The document discusses the fundamental principles of antennas, particularly focusing on microstrip antennas (MSAs) and their design objectives, highlighting their advantages such as low profile and integration ability, as well as challenges like low gain and narrow bandwidth. It details the use of a defected ground structure (DGS) design to enhance performance, presenting a novel equivalent circuit model for a microstrip patch antenna with a circular defect aimed at Ku-band applications. The results indicate a significant impedance bandwidth and gain, demonstrating that the proposed antenna design achieves effective radiation characteristics suitable for its intended frequency range.

1. 1
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

2. 2
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.

3. 3
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

4. 4
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

5. 5
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.

6. 6
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|)

7. 7
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

8. 8
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 16.00
Freq [GHz]
-35.00
-30.00
-25.00
-20.00
-15.00
-10.00
-5.00
dB(St(feed_T1,feed_T1))
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 : LastAdaptive
Freq='12.52GHz' Phi='0deg'
dB(rETotal)
Setup1 : LastAdaptive
Freq='12.52GHz' Phi='90deg'

9. 9
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