Design Multiband FSS using a unique method

7/23/2019 Design Multiband FSS using a unique method

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DESIGN OF MULTIBAND FREQUENCY SELECTIVE

SURFACES

By:

KALEEM ULLAH Reg. No:11PWELE3985

ABDUL RAHMAN ALI Reg. No:11PWELE3994

ADNAN Reg. No:11PWELE3981

Supervised By:

DR. SHAHID BASHIR

DEPARTMENT OF ELECTRICAL ENGINEERING

UNIVERSITY OF ENGINEERING AND TECHNOLOGY

PESHAWAR


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CONTENTS

CONTENTS ............................................................................................................................................ i

LIST OF FIGURES ............................................................................................................................. iii

LIST OF TABLES ............................................................................................................................... iv

AKNOWLEDGEMENTS .................................................................................................................... v

ABSTRACT ......................................................................................................................................... vi

Chapter-1 .............................................................................................................................................. 1

WHAT ARE FREQUENCY SELECTIVE SURFACES? ................................................................ 1

1.1 Introduction: .............................................................................................................................................. 1

1.2 Application of FSS: ................................................................................................................................... 8

1.2.1 Microwaves oven: .............................................................................................................................. 8

1.2.2 Stealth technology: ............................................................................................................................. 9

1.2.3 sub-reflector in multiband antenna: .................................................................................................... 9

1.2.4 Security ............................................................................................................................................. 10

1.2.5 Interference ....................................................................................................................................... 10

1.3 Governing factor of FSS: ......................................................................................................................... 11

1.3.1 Geometry of the element: ................................................................................................................. 11

Chapter-2 ....................................................................................................................................................... 13

DESIGNING THE PROPER STRUCTURE ................................................................................... 13

2.1 Dimensions of the element: ..................................................................................................................... 13

2.2 Accuracy required: .................................................................................................................................. 14

2.3 The stop band attenuation: ....................................................................................................................... 14

2.4 Response to different incident angles: ..................................................................................................... 14

Chapter-3 ....................................................................................................................................................... 16

METHODOLOGY ........................................................................................................................................ 16

3.1 Method of moment: ................................................................................................................................. 16


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3.2 Finite Element Method: ........................................................................................................................... 17

3.3 Finite difference time domain method: ................................................................................................... 17

3.4 Equivalent Circuit Method: ..................................................................................................................... 18

3.5 Analytical Parameter Adjustment Method (APAM):.............................................................................. 19

Chapter-4 ....................................................................................................................................................... 21

RESULTS ............................................................................................................................................ 21

4.1 Simulation results: ................................................................................................................................... 21

4.2 Prototype: ...................................................................................................................................... 26

4.2.1 Materials needed for prototype construction: ........................................................................ 26

4.2.2 Procedure for prototype construction: .................................................................................... 27

CONCLUSION ................................................................................................................................... 30

BIBLIOGRAPHY .............................................................................................................................. 31


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LIST OF FIGURES

1. Figure 1.1. FSS of cross dipoles elements ................................................................2

2.

Figure 1. 2. Some type of conducting elements of FSS sheet ...................................4

3.

Figure 1.3. Unit cell arrangements for (a) single-strip, (b) double-strip, and (c) triple-stripFSS element,(d) PWS basis function on a strip element ..........................................5

4. Figure 1.4. Characteristics of PBG unit and periodic sheet regarding reflection phase vs.

frequency...................................................................................................................6

5. Figure 1.5. Fractal triangle with dielectric substrate .................................................7

6. Figure 1.6. The basic four kinds of electromagnetic filter responses based on specific

geometry ...................................................................................................................9

7.

Figure 1.7 FSS used in a Multiband antenna as a sub reflector ..............................10

8.

Figure 1.8 Most known shapes of elements .............................................................11

9.

Figure 1.9 Modified Square Loop of the current research project ...........................12

10.Figure 2.1 Multipath Propagation of the Signals between two rooms .....................15

11. Figure 3.1 Evaluation of Electric fields .................................................................17

12.

Figure 4.1 two ports with MSL design in CST ........................................................21

13.

Figure 4.2 The normal incident of both TE wave ...................................................22

14.

Figure 4.3 Transmission parameters for TM wave at normal incident ...................23

15.

Figure 4.4 Transmission parameters for TE wave incident at 30 degrees ..............24

16. Figure 4.5 Transmission parameters for TM wave incident at 30 degree ..............24

17. Figure 4.6 Transmission parameters for TE wave incident at 45 degrees .............25

18. Figure 4.7 Transmission parameters for TM incident at 45 degrees ......................25

19.

Figure 4.8 a: Manually printed design on the copper clad FR4 Substrate ..............28

20.Figure 4.8 b: Manually design prototype box ..........................................................29


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LIST OF TABLES

Table 2.1 Element parameters for current project unit cell ...........................................13

Table 3.1 Effect of parameters on resonant frequencies ................................................19


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AKNOWLEDGEMENTS

We are very happy on the completion of our final year project. It was a great experience. We

would specially thank our supervisor Dr. Shahid Bashir for his great supervision and

encouragement throughout the project. His guidance led us to the completion of this project.

He never restricts us to explore new ideas. His enthusiasms made us believe that we can do

this and we did it. Thank you again for your great guidance and believing in us.

We also want to thank our parents who throughout our lives looked for us, believed in us and

never let us feel helpless. This success would not be possible without our parentsguidance

and help. From class first till graduation our parents cared for us and felt proud of us. We are

so much grateful to have you in our lives. Thank you so much.

We also want to thank the UET PESHAWAR who gave us an opportunity to study in such a

great university and provided us the better and quality education. We are grateful to all

teachers and friends who gave better ideas and helped us during our education career.


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ABSTRACT

This thesis shows a novel technique for designing multiband frequency selective surfaces. A

dual stop band for WLAN transmission for security and interference improvement with a

modified square loop is presented as an example of multiband FSSs which gave us

outstanding results regarding the bandwidth of stop band and very little effect of the different

incident angles. The bandwidth of stop band was noted up to 380 MHz (at 2.4 GHz) and 90

MHz (at 5.8 GHz). The methodology and the structure used is very simple and needs no

calculations unlike others which gives us the advantages of saving time to design for any

resonant frequency. The new method used here was developed during simulating different

designs and noting the effect of parameters change on the simulation results. So it was given

the name of Analytical Parameter Adjustment Method (APAM). This method uses only theinformation of wavelength of resonant frequency to determine the approximate dimension of

element. Other parameters can be adjusted easily to achieve the resonance at desired

frequencies. The shape of element was chosen to be a modified square loop. Which gave us

the more stop band bandwidth compared to other shapes. The design is tested for incident

waves for different angles. The computer simulation technology (CST) microwaves studio

was used to get the required results.


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Chapter-1

WHAT ARE FREQUENCY SELECTIVE SURFACES

1.1 Introduction:

Wireless technologies increasingly pervade our society. Technologies such as cellular

phones, wireless local area networks (WLANs) and Bluetooth devices have brought

mobility and edibility to our everyday lives and open new opportunities to industry.

Use of these wireless technologies has been expanding, especially for wireless

applications in the unlicensed frequency band. For example, WLAN systems have been

widely deployed in commercial and residential houses. However, one important issue

arising from the proliferation of wireless devices in modern communities is that radio

signals may naturally propagate beyond their intended receivers and cause interference

with other users. For instance, for two WLAN systems installed in close proximity,

signals may propagate from one system to another. This is referred to as the

interference between coexisting systems. This kind of interference does not only

degrade the system performance which causes packet losses and throughput reductions

[1], but also the systems security and privacy is compromised. Thats why it is

essential to decrease the interference or to develop techniques that would allow systems

to operate well even in the presence of interference. For this purpose research work has

been done for long time and different kinds of techniques are presented. We study here

the Frequency selective surfaces. Nowadays in the era of new technology we have to

work more and more to design the best of these surfaces to be compatible with modern

technology. As decades ago just simple and a single layer FSS sheets were used to

block or allow a single frequency, so in this era we have to design sheets that can

transmit-in or transmit-out multiple frequencies at same time. For this purpose new

software and technology have been developed to study the characteristics of different

designs to choose the best one for the respective purpose. FSS sheets used in stealth

technology to avoid radar detection and that used in multiband antenna have different

designs.


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Frequency Selective Surfaces (FSS) have been studied and researched for a long time

of over 50 years. With the recent interest in meta-materials it is perhaps time toconsider a current list of applications of FSS and of areas where new engineering

solutions and further research work would be of special value. So how should we

define an FSS? Pedantically, a single FSS should be a thin surface defined by a pattern

of conductor or resistive material on a generally curved surface with some structural

support, commonly a dielectric layer as shown inFigure 1.1

Figure 1.1 FSS of cross dipoles elements


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Of course multiple FSS structures may be, and often are, constructed using multiple

dielectric layers and/or FSS embedded within some composite. In addition, an FSS

may also feature non-thin components such as inductors, capacitors, diodes and

transistors which are bonded to the surface. Often these are referred to as circuit

analogue structures. The FSS may feature deliberate loss, as part of a radar absorbing

material, or may be designed to be as lossless as possible. They may be active,

featuring amplification structures, semi-active featuring biased diodes etc. or

completely passive. There may be connecting structures, such as vias, between FSS or

each FSS may be electrically isolated. They may also comprise arrays of fully three

dimensional elements. It soon becomes difficult to decide what to call such structures

and how they fit in with antenna array definitions, two or three dimensional meta-

materials, dichoric surfaces, etc.

With time passes the improvement is expected and in todays world of t echnology we

need to design multiband frequency selective surfaces. Combining many planar FSSs

together, and a dielectric layer in between them, will provide an added degrees of

freedom to design a filter with a desired spectral response. Such filters typically show

several resonances in their spectral responses.

Here two type of geometry are studied, an inductive which behaves as high pass filter

while the other is capacitive which behaves as low pass filter. So at the resonance the

inductive will present us total transmission and similarly the capacitive will present us

the total reflection. Common capacitive FSS are constructed from Periodic rectangular

Patches while inductive FFS from apertures. Metallic patches in rectangular shape

behave same as a capacitive circuit that of metallic apertures behave as inductive

circuit. So now we can define FSSs as surfaces which are the periodically assembled

arrays of certain shape conducting elements supported by dielectric used as filters to

electromagnetic waves. Some of certain shapes elements are shown in Figure 1.2.


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The FSSs have the ability to transmit in-band EM waves and reflect out-band EM

waves [2].FSSs are also mostly used to reduce the volume and also to improve the anti-

jamming capability of [3] some multiband electronic system [4]. FSS is also used to

work as sub reflectors in multiband antenna for the satellite communication so that a

single main reflector can share the different frequency bands [5]. To increase the

capabilities of multi-function and multi-frequency antennas, an FSS equipped sub

reflector is required to operate at multi-frequency bands [6]. For this purpose these

multiband FSSs are presented.

Frequency selective surface (FSS) filters [7] also improve the spectrum efficiency of

indoor wireless networks. its minimize eavesdrop by selectively isolating various

frequency bands. As WLAN uses mostly two bands of frequencies i.e. 2.4GHz and

5.8GHz. If both of them exist in an indoor environment, Furthermore, selectively

blocking either one or both of them would be useful.

Multiband FSSs can be obtained by many other techniques. A lot of research has been

done in this field. Some of them we have studied are mentioned.

Figure1.2. some type of conducting elements of FSS

sheet


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Multiband FSSs with Multi-ring Patch Elements [8] uses perfectly conducting rings for

resonance frequency. we studied a another type which had multiple strips combined

together. Different kind of reflectance properties and that transmittance properties of

different groups of strips were analyzed which we show in Figure 1.3.

It was shown that for a single strip structure the spectral properties were improved by

making elements in groups. If the length of strips were not equal the resonance

occurred to be in K-band region of frequencies.

In another paper Single-Layer FSS Designed by Genetic Algorithm and Geometry-

Refinement Technique [9] uses Genetic Algorithm to design the conducting element

shapes. In this paper a the optimization-design technique based on the Genetic

Algorithm into which the GRM is used. There is condition of the continuity of

elements. the adjacent conductors touch only at a point, because the GRM removes thecritical points. In this way the FSS can be designed without the point contacts of

conductors.

Realization of a New Kind of Frequency Selective Surface for Multi Band

Operations[10] Uses variation in capacitance, by cutting edge, to achieve multiple

Figure1.3. Unit cell arrangements for (a) single-strip, (b) double-strip, and (c) triple-strip

FSS element,(d) PWS basis function on a strip element


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frequency operations. These new AMC surfaces can tune the antenna to work with

different frequencies over a selected band. Instead of changing the physical

components like variable diodes are having the FET components between the patches

of FSS the changing of cuts with movable plates.

Also we studied the multiband artificial conductor which are magnetic in nature and

use FSS having high impedance [11] it use metamaterials to get a tunable multiband

FSS. In this paper the surfaces were designed for any wanted combination of operating

frequencies. Figure 1.4 shows the simulation of the design. in this paper GA

optimization technique was used to design synthesis of multiband AMC surfaces. the

geometry and size of the FSS unit cell, as well as the thickness and relative dielectric

constant of the substrate was obtained by using GA. for operation at GPS the optimized

dual and triband were also shown. And also same case is for cellular frequencies.

In another paper a fractal triangle was used to design FSS. The structure is composed

of three regular triangles thats areplaced into a larger one and many other nesting

Figure 1.4 Characteristics of PBG unit and periodic sheet regarding reflection phase vs. frequency


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triangle of different lengths and apex angles were placed inside in front of a

dielectric slab as shown in Figure 1.5.

The unit cell is composed a combination of fractal triangle elements of different size.

Three different resonant frequencies in a band of 0-20GHz were obtained by

simulation.

Using controllable triband characteristics a design was proposed. [12]. That proposed

design of FSS is composed of a stacked periodic array of square loops and

complementary apertures, respectively centered within a wire grid and an aperture grid,

which can create two transmission zeros and also three transmission poles.

Controllable triband performance is achieved, which allow the FSS to transmit the

signal at 4GHz while reflecting the signals at 6GHz and 9.5 GHz.

Frequency Selective Surface Design for Blinds applications [13]. In this paper, the

reconfigurable blind structure with frequency selective surfaces (FSS) for the building

applications are presented. Frequency transmission characteristics for different angles

of the proposed blind-FSSs are investigated and verified through simulation and

measurement. The simulation results show that we can implement the reconfigurable

blind-FSS for multiband wireless applications.

Figure 1.5 Fractal triangles with dielectric substrate


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For the validation of this paper, the simulated FSS structure is fabricated and measured

the transmission characteristics. Even though there are some discrepancies between

simulation and measurement results, the results show the possibilities of reconfigurable

resonant frequency for different angle of blind. Consequently, the applicability of FSS

structure is improved in consideration of more diverse communication environments

using the band adjustments of reflecting specific frequencies according to the different

angles of the FSS structure. Future studies should be performed to get more accurate

results of this study by fabricating the Blind-FSSs structure with higher resonant

frequency.

1.2 Application of FSS:

As present in introduction the FSSs are periodic structures assembled of apertures or

patches that behave like filters to electromagnetic waves. The conducting elements

either apertures or patches are assembled in a periodically repeating array of either in

one or two dimension [14-16]. Geometry of the elements decides the category of the

filter response. Generally there are four kind of filter responses based on the elements

geometry: a low pass, a high pass, a band stop and a band pass.

1.2.1 Microwaves Oven:

The most famous example of FSS is the metal perforated door of microwave oven that

is used for cooking the food. The FSS used in this door is very similar to the high pass

filter shown in Figure 1.6.The doors microwave oven blocks the 2.4 GHz microwaves

and allows the visible light which has very high frequency of 400 THz-700 THz to

pass. In this way the cooking condition is monitored. The door of microwave oven is

transparent by look but the FSSs blocks the internal microwaves


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Figure 1.6: The basic four kinds of electromagnetic filter responses based on specific geometry.

not to leave the microwaves oven,

1.2.2 Stealth Technology:

As we all know stealth technology is the most advanced military technology. It gives

the upper hand over the simple planes that can be detected by radar. Stealth technology

uses the FSSs structures to absorb the incoming radiation of radar and do not allow it to

reflect back to the radar. Thats how the radar cant detect the incoming planes.

1.2.3 Sub-reflector in multiband antenna:

FSSs are also used in the multiband antennas as sub reflectors. It reflects the unwanted

microwaves and thus allows only the useful waves and giving the antenna multiband

properties as shown in figure 1.7


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Figure 1.7 FSS used in a Multiband antenna as a sub reflector.

1.2.4 Security

FSSs are used to secure the private networks. This application is presented in this thesis

as well. It restricts the WLAN to a specific area i.e building or room. So that outside

that area WLAN cant be reached.

1.2.5 Interference

FSSs are also used to isolate an antenna from interfering with other antennas. The

radiation from other antennas can be blocked by putting the FSSs structure to the

isolated antenna.


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1.3 Governing factor of FSS:

The FSSs factors can be categorized into four main features i.e the geometry of

element, the conductivity of element, the dielectric substrate on which the elements are

mounted and the incident angle the FSSs faces. Each one is explained in this section.

1.3.1 Geometry of the element:

There is no restriction on the shape of the element. Based on Munk generally there are

four main groups of element geometry.

1) center connected geometry: like cross dipoles, Jerusalem crosses, dipoles, tripoles

etc [17-19]

2)

Solid interior types: this kinds of elements are usually apertures and patches such

square meshes and circular patches.[20]

3) Loop types geometry:shapes like rings and square loops [21-23]

4) Combination of the above three: sometimes the geometry of element is deduced

from the first three geometries. Combined two or three geometries to overcome the

defects of the FSSs performance of simple element[24-26]

Some of the most known shapes are illustrated in figure 1.8.

Figure 1.8 most known shapes of elements


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As we have designed the modified square loop(MSL) in this research the resonance

frequency response is determined by the parameters given as p,h,w and d as illustrated in

figure 1.9.

Figure 1.9 Modified Square Loop of the current research project

The dimension of the MSL d, the separation between elements p the width of cutting

area w, and the height of the cutting area h determines the resonant frequency while the

inter spacing between elements g control the angular stability of the FSSs.


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Chapter-2

DESIGNING THE PROPER STRUCTURE

As discussed before for the specific resonant frequency the the size of the element and

the structure, the stop band attenuation, the incident angle and the accuracy is

considered to meet the required results.

2.1 Dimensions of the element:

To get the required resonant frequency as we discussed before that the parameters of

element determine the resonant frequency. As in this project we need to block the

2.4GHz and 5.8GHz WLAN transmission. So we noted that by increasing the

dimension d both the resonant frequencies increases and vice versa. Also by increasing

the height the 2.4 GHz remains almost constant while the 5.8 GHz resonance decreases

and vice versa. Similarly by increasing the width w the both resonant frequencies

decrease and vice versa. We adjusted the dimension of the element as given in the table

2.1 given below.

Table 2.1 Element parameters for current project unit cell

Dimension Units(mm)

Each side of the square ,d 34

Height of the cut area, h 12.6

Width of the cut area, w 6.5

Inter element spacing , g 1

Thickness of substrate, ts 1.6

Thickness of element, te 0.08


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2.2 Accuracy required:

As we know the accuracy tolerance in the FSSs for the indoor wireless transmission is

higher than it is for the FSSs for antenna design. In antenna design even one percent of

deviation can affect the performance badly while in indoor wireless system such as

designed in this project there is concession for the accuracy as beside the attenuation

caused by element the signal also is attenuated by walls and other objects. So even if

the simulation does not offer the more accurate analysis of the attenuation it will not

affect the performance significantly. For the WLAN transmission it was noted the 25

dB attenuation is required to block the transmission [27]. The shift in resonance

frequency is not cared about as long as the attenuation level is 25 dB or more. If the

stop band can be achieved more the accuracy will be greater. With more stop band theshift in resonant frequencies, because of different incident angles, wont cause affect on

the blocking WLAN transmission as long as the transmission level is 25 dB or more.

2.3 The stop band attenuation:

As this project aim is to block the WLAN transmission we need to block the two

WLAN frequencies i.e. 2.4GHz and 5.8 GHz. At these two frequencies the resonance

occurs. At the resonance the FSSs behaves like the metal shield for WLAN

transmission. Theoretically the resonance attenuation is infinite but in practice the

attenuation is finite. However the FSSs can give the required stop band attenuation at

which the transmission is blocked. The minimum level of attenuation which can isolate

the wanted and unwanted transmission is known as minimum stop band attenuation. As

discussed before for the two rooms to be isolated requires 25dB attenuation.

2.4 Response to different incident angles:

Inside the building the signals undergo different reflections due to walls and other

objects in the buildings before it reaches the receiver. So the signals arrive at the FSSs

structure at different incident angles. It is there considered that the design should give

us at least 25 dB attenuation at different incident angles. The signal also loses energy

during reflections. Multipath reflection is the main cause of less energy signal. It is


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therefore important to consider the incident angles as well as the energy profile. The

profile can be obtained from the radio propagation simulation like study by Singh and

Chinni [28-29].

The Singh and Chinni presented that although the signals can undergo the change inangles up to 80 degrees. However if the signal undergo one reflection its energy is

decreases by 20 dB. Therefore higher order of reflection is not significant as shown in

Figure 2.1.

Figure 2.1: Multipath Propagation of the signals between two rooms.


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Chapter-3

METHODOLOGY

Over the past four decades different kinds of techniques for analyzing periodicstructures have been developed such as method of moment (MoM) [30], finite element

method [31], simple analytical equivalent circuit method (EC)[32-33] and the finite

difference time domain method (FDTD) [34-35]. Here in this project we used a novel

method for designing. A totally new idea which needs simple calculation and is time

saving. It was given a name of analytical parameter adjustment method (APAM). The

other methods will briefly be explained before introducing the APAM.

3.1 Method of moment:

It is the most known method used to analyze the FSSs. Different variation has been

developed in the MoM recently but the earlier work was done by Chen [36]. Chens

work is also known as the modal method or integral equation method. The C hens

method evaluates the flow of current on surface of conducting elements. For the

unknown current the tangential field at the surface of element is matched and an

integral equation is formed which finds out the unknown current. In the method of

moment as integral equation is reduced to form small algebraic equations which are

easy to solve.

A periodic greens function is used in MoM. It is computationally expensive to evaluate

greens function and solve the large set of linear equations. An example is given in

Figure 3.1 which show the solution of field scattered from a simple periodic structure.

Using Green's functions, E1;E2;E3 can shown as a function of I1; I2; I3 and the

corresponding distance to the observation point respectively. At the points B and C

Repeating a similar procedure, three simultaneous equations with three unknowns (i.e.

I1; I2; I3) can be achieved which brings up 3x3 matrix to calculate the unknown

current.


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Figure 3.1 Evaluation of Electric feilds

3.2 Finite Element Method:

To model the field scattered from a periodic structure the FEM is an alternative method

commonly used [37]. It is a numerical method where the unknown fields are

discretised by using the finite mesh of element [38]. Generally for two dimension

problem a triangular element is used while for 3D a tetrahedron is used. It has been

shown through studies that complementation of finite element method with proper

boundary conditions and the method of moment which imposes radiation conditions,

an infinite periodic structure can be analyzed by a hybrid finite element method[39-40].

The FEM is a proper technique to cope with sophisticated shapes and different material

but it needs larger computer resources and is thus computationally intensive.

3.3 Finite difference time domain method:

The above two techniques were time domain while the FDTD method is frequency

domain and thus can cover a larger and wide range of frequencies over a single

simulation run. It is a direct solution for the Maxwells time domain curl equations[41].

Space and time domains are used to discretize the analysis and thus in a leag-frog

manner the equations are solved to update the electromagnetic field within the


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considered computation domain. By gridding the computational domain the electric

field at a specific point will be updated which depends upon the previous magnetic

field stored on the both sides of the point. Similarly the magnetic field can be updated.

As the process goes on the electric and magnetic fields which are scattered from the

structure or which are within the structure can be determined [41-43].

3.4 Equivalent Circuit Method:

It is a simple method for analysis of FSSs. From analogy with transmission line the

transmission characteristics of FSSs can be determined. The circuit components of the

FSSs are evaluated on quasi-static Equivalent circuit approximation of conducting

strips as inductive or capacitive. Quasi-static EC approximation was presented by

Markowitz [45-46]. The analysis in EC method is limited to simple shape elements and

linear polarization as due to its scalar nature. The accuracy in EC methods vary from

case to case. The incident angle of the signal and the properties of dielectric substrate

are considered in EC method. The EC method offers less precise analysis as compared

to the other techniques.


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3.5 Analytical Parameter Adjustment Method (APAM):

All of the above techniques are computationally expensive and takes time to analyze an

FSS. Over and over experimentation brought us to a new technique for loop type

geometry. This new method of APAM for analyzing the FSSs parameters is very

simple and interesting. In APAM the dimension of the element is taken such that the

one forth part of the wavelength of the resonant frequency approximates the dimension

of square loop. Then the width and height can be adjusted by the check and simulate

process. As from simulation it is noted that when we decrease dimension d, the two

resonant frequencies (in case of this project) also shift to lower frequencies and vice

versa. If we decrease the height of the cut area the lower resonant (2.4 GHz) remain

approximately constant while the higher resonant frequency (5.8 GHz) shifts to higherfrequency significantly. Similarly if we decrease the width of the cut area both of the

resonant frequencies shift to higher frequencies. Also here the shift is significant.

This project aim is to provide at least 25 dB stop band attenuation to a dual band

transmission i.e. 2.4GHz and 5.8 GHz. First 2.4 GHz frequency is taken. The

wavelength of 2.4GHz frequency is 125mm. One forth of 125mm is 31.25. So first the

dimension d is taken to be 31.25mm. Then half the d which approximates 15 is taken

as h. and half the h which approximates 7 is taken w. after the simulation we changethe parameter according to the shift of resonant frequencies as discussed above. The

table for parameter vs. resonant frequencies is shown below.

Table 3.1: Effect of parameters on resonant frequencies

parameters action Resonant f1 2.4GHz Resonant f2 5.8 GHz

Dimension, d Decreas

e

A small Shift to lower

frequency

A small shift to

lower frequency

Height of cut area, h decrease Remains almost constant A big shift to higher

frequency

Width of cut area, w decrease A big shift to higher

frequency

A big shift to higher

frequency


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So looking at the behavior of parameter change Vs the resonant frequencies we can easily

adjust the parameter to the required resonant frequencies without any brainstorming

computations. During the experiment it was noted that the time needed for other

techniques was more than this method. So this method was preferred to be used in our

research project.


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Chapter-4

RESULTS

After taking everything into consideration finally the unit cell design was obtained with

parameter given in table 3.1. the results were categorized into two type, simulation

results and implementation results.

4.1 Simulation results:

The software used for simulation results is Computer Simulation Technology (CST)

Microwaves studio. The units used were mm for dimensions, GHz for frequency, dB

for attenuation and degree for angle. The design was created and simulated in CST tofind out the S parameters. Two ports names Zmin and Zmax were placed as excitation

sources to excite the FSS structure and observed the transmission on the other side of

FSS structure as shown in Figure

Figure 4.1 two ports with MSL design in CST


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Different angles were considered for simulation due to the reflection of signals and

coming at different angles at FSS structure. Some of the s parameter results for

different angles are given below.

A.

Normal signal incident:For normal incident angle of signal the unit cell resonated at exactly 2.4 GHz and 5.8

GHz. The boundary conditions were placed on unit cell. The Zmin and Zmax were

placed to measure the transmission characteristic. The incident angle was considered

from 0 to 45 degrees because beyond 45 degrees the energy of signal lowers

significantly. We first simulated the design with normal incident signal which exactly

simulated at 2.4 GHz and 5.8 GHz. The Figure 4.2 Shows the normal incident signal

simulation.

Figure 4.2: the normal incident of both TE wave.

As we can see from the Figure 4.2 the first resonance occur at 2.48 which the exact

resonant frequency for WLAN 2.4 GHz. At this resonance the attenuation is -43.46 dB

which is more than the required one. The second resonance occurs at 5.808 which is

the resonant frequency for the WLAN 5.8 GHz. As for higher WLAN a little

attenuation is required so the attenuation level at 5.8 GHz is -26.89 dB which is more

than the required attenuation. So for the normal incident the design is fully successful

and the WLAN transmission at the both resonant frequency will be totally block. The


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transmission parameter for both TE and TM at normal incident is same. TM incident

signal simulation result is shown in Figure 4.3.

Figure 4.3 Transmission parameter for TM waves incident at normal.

B. Signal incident at 30 degrees: The design presented in this is very insensitive to the

incident angle change. Even the when the angle changes from zero to 30 degrees the

bandwidth of stop band at 2.4GHz is still 380 MHz which is an outstanding result. It

was noted that even the resonant frequencies experience a small shift at 30 degrees

incident wave, the attenuation level is still more than 25 dB which will definitely be

help for blocking the transmission. Also after first order reflection the energy of the

signal decreases by 20 dB. The simulation result of TE incident signal at 30 degrees is

shown in Figure 4.4.


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Figure 4.4 Transmission parameter for TE waves incident at 30 degrees.

At 30 degrees incident the TM waves resonates the structure a bit different from the TE

waves. The 5.8 GHz resonant frequency is shift to higher frequency but still the

attenuation level is enough to the 5.8 GHz at 30 degrees. For TM waves at 30 degrees

the simulation result is shown in Figure 4.5.

Figure 4.5 Transmission parameter for TM wave incident at 30 degrees .


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Similarly at 45 degrees the attenuation level is again more than 25 dB at 2.4 GHz but

decrease at 5.8 GHz. The Figure 4.6 shows the incident TE wave at 45 degrees.

Figure 4.6 Transmission parameters for TE wave incident at 45 degrees.

The simulation results for TM waves shows a better result than the TE waves. For the

45 degrees the TM waves gives us a smooth and in level simulation results which

satisfies the required criteria. TM waves incident at 45 degrees is shown in figure 4.7.

Figure 4.7 Transmission parameters for TM waves incident at 45 degrees


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4.2 Prototype:

A rough design was manually produced at as a prototype for the project design. This

prototype verified the results of our software design. The prototype was checked for the

WLAN transmission at 2.4 GHz and 5.8 GHz and it blocked the WLAN transmission

completely while allowing the GSM signals to pass through. The prototype designing

procedure and the practical experiment details are given below.

4.2.1 Materials needed for prototype construction:

We used the following materials step by step for construct the required prototype for the

verification of simulation results.

a) Copper clad FR4 substrate sheets:

We used 12x6 inches copper clad FR4 sheets to make the prototype. The thickness of the

sheet was taken to be 1.6 mm as used in the simulation.

b) Isopropanol 99%:

Isopropanol is used for cleaning the copper clad FR4 substrate. The greasing level is

removed by using Isopropanol.

c) Ferric chloride:

It is used to etching the copper from the copper clad FR4 substrate. It is also known as

iron chloride.

d) Acetone:

It is used to remove the printed design surface after etching.

e)

Photopaper:The design is printed on photopaper using laser technology.

f) Iron:

The design on photopaper is printed on copper clad FR4 substrate by pressing through

iron.


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g) super glue:

The building shape box is made up from the printed FSS structures using super glue.

4.2.2 Procedure for prototype construction:

As according to the software the copper clad Fr4 substrate was used to for practical

design. First the unit cell design was printed on photo paper using laser printing. As the

copper clad FR4 comes with a greasing layer upon it for copper safety we had to

remove the greasing layer so that the printed design can stick to the copper. For that

purpose the copper clad FR4 was cleaned with Isopropanol 99 % to degrease the

copper layer. After degreasing the copper clad the design printed on photo paper was

printed on FR4 substrate by putting the printed photo paper on substrate and thenpressing by

iron for about 1 to 2 minutes. After that the FR4 sheet was placed in water for sometime

so that the photo paper can be removed easily with disturbing the design screen to FR4

sheet. Once the design came on the copper clad the copper under the design is covered

with carbon from the photo paper. After that The printed FR4 was placed in ferric

chloride solution for about 10 minutes to etch the unwanted copper on the substrate.

The copper required for the design will not be etched as it is covered with carbon. Only

the extra copper which is exposed to the ferric chloride solution was etched. After

removing the copper the substrate was cleaned with acetone, removing the carbon

above the copper to bring up the design surface containing copper that was covered by

the printed area. At last the sheet was cleaned with water that gave us the clean view of

the design screen to the FR4 substrate. That give us the one sheet we need for the cube

box. We did the same procedure to make the other five sheets required. Using the super

glue we arranged all the six structures in a cube shape so that to be resembled as a

building. The design printed sheet and a rough prototype design manually constructed

are given in Figure 4.8 a and Figure 4.8 b respectively.


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Figure 4.8 a: Manually printed design on the copper clad FR4 Substrate.


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Figure 4.8 b: Manually constructed Prototype box


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CONCLUSION

It is concluded that multiband frequency selective surfaces can be designed using

repetitive structures. The resonance, stop band bandwidth, attenuation, sensitivity at

different angles and accuracy purely depends upon the size, shape and inter element

spacing. By varying the dimension, shape and inter element spacing the results are

changed. We noticed that small size unit cells stick closely together will give better

results than larger size openly place elements in an aperture. We also noticed that

design sandwiched between substrates give better incident angle insensitivity than a

single side one. So to design a perfect FSS with larger stop band bandwidth and better

resonance characteristics at different incident angle the FSSs structure should becomposed of small size elements, closely placed together with on either side the

substrate.


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Design Multiband FSS using a unique method