab use at CTU in Prague. The woven fabric consists of silver nano particles attached to the thread of fiber when being constructed and then woven

fabric manufacturing process is similar to that of paper manufacturing process.

The material used is Cu-Ni with the thickness measured in the lab is 0.14 mm.

**Description** copper + nickel plated non-woven polyamide fabric

**Roll widths** 102 cm ± 2 cm

180 Radio Frequency Identification from System to Applications

**Surface resistivity** Max average 0,02 Ohm/square **Shielding effectiveness** 70-90 dB from 50 MHz to 1 GHz **Purpose** conductive fabric for general use **Temperature range** -30 to 90 (degree centigrade)

**Table 3.** Technical specification of Cu-Ni textile material [7]

to form a conductive textile, Figure 9.

**Name** Betex

**Number of fiber threads per centimeter** 20 **Surface Resistivity** 1.19Ω /sq.

**5.1. Electrical resistance and resistivity of textile materials**

**Table 4.** Technical specification of Betex textile material

**Materials Used** Shiledex (60%), Polyster (40 %)

whereσ is electrical conductivity [S/m], ρ is electrical resistivity [Ω m].

Electrical conductivity of textile materials is calculated from electrical resistivity as:

σ = 1/ρ (2)

Electrical resistivity can be obtained via resistance mesurement [8-10]. We differentiate surface and bulk electrical resistance. Surface resistance is defined as the ratio of a DC voltage *U* to the current *IS* which flows beteen two specific electrodes. The electrodes are placed on the same side of measured material and it is assumed all currents flows only between electrodes and do not penetrate into the bulk of material [8].

Bulk resistance or electrical resistance takes into account all currents flowing in the material, not only on the surface. It can be measured by RLCG bridge or DC power source (showing voltage and current values).

#### **5.2. Resistance modelling**

The textile material can be modelled as finite grid of resistors. However, it assumes only woven fabric [11]. The example is depicted in Figure 10.

**Figure 10.** Equivalent circuit diagram of Betex textile material

Measurement of resistance (bulk or surface) is based on placing two square electrodes on two ends of the woven textile material. The structure can be interpreted as series-parallel connec‐ tion of resistors. The battery represents electrodes and resistors the textile fibres.

The resultant resistance of this model can be calculated as series-parallel connection of resistors

where *n*, *r* represents number of squares in "horizontal" direction and *s* in "vertical" direction.

Considering Betex sample and setup measurement, the Betex sample reach dimensions 10 x 3 cm, 25 threads/cm in warp and 20 threads/cm in weft. Parameters *n* and *s* are then equalled

<sup>59</sup> <sup>=</sup> <sup>249</sup>*R*<sup>1</sup>

The parameter *R1* represents a resistance element of used fiber which forms the whole fabric. It can be calculated from the dimensions of textile structure with the aid of fiber diameter measurement. *R1* is set to 0.97Ω and *R=4.09Ω.* It meands the structure is very conductive.

<sup>6</sup> =2*R*1 (3)

RFID Textile Antenna and Its Development http://dx.doi.org/10.5772/53521 183

<sup>s</sup> , n, r, s ∈N (4)

*r* =10 · 25 - 1=249 (5)

*s* =3 · 20 - 1=59 (6)

<sup>59</sup> =4.22*R*1 (7)

<sup>6</sup> <sup>=</sup> <sup>12</sup> *<sup>R</sup>*<sup>1</sup>

*R* =∑ 1 12 *R*1

*R* = ∑ n=1 r *R*1

as:

to:

Formula (3) can be generalized as:

Resultant resistance is equalled to:

**Figure 13.** Measurement setup

*R* = ∑ n=1 r *R*1 <sup>s</sup> = ∑ n=1 249 *R*1

The equivalent circuit diagram can be simplified with respect to basic physical laws. Equipo‐ tential points in this diagram are in all individual "vertical" resistor connections. The resistors placed between the points with same potential can be eliminated because they are equalled to zero. The voltage probes are placed in the equipotential points in Figure 11. The results are depicted in Figure 12. All probes reach the same value and therefore the resistors can be eliminated.

**Figure 11.** Simplified equivalent circuit model

**Figure 12.** Result values of placed voltage probes.

The resultant resistance of this model can be calculated as series-parallel connection of resistors as:

$$R = \sum\_{1}^{12} \frac{R \, 1}{6} = \frac{12 \, R \, 1}{6} = 2R \, 1 \tag{3}$$

Formula (3) can be generalized as:

Measurement of resistance (bulk or surface) is based on placing two square electrodes on two ends of the woven textile material. The structure can be interpreted as series-parallel connec‐

The equivalent circuit diagram can be simplified with respect to basic physical laws. Equipo‐ tential points in this diagram are in all individual "vertical" resistor connections. The resistors placed between the points with same potential can be eliminated because they are equalled to zero. The voltage probes are placed in the equipotential points in Figure 11. The results are depicted in Figure 12. All probes reach the same value and therefore the resistors can be

tion of resistors. The battery represents electrodes and resistors the textile fibres.

eliminated.

**Figure 11.** Simplified equivalent circuit model

182 Radio Frequency Identification from System to Applications

**Figure 12.** Result values of placed voltage probes.

$$R = \sum\_{n=1}^{r} \frac{R1}{s}, \text{ n, r, s } \in \mathbb{N} \tag{4}$$

where *n*, *r* represents number of squares in "horizontal" direction and *s* in "vertical" direction.

Considering Betex sample and setup measurement, the Betex sample reach dimensions 10 x 3 cm, 25 threads/cm in warp and 20 threads/cm in weft. Parameters *n* and *s* are then equalled to:

$$r = 10 \cdot 25 \text{ - 1} = 249 \tag{5}$$

$$\mathbf{s} = \mathbf{3} \cdot \mathbf{20} \text{ - 1} = \mathbf{59} \tag{6}$$

Resultant resistance is equalled to:

$$R = \sum\_{n=1}^{r} \frac{R \, 1}{s} = \sum\_{n=1}^{249} \frac{R \, 1}{59} = \frac{249R \, 1}{59} = 4.22R \, 1 \tag{7}$$

The parameter *R1* represents a resistance element of used fiber which forms the whole fabric. It can be calculated from the dimensions of textile structure with the aid of fiber diameter measurement. *R1* is set to 0.97Ω and *R=4.09Ω.* It meands the structure is very conductive.

**Figure 13.** Measurement setup

#### **5.3. Resistance measurement**

The Betex sample is measured by RLCG bridge with respect to its calculated resistance. DC power source can cause sample damage at low voltage values (10 V corresponds to approx. 10 A). The measurement setup is depicted in Figure 13.

**Parameter** Cu-Ni BETEX **Frequency** 869 MHz 869 MHz **Surface resistivity** 0.02 Ω/sq. 1.19 Ω/sq. **Thickness** 0.14 mm 0.35 mm **Conductivity** 357143 S/m 2381 S/m

**Parameter** TAG1 TAG2

**Thickness of the Substrate** 4 mm 4 mm

357143 S/m

**Substrate Material** Fleece Fabric Fleece Fabric

Cu-Ni conductive textile has high conductivity and the designing with this conductivity gives better radiation performance then Betex textile which has low conductivity. The simulation of two antennas with two different materials having same dielectric substrate and the same

(a) (b)

**Figure 14.** Simulated H-slot antenna for radiating element having surface resistivity = 0.02(a), and surface resistivity

As expected, the tag with high conductive material is giving better radiation performance. When a comparison made between this two tag's simulations, a high reading distance and high radiation efficiency are achieved from TAG1 which is designed with high conductive

textile. This can be shown by the plot obtained from the simulation, Figure 15.

BETEX, conductivity of

RFID Textile Antenna and Its Development http://dx.doi.org/10.5772/53521 185

2381 S/m

**Table 5.** Specifications for the conductive textile radiating element

**Conductive Material** Cu-Ni, conductivy of

**Table 6.** Specification for simulating antenna (TAG1 and TAG2)

*6.1.1. Antenna layouts and designs*

=1.19(b)

dielectric constant is shown in Figure 14.

The measurement of Betex sample shows the resultant resistance is approx. 4Ω which confirms the modelling results.
