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Leonardo Electronic Journal of Practices and Technologies

ISSN 1583-1078

Issue 28, January-June 2016

p. 211-224

211 http://lejpt.academicdirect.org

Bandwidth enhanced electromagnetic bandgap structure structured closed

ground monopole antenna

Modali S. S. S. SRINIVAS1, Tottempudi Venkata RAMAKRISHNA1, Boddapati T. P.

MADHAV1,*, Sathuluri Venkata RAMA RAO2, Shaik ASHRAF ALI2

1 Department of ECE, K L University, AP, India

2 Department of ECE, NRI Institute of Technology, Pothavarapadu, Krishna District, AP, India Emails: [email protected]; [email protected]; [email protected];

[email protected]; [email protected] * Corresponding author, Phone: +91-9908243452

Received: May 7, 2016 / Accepted: July 17, 2016 / Published: July 31, 2016

Abstract

The primary barrier to implement microstrip patch antennas in modern

broadband communication systems are their narrow bandwidth. For

broadband antennas some of the characteristics are very essential like feed

impedance matching, patch geometry optimization and suppression of surface

waves etc. To improve impedance bandwidth low permittivity substrate with

increased thickness is required. But by taking low permittivity with substrate

thickness, the surface wave related problems will be raised. To overcome this

problem a coplanar wave guide fed square patch monopole antenna with

closed ground structure is proposed in this paper and electromagnetic band

gap structure is added to the antenna design to enhance the impedance

bandwidth, this being the aim of the present work. Antenna with square patch

and closed ground structure is designed to resonate at dual band and by adding

Electromagnetic Bandgap Structure (EBG) structure without closed ground

structure in the design the modified antenna is resonating at triple band. To

enhance the bandwidth and to suppress the surface waves related problems,

we incorporated EBG structure and closed ground structure in the proposed

antenna model. The proposed model attained bandwidth more than 10.7 GHz

with impedance bandwidth of 82.3%. In this design the HFSS electromagnetic

Bandwidth enhanced electromagnetic bandgap structure structured closed ground monopole antenna

M. S. S. S. SRINIVAS, T. V. RAMAKRISHNA, B. T. P. MADHAV, S. V. RAMA RAO, S. ASHRAF ALI

212

simulator tool results are in good agreement with fabricated antenna measured

results over ZNB 20 vector network analyzer.

Keywords

Bandwidth Enhancement Closed Ground; Coplanar Waveguide (CPW);

Electromagnetic Bandgap Structure (EBG); Monopole Antenna; Vector

Network Analyzer (VNA)

Introduction

Microstrip patch antennas are one of the attractive candidates for modern

communication systems to transmit and receive the data from 1m to 104 m. When used with

microwave integrated circuits, the fabrication becomes simpler and low profile designs can be

generated for numerous communication and military applications [1-2]. Due to surface waves,

the microstrip patch antennas are suffering with low bandwidth and gain. Generally a

conventional microstrip patch antenna consisting of patch on the surface of substrate material

and ground plane at bottom side. The ground plane supports the propagation of TM surface

waves. These waves travel along the surface that causes unnecessary end–fire radiations. With

the image currents being out of phase it further causes weakening of the radiation pattern [3-

5]. In addition in many of the stated applications there would be a multi-frequency operations

requirement. So, the enhancement of the bandwidth and the achievement of multi frequency

operation are major challenges for the microstrip patch antennas.

Surface waves are TM (Transverse magnetic) and TE (Transverse electric) modes of

the substrate. These modes are characterized by waves attenuating in the transverse direction

(normal to the antenna plane) and having a real propagation constant above the cutoff

frequency. The phase velocity of the surface waves is strongly dependent on the substrate

parameters h and εr. The lowest order TM-type mode has no cut off frequency [6-8]. It is

designated as the TM0 mode .The cutoff frequencies for the higher order TMn and TEn modes

are given by:

1h4ncf rc −ε= (1)

where c is the velocity of light in free space, h is the substrate thickness and n=1, 3, 5... for

TEn modes and n=0, 2,4,...for TMn modes. It will even propagate on very thin substrates


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having low dielectric constant values at nearly the velocity of light. The field distribution of

the TE1 mode is such that it can propagate below the patch metallization, and can always be

excited above the cut-off frequency. The dispersion relations for the TMn and TEn modes are

given below (where μ02 = β2 - k0

2 and μ2 = β2 - k02εr):

÷ for TMn modes (n = 0, 2, 4, ...): εr·μ0·h + μ·h·tan(μ·h) = 0

÷ for TEn modes (n = 1, 3, 5, ...): μ0·h + μ·h·cot(μ·h) = 0.

However, one needs to know the exact value of β for the analysis of microstrip

antennas. The exact value of is generally obtained using a root searching algorithm. The

initial value of β is still needed in a closed form for root searching. For the TM0 mode in

lossless substrates, the radial propagation constant β is real, and is bounded by 1 < β/k0 < εr. If

the substrate is electrically thin, β/k0 ≈ 1. The value of β/k0 can then be obtained by assuming

β/k0 = 1 + δ:

∑∞

=

=δα−δ+δε0n

nn

2r 02

(2)

where the infinite sum in (1) is a Taylor’s series for )zhktan(z 2r0

2r −ε−ε around the

point z = β/k0 = 1. In (1):

⎥⎦

⎤⎢⎣

⎡+−=α=α

)hsk(coshsk)hsktan(

s1),hsktan(s

02

00100

(3)

Retaining only the dominant term α0 in (1), one can obtain [5, 6]:

1s r −ε= (4)2

0r

r0 hk1

211k/ ⎟⎟

⎠

⎞⎜⎜⎝

⎛ε−ε

+≈β (5)

Two-term Taylor approximation in (1) yields(5):

21

2r

2010

2rr

2r10

0

21

k α−εα+αα−εε+ε−αα

+=β

(6)

This expression gives good accuracy for sh < λ0/4. If the substrate has moderate losses

defined as εr·(1-j·tan(δ)), then the surface wave propagation constant will also be complex

valued defined as βr = (βr-j·βi) with βi > 0. The approximate expression for β is found as

(where the value of β is defined in (5) or (6)):

])/hk(tan[)1(, 2r0rir εδ−ε≈ββ=β (7)

For a multilayered substrate consisting of N different layers, a general formula for β/k0



214

for the TM0 mode has been obtained (7) it is a generalized function of (4) and is given as: 2N

1ii0

ri

ri0 hk1

211k/ ⎥

⎦

⎤⎢⎣

⎡ε−ε

+=β ∑=

(8)

where and are the constituent parameters of the i th layer. It is also of interest to know the

value of the substrate thickness such that only the surface wave more propagates and all other

modes are below cutoff. For the single-layer substrate one obtains from (a) for n=1:

141h

r0 −ε<

λ

(9)

The condition for the double-layer substrate case is obtained as follows:

11)1hktan()1hktan(

1r

2r2r201r10 −ε

−ε≤−ε−ε

(10)

Two solutions to the surface wave problem are available now. One approach is based

on the micromachining technology in which part of substrate beneath the radiating element is

removed to realize a low effective dielectric constant environment for antenna. In this case the

power lost through surface wave excitation is reduced and coupling of power to the space

engineering. In this case, the substrate is periodically loaded so that surface wave dispersion

diagram presents a forbidden frequency range about antenna’s operating frequency [9-11].

Because surface waves cannot propagate along the substrate, an increased amount radiated

power couples to the space wave. Also, other surface wave coupling acts like mutual coupling

between array elements and interference with on hard systems are now absent.

The second solution can be addressed through electromagnetic bandgap structures

(EBG). The EBG structures which are used in the microstrip patch antennas can reduce the

surface wave problems and improve the gain and radiation efficiency of the antennas [12-15].

EBG is also known as an artificial magnetic conductor which can reduce the side lobes and

back lobes levels with its typical structure in the design. Antennas can be designed using EBG

substrate, which are selective in supporting surface waves without reversing the phase of the

reflected waves [16-18]. At the same time, it provides very high impedance within a specified

frequency range. Also microstrip patch antenna using electromagnetic bandgap can provide a

directive radiation pattern with increase in the directivity of the main lobe and by minimizing

the radiation in undesired direction. In this article the properties of electromagnetic bandgap

structures are utilized in substrate design and with the help of closed ground structure.


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Material and method

The design of the antenna will start with mathematical formulation and which is based

on the dielectric material permittivity and resonant frequency (see the Flow Chart).

Flow Chart: Algorithm for the proposed work

Design and simulation is done with commercial electromagnetic tool High Frequency

Structure Simulator (HFSS), which is based on finite element method.

A small portion of radius 1mm is removed in the form of cylinder on the FR4 substrate

and filled with another dielectric material PEC. The substrate is the sandwich of FR4 and

PEC, which forms the EBG structure in the substrate. Square patch and ground plane are

designed with copper material of thickness 0.05 mm.

An impedance of 50 ohms is attained with the coplanar wave guide feeding technique

in the design. The optimized antenna model after simulation is fabricated on FR4 substrate

with thickness 4.4. The prototyped antenna is tested using vector network analyzer for

validation.

Antenna geometry

Figure 1 shows the square patch antenna with closed ground structure on FR4

substrate. The overall dimension of the antenna is around 40x40x1.6 mm (Table 1). Figure 2

shows the EBG structured antenna without closed ground structure. PEC material is inserted

in the FR4 substrate adjacent to patch in the cylindrical shape with height of 1.6 mm.

Mathematical formulation to calculate the dimensions of the antenna design

Design and simulation of the antenna models using HFSS Tool

Optimization of the antenna models using HFSS and prototyping on FR4 substrate

Measuring antenna S-parameters using ZNB 20 VNA

Comparing measured results with simulation results for validation



216

Figure 1. Antenna Iterations, (a) Square Patch with Closed Ground Structure,

(b) EBG without Closed Ground Structure, (c) EBG with Closed Ground Structure

Figure 2. Proposed EBG Antenna with Closed Ground Structure

Table 1. Antenna dimensions

Parameter Sw SL Pw PL Fw G rDimensions in mm 40 40 6 6 3 0.5 1

The overall area occupied by the ground structure in the proposed antenna model is

40% less in compared with basic antenna model given in Figure 1(a).

Results and discussion

Designed antenna models are simulated using HFSS tool and their |S11|, radiation

pattern and field distributions are presented in this section.

Figure 3 shows the proposed EBG antenna with closed ground structure and shows the

return loss curve for the square patch antenna with closed ground structure. The basic antenna

model is resonating at triple band with impedance bandwidth of 18% at 12.5 GHz, 6% at 16.5

GHz and 5.5% at 18.5 GHz.

Figure 4 shows the return loss curve for the EBG structured antenna without closed

ground plane.


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Figure 3. Return Loss Vs Frequency of Square Patch with Closed Ground Structure

Figure 4. Return Loss Vs Frequency of EBG without Closed Ground Structure

The designed antenna is resonating at dual band with impedance bandwidth of 20% at

1.5 GHz and 22% at 17.5 GHz. The proposed EBG structured antenna with closed ground

plane is shown in Figure 5.

Figure 5. Return Loss Vs Frequency of EBG with Closed Ground Structure

Proposed antenna is resonating in wideband from 8 GHz to 19 GHz with bandwidth of



218

11 GHz. By adding EBG structure to closed ground plane we attained impedance bandwidth

of 73% at center frequency of 14 GHz.

Figure 6, 7 and 8 shows the radiation pattern of the antenna models at resonating

frequency, where significant consistency is achieved.

Figure 6. Radiation pattern of the square patch antenna with closed ground structure in E,

H-plane and 3D

Figure 7. Radiation pattern of the EBG antenna without closed ground structure in E, H-

plane and 3D

Figure 8. Radiation pattern of the proposed EBG antenna with closed ground structure in E,

H-plane and 3D The antenna displays good omnidirectional radiation patterns. Within the operating

frequency band, nearly symmetric and equal radiation patterns in the E- and H-planes are

achieved. In addition, the measured cross-polarization levels in E- and H-planes are generally


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low across the whole frequency bandwidth.

Radiation pattern is highly distorted due to the asymmetry in the configuration. The

three dimensional view of the radiation pattern of the proposed antenna with antenna

placement in transparent scenario can be observed from Figure 9.

Figure 9. 3D view of the proposed antenna

The radiation field strength in different directions can be observed from this

appearance. Asymmetry in the structure induces an asymmetric field distribution in the

ground plane as shown in Figure 10.

Figure 10. E-Field distribution of the proposed antenna at 14GHz



220

Feed line and lower half of the patch surface is showing more field intensity compared

with ground plane. The proposed antenna and base model are fabricated on FR4 substrate of

dielectric constant 4.4 and thickness 1.6 mm.

Figure 11 shows the prototyped antennas of model 1 and model 3.

a) b) Figure 11. Prototyped Antennas, (a) Square Patch Closed Ground Antenna,

(b) EBG structured Closed Ground Antenna

A small spot on substrate i.e., EBG can be observed on both sides of the patch.

Coplanar wave guide feeding is used in the design and SMA connector is connected to

support 50 ohm impedance in the fabricated models.

ZNB 20 Vector network analyzer is used to analyze the reflection coefficient results of

the prototyped antenna. Figure 12 shows the measured reflection coefficient of the proposed

EBG structured closed ground antenna.


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Figure 12. Measured Return loss of proposed antenna on ZNB 20 VNA

Antenna is showing similar kind of reflection coefficient results of simulated model

and good agreement can be observed between HFSS and VNA. Antenna is showing almost 11

GHz bandwidth in the desired operating band.

Conclusions

The fabricated antenna results on VNA are in good agreement with the simulated

results. Antenna 1 with square patch and closed ground structure is resonating at triple band

with impedance bandwidth of 18% at 12.5 GHz, 6% at 16.5 GHz and 5.5% at 18.5 GHz.

Antenna 2 with EBG structure and without closed ground plane is resonating at dual band

with impedance bandwidth of 20% at 1.5 GHz and 22% at 17.5 GHz. The proposed EBG

structured antenna with closed ground plane is resonating in the wideband from 8 GHz to 19

GHz with bandwidth of 11 GHz. By adding EBG structure to closed ground plane we attained

impedance bandwidth of 73% at centre frequency of 14 GHz.



222

The proposed antenna model is showing gain more than 4 dB in the operating band

with efficiency greater than 85%. The results obtained from the current antenna are most

suitable for the stable gain wideband communication systems.

Acknowledgements

Authors like to express their gratitude towards the department of ECE and

management of K L University and management of NRI Institute of Technology for their

support and encouragement during this work. Further Madhav likes to express his gratitude

towards FIST grant from DST (SR/FST/ETI-316/2012).

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