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Design and simulation of antenna
Degree Project Design and Simulation of Microstrip Phase Array Antenna using ADS Supervisor: Prof. Sven-Erik Sandström Department of Computer Science, Physics and Mathematics Submitted for the degree of Master in Electrical Engineering Specialized in Signal Processing and Wave Propagation Muhammad Kamran Khattak Osama Siddique Waqar Ahmed 2011-05-17 Subject: Master Thesis Level: Second Course code: 5ED06E Acknowledgement We would like to thank our supervisor Dr. Sven-Erik Sandström for his kind support, patient guidance and co-operation in making this work possible. We would like to thank the Department of Computer Science, Physics and Mathematics, Linnaeus University, Sweden, for providing best educational facilities. We would like to thank the Swedish government for providing free education, education facilities and co-operation. We would like to thank all our friends and siblings whose company will always be cherished. And last, we would like to thank our parents who are the source of our very existence. Without their support, accomplishing this goal was never possible. We thank our parents from heart and soul. ii Abstract The aim of this project is to design a microstrip phase array antenna in ADS (Advance Design System) Momentum. The resonant frequency of which is 10 GHz. Two circular patches with a radius of 5.83 mm each are used in designing the array antenna. RT-DURROID 5880 is used as a substrate for this microstrip patch array design. These circular patches are excited using coaxial probe feed and transmission lines of particular lengths and widths. These transmission lines perfectly match the impedance of the circular patches. Various parameters, for example the S- parameters, two dimensional and three dimensional radiation patterns, excitation models, gain, directivity and efficiency of the designed antenna are obtained from ADS Momentum. Key words: Microstrip phase array antenna, Circular patch antenna, ADS Momentum. iii Table of Contents 1 Introduction ................................................................................................................................................................1 1.1 Thesis Approach ..................................................................................................................................................1 1.2 Objective .............................................................................................................................................................1 1.3 Thesis Organization .............................................................................................................................................2 2. Literature Review ....................................................................................................................................................3 2.1 Basic Antenna Terminology ................................................................................................................................3 2.1.1 Radiation Pattern..........................................................................................................................................3 2.1.2 Radiation Pattern of a dipole antenna ..........................................................................................................4 2.1.3 Directivity ....................................................................................................................................................4 2.1.4 Gain..............................................................................................................................................................4 2.1.5 Aperture Efficiency ......................................................................................................................................5 2.1.6 Beamwidth ...................................................................................................................................................5 2.1.7 Input Impedance...........................................................................................................................................6 2.1.8 Polarization ..................................................................................................................................................6 2.1.9 Antenna Efficiency ......................................................................................................................................6 2.1.10 Beam Efficiency.........................................................................................................................................6 2.1.11 Bandwidth ..................................................................................................................................................7 2.1.12 Antenna Radiation Efficiency ....................................................................................................................7 2.1.13 Return Loss ................................................................................................................................................7 2.2 Basics of Transmission Line Theory ...................................................................................................................7 2.2.1 Wave propagation on a transmission line ....................................................................................................8 2.2.2 Phase velocity ..............................................................................................................................................8 2.2.3 Voltage reflection coefficient (Г) .................................................................................................................8 2.2.4 Standing wave ratio (VSWR) ......................................................................................................................8 2.2.5 Transmission lines with some special lengths..............................................................................................9 2.2.6 Charactereristic Impedence ..........................................................................................................................9 2.2.7 The Smith Chart ......................................................................................................................................... 10 2.2.8 S-parameters .............................................................................................................................................. 11 2.4 Antenna Arrays ................................................................................................................................................. 12 2.4.1 Broadside Array ........................................................................................................................................ 13 2.4.2 End-Fire Array ........................................................................................................................................... 13 2.5 Mutual Coupling in Antenna Array .............................................................................................................. 14 2.6 Microstrip Antennas .......................................................................................................................................... 15 2.6.1 Introduction ................................................................................................................................................ 15 iv 2.6.2 Rectangular Patch ...................................................................................................................................... 16 2.6.3 Feed Models ............................................................................................................................................... 18 2.6.4 Microstrip Line Feed .................................................................................................................................. 18 2.6.5 Coaxial Probe Feed .................................................................................................................................... 18 2.6.6 Aperture- coupled Feed .............................................................................................................................. 19 2.7 Photonic crystals in microstrip antenna substrates ............................................................................................ 22 3. ADS Momentum Overview ..................................................................................................................................... 23 3.1 Introductions to ADS Momentum ..................................................................................................................... 23 3.2 Applications of Momentum ............................................................................................................................... 23 3.3 Method of Calculation ....................................................................................................................................... 24 3.4 Working with ADS Momentum ........................................................................................................................ 25 3.5 Theory of Operation for Momentum ................................................................................................................. 27 3.6 Method of Moment Technology ........................................................................................................................ 28 3.7 Simulation Techniques Used in ADS ................................................................................................................ 31 3.8 Block Diagram of ADS Momentum Simulation ............................................................................................... 32 4. Design and Analysis ................................................................................................................................................ 33 4.1 Design of a Rectangular Patch Antenna ............................................................................................................ 33 4.2 Gain and Directivity .......................................................................................................................................... 35 4.3 Design of the Circular Patch.............................................................................................................................. 37 4.3.1 Resonant Frequency ................................................................................................................................... 37 4.3.2 Radius of the Patch .................................................................................................................................... 38 4.3.3 Feed Point Location ................................................................................................................................... 38 4.4 Proposed Design of a Single Circular Patch Antenna ....................................................................................... 39 4.4.1 Gain and Directivity: .................................................................................................................................. 40 4.4.2 S11 Parameters: ........................................................................................................................................... 41 4.4.3 Efficiency ................................................................................................................................................... 43 4.5 Proposed Design for the Circular Patch Array Antenna .................................................................................... 43 4.5.1 Directivity and Gain ................................................................................................................................... 45 4.5.2 S11 Parameters ............................................................................................................................................ 47 5. Conclusion ............................................................................................................................................................... 50 5.1 Conclusion Summary ........................................................................................................................................ 50 5.2 Future work ....................................................................................................................................................... 50 References ................................................................................................................................................................... 51 v Table of Figures Figure 1: Radiation pattern of a Dipole antenna. ............................................................................ 3 Figure 2: Antenna Beamwidth. ....................................................................................................... 5 Figure 3: The Smith chart. ............................................................................................................ 10 Figure 4: Mutual Coupling Mechanism. ....................................................................................... 14 Figure 5: Different shapes of microstrip patch. ............................................................................ 16 Figure 6: Rectangular microstrip patch antenna. .......................................................................... 16 Figure 7: Fringing effects in the microstrip patch antenna. .......................................................... 17 Figure 8: Microstrip feed line designed in ADS. .......................................................................... 18 Figure 9: Coaxial probe feed......................................................................................................... 19 Figure 10: Aperture-coupled feed. ................................................................................................ 20 Figure 11: Proximity-coupled feed. .............................................................................................. 20 Figure 12: Types of feed. .............................................................................................................. 21 Figure 13: Stepwise simulation of ADS Momentum. ................................................................... 24 Figure 14: Layout window of ADS Momentum. .......................................................................... 25 Figure 15: Different parameters in ADS Momentum. .................................................................. 26 Figure 16: Output S-parameter curves. ......................................................................................... 27 Figure 17: Discretization of the surface current using rooftop basis function. ............................. 29 Figure 18: Mesh representation in the form of L and C. .............................................................. 30 Figure 19: Block diagram of ADS Momentum simulation. ......................................................... 32 Figure 20: Rectangular patch designed in ADS Momentum layout. ............................................ 33 Figure 21: Magnitude of S11 in dB. ............................................................................................... 34 Figure 22: S11-parameter shown in Smith chart ............................................................................ 35 Figure 23: Gain and Directivity of the rectangular patch. ............................................................ 36 Figure 24: 3D graph of the far field radiation. .............................................................................. 36 Figure 25: Design of single circular patch antenna in ADS Momentum. ..................................... 39 Figure 26: Excitation of circular patch antenna in ADS Momentum. .......................................... 40 Figure 27: Gain and Directivity of single circular patch antenna in ADS Momentum. ............... 40 Figure 28: 3D view of the directivity of the single circular patch simulated in ADS Momentum. ....................................................................................................................................................... 41 Figure 29: Magnitude vs Frequency graph of input reflection coefficient. .................................. 42 Figure 30: S11-parameter of a single circular patch antenna on a Smith chart. ............................ 42 Figure 31: Efficiency of single circular patch antenna simulated by ADS Momentum. .............. 43 Figure 32: Circular patch phase array antenna designed in ADS Momentum. ............................ 43 Figure 33: 3D view of circular patch microstrip phase array antenna. ......................................... 45 Figure 34: Gain and Directivity graphs of circular patch microstrip phase array antenna. .......... 45 Figure 35: 3D Directivity of circular patch microstrip phase array antenna. ............................... 46 Figure 36: 3D radiation pattern shown by EMDS. ....................................................................... 47 Figure 37: Magnitude vs Frequency graph of S11 parameter. ....................................................... 47 Figure 38: Phase vs Frequency graphs of S11 parameter. ............................................................. 48 vi Figure 39: S11 parameter plotted on the Smith chart..................................................................... 48 Figure 40: Efficiency of the circular patch microstrip phase array antenna. ................................ 49 Figure 41: Radiated power of the circular patch microstrip phase array antenna. ....................... 49 vii 1 Introduction 1.1 Thesis Approach The thesis comprises design of a microstrip phase array antenna using circular patches. This antenna will have the main beam in the broadside direction with a specified beam width. The designed antenna consists of an array antenna with two circular patches. These two circular patches are connected to the quarter-wave transmission line through two transmission lines with specified length and width depending on the impedance of the circular patches. A coaxial probe is connected to the quarter-wave transmission line which will excite the system i.e. the antenna. The design is implemented and analyzed in ADS Momentum. ADS Momentum is a 2.5D simulator which is used to solve complex electromagnetic circuits. It can build passive electromagnetic circuits and the simulation shows the S-parameters of the designed system. ADS Momentum takes care of the electromagnetic coupling effect. It also provides 2D and 3D visuals of output parameters, for example the radiation pattern and the directivity of the antenna. 1.2 Objective Design and simulate a microstrip phase array antenna in ADS Momentum with a main beam in the broadside direction with specified beam width. It operates at 10 GHz (Resonant Frequency) with RT-DURROID 5880 as a substrate. The array antenna will consist of two circular patches in a linear fashion, having radius of 5.49 mm each. The height of each of the patch is 17.4 µm. The thickness of the substrate is 0.787 mm for both the patches. Two transmission lines are used to connect these patches to quarter-wave transmission lines. The impedance of each transmission line is required to be 200 Ω. The length of each transmission line is 462 mil (11.72 mm) and the width of each transmission line is 2.39 mil (0.06095 mm). The electrical length of each line is 180º. A quarter-wave transmission line is used to match the impedance of the system. The impedance of the line is 50 Ω. The length calculated to get 50 Ω impedance is 5.428 mm (213.70 mil) and the width is 2.419 mm (95.23 mil). The electrical length of the quarter wave transmission line is 90º. 1 This corporate feed network is excited by coaxial probe feed. A 50 Ω coaxial probe is connected to the quarter wave transmission line. The antenna is designed and simulated in ADS Momentum. 1.3 Thesis Organization Chapter 1 consists of an introduction and the objectives of the thesis. Chapter 2 represents a literature review and the prerequisite knowledge required in the design and simulation of the antenna. Chapter 3 gives a short overview of ADS Momentum. Chapter 4 is related to the design of the microstrip phase array antenna. Chapter 5 concludes and suggests future work that can be done in this field. 2 2. Literature Review 2.1 Basic Antenna Terminology Antennas radiate and receive electromagnetic waves which are converted into current after reception. Some of the basic characteristics of antennas are discussed below. 2.1.1 Radiation Pattern The antenna radiation pattern, or antenna pattern, is defined as ``a mathematical function or a graphical representation of the radiation properties of antenna as a function of space coordinates”. Radiation properties include power flux density, radiation intensity, field strength, directivity, phase or polarization. A trace of received electric or magnetic field at a constant radius is called amplitude patten. A graph of the spatial variation of the power density along a constant radius is called an amplitude power pattern [1]. The radiation pattern can be presented in two forms : Azimuth Pattern Elevation Pattern The top view of the energy radiated by an antenna is known as Azimuth Pattern while the graphical side view is called an Elevation. The combination of these two terms is known as 2D pattern of the field produced [1]. The basic radiation pattern of a dipola antenna is shown in Fig. 1. Figure 1: Radiation pattern of a Dipole antenna. 3 2.1.2 Radiation Pattern of a dipole antenna Following are some important terms for an antenna [1]: Field Pattern (in linear scale) represents a plot of the magnitude of the electric or magnetic field as a function of angular space. Power Pattern (in linear) typically represents a plot of the square of the magnitude of the elecrtric or magnetic field as a function of the angular space. Power Pattern (in decibels) represents the magnitude of the electric or magnetic field in decibels, as sa function of the angular space. 2.1.3 Directivity The ratio of the radiation intensity in a given direction to the radiation intensity avreaged over all directions [1]. Mathematically directivity can be expressed as (1) The directivity of a non-isotropic source is equal to the ratio of the its radiation intensity in a given direction over that of isotropic source (2) The partial directivity of an antenna for a given polarization is, the part of the radiation intensity corresponding to that polarization, divided by the total radiation intensity averaged over all directions. 2.1.4 Gain The gain of the antenna is related to the directivity of the antenna. Gain takes into account the directional capabilities as well as the efficiency of the antenna [1]. The gain of an antenna (in a given direction) is defined as “the ratio of the intensity, in a given direction, to the radiation intensity that would result if the power fed to the antenna were radiated isotropically”. The radiation intensity corresponding to the isotropically radiated power is equal to the power from the generator, to the antenna divided by 4π [1]. 4 Mathematically this can be expressed as radiation intensity ( ) Gain = 4π = 4π (dimensionless) (3) total input (accepted)power 2.1.5 Aperture Efficiency The ratio of the maximum effective area to the physical area. 2.1.6 Beamwidth The beamwidth involves a trade-off because the side lobe level increases as the beamwidth decreases and vice versa. The beamwidth is also used to describe the capability of the antenna to distinguish between two adjacent radiating sources or radar targets [1]. The beamwidth of an antenna is defined as the angular separation between two identical points on opposite sides of the pattern maximum. There are a number of beamwidths in the antenna pattern. One of the most widely used beamwidths is the Half-Power Beamwidth (HPBW) [1]. Figure 2: Antenna Beamwidth. 5 2.1.7 Input Impedance The impedance presented by an antenna at its terminals or the ratio of the voltage to current at a pair of terminals [1]. ZA = RA + jXA (5) RA is the real part and XA is the imaginary part. The resistive part relates to the power dissipation, while the imaginary (reactive) part relates to power stored in the near field of the antenna. 2.1.8 Polarization The polarization is the orientation of the electric field far from the source [2]. It describes the time-varying direction and relative magnitude of the electric field vector. Polarization for an antenna in a given direction is defined as the polarization of the E-field transmitted (radiated) by the antenna. When the direction is not stated the polarization is taken to be the polarization in the direction of maximum gain. The polarization of a wave radiated by an antenna, in a specified direction, at a point in the far field, is defined as the polarization of the plane wave which is used to represent the radiated wave at that point [1]. Polarization may be classified as linear, circular, elliptical, circular left hand, circular right hand, elliptical right and elliptical left hand. 2.1.9 Antenna Efficiency The total antenna efficiency eo is used to take into account losses at the input terminals of the antenna. Such losses may be caused by: Reflections because of the mismatch between transmission line and antenna. I2R losses (conductive and dielectric). In general, overall efficiency can be written as: (6) eo = total efficiency, er = reflection mismatch efficiency, ec = conduction efficiency, ed = dielectric efficiency. 2.1.10 Beam Efficiency A parameter used to judge the quality of transmitting and receiving antennas is the beam efficiency [1]. 6 ( ) ( ) (7) ����1 is the half angle of the cone in the above equation. 2.1.11 Bandwidth The bandwidth of an antenna is defined as “the range of frequencies within which the performance of the antenna, with respect to some characteristics, conforms to a specified standard”. The bandwidth can be considered to be the range of frequencies on either side of the center frquency where the antenna characteristics are close to those at the center frequency [1]. 2.1.12 Antenna Radiation Efficiency The conduction dielectric efficiency is defined as the ratio of the power delivered to the radiation resistance Rr, to the power delivered to Rr and RL. The resistance RL is used to represent the conduction-dielectric losses [1]. 2.1.13 Return Loss The characterization of the input and output signal can be shown in a more convenient way in the form of return loss when a load is mismatched [3]. This means that all the source power is not delivered to the load. This loss of power is known as “return loss” and can be represented as: | |( ) (8a) Where | | (8b) | Г|= Magnitude of reflection coefficient, Vo = Reflected voltage, Vin = Incident voltage, ZL and Zo are the load and characteristic impedances. 2.2 Basics of Transmission Line Theory Transmission lines and waveguides are conduits for transporting RF signals between elements of a system. For example transmission lines are used between an exciter output and transmitter input, between the transmitter input and its output and between the transmitter output and the antenna [4]. Transmission lines are complex networks containing the equivalent of all the three 7 basic electrical components: resistance, capacitance, and inductance. Hence, transmission lines must be analyzed in terms of an RLC network [4]. 2.2.1 Wave propagation on a transmission line The wavelength of the travelling waves is defined as the distance between two successive points of equal phase on the line at a fixed instant of time [4]. (9) 2.2.2 Phase velocity The phase velocity of the wave is defined as the speed at which a constant phase point travels down the line. (10) 2.2.3 Voltage reflection coefficient (Г) The amplitude of the reflected voltage wave, normalized to the amplitude of the incident voltage wave, is defined as the voltage reflection coefficient [1]. (11) The average power flow is constant at along the line and the total power delivered to the load is the difference between incident power and the reflected power. If Г =0, maximum power is delivered to the load while no power is delivered for Г =1 (all the incident power is reflected back from the load) [1]. 2.2.4 Standing wave ratio (VSWR) When a tranmission line is not matched to its load some of the energy is absorbed by the load and some is reflected back down the line towards the source. The intereference of the incident and reflected wave creates standing waves on the transmission line [4]. As the magnitude of the reflection coefficient Γ increases, the ratio of Vmax to Vmin also increases. A measure of the mismatch of the line is called the standing wave ration (SWR), also know as the volatage standing wave ratio (VSWR) [1]. 8 (12) One has, 1<VSWR<∞ , where 1 implies a matched load. 2.2.5 Transmission lines with some special lengths There are some special transmission lines. For example: l= /2, Zin = ZL This means that half a wavelength of the transmission line does not alter or transform the impedance regardless of the characteristic impedance of the line. If the line is a quarter wavelength long i.e. l= /4 +n /2, for n = 1,2,3...N, the input impedance is given by, Zin= / ZL (13) Such a line is called quarter wave transformer because it has the effect of inverting the load impedance [1]. 2.2.6 Charactereristic Impedence The charactereristic impedence for a specific type of line is a function of the conductor size, the conductor spacing, the conductor geometry and the dielectric constant of the insulating material used between the conductors [4]. The transmission line is an RLC network with a charactereristic impdence Z0. Z0 is a function of line per unit lenght parameters, resistance R, conductance G, inductance L, capacitance C. Z0 = √ (14a) If X>>R √ (14b) 9 2.2.7 The Smith Chart The mathematics of transmission lines becomes cumbersome at times, especially when dealing with complex impedances and nonstandard situations. In 1939, Phillip H. Smith published a graphical device for solving these problems, the Smith Chart. It consists of a series of overlapping orthogonal circles that intersect each other at right angles. These sets of orthogonal circles make up the basic structure of the Smith chart and are shown in Fig. 3. The following is a brief description of the Smith chart and how it works [4]. Figure 3: The Smith chart. 10 2.2.7.1 The normalized impedance line A baseline bisects the Smith chart outer circle and forms the reference of measurements made on the chart. The Complex impedance contains both resistance and reactance and is expressed mathematically as: Z= R jX (15) R is the resistive component of the impedance and X is the reactive component of the impedance [4]. The pure resistance circle represents the situation where X=0, and the impedance is therefore equal to the resistive component only. To make the Smith chart universal, the impedances along the pure resistance line are normalized with reference to system impedance (Z0). The actual impedance it is divided by the system impedance. The pure resistance line is structured such that the system standard impedance is at the center of the chart and has a normalized value of 1.0 [4]. 2.2.7.2 The constantresistance circles The isoresistance circles, also called the constant resistance circles represent points of equal resistance. These circles are all tangent to the point at the right hand extreme of the pure resistance line and are bisected by that line. 2.2.7.3 The constantreactance circles The circles above the pure resistance line represent the inductive reactance (+X) while the circles below the pure resistance line represent capacitive reactance (-X). The outermost circle is called the pure reactance circle. Points along this circle represent reactance only. 2.2.8 S-parameters The S-parameters are very important in microwave design for describing the behavior of electrical devices. Most of the electrical properties i.e. gain, return loss, power, VSWR etc relates to the S-parameters. The S-parameters can be observed by sending a signal through an input port and observing the response on an output port. The term impedance is of great importance while calculating the S-parameters because the system should be matched properly, otherwise reflection which will give rise to standing waves and the system will not produce the desired output. The S-parameters S11 and S22 represent input and output reflection while S21 is the forward transmission coefficient (gain) and S12 is the reverse transmission coefficient (isolation). 11 2.4 Antenna Arrays Usually the radiation pattern of a single element is relatively wide, and each element provides low values of directivity (gain). In many applications, it is necessary to design antennas with very high directive characterristics (very high gain) to meet the demands of long distance communication. This can only be accomplished by increasing the electrical size of the antenna [1]. Enlarging the dimensions of single elements often leads to more directive characteristics. Another way to enlarge the dimensions of the antenna, without increasing the size of individual element, is to form an assembly of radiating elements in an electrical and geometrical configuration. This new antenna antenna formed is referred to as an array [1]. The antenna arrays are of vast importance and are widely used nowadays for various purposes like military, missiles and satellite communication. There are different forms of antenna arrays linear, circular, planar etc. The radiation pattern of an array antenna is mostly considered in the far field, where the field depends on two parameters. One is the distance r of the reciever and the other deals with the spherical coordinates θ and φ. The radiation pattern of an antenna can be calculated by : Array Pattern = Array element pattern * Array factor(AF) (16) The array factor determines the overall radiation pattern of the array while the element pattern describes radiation pattern of the individual element [5]. The array factor can also be defined as “The function of the total number of elements, their spacing and the phase difference between each element” [6]. The array factor for a uniform antenna can be written mathematicaly as: ⁄ ⁄ (17) One may normalise the array factor so that the maximum value is equal to unity. ⁄ ( ) ⁄ (18) 12 Antenna array design involves two broader concepts: Broadside Array End-Fire Array 2.4.1 Broadside Array In a broadside array, the radiators are along a straight line producing a beam perpendicular to the line [5]. For an optimal design, the maxima of the single element as well as of the array should be directed toward θ= 900 and the phase angle is zero. ( ⁄ ) ( ) ⁄ (19) ( ) Where and The requirements for the single element can be met by a judicious choice of the radiators, and those of the array factor by the proper separation and excitation of the individual radiators [1]. 2.4.2 End-Fire Array A linear array whose direction of maximum radiation is along the axis of the array. It may either be unidirectional or birectional. The main beam will either be at θo= 0o or 180o. (20) For θo= 0o or 180o (21) Which gives ( ) (22) ( ⁄ ( ) ( ) ⁄ (23) ( ( ) 13 2.5 Mutual Coupling in Antenna Array One of the basic characteristics of an antenna array appears when two or more elements are located near to each other and effect each other [1]. The amount of coupling depends on the following: Radiation characteristics. Actual separation between elements . Relative orientation of elements. The mutual coupling between two radiating elements depends upon the distance between them. If they are close to each other the mutual coupling will be greater. Thus energy is transferred between elements and this is called mutual coupling. One can say that the electromagnetic coupling between the elements is mutual [7]. The transmitting mode coupling can be shown with the help of Fig. 4. Two antennas, A and B are placed relative to each other. Antenna A is excited by a source and radiates. When this radiation reaches antenna B, it excites antenna B and rescatteres some the energy back to antenna A. Antenna A recieves the energy again and so on. The total contribution that an element makes to the far field pattern does not depend on its own excitation from the generator only, but also upon the total parasitic excitation due to which coupling is introduced to other generators [8]. The mutual coupling phenomenon is reciprocal in nature. If one antenna is used as a transmitter and the other as a reciever or vice versa. Both is the same. A B Figure 4: Mutual Coupling Mechanism. 14 Since mutual coupling in phased array antennas can affect the radiation pattern so consideration should be given to this mechanism. 2.6 Microstrip Antennas 2.6.1 Introduction For applications where size, weight, cost, performance, ease of installation and aerodynamics are constraints, low profile antennas are needed. Aircraft, spacecraft, satellite and missile applications and recently mobile radio and wireless communications demands this [1]. To meet these requirements microstrip antennas can be used. These antennas are low profile, suited to planar and non planar surfaces, simple and inexpensive to manufacture, mechanically robust when mounted on rigid surfaces, compatible with MMIC designs, and when the particular patch shape and mode are selected, they are very versatile in terms of resonant frequency, polarization, pattern and impedance [1]. Major operational disadvantages of microstrip antennas are their low efficiency, low power, high Q, poor polarization purity, poor scan performance, spurious feed radiation and narrow frequency bandwidth which is typically only a fraction of a percent or at most, a few percent [1]. Microstrip antennas also exhibit large electromagnetic signatures at certain frequencies outside the operating band and are rather large physically at VHF and possibly UHF frequencies. In large arrays there is a trade-off between bandwidth and scan volume [1]. The idea of the microstrip antenna was introduced in 1953 by G.A Deschamps and it received considerable attention by 1973. In 1970, Howell and Munson defined a transmission model for microstrip antennas. Microstrip antenna patch elements are the most common form of printed antennas. These antennas are quite cheap, light weight and give good results. The microstrip patch can have different shapes like circular, rectangular or square as shown in Fig. 5. 15 Figure 5: Different shapes of microstrip patch. 2.6.2 Rectangular Patch The rectangular patch is by far the most widely used configuration. A basic form of rectangular patch is shown in the Fig. 6. Figure 6: Rectangular microstrip patch antenna. The patch of a microstrip antenna is usually made of a conducting material. The patch is parallel to the ground plane. In between the patch and the ground plane there is substrate with a dielectric constant whose value depends on the substrate used. The inside of the rectangular patch is shown in Fig. 7. 16 Figure 7: Fringing effects in the microstrip patch antenna. Because the dimensions of the patch are finite, the fields at the edges of the patch undergo fringing [1]. The amount of fringing is a function of the dimensions of the patch and the height of the substrate. For the principal E-plane (xy-plane), fringing is a function of the ratio of the length of the patch L to the height h of the substrate (L/h), and the dielectric constant ϵ r of the substrate [1]. Most of the electric field lines reside in the substrate and parts of some line exist in air. As W/h >> 1 and εr >> 1, the electric field lines concentrate to the substrate. Fringing in this case makes the microstrip line look wider electrically compared to its physical dimensions [1]. The resonant length can be calculated using: √ L is resonant lenght, is the wavelength in printed circuit board, is wavelenght in free space and εr is the dielectric constant. The effective dielectric constant can be calculated by the formula: * + (22) Some of the electric field rests inside the substrate while some extends outwards due to fringing. Because of the fringing field between the edge of the patch and the ground plane, the patch radiates. To make antennas efficient, thick dielectric substrates with low dielectric constant are suitable. This gives larger bandwidth efficiency and desirable radiation. Due to large bandwidth, the size of the antenna will be very large, which is not wanted. To get rid of this problem, a thin dielectric substrate with high reduces the bandwidth but a trade-off has to be made. 17 2.6.3 Feed Models There are many configurations that can be used to feed microstrip antennas. The four most popular feed models are microstrip line, coaxial probe, aperture coupling and proximity coupling [1]. 2.6.4 Microstrip Line Feed The microstrip feed line is also a conducting strip, usually of much smaller width compared to the patch. The microstrip feed line is easy to fabricate, simple to match by controlling the inset position and rather simple to model [1]. However, as the substrate thickness increases, surface waves and spurious feed radiation increases which for practical designs limit the bandwidth [1]. Figure 8: Microstrip feed line designed in ADS. 2.6.5 Coaxial Probe Feed The inner conductor of the coax is attached to the radiation patch and the outer conductor is connected to the ground plane [1]. The coaxial probe feed is also easy to fabricate and match, and has low spurious radiation. However, it also has narrow bandwidth and it is more difficult to model, especially for thick substrates [1]. 18 Figure 9: Coaxial probe feed. 2.6.6 Aperture- coupled Feed The most difficult technique to fabricate is the aperture coupled feed. Having a narrow bandwidth, it is however somewhat easier to model and has moderate spurious radiation [1]. The aperture coupling consists of two substrates separated by a ground plane. On the bottom side of the lower substrate there is a microstrip feed line whose energy is coupled to the patch through a slot in the ground plane separating the two substrates. This arrangement allows independent optimization of the feed mechanism and the radiating element. Typically a high dielectric material is used for the bottom substrate, and a thick low dielectric constant material is used for the top substrate [1]. 19 Figure 10: Aperture-coupled feed. The main disadvantage of such a design is that it requires complex multiple layers. 2.6.7 Proximity-coupled Feed Proximity-coupled Feed is sometimes called an electromagnetic coupling scheme. It consists of two layers on top of each other. There is no ground plane in such an antenna. The microstrip feed line is in between the two substrates and the radiation patch is on the top of the substrate as show in Fig. 11. Of the four feeding models, the proximity coupling has the largest bandwidth and it is fairly easy to model, having low spurious radiation [1]. However, its fabrication is more difficult. The length of the feeding stub and the width- to-line ratio of the patch can be used to control the match [1]. Figure 11: Proximity-coupled feed. 20 2.6.8 Arrays and Feed Networks Arrays are very versatile and are used, among other things, to synthesize a required pattern that cannot be achieved with a single element. In addition, they are used to scan the beam of an antenna system, increase directivity, and perform various other functions which would be difficult with any one single element [1]. The elements can be fed by a single line called the series-feed network or by multiple lines called corporate-feed network [1]. Among all the feeding techniques, corporate feed is mostly used in scanning, phased multiple beam or shaped- beam arrays. With this method, the designer has more control of the feed of each element (amplitude and phase) and it is ideal for scanning phased arrays, multiple beam arrays, or shaped-beam arrays [1]. While designing an array, the feed point and the distance between each patch is kept constant in order to provide equal phase patch excitation. A series feed network is easy to fabricate and implement as compared to corporate feed network. The disadvantage of using series feed is that it gives phase delay and hence it is not preferred for the phase scanning arrays [9]. These phase shifts are frequency dependant due to which beam scanning is dependent on the frequency [9]. Corporate feed networks provide flexible phase control of each array element. It is suitable for phase scanning as it is less affected by the frequency scan [9]. The most common form of corporate feed network is the Wilkinson Power divider rule. (a) Series Feed. (b) Corporate Feed. Figure 12: Types of feed. 21 2.7 Photonic crystals in microstrip antenna substrates During the past decade, a new technology has emerged which has become the key to developing ultra-wideband microstrip antennas. This technology manipulates the substrate in such a way that the surface waves are completely forbidden from forming, hence resulting in improvements in the antenna efficiency and bandwidth, while reducing the side lobes and electromagnetic interference levels. These substrates contain so called photonic crystals [10]. The patch antennas on high ԑ substrates are not efficient radiators due to losses. The patch antenna having a narrow frequency bandwidth results in reduced gain and efficiency at high frequencies. Patch antennas also have an unacceptably high level of cross polarization and mutual coupling within the array environment. Therefore, much effort has been made recently to realize high efficiency patch antennas on high permittivity substrates at high frequencies [11]. A PGB crystal is a periodic structure that forbids the propagation of electromagnetic waves within a particular frequency band, called the band gap, thus permitting controls of the behavior of the electromagnetic waves other than the conventional guiding and filtering structures [10]. The photonic crystals are a class of periodic metallic, dielectric or composite structures that exhibit a forbidden band (band gap) of frequencies in which waves, incident at various directions interfere destructively and thus are unable to propagate [12]. Based on the spatial periodicity of the crystal structure, the band gaps can be in one, two or three-dimensional planes, with a level of complexity that increases with the number of dimensions. The three-dimensional nature of the band gap rejects incident energy from all directions around a unite sphere like a high efficiency reflector or mirror. In a 2-D photonic crystal fiber the band gap exists only within a plane, thereby allowing propagation along one axis only. This is the ideal scenario for microstrip antenna design, since the rejection plane could be in the plane of the patch and thus prevent surface wave formation [12]. 22 3. ADS Momentum Overview 3.1 Introductions to ADS Momentum Momentum is a part of Advance Design System and it provides the simulation tools required to evaluate and design products of modern communication systems. Momentum is an electromagnetic solver in the form of a simulator that computes the S-parameters for general planar circuits which includes microstrip, slotline, stripline, coplanar waveguides and many other topologies. Multilayer communication circuits and printed circuit boards can also be simulated in ADS Momentum with accurate results. Momentum is a complete tool for prediction of the performance of high frequency circuit boards, antennas and integrated circuits [13]. The ADS Momentum optimization tool extends Momentum capability to a real design automation tool. The Momentum Optimization process varies geometry parameters automatically to help in achieving the optimal structure that for the circuit or device performance goals. Momentum optimizations can be done by using layout components (parameterized) from the schematic page. One of the great advantages that Momentum possesses is the 3-dimensional interface that it provides for the user during simulations and results. Momentum is a 2.5D solver that can do both 2D and 3D computations. For example while computing the antenna parameters, Momentum provides both 2D and 3D graphs of the directivity and the far-field radiation patterns of the antenna. 3.2 Applications of Momentum ADS Momentum can be used as follows [13]. ADS Momentum is applicable when no analytical model exists for the circuit. Momentum co-simulates with ADS and performs the required tasks. ADS Momentum can be used to determine coupling effects. ADS Momentum can calculate narrow resonances within the circuit model which cannot be found with analytical models. 23 ADS Momentum can be used to display the radiation patterns and far field radiation plots for antennas etc. ADS Momentum can show the current pattern and current densities within the circuit. Momentum can be used for the CPW (Co Planar Waveguides) results with no slot mode. Momentum can be used to optimize or modify the geometry of the passive layouts to achieve the desired results. 3.3 Method of Calculation The method of simulation that is used by ADS Momentum is called the Method of Moments which is based on the integral formulation of Maxwell’s equations, simulating the circuit with matrix equations. Fig. 13 shows the stepwise simulation of a circuit by ADS Momentum. Where a known circuit is first simulated and then divided into mesh strip, wires with rectangles and triangles (arbitrary surface meshes). The next step is to model the surface current in each current cell i.e. linear distribution. The final step is to solve a mesh matrix equation and calculate S-parameters. Figure 13: Stepwise simulation of ADS Momentum. 24 3.4 Working with ADS Momentum A short literature on how one should start using ADS Momentum is given below. As with every simulator, working with the ADS Momentum is a stepwise process. Some of the steps are given below. Step 1: Step 1 shows the startup of the ADS Momentum. Momentum starts in the ADS layout window as shown in Fig. 14. Figure 14: Layout window of ADS Momentum. A number of options can be seen on the task and menu bar. We can use different kinds of microwave components depending on our requirement. Fig. 14 shows the mapping of microstrip patches of varying lengths in the ADS layout window. Ports are connected on both sides of the circuit, making it a two port network for the S-parameter calculation. 25 Step 2: The Microstrip patch antennas cannot be designed without a substrate. In order to define a particular substrate we can create our own substrate or we can use the built in substrates defined in ADS momentum. Figure 15: Different parameters in ADS Momentum. Fig. 15 shows the different parameters which need to be set before simulating any design in the ADS Momentum layout. After substrate definition, we will use ports calibration. Port is necessary in the optimization of any design because it serves as an input to the system. The next parameter in the list is Mesh setting. We can change the mesh frequency in order to synchronize it with the input resonant frequency. Design cannot work if the mesh frequency is not synchronized with the input resonant frequency. Step 3: The final step is to calculate the S-parameters. Depending on the number of ports used in the network, ADS Momentum will provide the related S-parameters. Fig. 16 shows an example of output curves of S-parameters calculated by ADS Momentum. 26 Figure 16: Output S-parameter curves. 3.5 Theory of Operation for Momentum Momentum is based on a numerical discretization technique called the Method of Moments. This technique is used to solve the Maxwell equations for planar structures embedded in multilayer dielectric substrates. Momentum uses two different modes of simulation which are based on the Method of Moments. The first one is the microwave, or full wave, mode of simulation and the second one is the RF, or quasi-static, mode of simulation. The application and formulation of the Green’s function is the main difference between these two methods. Momentum, or the full wave simulation mode, uses the full wave Green’s function. The Full wave Green’s function is frequency dependant and it fully characterizes the substrate without making any further approximations. This formulation results in the L and C elements that are complex and frequency dependant as shown in Fig. 13. The RF, or quasi-static, mode uses a 27 frequency independent Green’s function which results in L and C elements which are complex but frequency independent. As this mode is not frequency dependant, the quasi-static mode only approximates the solution of the network (L and C) for the first frequency simulation point and hence the RF mode runs much faster than the Momentum mode. The simulations also show that the quasi-static mode should be used for structures that are smaller than half a wavelength [13]. Both the engine modes use the star-loop basis function that ensures a stable solution at all frequencies. Both the modes use the mesh reduction algorithm which helps in reducing the number of unknowns when dividing the design into polygonal meshes. This function can be turned on or off [13]. Excitation of the networks is fed through the input port. The currents in the equivalent network model are given by unknown amplitudes in the rooftop expansion model. The amplitudes are obtained by solving for the unknowns in the rooftop expansion. The S-parameters are extracted with the help of the port calibration process. 3.6 Method of Moment Technology The method of moments (MoM) was first applied by R.F. Harrington who worked extensively on the method and successfully applied it to electromagnetic field problems. It is based on the theory of weighted residuals and variational calculus. In the MoM, Maxwell’s equations are transformed into integral equations before discretization. Momentum uses the mixed potential integral equation (MPIE) formulation [14]. This method expresses the electric and magnetic fields with a combination of the scalar and the vector potential. The electric and magnetic surface currents in the design network are the unknowns in the planar circuit. From electromagnetics one has the integral equation, ∬ ⃡( ⃗ ⃗ )⃗⃗( ⃗ ) ⃗⃗⃗⃗⃗( ⃗) (23) 28 Here, ⃗( ⃗) represents the unknown surface current and ⃗⃗ ( ⃗) represents the known excitation of the problem. The Green’s dyadic of the layered medium acts as an integral kernel. The unknown surface currents are discretized by meshing the planar metallization pattern and applying an expansion in a finite number of sub-sectional basis functions B1( ⃗)…., BN( ⃗) [14]: ⃗( ⃗) = ∑ ⃗⃗ ( ⃗) (24) The rooftop functions are used in planar EM simulators. These standard basis functions are defined over rectangular, triangular and polygonal cells in the mesh. Each rooftop is associated with one edge of the mesh and represents current with continuous density as shown in Fig. 17. Ij, determine the current elements that corresponds to the edges of the mesh [14]. Figure 17: Discretization of the surface current using rooftop basis functions. Eq. 23 is discretized by inserting the rooftop expansion of Eq. 24. We can write: 29 =∑ j(r) or ZI = V (25) ∬ ⃗⃗ ( ⃗) ∬ ⃡ ( ⃗ ⃗ ) ⃗⃗ ( ) (26) ∬ ⃗⃗⃗⃗( ⃗) ⃗⃗ ( ⃗) (27) The matrix Z is known as the interaction matrix since the elements in this matrix describe the electromagnetic interaction between the rooftop basis functions. The vector V represents the discretized contribution of the excitation applied at the ports of the circuit [14]. The final values of L and C in the network can be written as: ∬ ⃗⃗⃗⃗( ⃗) ∬ ⃡⃗⃗⃗⃗⃗( ⃗ ⃗ ) ⃗⃗ ( ⃗ ) (28) ∬ ⃗⃗⃗⃗( ⃗) ∬ ⃡⃗⃗⃗⃗( ⃗ ⃗ ) ⃗⃗⃗⃗(⃗⃗⃗) (29) Eq. (28) and Eq. (29) gives a physical interpretation to the interaction matrix, as shown in Fig. 18. Figure 18: Mesh representation in the form of L and C. The whole discussion is explained diagrammatically in Fig. 17. 30 3.7 Simulation Techniques Used in ADS In addition to the “auto-select mode”, ADS uses three different matrix solution techniques which are explained as following. 1. Direct Dense. 2. Iterative Dense. 3. Direct Compressed. 3.7.1 Direct Dense Method In this method, the matrix N is stored in a dense matrix format which requires memory space of order N2. This matrix is then solved by the direct matrix factorization technique. This method requires N3 order for solution (computer time). The direct dense matrix solver has a predetermined number of operations. The main disadvantage of this method is that it requires cubic computer time to solve dense matrices of the order N. Hence it requires larger time for complex problems [13]. 3.7.2 Iterative Dense Method In this method, the matrix N is stored in a dense matrix format which requires N2 memory space while the matrix N is solved using iterative matrix solver technology. This solution method requires N2 order to solve the matrix, hence scaling the computer time to quadratic (N2) from cubic (in the direct dense method). This yields shorter simulation time for larger problem sizes [13]. Convergence of the iterative technique is the main drawback of this method of simulation because iterative methods do not converge quickly in large and complex problems. ADS monitors the convergence rate and automatically jumps to the direct dense method of simulation when it detects stagnancy in the convergence [13]. 3.7.3 Direct Compressed Method Direct compressed method (DCM) is one of the latest techniques used for matrix solution. DCM is also known as the FMM (Fast Multipole method) and is considered to be one of the top ten 31 algorithms of the 20th century. In this method, the matrix is stored in a compressed matrix form which requires NlogN memory space while it is solved using direct compressed matrix factorization technique. The direct compressed factorization technique requires (NlogN)1.5 computer time [20]. The computer time that this method requires is linear logarithmic with matrix size N which makes this method most useful for the solution of large and complex problems. This method reduces both the simulation speed and the memory allocation space required for the simulation [13]. All these three type of methods are available in ADS. Generally the default settings of the ADS are set to auto-mode but user can change the type of simulation manually. ADS is sensitive to the type of problem and chooses proper simulation method according to the problem, so the preferable way to use ADS is to keep the settings on auto-mode. 3.8 Block Diagram of ADS Momentum Simulation The Block diagram in Fig. 20 shows how ADS Momentum simulates its designs and provides the outputs. Figure 19: Block diagram of ADS Momentum simulation. 32 4. Design and Analysis Before going into the details of the project, it is useful to perform a simple test with the ADS Momentum to validate the simulations of ADS Momentum with known results. For this purpose, an example from the book of “Antenna Theory” [1] was simulated in ADS momentum. All the results were calculated mathematically and then fed to the ADS Momentum design guide to get the visuals of the far field and other graphical results. 4.1 Design of a Rectangular Patch Antenna A rectangular patch with TMX010 mode is designed in ADS Momentum. The length of the patch is 0.906 cm, the width of the patch is 1.186 cm and the height of the patch is 0.1588 cm. Permittivity of the substrate is 2.2 and the resonance frequency is 10 GHz. RT Durroid 5880 is used as a substrate with a substrate height of 0.787 µm. The rectangular patch was energized using coaxial probe feed. The mathematical solution is available in [1]. This patch was designed in ADS Momentum. After design, the patch was simulated in ADS Momentum to get the directivity, the gain curves along with the 3D visuals of the far field radiation and the 3D view of the designed antenna patch. Fig. 20 shows the design of the single rectangular patch in ADS Momentum environment. Figure 20: Rectangular patch designed in ADS Momentum layout. 33 Fig. 21 and Fig. 22 show the shows the simulation results of the rectangular patch in ADS Momentum. Fig. 21 shows the behavior of the S11 parameter or the input reflection coefficient over a range of frequencies. It is clear from the figure that the patch resonates at 10 GHz and has minimum loss at the resonant frequency i.e. -3 dB. m1 freq=10.35GHz dB(example_14_1_mom_a..S(1,1))=-3.077 Min S11 0 -1 Mag. [dB] -2 m1 -3 -4 0 2 4 6 8 10 12 14 16 Frequency Figure 21: Magnitude of S11 in dB. Fig. 22 shows the same input reflection coefficient (S11) result in the Smith chart. We can see from the Smith chart that the impedance of the system is also resistive at the input. The marker shows the impedance of the system at the resonant frequency. 34 Figure 22: S11-parameter shown in Smith chart 4.2 Gain and Directivity One of the main features of the ADS Momentum is that it can give us both the 2D and 3D graphs of the gain and directivity of the system. Fig. 23 shows the gain and directivity of the rectangular patch simulated in ADS Momentum. The lower graph line, the Gain of the system is approximately -20 dB while the upper line, the directivity of the system, is approximately 10 dB. 35 Power Gain Directivity 20 0 Mag. [dB] -20 -40 -60 -100 -80 -60 -40 -20 0 20 40 60 80 100 THETA Figure 23: Gain and Directivity of the rectangular patch. Similarly ADS Momentum simulates the three dimensional view of the directivity, or the far field radiation pattern, of the rectangular patch microstrip antenna as shown in Fig. 24. It can be seen that the far field radiation is not broad side but almost isotropic. As it is a single patch antenna, it does not possess great directivity. Figure 24: 3D graph of the far field radiation. 36 4.3 Design of the Circular Patch The design of the circular patch requires some constraints. Following are some design parameters which should be calculated while designing the circular patch. 4.3.1 Resonant Frequency The resonant frequency of the circular patch can be analyzed with the cavity model. The cavity model consists of the electrical conductors above and below the cavity while a perfect magnetic conductor having cylindrical shape and radius a in between the two electrical conductors represents the value of the cavity [1, 9, 15]. The resonant frequency for TMzmn0 mode is: (fr)mn0 = ( ) ( ) (30) in Eq. (30) is the nth zero of the Bessel function Jm(X) which determines the resonant frequency that is different for different modes of operation. Following are values of the [1]. = 1.1841 = 3.0542 = 3.8318 = 4.2012 With the values of the zeros of the Bessel function in Eq. (30) we can show, (fr)mn0 = (31) √ For the mode that is used in the design one has, (fr)110 = (32) √ Eq. (32) gives the resonant frequency for the circular patch in the cavity model. Here ae is the radius of the patch while c is the velocity of light in vacuum. 37 4.3.2 Radius of the Patch In designing a rectangular patch, we account for the length and width of the patch. Changing these two parameters will change the mode of operation of the rectangular patch. In the circular patch, we have only one degree of freedom and that is the radius of the patch. To calculate the radius of the patch we have to include fringing in the circular patch. Fringing is the effect which makes the patch electrically larger than geometrical patch. Due to this phenomenon, the effective radius ae is introduced. Eq. 33 shows the expression for effective radius for the circular patch [1, 2, 15]. ae = a√, * ( ) +- (33) Here the original radius a is given by a= (34) √, * ( ) +- F= (35) √ 4.3.3 Feed Point Location In the design and excitation of the circular patch, the feed point location is one of the most important parameters [15]. The impedance of the circular patch antenna is almost zero at the center of the patch while it is about 200-300 ohms at the edges of the patch. Impedance matching can only be obtained by locating the feed point so that the overall system impedance equals 50 ohms. According to Karmakar [9], the mathematical expression for the feed point location for the TM110 is given below: = a/3 (36) Eq. (36) is the location of the feed point. 38 4.4 Proposed Design of a Single Circular Patch Antenna To design a circular patch antenna in ADS Momentum, we need to know all the parameter values [15, 16, 17]. All the needed design parameters were calculated using the known equations. RT DURROID 5880 was used as a substrate with relative permittivity ԑ equal to 2.2. The height of the substrate H is equal to 0.787µm. The loss tangent for this substrate is equal to 0.0009. The radius of the circular patch calculated using Eq. 33 was equal to 5.83 mm. The feed point distance from the center of the circle is equal to 1.83 mm. We used coaxial probe feed to excite the circular patch antenna with the input impedance equal to 50 ohms. Fig. 25 shows the design of this circular patch in ADS Momentum. Figure 25: Design of single circular patch antenna in ADS Momentum. After designing the circular patch in the ADS Momentum environment, the patch was simulated to check the performance of the patch. Figures (26a) and (26b) shows the three dimensional design of the circular patch antenna before and after excitation, respectively. 39 (a): Circular Patch before excitation (b) Circular patch after excitation Figure 26: Excitation of circular patch antenna in ADS Momentum. 4.4.1 Gain and Directivity: Fig. 27 shows the gain and directivity curves of the single circular patch antenna simulated by ADS momentum. Figure 27: Gain and Directivity of single circular patch antenna in ADS Momentum. The lower curve in Fig. 27 represents gain while the upper curve represents the directivity of the patch. As visible from the figure, the gain of the single circular patch antenna is 5 dB approximately while the directivity of the patch is approximately 8 dB. 40 ADS Momentum can also simulate the three-dimensional graph of the single circular patch antenna along with two dimensional graphs as shown in Fig. 27. Fig. 28 shows the 3D graph of the far field radiation of the antenna. Figure 28: 3D view of the directivity of the single circular patch simulated in ADS Momentum. From Fig. 28, it is clearly visible that the single circular patch antenna has the main beam in the 90º to 270º range i.e. Perpendicular to the axis of the patch. The single circular patch antenna is not an efficient antenna so the main lobe is wide. 4.4.2 S11 Parameters: ADS Momentum simulations help us in gathering information about the reflection coefficients of the antenna. We are using only one probe, so all the coefficients of the S-matrix will be zero accept the S11 parameter which is the input reflection coefficient [1]. We can easily understand the performance of the circular patch antenna from the S11 Parameter graph. Fig. 29 shows the graph of S11 parameter simulated by ADS Momentum. 41 Figure 29: Magnitude vs Frequency graph of input reflection coefficient. Fig. 29 clearly indicates that the single circular patch antenna resonates at 9.875 GHz having a minimum magnitude of approximately -5 dB. ADS Momentum also simulates the graph for the input reflection coefficient on the smith chart as shown in Fig. 30. The single circular patch antenna resonates at 10 GHz with minimum impedance at that particular point which is also indicated by the m2 marker on the Smith chart. Figure 30: S11-parameter of a single circular patch antenna on a Smith chart. 42 4.4.3 Efficiency Fig. 31 shows the efficiency of the single circular patch antenna simulated by ADS. Figure 31: Efficiency of single circular patch antenna simulated by ADS Momentum. 4.5 Proposed Design for the Circular Patch Array Antenna Fig. 32 shows our final project design. The array consists of two circular patches excited with coaxial probe feed, designed and simulated in ADS Momentum. Figure 32: Circular patch phase array antenna designed in ADS Momentum. 43 Each of the circular patches has the radius of 5.83 mm. The separation between the two circular patches is 12 mm from center to center. The impedance of the circular patch is 200-300 ohms at the edge of the patch while this impedance decreases to zero towards the center of the patch. The two circular patches are excited with a coaxial probe feed through the corporate feed method i.e. The Wilkinson power divider rule. The patches are connected to the coaxial probe via two transmission lines. These two transmission lines are terminated by a quarter wave transmission line. In order to make the circular patch array antenna radiate, the impedance of the system should be matched and it should not exceed 50 ohms [13, 15]. For this purpose the length and width of the transmission lines were calculated with the software called Line Calc, which is available in ADS. For a 200 ohms transmission line, the length is equal to 11.72 mm and the width is equal to 0.06096 mm. Both these transmission lines are terminated on the 100 ohms termination point on the quarter wave transmission line. The length of the quarter wave transmission line is equal to 5.428 mm and the width of the quarter wave transmission line is equal to 2.419 mm. The 50 ohms coaxial probe is connected to the other termination point of the quarter wave transmission line. With these arrangements, the impedance of the system is matched to approximately 50 ohms. RT-Durroid 5880 is used as the substrate for the circular patch microstrip phase array antenna. This substrate is used worldwide for the design of the microstrip phase array antenna. The thickness of the substrate is very important in the design of the microstrip antenna because the beamwidth changes with the thickness of the substrate [16, 18, 19]. For a 10 GHz resonating frequency, the thickness of the substrate is 0.787 mm while the thickness of the circular patches is 17.8 µm. The thickness of the patch should not exceed the thickness of the substrate. The microstrip phase array antenna was simulated after design in ADS Momentum. Following were the different simulation results of the proposed design. 44 The 3D view of the design simulated by the ADS Momentum is shown in Fig. 33. Figure 33: 3D view of circular patch microstrip phase array antenna. 4.5.1 Directivity and Gain ADS Momentum provides both the 2D and 3D graphs of the gain and directivity of the microstrip phase array antenna. The 2D graphs are shown in Fig. 34. Figure 34: Gain and Directivity graphs of circular patch microstrip phase array antenna. From Fig. 34, it is clear that the magnitude of the gain and directivity of the system is approximately 5 dB. Maximum results are obtained from 60º to 120º. The 3D graph of the radiation pattern of the designed antenna is shown in Fig. 35. 45 Figure 35: 3D Directivity of circular patch microstrip phase array antenna. Fig. 35 shows an accurate three dimensional graph of the radiation pattern of the microstrip phase array antenna. The antenna is radiating broadside i.e. Perpendicular to the axis of the patch. The main beam is sharp in between 90º and 270º. This graph shows that the design is working well and that it has achieved the desired results. A more advanced 3D curve is obtained from the ADS Momentum using a feature called the Electro Magnetic Design Solver showing a different presentation method of the 3D curves. The Graph obtained by EMDS is shown in Fig. 36. 46 Figure 36: 3D radiation pattern shown by EMDS. EMDS in Fig. 36 shows main lobes on 90 º and 270 º with nulls at 0 º and 180 º. 4.5.2 S11 Parameters As we have used only one probe feed in our design, we will find only the input reflection coefficients or the S11 parameters of the micro strip phase array antenna. The simulation results of the ADS Momentum are shown in Fig. 37. Figure 37: Magnitude vs Frequency graph of S11 parameter. 47 Figure 38: Phase vs Frequency graphs of S11 parameter. Figure 39: S11 parameter plotted on the Smith chart. Figures 37, 38, and 39 give us clear information about the performance of our design. In Fig. 37 we can see that the designed antenna resonates at the desired frequency which is 10 GHz. At the resonant frequency the input reflection coefficient has the minimum magnitude which is about -16 dB. In Fig. 39, the input reflection coefficient is shown on the Smith chart where the marker clearly indicates that the microstrip phase array antenna resonates at 10 GHz having the minimum impedance over the straight resistance line at the resonating frequency. The graphs in the three figures verify the performance of the designed antenna to a great extent. 48 The efficiency of this antenna is shown in the Fig. 40. The efficiency of array antenna is far better than the efficiency of single circular patch antenna. The efficiency of the array antenna is 90% while the efficiency of single circular patch antenna was 60%. Figure 40: Efficiency of the circular patch microstrip phase array antenna. The power radiated by the designed antenna is shown in Fig. 41. Figure 41: Radiated power of the circular patch microstrip phase array antenna. 49 5. Conclusion 5.1 Conclusion Summary All the simulation results show that the microstrip phase array antenna performs better then the single circular patch or the single rectangular patch antenna. The radiation pattern of the microstrip array antenna is far better than the single circular or the single rectangular patch. The main beam is in the broadside direction between 90º and 270º with nulls at 0º and 180º. Similarly the efficiency, directivity and gain of the array patch antenna is better than the single patch. All these simulations lead to the conclusion that the number of patches in an array is directly proportional to the efficiency, directivity and gain of the antenna. If we increase the number of elements in the array, the radiation pattern will improve further. Another important conclusion that was deduced from all the experiments that were made during the design of this antenna was that impedance matching is very important. Effic ient results were only obtained when the impedance of the system was perfectly matched to 50 Ω. 5.2 Future work Antenna technology is a vast field. Every day new research is published. A few design parameters were taken into consideration while designing this antenna. Further improvements can be made in the following areas. The gain, directivity, radiation pattern and efficiency can be improved by using 2n array elements in the microstrip phase array antenna. We have used only two circular patches. The beam of the circular patch phase array antenna can be steered using phase shifters in the design. Recent research involves the use of photonic band gap crystals in the substrate to improve the band width of the antenna. Instead of the ADS Momentum, the HFSS simulator can be used for design and simulation. 50 References [1] C. A. Balanis, Antenna Theory, 3rd edition, John Wiley, New York, 2005. [2] N. J. Kolias, R. C. Compton, J. P. Fitch and D. M. Pozar, Antenna, CRC Press, 2000. 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Wandzura, “The Fast Multipole Method for the Wave Equation: A Pedestrian Prescription,” IEEE AP-35, pp 1-12, June 1993. 52 SE-351 95 Växjö / SE-391 82 Kalmar Tel +46-772-28 80 00 dfm@lnu.se Lnu.se