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1. INTRODUCTION

1.1 Basic Theory of Antennas

An antenna is an electrical transducer which transforms electric currents in to electromagnetic

waves. In general, any flow of electric charges produces disturbances in the electromagnetic

field which propagate as a wave, with certain speed depending on the medium. The reverse

process is possible and accounts for the capability of antennas to receive electromagnetic

radiation. In free space, for instance, the speed is that of the light. This result is a

consequence of Maxwell Equations which govern all electromagnetic phenomena, and

similarly, makes up the fundamentals of antenna theory.

Over the years antennas evolved from simple metal sticks, properly called dipoles, to quite

complex structures, ranging from several meters to a few millimeters in dimension. This

diversity in antenna dimensions, extending as well over the design technology, has been

motivated mainly by the need of telecommunications to send and receive higher amounts of

information, which implies increasing the frequency of the carrier channel. The way the

frequency relates to the size of the antennas is in fact one of the consequences of Maxwell

Equations, which apply a constraint on the antenna structures for the optimal emission and

reception of electromagnetic waves. Thus, recalling the discussion from the previous

paragraph, the wave length (λ) of an electromagnetic wave in a medium is:

√

In the equation, εr is the relative permittivity of the medium, ƒ is the frequency of the wave,

and c is the speed of light in free space. This equation states that as the frequency increases

the wave length decreases proportionally and so do the dimensions of the antenna emitting or

receiving such a wave. For comparison, figure 1.1 shows multiple Microwave antennas and

figure 1.2 shows an AM radio broadcast antenna which deals with smaller frequencies,

meaning larger wave lengths.

Fig. 1.1 Several microwave antennas Fig. 1.2 Radio broadcast antenna

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1.2 Two Main Qualities of Antennas

In general there are two main measures that define and qualify an antenna, the Resonant

Capability and the Radiation Capability. The Resonant Capability is the most important

characteristic of any antenna. It determines the specific frequency of electromagnetic waves

(EM Wave) at which the antenna structure favors the establishment of the varying electric

field lines which induce a potential difference between isolated parts of the antenna structure.

This phenomenon occurs when the dimensions of the antenna structure allow half, full waves,

or several periods of them. Then, the varying potential difference that those EM waves

originate is emphasized from among the infinite number of frequencies traveling in the space.

The Radiation Capability relates, in a single subject, two other important characteristics of an

antenna. Readily, these two qualities are Antenna Efficiency (Antenna Gain) and Radiation

Pattern. Together, Antenna Efficiency and Radiation Pattern talk about the way antennas use

the energy supplied to radiate it all, in any or a particular direction, ideally, or most of it in

the best real life scenarios. Radiation Capability, despite the short description given in this

paragraph, involves a whole lot of branches of the Engineering, Physics, and Chemistry

fields, for its analysis and development of improving technologies. As for instance, important

research and experimentation is being performed in private and academic Microwave and

Electromagnetic laboratories about the implementation of Double Negative Materials (DNG

Materials), Meta materials indeed, for the gain improvement and size reduction of Microwave

Antennas. Keeping the focus to this project, it is important to mention that the technology

employed in the design of the multiband antenna, MSA, has lately become much popular

with in this group of Microwave Antennas.

1.3 Remarkable Advantages and Disadvantages of Microstrip Antenna Technology

MSA’s, as discussed earlier, are intrinsically light and small sized elements that can be

fabricated with quite inexpensive materials, even in the retail market. Moreover, they are

easily implemented with Microwave Integrated Circuits, as they are popularly built on

Printed Circuit Boards (PCB’s). And, in favor of the project, they are capable of dual and

triple frequency operation, as explained on the previous paragraphs.

However, compared with most popular antenna structures for wireless applications, MSA’s

suffer from two parameters which make them not the first option if high enough gain and

wide band response is a requirement.

Nevertheless, these disadvantages vary from geometry to geometry, which opens the

possibility to employ computer aided simulations. Then, through a series of parametric

analyzes, we may find the most convenient shape and dimensions, depending on the set goals

and expectations for the project. This is in fact the approach followed in the design of the

Multiband MSA.

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2. DESIGN OBJECTIVES

Microstrip antennas are widely used due to their low profile, ease of fabrication, and

capability of integration with other devices. Conventional Microstrip antenna has

disadvantages of narrow bandwidth, low gain and cross-polarized radiation characteristics.

Thus, enhancement of bandwidth and gain of Microstrip patch antenna (MPA) has become a

challenging current research area. MPA with defected ground structure (DGS) has been

reported for improving the performance of the patch antenna. The MPA with DGS has been

reported for suppressing the cross-polarization level. Dual band with circular polarization in

both planes has been achieved by using truncated DGS MPA. Rotated square-shape defect in

the ground plane has been reported for enhancing the impedance bandwidth up to 50% with

gain ranging from 3 dBi to 6 dBi. Several studies have been reported for DGS with

Microstrip-line. DGS has been presented for reducing the size of low pass filter with wide-

stop-band characteristics. Size reduction and harmonic suppression of Microstrip branch-line

coupler by using DGS has been reported. A complimentary split ring resonator loaded ground

plane for low-pass filter with compact size and wide stop band characteristics, and an

equivalent circuit model has been presented. In present endeavor, a Microstrip linefed MPA

with DGS is proposed for Ku-band applications. A circular shaped defect is integrated in the

ground plane. A novel equivalent circuit model is presented for calculating the input

impedance of the proposed MPA. The simple designing equations are presented and

characteristics of the proposed antenna are analyzed by Finite Element Method (FEM) on

Ansoft HFSS v.13.

Fig. 2.1 Top and bottom view of proposed antenna

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3. SOFTWARE AIDED DESIGN

The schematic of the proposed MPA is shown in Fig.3.1. A circular slot of radius a is

integrated in the ground plane and an open ended Microstrip line of width wml and length

Lml is placed on the opposite side of the ground plane. The radius of the circular slot is

chosen, same as the radius of simple circular Microstrip patch antenna (CMPA). By making a

circular defect of same radius in the ground plane, the new structure shows wide band

characteristics.

Fig. 3.1: Structure of the proposed antenna

Table 3.1: Design specifications

Parameter (mm) Parameter (mm)

a 5.50 Lml 27.41

r1 1.50 h 1.6

wml 2.30 D 36

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The distance between the open-end-edge of Microstrip-line and centre of the circular slot is

referred as feeding distance r1and it controls the return loss level of antenna. The design

frequency for proposed structure is considered as 10 GHz.

3.1 Equivalent circuit model

The modeling of equivalent circuit is divided in four parts; modeling of length l1and l2of

Microstrip line, modeling of circular slot, modeling of fringing capacitance Cf due to

discontinuity of Microstrip line, and modeling of coupling capacitance Cp between circular

slot and Microstrip line. Circular Slot impedance is modeled by an ideal transformer with the

Microstrip line, as shown in Fig. 3.1.

Fig. 3.2: Equivalent circuit model of the proposed antenna

3.1.1 Modeling of Microstrip line

A lossy Microstrip line is represented by a tank circuit of parameters, series resistant R, series

inductance L, shunt conductance G, and shunt capacitance C. These parameters R, L, G and

C are in per unit length and calculated for length l1and l2of Microstrip line.R1, L1, G1, C1and

R2, L2, G2, C2are corresponding to the Microstrip line of length l1and l2 respectively. Losses

in Microstrip line are due to imperfect conductor and dielectric, and is characterized by

attenuation constant α, determined by

α=αc +αd

Where αc and αd are referred as conductor loss and dielectric loss respectively.

3.1.2 Modeling of circular slot

The circular defect of diameter 2a is considered as a rectangular defect of length l and width

w. The width of the rectangular defect is taken to be the same as the diameter of circular

defect and length of the corresponding rectangular defect is calculated by equating the area.

The input impedance offered by a rectangular slot can be calculated from the input

impedance of a cylindrical dipole using Babinate’s principle.

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3.1.3 Modeling of fringing capacitance

Due to the open-ended discontinuity, a fringing field of length l is developed at the end-edge

of Microstrip line. A fringing capacitance Cf is used to model this fringing field and defined

as

√

3.1.4 Modeling of coupling capacitance

coupling between the two circuits is modeled by a coupling capacitance Cp and given by

( ) √( ) (

)

Where Cs is the slot capacitance, calculated from reactive part of the slot impedance given in

Eq. (10), and the coupling coefficient Ck between two networks is determined by

√

Where Q1is the quality factor of the Microstrip line and Q2is the quality factor of the slot.

3.1.5 Total Impedance

The parallel slot impedance is given by

Z’slot=N. Zslot

The parallel slot impedance Z’slot, impedance Zp offered by coupling capacitance Cp, and

impedance Zf due to fringing capacitance Cf are added in parallel with impedance Z of

Microstrip line. Where Z is the impedance offered by Microstrip line and determined by

adding the impedances Z1and Z2in series. The equivalent circuit model of the proposed

structure is shown in Fig. 3.2.

Reflection coefficient, VSWR and Return loss is calculated as

( )

S11 =20 log (|r|)

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4. RESULTS

The proposed structure is analyzed on Ansoft HFSS v.13.FR4_epoxy substrate of dimension

36 mm*36 mm*0.762 mm is used to fabricate the prototype. The dielectric constant εr, loss

tangent, and thickness h of the substrate are as 2.2, 0.0009 and 1.6 mm respectively.

Fig. 4.1: Patched Antenna designed on HFSS v13

Fig. 4.2: Patch Antenna Covered By Airbox

The Microstrip-line-fed antenna with circular shape defect inthe ground plane is designed.

Accurate design-ing equations are presented for wideband Microstrip antennafor the

proposed structure. A novel equivalent circuit model ispresented for proposed prototype. An

impedance bandwidth of56.67% ranging from 9.8 GHz to 17.55 GHz has been achieved.

Thesimulated, measured and theoretical results are in good agree-ment. A gain of 5–12.08

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dBi is achieved in impedance bandwidth.The antenna produces good radiation pattern in the

impedancebandwidth range. The Cross-Polarization levels are below −35 dBand −20 dB in

H-Plane and E-Plane respectively. The isolations between the Co-Polar and Cross-

Polarization levels are about 35 dBin H-Plane and 15 dB in E-Plane. Thus, designed antenna

is a suitable candidate for Ku-band applications with good radiation characteristics.

Fig. 4.3: Retain loss curve

Fig. 4.4: Radiation pattern

10.00 11.00 12.00 13.00 14.00 15.00 16.00Freq [GHz]

-35.00

-30.00

-25.00

-20.00

-15.00

-10.00

-5.00

dB

(St(

fee

d_

T1

,fe

ed

_T

1))

HFSSDesign1XY Plot 1 ANSOFT

m 1

Curve Info

dB(St(feed_T1,feed_T1))Setup1 : Sw eep

Name X Y

m 1 12.5226 -31.9354

-23.00

-16.00

-9.00

-2.00

90

60

30

0

-30

-60

-90

-120

-150

-180

150

120

HFSSDesign1Radiation Pattern 5 ANSOFT

Curve Info

dB(rETotal)Setup1 : LastAdaptiveFreq='12.52GHz' Phi='0deg'

dB(rETotal)Setup1 : LastAdaptiveFreq='12.52GHz' Phi='90deg'

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REFERENCES

[1] M.K. Khandelwal et al./Int.J.Electron.Commun.(AEÜ)68(2014)951–957

http://dx.doi.org/10.1016/j.aeue.2014.04.017

[2] A Novel Neural Network for the Synthesis of Antennas and Microwave Devices

Heriberto Jose Delgado, Member, IEEE, Michael H. Thursby, Senior Member, IEEE,

and Fredric M. Ham, Senior Member, IEEE IEEE TRANSACTIONS ON NEURAL

NETWORKS, VOL. 16, NO. 6, NOVEMBER 2005

[3] Design of a Widebland Microstrip Antenna and the Use of Artificial Neural Networks

in Parameter Calculation Dipak K. Neog1, Shyam S. PattnaiK, Dhruba. C. Panda2,

Swapna Devr, Bonomali Khuntia3, and Malaya Dutta4 IEEE Antennas and

Propagation Magazine, Vol. 47, No.3, June 2005