BROADBAND CB CPW BASED MICROSTRIP ANTENNA …

e-ISSN: 2582-5208 International Research Journal of Modernization in Engineering Technology and Science

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BROADBAND CB CPW BASED MICROSTRIP ANTENNA INCORPORATED

WITH PARASITIC PATCH AND DEFECTED GROUND STRUCTURE Hepzipha Angel S*1, Dr. T. Jayasree *2

*1 Department of Electronics and Communication Engineering, Government College of Engineering,

Tirunelveli, Tamil Nadu, India.

*2 Assistant Professor, Department of Electronics and Communication Engineering, Government

College of Engineering, Tirunelveli, Tamil Nadu, India.

ABSTRACT

This paper presents a design of compact CB-CPW based microstrip Ultra Wide Band(UWB) antenna on a low loss

Rogers RT 5880 substrate for broadband wireless communication. The proposed design has defected ground structure,

transmission line, CPW, circular parasitic patch. The antenna has a compact size of 40 40 1 mm3. It achieves less

than -10 dB return loss characteristics in the frequency ranging from 2.78 GHz to 12 GHz which includes S band

(2.78 to 4GHz) C band (4 GHz to 8 GHz) and X band (8 GHz to 12 GHz). Overall antenna characteristics such as

antenna gain, Voltage Standing Wave Ratio (VSWR), radiation patterns and radiating efficiency across the band of

operation are observed to be good.The designed antenna has a highest peak gain of 6.12 dB and a maximum directivity

of 8.13 dBi at 11 GHz. The design and simulation of the proposed antenna are carried out using CST Microwave

Studio Simulation tool.

Keywords: Microstrip antenna, CB-CPW, parasitic patch, bandwidth, gain, directivity.

I. INTRODUCTION

Wireless technologies or communications plays a vital role in our daily routine lives. These communication techniques

have provided so much freedom that anyone can communicate with others without any geographical limitations.

Antennas play a prominent role in wireless communication systems, as they radiate electromagnetic waves.

There are many types of antenna, among them Microstrip antennas (MSA) are widely used in various applications,

especially in wireless communications. Microstrip or patch antennas are flat metallic patches printed over a dielectric

medium and have a ground plane connected to these two upper layers. It has many advantages, such as low profile,

lightweight, conformity, low fabrication costs, simplicity of manufacture, and the capability to be integrated with

microwave integrated circuits [1]. However, their application in many systems is restricted because they have

inherently narrow bandwidths.

High-data rate transmission can be obtained by increasing the bandwidth of antenna. To overcome the inherent

limitation of narrow impedance bandwidth of MSA, many techniques have been suggested for bandwidth

improvement such as impedance matching, increasing antenna volume, using coplanar waveguide structures or multi-

layer elements. Inclusion of slots and defected ground structure improves the bandwidth [2]-[5]. RT Duroid 5880

substrate with dielectric constant of 2.2 can be used to achieve wideband operating frequency rangeas it have low

dielectric constant[6]. Coplanar waveguide feed (CPW) improves impedance matching characteristicand broadens the

impedance bandwidth. It also achieves lower dispersion and attenuation [7]-[11].CB-CPW structure achieves

broadband effects and adds high mechanical strength to the antenna [13]-[16].

Low profile MSAs can also be used for wideband applications by employing such bandwidth improvement techniques

efficiently. In this paper, a Conductor Backed Coplanar Waveguide(CB-CPW) structure based Microstrip antenna

(MSA)is optimized to achieve wide bandwidth.

II. METHODOLOGY

The methodology used here to achieve widebandwidth is Conductor Backed Coplanar waveguide (CB-CPW)

structure. It is a kind of electrical planar transmission line which can be fabricated using printed circuit board

technology [15]. Conductor-backed coplanar waveguide (CB-CPW) is also known as coplanarwaveguide with

ground (CPWG). The difference between traditional CPW and CB-CPW is the ground plane.

The CB-CPW structure is shown in Figure 1. The CB-CPW structure have easy heat dissipation and high mechanical

strength characteristics and also provides good transmission performance like Coplanar Waveguide (CPW)




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structure.The CB-CPW structure can produce resonance phenomena. The CB-CPW structure is composed of the

following 3 layers.

Ground layer

Dielectric substrate layer

Radiating layer

Figure 1: Structure of Conductor Backed Coplanar Waveguide

Ground layer covers the entire back-face of the substrate. This layer is made up of copper.The ground layer serves as a

third return conductor. Under cavity model theory, the ground plane (ground layer) at both ends of the transmission

line and the back-face of the substrate can be considered as two patch resonators which are surrounded by ideal

magnetic walls. Defected ground structure(DGS) disturbs the current distribution at the ground layer which results in

bandwidth enhancement.

Dielectric substrate plays a significant role in antenna design, influencing its overall performance. Hence, in order to

design a CB-CPW based MSA one should know about the various available substrates with their characteristics.

Comparing with other dielectric substrates RT Duroid is found to have very low dielectric constant which can be used

to obtain wide bandwidth. RT Duroid is a Glass Microfiber Reinforced PTFE (polytetrafluoroethylene) composite

produced by Roger Corporation. RT Duroid has the most negligible electrical loss of any reinforced PTFE material,

low moistness, are isotropic, and have uniform electrical properties.

The radiating layer is the top layer of the CB-CPW structure. This layer is made up of a conductive material such as

copper. It consists of a microstrip line unit surrounded by coplanar ground (conductive material) as shown in

Figure 1. The microstrip line is excited by the electromagnetic energy and this energy is coupled to the coplanar

ground thus radiation takes place.

The resonant frequency [14] and the impedance of the CPW structure can be calculated as follows:

fmn =

√ [ (m/wg)

2 + (n/lg)

2 ]

Z0 = √

Where

fmn = resonant frequency

Z0 = impedance

C = speed of light

εr = relative permittivity of the material

m,n = factors of the resonant mode

wg,lg=width and length of the ground plane at both ends of CPW respectively

L, C= distributed capacitance and distributed inductance of the transmission line respectively




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III. MODELING AND ANALYSIS

The proposed antenna design is based on the Conductor Backed Coplanar Waveguide structure (CB-CPW). RT

Rogers 5880 with dielectric constant 2.2 is used as dielectric substrate. The microstrip line and ground layer are made

of conductive material such as copper. The antenna has a compact size of 40 40 1 mm3.

Four CB-CPW antennas with different geometrical shapes are tested to analyze the impact of different shapes on

bandwidth of the antenna. As shown in Figure 2, Antenna 1 is a triangle cut off from the common surface, Antenna 2

is a pentagonal cut off from the common surface, Antenna 3 is a hexagonal cut off from the common surface and

antenna 4 is an octagonal cutoff. All the four antenna have the same grounding structure as shown in Figure 3. The

dimensions of the four antennas are given in table 1.

(a)Antenna 1 (b)Antenna 2

(c)Antenna 3 (d)Antenna 4

Figure-2: Comparison of four antenna geometric shapes. (a) Antenna 1, (b) Antenna 2 and (c) Antenna 3 (d)

Antenna 4 respectively correspond to the triangle, pentagon, hexagon, octagon to be cut off.




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Figure 3:Bottom view of antenna 1, 2, 3, 4

Table 1:Dimensions of different antenna shapes

The four different geometrical shaped antenna were simulated and the return loss characteristics of all the

four antenna were analyzed.The return loss characteristics of the four antennas is shown below in Figure 4. Comparing

the return loss characteristics of the four structures, Antenna 3 has the best effect.

Figure 4:Return loss(S11) comparison of four antenna shapes

Antenna

No

Antenna dimensions in mm

W/L H a b c d e f g h R1 R2

1 40 1 30.02 - 2.4 2 20 12 10 5 2 5

2 40 1 17.63 - 2.4 2 20 12 10 5 2 5

3 40 1 14 - 2.4 2 20 12 10 5 2 5

4 40 1 7.07 19 2.4 2 20 12 10 5 2 5




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From Figure 4 it is clear that antenna 4 has the best return loss characteristics and achieves a bandwidth of 8.6 GHz

from 3.6 GHz to 12.2 GHz. Since UWB ranges from 3.1 GHz to 10.6 GHz, a parametric analysis is carried out to

optimize the design in order to achieve wide bandwidth. Antenna 4 shows best performance after optimization but at

3.1 GHz the return loss is above -10dB. To bring it down the antenna is incorporated with a circular parasitic patch.

The optimized proposed antenna design is shown in Figure 5.

(a)Top view (b) Side view (c) Bottom view

Figure 5:Illustration of proposed antenna design

The specific dimensions and optimized values of the proposed antenna design are shown in Table 2.

Table 2.Dimensions of the proposed design

Parameters W/L H a b c d e f g h S R1 R2

Dimensions(mm) 40 1 7.07 19 2.4 2 20 12 10 4 7 2 5

IV. RESULTS AND DISCUSSION

A. RETURN LOSS

A conductor backed coplanar waveguide based microstrip antenna is analyzed in this paper using CST Microwave

Studio software. S11 represents the return loss of the antenna, this parameter measures how much input power given

to device is reflected back at input port. The proposed design has return loss under -10 dB over the frequency range of

2.78 GHz to 12 GHz as shown in Figure 6.

Figure 6:Return loss (S11) of designed antenna




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B. VSWR

VSWR stands for Voltage Standing Wave Ratio, and is also referred to as Standing Wave Ratio(SWR). VSWR is a

function of the reflection coefficient, which describes the power reflected from the antenna. The smaller the VSWR is,

the better the antenna is matched to the transmission line and more power is delivered to the antenna. The minimum

VSWR is 1.0. In this case, no power is reflected from the antenna, which is ideal. For good functioning, VSWR should

be less than 2. The VSWR of the proposed wideband antenna is below 2 from 2.78 GHz to 12 GHz as shown in

Figure 7.

Figure 7:VSWR characteristics

C. RADIATION PATTERN

The radiation pattern of an antenna is the important characteristics of antenna because it indicates the distribution of

the energy radiated by an antenna in the space.

(a) (b) (c)

Figure 8:2D radiation pattern (a) 5.8 GHz (b) 10 GHz (c) 11GHz

The 2-D radiation pattern (E-plane and H-plane) was measured at 5.8 GHz, 10 GHz, 11 GHz, respectively and is

shown in Figure 8. At 5.8 GHz frequency, the E-plane of the antenna shows the quad-directional radiation patterns (4

beam) and, in the H-plane of antenna shows the dipole type of radiation patterns. At 10 GHz frequency, the E-plane of

the antenna shows the quad-directional radiation patterns and, in the H-plane of antenna shows the dipole type of

radiation pattern. At 11 GHz, E-plane shows omnidirectional and the H-plane shows quad-directional radiation

pattern. Therefore, the designed antenna achieves multi-beam performance.




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D. GAIN

Antenna gain is the ability of the antenna to radiatemore or less in aparticular direction. The relationship between

simulation gain and frequency of the proposed antenna is shown in Figure 9. The gain of 5.8 GHz, 10 GHz, and

11GHz is 2.95 dB, 5.82 dB and 6.12 dB, respectively, among which the peak gain occurs at 11GHz.

Figure 9: Gain of designed antenna

E. DIRECTIVITY

Directivity is the measure of the concentration of the antennas radiation pattern in a particular direction. Figure 10

shows the simulation data of proposed antenna directivity in the range of 2.78 GHz to 12 GHz. Maximum directivity

of 8.13 dB is obtained at 11.1GHz.

Figure 10:Directivity of designed antenna




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F. SURFACE CURRENT DISTRIBUTION

Figure 11 shows the Surface Current Distribution of the proposed antenna for frequencies 5.8 GHz, 10 GHz, 9 GHz

and 11 GHz. We observe that the maximum current intensity is 99.2 A/m, 101 A/m, and 110 A/m at 5.8GHz, 10GHz,

and 11 GHz respectively. Another observation is that the current intensity increases around the transmission line and

at the edges of octagonal cut off.

Figure 11:Surface current distribution of the designed antenna at 5.8 GHz, 10GHz, and 11GHz.

G. COMPARISON OF PROPOSED ANTENNA WITH OTHER REPORTED ANTENNAS

Table 3.Comparison of proposed antenna with other reported antennas

Reference No Size(mm3) Operating band(GHz)

[2] 50×50×1 1.71-3.66

[5] 46×52×1.6 2-6

[6] 36×40×1.6 3.4-11

[9] 43×29×1.6 5.1-5.9

[10] 36×36×1.6 2.63-10.44

[15] 40×40×1 4.8337-8.1714

Proposed 40×40×1 2.78-12

V. CONCLUSION

Microstrip antennas have many advantages such as small size, low cost etc., but their major disadvantage is achieving

wide bandwidth. In this work a low profile microstrip antenna is modified to achieve wider operating frequency range.




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The proposed design is based on CB-CPW structure. The CB-CPW based microstrip antenna is designed and

simulated in CST Microwave Studio software. Initially four different CB-CPW structures are designed and their return

loss characteristics were tested. The antenna with octagonal cutoff from the common surface shows best return loss

characteristics. The octagonal cut off antenna structure is further modified by incorporating a circular parasitic patch

to further enhance the bandwidth. Defected ground structure (DGS) is also used to obtain broadband effects. The

antenna has a compact size of 40 40 1 mm3. Parametric analysis is carried out to determine the relation between

bandwidth and chosen dimensional parameters. The proposed optimized design operates over a wide frequency range

from 2.78 GHz to 12 GHz.

VI. REFERENCES

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BROADBAND CB CPW BASED MICROSTRIP ANTENNA …