THIẾT KẾ MẢNG ANTEN TUYẾN TÍNH ĐẶT BÚP KHÔNG ĐƠN BẤT KỲ BẰNG PHƯƠNG PHÁP NÉN BÚP PHỤ
Nguyễn Việt Dũng
Khóa QH-2014-I/CQ , ngành Công nghệ kỹ thuật Điện tử – Truyền thông
Tóm tắt Khóa luận tốt nghiệp:
Trong khóa luận này, một mảng ăng ten có ứng dụng chống nhiễu được thiết kế bằng phương pháp đặt điểm không. Thuật toán Bat được áp dụng vào thiết kế hệ thống chia tín hiệu. Đầu tiên, một hệ thống chia tín hiệu cho 10 phần tử được thiết kế. Tiếp theo, phần tử bức xạ được thiết kế và kết nối với hệ thống chia tín hiệu. Các kết quả đạt được cho thấy mảng ăng ten đặt điểm không tại -48o, độ lợi của ăng ten 12.7dB, mức búp phụ -17dB, điểm không tại -48o là -41.2dB.
Từ khóa: thiết kế ăng ten, mảng ăng ten, nhiễu
DESIGN OF AN ULA ANTENNA WITH ARBITRARY SINGLE NULLS IN SIDELOBES
Nguyen Viet Dung
QH-2014-I/CQ , Faculty of Electronic and Telecommunication
Abstract
The thesis concentrates on the design of antenna arrays to suppress interferences by using nulls placing method. The Bat Algorithm has been applied to this antenna array for the feeding networks. Firstly, a feeding network for 10×1 antenna array has been designed and simulated. Secondly, radiating elements have been designed and connected with the feeding networks. As the results, the antenna array has been designed for the nulls at -48°. The gain of antenna array is 12.7dB, the sidelobe level is -17dB and the null at -48° can be reduced to maximum -41.2 dB, respectively.
Keyword: microstrip antenna design, antenna array, interference .
AUTHORSHIP
“I hereby declare that the work contained in this thesis is of my own and has not been previously submitted for a degree or diploma at this or any other higher education institution. To the best of my knowledge and belief, the thesis contains no materials previously published or written by another person except where due reference or acknowledgement is made.”
Signature:
SUPERVISOR’S APPROVAL
“I hereby approve that the thesis in its current form is ready for committee examination as a requirement for the Bachelor of Electronics and Telecommunications degree at the University of Engineering and Technology.”
Signature:
ACKNOWLEDGEMENT
Writing thesis gives me an opportunity to formal thank those who have supported and helped me during my years of study at University of Engineering and Technology especially this thesis implementation.
Foremost, I would like to express my sincere gratitude to my supervisor for the continuous support of my research and thesis, for his patience, motivation, enthusiasm, and immense knowledge.
Secondly, I am highly thankful to my group members and my classmates for their helps with both technical and nontechnical support in order to help me not only my study but also my work on this thesis
I am greatly thankful to my family and my friends for their supports,. Without their encouragement, motivation, understanding and sacrifices it would have been impossible for me to complete this work.
This work has been partly supported by Vietnam National University, Hanoi (VNU), under Project No. QGTD. 13. 05.
Abstract
The thesis concentrates on the design of antenna arrays to suppress interferences by using nulls placing method. The Bat Algorithm has been applied to this antenna array for the feeding networks. Firstly, a feeding network for 10×1 antenna array has been designed and simulated. Secondly, radiating elements have been designed and connected with the feeding networks. As the results, the antenna array has been designed for the nulls at -48°. The gain of antenna array is 12.7dB, the sidelobe level is -17dB and the null at -48° can be reduced to maximum -41.2 dB, respectively.
Keywords: microstrip antenna design, antenna array, interference
TABLE OF CONTENTS
List of Figures vii
List of Tables ix
ABBREVATIONS 1
INTRODUCTION 2
ANTENNA ARRAYS AND THE PROCEDURES TO IMPROVE THE PERFORMANCE 4
2.1. Introduction to Antenna Array 4
2.2. Fundamental parameters of antenna array 5
2.2.1. Radiation Pattern 5
2.2.2. Array Factor 7
2.2.3. Main Lobe 10
2.2.4. Sidelobes 11
2.2.5. Antenna Effeciency 11
2.2.6. Directivity and Gain 12
2.3. Linear Antenna Array 14
2.4. Improving performance procedures 16
2.4.1. Chebyshev Distribution 16
2.4.2. BAT Algorithm 17
2.5. Microstrip Patch Antenna 19
2.6. Design Techniques 21
2.6.1. The Feeding Network 21
2.6.1.1. Feeding Networks 21
2.6.1.2. Series Feeding Networks 22
DESIGN AND SIMULATION OF THE ANTENNA ARRAY 25
3.1. Introduction 25
3.2. Requirements 26
3.3. Design and Simulation Software 27
3.4. Design and Simulation of Antenna Samples 27
3.4.1. Feeding Networks Model 27
3.4.1.1. Simulation Results of Feeding Networks 27
3.4.2. Radiating Elements 29
3.4.3. 10×1 Antenna Array 30
3.4.3.1. 10×1 Antenna Model 30
3.4.3.2. 10×1 Antenna Array Results 30
CONCLUSIONS 34
List of Publications 35
References 36
List of Figures
Figure 2.1 Radiation Lobes of an Antenna Pattern in 3D 6
Figure 2.2 – Directional Antenna Pattern 7
Figure 2.3 – Near field of the array 8
Figure 2.4 – Phase difference between two elements on the x axis. 9
Figure 2.5 – Main (or major) lobe 10
Figure 2.6 – Side Lobes and Side Lobe Level 11
Figure 2.7 – A uniformly spaced linear array 14
Figure 2.8 – The beam pattern of an 8-element linear array. 15
Figure 2.9 –Chebyshev patterns maintain SLL at a -20 dB. 16
Figure 2.10 – Flowchart of the proposed beamformers 19
Figure 2.11 – Basic microstrip patch antenna 21
Figure 2.12 – Patch shapes 22
Figure 2.13 – Single patch dimension 23
Figure 2.14 – Patch with extension 24
Figure 2.15 – Series feed 26
Figure 2.16 – The equivalent capacitor of shunt stub 27
Figure 2.17 – The feed with shunt stubs model and the equivalent circuit 27
Figure 3.1 – Antenna array design procedure 29
Figure 3.2 – Feeding networks model 30
Figure 3.3 – S-parameter of the feeding network 31
Figure 3.4 – Phase of output ports 31
Figure 3.5 – Patch antenna model 32
Figure 3.6 – 10×1 antenna array model 33
Figure 3.7 – Antenna array with parameters 33
Figure 3.8 – S-parameter of 10×1 antenna array 34
Figure 3.9 – Radiation pattern in 3D model 35
Figure 3.10 – Uniform array antenna 35
Figure 3.11 – Comparing with uniform pattern at 5.7 GHz 36
List of Tables
Table 3.1 – Requirements of output signal with the interference from -48o 29
Table 3.2 – Comparing the results between simulated and theory 32
Table 3.3 – The antenna metrics 32
Table 3.4 – Parameter of antenna array 33
ABBREVATIONS
SIR Signal to Interference Ratio
SLL Sidelobe Level
CST CST Microwave Studio Software
MPA Microstrip Patch Antenna
RF Radio Frequency
ULA Uniform Linear Array
DSPD Double-Side Printed Dipole
PCB Printed Circuit Board
Introduction
Over the last few years, antenna arrays have been widely used in mobile, wireless and radar communications systems to improve signal quality, increasing system coverage, capacity and link quality. The performance of these systems depends firmly on the antenna array design.
Because of the increasing of the wireless devices that make the electromagnetic environment is polluted. In telecommunications, interference is anything which modifies, or disrupts a signal as it travels along a channel between a source and a receiver. Hence, the antenna array plays an important role in the system which have to meet the low sidelobe levels, nulls placing to suppress interferences. One of the methods is sidelobe level reduction or nulls placing when the direction of interference have been defined. Methods of nulls placing, sidelobe level reduction which have been studied extensively in the past [8-12] and some models which reduce sidelobe level has been fabricated recently [5-7]. Bat is a algorithm that has been developed by Xin-She Yang in 2010 and Bat Algorithm allows to design beamforming antennas by controlling the weights [8-10]. Nulls placing include controlling the amplitude-only, the phase-only, the position-only, and the complex weights (both the amplitude and phase) of the array elements. In this thesis, the amplitude-only method has been chosen to implement.
In parallel with the development of antenna technology, microstrip antenna arrays are being used commonly due to their advantages such as low cost, light weight and easy to fabricate base on PCB technology.
In this thesis, a microstrip antenna sample for maximizing signal-to-interference ratio applications has been researched, designed and simulated. The antenna is placed on a Roger 5870 lossy substrate, which has the permittivity of ε_r= 2.33, the thickness h = 1.575mm and the 3D model size is 380mm x 35mm x 1.645mm. They are designed and simulated on CST Microwave Studio Software.
This thesis is divided in four following chapters:
Chapter 1: Introduction.
Chapter 2: Antenna arrays and the procedures to improve the performance.
Some fundamental parameters of antenna, microstrip antenna design technique and optimization algorithm will be shown in this chapter.
Chapter 3: Design and simulation of antenna.
Chapter 4: Conclusions
The final chapter gives conclusions and recommendations of future research.
This thesis has been done at the Wireless Communication Room, Faculty of the Electronics and Telecommunications under the supervision of Assoc. Prof. Dr. -Ing. Truong Vu Bang Giang.
ANTENNA ARRAYS AND THE PROCEDURES TO IMPROVE THE PERFORMANCE
Introduction to Antenna Array
There are several types of antennas: wire and aperture antennas; resonant antennas (such as dipole and patch antennas) and traveling wave antennas (such as Yagi–Uda and periodic antennas). They can all be classified as single-element antennas. Once the frequency is given, everything (the radiation pattern, input impedance, etc.) is fixed. Besides, the gain of single-element antenna also is limited. Hence, they lack flexibility and efficiency. A high-gain antenna means that the aperture size of the antenna has to be very large, which may be a problem in practice. Also, sometimes we need to be able to control the antenna radiation pattern, for example interference applications. A single-element antenna can not good enough to meet such a requirement. In this case, an antenna array could be a good solution.
An antenna array consists of more than one antenna element and these radiating elements are strategically placed in space to form an array with desired characteristics, which are achieved by varying the amplitude and the phase and relative position of each radiating element. The total radiated field is determined by vector addition of the fields radiated by the individual elements. The total dimensions of the antenna are enlarged without increasing the size of the individual element. The main advantages of an array are:
• The flexibility to fix a desired radiation pattern
• The high gain
• Allow scanned beam
The main drawbacks are:
• Tomplicated feeding network
• The bandwidth limitation
Fundamental parameters of antenna array
Radiation Pattern
The radiation pattern of the antenna is a graphical representation of the radiation properties of the antenna in space. In this, the radiation properties contain directivity, phase, polarization, radiation intensity and so on.
The radiation pattern can be represented in three dimensional graph of horizontal or vertical cross sections or two dimensions with linear plot, polar plot. It includes several various parts as lobes which are parts of the radiation pattern bounded by regions of weak radiation intensity. The radiation lobes include two kinds, namely major (or main) lobes and minor lobes.
Figure 2.1 Radiation Lobes of an Antenna Pattern in 3D [2]
A major (or main) lobe is a part the radiation lobe with the maximum radiation. Base on the applications, an antenna can include one or more than one the major lobe such as split-beam antennas. The minor lobes are unexpected such as side lobes, and back lobes. A side lobe is a radiation lobe which is adjacent to the main lobe and occupies the hemisphere in the direction of the main beam. Generally, it is the largest lobes of the minor lobes. A back lobe is kind of minor lobe whose axis creates an angle (approximately 180 degree) with the main beam of antenna. The back lobe occupies the hemisphere in a direction opposite to the major lobe.
In addition, basing on the direction of pattern, the radiation pattern can be distinguish in three forms, namely isotropic pattern, directional pattern and omni-directional pattern.
Figure 2.2 – Directional Antenna Pattern [18]
The isotropic radiator is a hypothetical lossless antenna having same radiation in all directions that is not physically reliable. A directional antenna is antenna having the property of radiating and receiving electromagnetic waves more effectively in some directions than in others. And, the omni-directional antenna is antenna with an essentially non-directional pattern in a given plane and a directional pattern in any orthogonal plane. It is a special case of the directional antenna.
Array Factor
The array factor represents the far-field radiation pattern of an array of isotropically radiating elements. A single isotropic point source transmits a field as derived. If that point source transmits to an array of point sources, then the output of the array is proportional to the weighted sum of the received signal from each element in the array [1].
(2.1)
Where = distance from element n to the point at . The phase of the received signal at the element is positive, because the signal is traveling toward the element. A transmit array has a minus sign in the phase, because the radio waves is going away from the antenna. Figure 2.3 is a diagram of a point source transmitting to an array of point sources. When the array is very far from the point source, then all the Rn in the denominator of (2.2) are approximately the same. Consequently, the field is proportional to the sum of the weighted phase factors [5].
(2.2)
Most arrays are either linear or planar. A linear array has all of its elements lying along a straight line. In order to make calculations easy, assume that the array lies along the x, y, or z axes. The phase reference, or point of zero phase, is chosen to be either the first element or the physical center of the array. The origin of the coordinate system is placed at the phase center. An incident plane wave arrives at all of the elements at the same time when the incident field is normal or broadside to the array. When the plane wave is off-normal, then the plane wave arrives at each element at a different time. Thus, the phase difference between the signals received by the elements is accounted for by an appropriate phase delay before summing the signals to get the array output. An example of the phase difference between two elements along the x axis is shown in Figure 2.4. If the incident wave vector is in the x-y plane, then the phase is a function of . If the incident wave vector is in the x-z plane, then the phase is a function of . The array factor or antenna pattern due to isotropic point sources is a weighted sum of the signals received by the elements [1].
Figure 2.3 – Near field of the array [1]
Figure 2.4 – Phase difference between two elements on the x axis [1]
(2.3)
where N is the number of elements, is the complex weight for element n, , is the location of element n, is the direc-tion in space, and
■( along x axis@ along y axis @ along z axis)
Equations (2.2) and (2.3) are the same when . The definition of the variable, u, depends upon which plane contains the array and the incident field vector. In digital signal processing terms, this equation is a spatial finite impulse response (FIR) filter.
A planar array has all of its elements in the same plane. By convention, the elements of a planar array usually lie in the x-y plane with the z axis pointing away from broadside. Thus, is measured from broadside and is often called the elevation angle, while is measured from the x axis and is often called the azimuth angle.. Of course, the array can lie in the x-y, x-z, or y-z planes. The definition of depends upon the plane in which it lies.
■(x-y plane@x-z plane @y-z plane) (2.4)
where
location of element n
The array factor is a function of amplitude weights, phase weights, distance between each element, and frequency. This chapter examines the effects on the array factor due to varying these variables. The next chapter provides recipes for finding the values of these variables that produce a desired array factor [1].
Main Lobe
The main lobe of an antenna radiation pattern is the lobe containing the direction of maximum radiation power.
Figure 2.5 – Main (or major) lobe
Sidelobes
Sidelobes are lobes in any direction other than that of the main lobe. For a linear array with uniform weighting, the first sidelobe (i.e… the one nearest the main lobe) in the radiation pattern is about 14 dB below the peak of the main lobe.
Figure 2.6 – Side Lobes and Side Lobe Level
The Sidelobe Level (SLL) is another important parameter used to characterize radiation patterns. The sidelobe level is the maximum value of the sidelobes (away from the main beam) [2].
Antenna Effeciency
Besides some above antenna parameters, we can also determine some other efficiency such as the total antenna efficiency, voltage standing wave ratio (VSWR) or return loss (RL) of antenna and etc.
The total antenna efficiency e0:
(2.5)
where : the total efficiency
: the reflection efficiency (2.6)
: the conduction efficiency
: the dielectric efficiency
: the reflection coefficient at the input
The total antenna efficiency is used to take into account losses at the input terminals and within the structure of the antenna [2]. The losses can contains some types such as the reflections because the mismatch between the transmission line and the antenna, the conduction and dielectric losses.
Beam efficiency which is a type of antenna efficiency represents the quality of transmitting and receiving antennas. The beam efficiency is the proportion between the power transmitted (or received) within a fix angle and the total power transmitted (or received) of the antenna. Generally, the main beam is the beam which has the majority power, so the beam efficiency can be also the ratio between the power of the main lobe and the total power.
The VSWR is classified as the ratio of the magnitude of the maximum voltage on the line to the magnitude of the minimum voltage on the line [3]:
The return loss: (2.7)
Directivity and Gain
The directivity of antenna is the ratio between the radiation intensity in given direction and the average radiation intensity in all other directions with the average radiation intensity is equal to the total power divided by 4π.
(2.8)
where
D: directivity of antenna
U: radiation intensity in given direction [W/ solid angle]
U0: average radiation intensity in all direction [W/ solid angle]
Prad: total radiated power of antenna [W]
The value of directivity D is equal to or greater than zero and equal to or less than the maximum directivity: 0 ≤ D ≤ D0
One of important parameter which evaluates the performance of an antenna is the antenna’s gain. The gain of antenna is the ratio between radiation intensity in given direction and the radiation intensity which is obtained if the power accepts by antenna to iso-tropically radiate. Note that, the iso-tropically radiated power is equal to the total power in input over 4π.
The total radiated power is related to the total input power, as:
(2.9)
where ecd is the radiation efficiency (the conduction – dielectric efficiency).
The antenna radiation efficiency is determined by the ratio of the power delivered to the radiation resistance Rr to the power delivered to Rr and RL in equation 2.10.
(2.10)
The gain of antenna is directly related to the antenna directivity as:
(2.11)
where G0 is the maximum gain of the antenna.
Linear Antenna Array
Figure 2.7 – A uniformly spaced linear arra [4]
Consider a uniformly spaced linear array with K identical isotropic elements as depicted in Figure 2-9. Each antenna element is weighted with a complex weight V_k with k=0,1,…,K-1, and the interelement spacing is denoted by d. The differential distance along the two ray paths is dsinθ. The phase of the signal at the origin is arbitrarily set to zero, the phase lead of the signal at element k relative to that at element 0 is κkdsinθ, where κ=2π/λ and λ= wavelength. Summing all the element outputs together then we get array factor [4]:
F(θ)=V_0+V_1 e^jκdsinθ+V_2 e^j2κdsinθ+⋯=∑_(k=0)^(K-1)▒〖V_k e^jκkdsinθ 〗 (2.12)
which also can be written as:
F(θ)=V^T v (2.13)
where:
V=[V_0 V_1… V_(K-1) ]^T (2.14)
is the weighting vector and:
v=[1 e^jκdsinθ… e^(jκ(K-1)dsinθ) ]^T (2.15)
is the array propagation vector which contains the information on the angle of arrival of the signal. If the complex weight is
V_k 〖=A〗_k e^jkα (2.16)
where the phase of the k^th element leads that of the 〖(k-1)〗^th element by α, the array factor becomes:
F(θ)=∑_(k=0)^(K-1)▒A_k e^(j(κkdsinθ+kα)) (2.17)
If α=-κkdsinθ_0, a maximum response of F(θ) will result at the angle θ_0. Therefore, the antenna beam has been steered towards the wave source [4]. For example, in Figure 2-8, F(θ) for an eight-element linear array is depicted.
Figure 2.8 – The beam pattern of an 8-element linear array
Improving performance procedures
Chebyshev Distribution
For the sidelobe level reduction methods, Chebyshev Algorithm is a typical procedure to improve the performance of the antenna. Chebyshev pattern [15] is used to maintain the sidelobe level of a radiation pattern under a prescribed value. If an interfering source arrives from a direction in space out of the main lobe, its undesired contribution is minimized by low sidelobe level. The method is applicable to any antenna array [16]. Figure 2-11 indicates an array with 8×8 elements, left-hand circular polarization and sidelobe level set to 20 dB lower than the main lobe.
Figure 2.9 –Chebyshev patterns maintain SLL at a -20 dB [15]
The chebyshev patterns are given by a matrix.
For an even number of rows and columns M=2L, and m,k from 1 to L: [15]
w_(m,k)=∑_(s=max(m,k))^L▒〖(-1)^(L-s)×2(2L-1)/(L+s-1)×C_(2s-1)^(L+s-1) C_(s-m)^(2s-1) C_(s-k)^(2s-1) (z_0/2)^(2s-1) 〗 (2.18)
For an odd number of rows and columns M=2L+1, and m,k from 0 to L: [15]
w_(m+1,k+1)=∑_(s=max(m,k))^L▒〖(-1)^(L-s)×2(2L-1)/(L+s-1)×C_2s^(L+s) C_(s-m)^2s C_(s-k)^2s (z_0/2)^(2s-1) 〗 (2.19)
Where:
z_0=cosh(1/(2N-1)×acosh(SLL)) (2.20)
And
SLL=〖10〗^((SLL dB)/20)>0 (2.21)
C_p^q denotes the binomial coefficients.
This computes only a part of the weight coefficients. The rest of the matrix can then be obtained by mirroring the coefficients along both rows and columns of the weight matrix. For instance, the weight matrices before mirroring, for 4×4 and 5×5 arrays are filled (F) as below:
W_(4×4)=[█(F F- -@F F- -@- – – -@- – – -)] and W_(5×5)=[█(F F F- –@F F F– -@- – – – -@- – — -)] (2.22)
BAT Algorithm
For the null setting methods, there are several optimization techniques which have been used. The classical optimization techniques used for the array pattern synthesis are more limitations. Recently, in order to resolve these issues, various evolutionary optimization algorithms based on nature-inspired optimization. This approaches provides promising and effective for problem solving in machine intelligence, data mining and resource management. These algorithms such as the genetic algorithm (GA), ant colony optimization, particleswarm optimization (PSO), differential evolution,…
The Bat Algorithm (BA) is a new evolutionary computation technique based on the bat behavior of using echolocation to detect prey, avoid obstacles, and locate their roosting crevices in the dark. BA better than PSO and GA optimization in terms of convergence, robustness and precision. BA is a promising optimization tool for adaptive beamforming in terms of computation time.
In our proposal, the amplitudes of excitation for array elements are the only controlling parameters, and the main aim is to synthesize array patterns with nulls imposed on directions of interferences.
This section isn’t concentrated on calculation, it only shown the steps to build array element weights. Three steps to get the weights of an antenna array that has been presented [8]:
Initializations (I):
Setting the input data such as: number of array elements (N), Direction of Arrival (DOA) of Interferences; the termination condition such as maximum number of iterations (Max_I) or the desired value of objective function (Threshold); and the radiation pattern of array element.
Defining the objective function, the array factor is chosen in accordance with a particular pattern nulling technique.
Mapping solutions (sets of weights) to locations (x) of bats in the population during the optimization process.
Figure 2.10 – Flowchart of the proposed beamformers [8]
Finding the best solution (F):
The beamformer consecutively calculates and searches for the current best solution based on the BA. The operation is finished when the termination criterion is satisfied. Then, the final best solution is obtained.
Building of array element weights (B):
From the final best solution, the beamformer calculates the corresponding weights excited at each element of ULA. These parameters will be used for pattern nulling.
Microstrip Patch Antenna
Antennas are essential parts in communication systems, especially wireless communications. An antenna is a device used for radiating and receiving electromagnetic waves in free space. Antennas are frequency – dependent devices. Each antenna is designed for a certain frequency band. Beyond the operating band, the antenna can reject the signals in other bands. Therefore, we might look at the antenna as a band – pass filter and a transducer [14].
Antenna is an important branch in microwave engineering, so the antenna size is certainly exact to operate at a given frequency band. Without proper design of the antenna, the signal, which generated by the Radio Frequency (RF) system, will not be transmitted and no signal can be detected at the receiver. In order to suit the devices and meet the resonant frequency in different applications, many different antenna types have been design with various applications such as mobile, broadcast, radar, wireless LANs and satellite, etc. One of the types of antenna is the microstrip patch antenna (MPA). MPAs are increasing in popularity for use in wireless applications due to their low – profile structure. Therefore they are extremely compatible for embedded antennas in handheld wireless devices such as smart phone, pagers, etc… The telemetry and communication antennas on missiles need to be thin and conformal and are often Microstrip patch antennas. Another area where they have been used successfully is in satellite communication.
The advantages of MPA are given below:
Light weight and low volume
Low profile planar configurations
Easily integrated with microwave integrated circuits (MICs)
Capable of dual and triple frequency operations
Support both linear as well as circular polarization.
However, they still have a number of disadvantages such as narrow bandwidth, low efficiency, low gain, low power handling capacity, etc… Along with the antenna technology developments, these cons have been reduced and improved gradually.
Design Techniques
The Feeding Network
For an antenna array, microwave signal divider or feed network is often used to regulate the amplitude and phase feed requirements of the radiating elements (patches) to control the beam scanning properties. Thus selecting, optimizing and implementing the feed network forms a critical part of the antenna array design. Amongst the most common and well-known feeding techniques, the corporate feeding is widely used in scanning phased, multi-beam or shaped-beam arrays. A few other type of feeding techniques are series feed, space feed, and hybrid feed. The following section briefly introduces the common feed networks.
Feeding Networks
In general, radiation characteristics of an array can be determine one the aperture distributions is known. The amplitude and phase distribution at each element is usually determined from the intended application, for example, low side lobe and beam direction. The means of excitation of the radiating elements are thus an essential and important factor which must be carefully considered so that the required distribution is realized.
Existing methods that have been employed to feed micostrip antenna arrays can be categorized into parallel and series feeds, which refer to geometries rather than to actual equivalent circuits. The parallel or corporate feed has a single input port and multiple feed lines in parallel constituting the output ports. Each of these feed lines is terminated at an individual radiating element. The second type of feed system is series feed. It usually consist of a continuous transmission line from which small proportions of energy are progressively coupled into the individual element disposed along the line by various means including proximity coupling, direct coupling, probe coupling, or aperture coupling. The series feed constitutes a traveling-wave array if the feed line is terminated in a matched load, or a resonant array if the termination is an open or a short circuit. Both types of feeds can be realized as either coplanar with the radiating elements or in a separate transmission line layer. The feed networks in general have certain undesirable characteristics that must be carefully monitored in order to minimize any adverse effects on array performance [2].
These characteristics include conductor and dielectric losses, surface wave loss and spurious radiation due to discontinuities such as bends, junctions and transitions. These losses constitute the overall insertion loss of the feed affecting the maximum obtainable gain of the array [13].
Series Feeding Networks
Series-fed arrays can be conveniently fabricated using photolithography for both the radiating elements and the feed network. However, this technique is limited to arrays with a fixed beam or those which are scanned by varying the frequency, but it can be applied to linear and planar arrays with single or dual polarization. Also any changes in one of the elements or feed lines affect the performance of the others. Therefore in a design it is important to be able to take into account these and other effects, such as mutual coupling, and internal reflections. Series feeding network is more complex, although it uses up less space and is commonly used in arrays with a fixed beam. In this class of feeder, as the wave travels through the microstrip line, it is attenuated because of power radiated from each element of the array. These losses are to be accounted for when determining the element excitations. The connecting transmission line lengths and mutual coupling effects determine the phase of each element [2].
Figure 2.15 – Series feed
In this work, an untransposed configuration series fed network has been used with different shunt stubs that have been added to the feed line. These stubs serve as shunt capacitors and play as impedance matching to control the output amplitude excited at each element. The operation and the equivalent circuit of the stub in the antenna are given in the Figure 2-16. According to [5], the equivalent shunt capacitor of shunt stub can be calculated by the equation:
Y_in=jY_c tan(2π/λ_g l) ≃ jY_c (2π/λ_g l)=jω((Y_c l)/V_p ) (2.27)
where
((Y_c l)/V_p )=C (2.28)
Figure 2.16 – The equivalent capacitor of shunt stub
Due to the effect of shunt stubs in the impedance matching point, the S parameters at each port related to the input port will be easily handled. It means that the energy flowing out each output port can be controlled. Then, the model and equivalent circuit of the feed with the shunt stubs can be modelled as given in Figure 2.17. Therefore, by properly adjusting the shunt stubs, the coefficients can be obtained.
Figure 2.17 – The feed with shunt stubs model and the equivalent circuit
DESIGN AND SIMULATION OF THE ANTENNA ARRAY
Introduction
The feeding networks and the antenna array will be designed and simulated. First, the 10×1 feeding networks will be designed that have outputs signal proportional with the calculated weights from BA. Then, the radiating element will be design and connect to the feeding networks. The results of feeding networks and antenna model have been compared, analyzed and evaluated. Simulation results were including output signals of power divider, S parameters and farfield radiation patterns of antenna array. The simulation results of proposed array are given and compared with the uniform array results.
Figure 3.1 – Antenna array design procedure
Requirements
The interference signals can come from one or more than one direction. This thesis only focus on one of the most cases, that is one direction arbitrarily and the angle of interference is not wide.
Suppose that the angle of unwanted signal is -48 degree, apply the steps in section 2.4, the weights of antenna array will be calculated. These weight also can be calculated to output signal by using equation [10]:
〖 P〗_(n(dB))=10log_10 ((U_n^2)/(∑_(i=1)^10▒U_i^2 )) (2.23)
The weights and output signal requirements of the feeding network which are summarized in Table 3.1
Table 3.1 – Requirements of output signal with the interference from -48o
Port No. P2-11 P3-10 P4-9 P5-8 P6-7
Amplitude 0.2093 0.3854 0.6491 0.7696 0.9392
Signal (dB) -19.792 -14.489 -9.961 -8.482 -6.752
Design and Simulation Software
CST MICROWAVE STUDIO is a full-featured software package for electromagnetic analysis and design in the high frequency range. CST offers a wide range of software products to address simulation challenges in the core markets microwaves & RF, EDA & electronics, EMC/EMI, charged particle dynamics, and statics & and low frequency.
Design and Simulation of Antenna Samples
Feeding Networks Model
As the weight cofficients have been defined, the next step is to design the feeding network that has amplitude outputs proportional to the obtained weights. In order to do that, some stubs have been used by adding to the feedline. These stubs can be calculated in section 2.6.2.2. As mention before, the feeding networks places on Roger5870 substrate, which has dielectric constants of 2.33, and the thickness of h = 1.575 mm.
Figure 3.2 – Feeding networks model
Simulation Results of Feeding Networks
The results of this design have been shown in the figure 3.3 and 3.4. Figure 3.3 is the result of simulated S-parameters and figure 3.4 is the phases of the feeding network.
Figure 3.3 – S-parameter of the feeding network
Figure 3.4 – Phase of output ports
The difference between the results of simulated and theory that will be shown in table 3.2. It is clear that the simulation meets quite well with the theory.
Table 3.2 – Comparing the results between simulated and theory
Port Number P2-11 P3-10 P4-9 P5-8 P6-7
Theory (dB) -19.792 -14.489 -9.961 -8.482 -6.752
Simulation (dB) -20.562 -14.891 -9.341 -9.276 -8.147
Difference 0.77 0.402 0.62 0.794 1.395
Radiating Elements
The single element microstrip antenna (DSPD) is designed in a rectangular shape and is powered by feeding method. The design method has mentioned in the reference of DSDP antenna [6]. The antenna operates at center frequencies of 5.6 GHz.
These parameters will be shown in Table 3.3 below:
Table 3.3 – The antenna metrics
Resonant Frequency 5.6 GHz
Dielectrical constant ( r)
2.33
Dielectrical Thickness (h) 1.575 mm
Figure 3.5 – Patch antenna model
10×1 Antenna Array
10×1 Antenna Model
After designing the feeding networks and radiating elements, 10×1 antenna array will be designed by connecting the feeding networks with radiating elements. There are some stubs that have been adjusted to obtain the best result.
Figure 3.6 – 10×1 antenna array model
The parameters of simulated antenna array that will be presented in the table 3.4.
Table 3.4 – Parameter of antenna array
Parameters Value (mm) Parameters Value (mm)
W_1 2 st4 1.25
W_2 0.8 st5 1.9
st1 4 d 39.5
st2 0.5 L 35
st3 2 W 380
Figure 3.7 – Antenna array with parameters
10×1 Antenna Array Results
The simulated results of 10×1 antenna array that have been presented in this section. The return loss, pattern of antenna and the comparision between designed model with uniform model will be shown in the figure 3.8 to figure 3.9.
Figure 3.8 – S-parameter of 10×1 antenna array
The simulation antenna which has been designed to work on a frequency range with the center frequency is 5.52 GHz. The best result of pattern that have obtained is 5.7 GHz. The gain of designed antenna array at 5.7 GHz is only 12.7 dB. The sidelobe level of this array is about -17dB.
Figure 3.9 – Radiation pattern in 3D model
In order to see clearly the results, a uniform antenna array that has made to compare with 10×1 antenna array designed model. The inputs of uniform array is fed the signal by using the excitation list – a tool in CST Studio Suite that allow to design array antenna without feeding network.
Figure 3.10 – Uniform array antenna
Figure 3.11 – Comparing with uniform pattern at 5.7 GHz
The point at -48 degree (inteference angle) in pattern with no feed that is -20.2dB and this point is reduced to -41 dB in designed model.
CONCLUSIONS
This thesis has proposed the procedure to design a microstrip antenna array with interference suppression application using DSPD antenna.
The linear antenna array comprises of a 10 elements, which is constructed from the Roger Duroid 5870tm substrate. The proposed model has provided a good simulation results with the sidelobe level of -17dB, the gain of 12.7 dB, single null in -48o .
The simulated results met the requirements of the applications.
In future work, this antenna array model need to be further optimized in order to have better results in term of gain and fabricated.