Design and Analysis of Microstrip Leaky wave Antennas Dual Degree Project Stage 1 Report Submitted in partial fulfillment of the requirements for the degree of Master of Technology by Ayush Ambadas Dahale (Roll No. 18D070041) Under the guidance of Prof. Jayanta Mukherjee Department of Electrical Engineering Indian Institute of Technology Bombay 2022-2023 Declaration I declare that this written submission represents my ideas in my own words. I have cited and referenced the original sources where others’ ideas or words have been included. I also declare that I have adhered to all academic honesty and integrity principles and have not misrepresented, fabricated, or falsified any idea/data/fact/source in my submission. I understand that any violation of the above will result in disciplinary action by the Institute. In addition, it can also evoke penal action from sources that are not correctly cited or from whom proper permission has not been taken when needed. Ayush Amabadas Dahale Electrical Engineering IIT Bombay Acknowledgement I would like to express my gratitude to Prof. Jayanta Mukherjee for his valuable guidance. Also, I would like to thank Ms Vishakha Pandey and all my peers from the Integrated systems laboratory for their constant support, guidance and encouragement through the project. Ayush Ambadas Dahale Electrical Engineering IIT Bombay Abstract Although Leaky Wave Antennas have been in existence since the 1940s, This field has seen immense growth in the last decade as well as in recent years, with most of the recent work being in the area of planar Leaky Wave Antennas, which are very low profile and the ease of manufacturing is significant. Some of the recent developments in the Leaky Wave Antennas field have been related to the metamaterials area, which has provided new inspiration for novel designs. Various designs that overcome the stopband problem at the broadside and allow for continuous beam scanning through the broadside have been reviewed. Other developments that have been reviewed include LWAs that can scan to endfire. Since the 1970s, Microstrip Leaky Wave Antennas(MLWA) have undergone vigorous development and are now widely used in a wide range of applications, including satellite communication, target tracking, cruise control, and collision avoidance. In the field of MLWA, it is now widely accepted that the fundamental EH0-mode of the Microstrip line(MSL) is a limited mode that is unable to produce the intrinsic leaky-wave radiation from the guided-wave MSL after several decades. All MLWAs must therefore be implemented using these higher-order modes. In this thesis, we investigated the various designs of microstrip leaky wave antennas, analyzed their feasibility of implementation and studied various excitation modes. A thorough literature survey was performed on various designs. Designs were analyzed for various performance parameters by performing simulations in Anysys HFSS. In addition, we suggest improvements that can be made to the analyzed designs to improve performance parameters. KEYWORDS: Leaky-Wave Antennas, Microstrip line Contents List of Figures 3 1 Introduction 5 1.1 Organization of the Report . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Microstrip Leaky Wave Antenna 5 6 2.1 Description of Microstrip Antenna . . . . . . . . . . . . . . . . . . . . . . 6 2.2 Radiation Patterns of Microstrip Antenna . . . . . . . . . . . . . . . . . 7 2.3 Advantages of Microstrip Antennas . . . . . . . . . . . . . . . . . . . . . 8 2.4 Disadvantages of Microstrip Antennas . . . . . . . . . . . . . . . . . . . . 8 2.5 Applications of Microstrip Antennas 9 . . . . . . . . . . . . . . . . . . . . 3 Related Work 3.1 Periodic Half-Width Microstrip Leaky-Wave Antenna With a Backward to Forward Scanning 3.2 10 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 3.1.1 Design Methodology . . . . . . . . . . . . . . . . . . . . . . . . . 11 3.1.2 Design Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 3.1.3 Major Strengths . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 The Half-Width Microstrip Leaky Wave Antenna With the Periodic Short Circuis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 3.2.1 Design Methodology . . . . . . . . . . . . . . . . . . . . . . . . . 14 3.2.2 Design Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 3.2.3 Major Strengths . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 4 Implementation 4.1 19 Microstrip Leaky-Wave Antenna With I-Shaped Slots . . . . . . . . . . . 19 4.1.1 Design Methodology . . . . . . . . . . . . . . . . . . . . . . . . . 19 4.1.2 Design Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 2 5 Conclusion 24 6 Future Scope 25 3 List of Figures 2.1 Top View of Patch Antenna . . . . . . . . . . . . . . . . . . . . . . . . . 6 2.2 Side View of Microstrip Antenna . . . . . . . . . . . . . . . . . . . . . . 7 2.3 Radiation Pattern 8 3.1 3-D view of the proposed periodic half width MLWA with backward to . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . forward scanning capabilities . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 11 Layout of periodic half width MLWA with backward to forward scanning capabilities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3 The measured radiation patterns for 4.4 GHz, 4.9 GHz, 5.3 GHz 3.4 The measured radiation patterns for 6.2 GHz, 6.7 GHz, 8 GHz 11 . . . . 12 . . . . . 13 3.5 reflection co-efficient (S11 ) . . . . . . . . . . . . . . . . . . . . . . . . . . 13 3.6 The 3D view of the half-width microstrip LWA with the periodic short circuits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 3.7 The layout of the proposed half-width microstrip LWA . . . . . . . . . . 15 3.8 Copolarization patterns the following frequencies 4.6GHz, 5GHz, 5.2GHz 16 3.9 Copolarization radiation patterns the following frequencies 6.5GHz, 7.5GHz, 8.5GHz 16 3.10 Copolarization radiation patterns the following frequencies 4.6GHz, 5GHz, 5.2GHz 17 3.11 Crosspolarization radiation patterns the following frequencies 6.5GHz, 7.5GHz, 8.5GHz 17 3.12 Reflection co-efficient (S11 ) . . . . . . . . . . . . . . . . . . . . . . . . . . 18 4.1 EH2 -mode microstrip LWA with an array of I-shaped slots . . . . . . . . 20 4.2 Feeding structure: Configuration of the EH0 -to-EH2 mode converter . . 20 4.3 The 3-D view of Microstrip Leaky-Wave Antenna With I-Shaped Slots . 20 4.4 Photograph of the design EH2 -mode LWA with I-shaped slots . . . . . . 21 4.5 Photograph of the design EH2 -mode LWA with I-shaped slots . . . . . . 21 4.6 Photograph of the design EH2 -mode LWA with I-shaped slots . . . . . . 22 4.7 Photograph of the design EH2 -mode LWA with I-shaped slots . . . . . . 22 4.8 Photograph of the design EH2 -mode LWA with I-shaped slots . . . . . . 23 4 Chapter 1 Introduction . 1.1 Organization of the Report The background and motivation behind the selected topic and the current objectives and scope of the work have been listed above in the introductory chapter. The second chapter discusses microstrip antennas and the method used to design the microstrip antennas.. The third chapter walks through the design process of antennas that were analysed in the course of the project through a literature review. The fourth chapter walks through the simulated results of the designed antennas. The final chapter discusses the conclusions and improvements that can be made to the models in future work. 5 Chapter 2 Microstrip Leaky Wave Antenna 2.1 Description of Microstrip Antenna Microstrip antennas also called patch antennas are becoming very popular owing to their ease of production as they can be directly printed onto the circuit board. Consider the figure below, The microstrip consists of the following components. The patch antenna, microstrip transmission line, ground plane and a substrate. The substrate is made of some dielectric circuit board, whereas rest of the three components are made up of high conductivity metal just like a copper. The frequency of operation of the antenna shown above is determined by length L. The centre frequency will be approximately given by Figure 2.1: Top View of Patch Antenna 6 Figure 2.2: Side View of Microstrip Antenna fc ≈ c √ 2L εr = 1 √ 2L ε0 εr µ0 According to the equation above, the microstrip antenna’s length inside the dielectric (substrate) medium should be equal to one-half of a wavelength. The microstrip antenna’s width W regulates the input impedance. The bandwidth can also be expanded by using larger widths. 2.2 Radiation Patterns of Microstrip Antenna The normalised radiation pattern of microstrip antennas is approximately given by Eθ = Eφ = − sin sin kW sin θ sin ϕ 2 kW sin θ sin ϕ 2 kW sin θ sin ϕ 2 kW sin θ sin ϕ 2 cos cos kL sin θ cos ϕ cos ϕ 2 kL sin θ cos ϕ cos θ sin ϕ 2 In above equations the k is the free space wavenumber given by 2π/λ and the magnitude q of the feild is given by f (θ, ϕ) = Eθ2 + Eϕ2 . The figure below represents the radiation pattern of the microstrip antenna. It is observed that a very high-quality factor is offered by a patch antenna. A large Q results in a narrow bandwidth and low efficiency. However, this can be compensated by increasing the thickness of the substrate. However, the increase in thickness beyond a certain limit will cause an unwanted loss of power. 7 Figure 2.3: Radiation Pattern 2.3 Advantages of Microstrip Antennas The antenna is lightweight and compact. It provides a simple fabrication procedure. It is simple to install because of the low volume and tiny dimensions. It allows for simple device integration. It is capable of operating at dual and triple frequencies. The construction of the antenna’s arrays is simple. In comparison to rigid surfaces, it provides a high level of toughness. 2.4 Disadvantages of Microstrip Antennas The disadvantages of microstrip antennas are as follows, The efficiency of the antenna is low. These antennas exhibit extremely delicate behaviour when exposed to environmental conditions. These have a narrow bandwidth, poor gain, and low power handling capacity. They are more susceptible to false feed radiation. In microstrip antennas, there are the increased conductor and dielectric losses. 8 2.5 Applications of Microstrip Antennas Microstrip antennas’ low profile allows for widespread application in wireless communications. This explains why these antennas display compatibility with mobile phones and other handheld devices like pagers. These antennas are utilised as communication antennas on missiles because of their thin structure. Microstrip antennas are used in microwave and satellite communications due to their small size. One of the main benefits of microstrip antennas is GPS or the Global Positioning System. because it makes monitoring trucks and marines simple. Phased array radars that can manage bandwidth tolerance up to a certain percentage also use these antennas. 9 Chapter 3 Related Work In this chapter, we look at various design processes and methodologies of antennas that were analysed in the course of the project through a literature review. We have analyzed the following designs for microstrip leaky wave antennas. • Periodic Half-Width Microstrip Leaky-Wave Antenna With a Backward to Forward Scanning Capability • A Reconfigurable Microstrip Leaky-Wave Antenna With a Broadly Steerable Beam • The Half-Width Microstrip Leaky Wave Antenna With the Periodic Short Circuits • Compact Microstrip Patch Array Antenna With Parasitically Coupled Fee • Microstrip Leaky-Wave Antenna With I-Shaped Slots Each design has been analyzed on the basis of the innovation of the design and its contribution to the field of microstrip leaky wave antennas, the design process, major strengths and weaknesses of the design and the scope for improvements that exist. 3.1 Periodic Half-Width Microstrip Leaky-Wave Antenna With a Backward to Forward Scanning This study introduces the periodic half-width microstrip leaky wave antenna (MLWA) with a backward to forward scanning capability. The proposed antenna consists of a string of MLWAs that are half the width. On the different sides of the transmission line, the radiating periods occur. The periodic construction radiates the slow wave out along the edge while the radiating period operates in the first higher-order mode’s cutoff zone. 10 3.1.1 Design Methodology Ten radiating segments make up the periodic half-width MLWA as depicted in Figure below. The proposed antenna is constructed on a substrate that has a relative permittivity of ϵr = 2.65 and a thickness of h = 0.8mm. The length L + l′ = 225mm long and width W = 21mm wide. For the transmission s = 2.2mm and line impedence is 50 Ω. The width of each radiating segment is close to λ0 /4, which is a half of the conventional MLWA. A short connecting the antenna patch and the ground plane is created by integrating each of the exterior nonradiating edges with a set of shorting pins. The radius of these pins is a = 3mm and the distance between shorting pins is r = 0.5mm. Figure 3.1: 3-D view of the proposed periodic half width MLWA with backward to forward scanning capabilities Figure 3.2: Layout of periodic half width MLWA with backward to forward scanning capabilities 11 3.1.2 Design Analysis To analyze the design to greater details, we measured the radiation patterns of ye proposed periodic half width MLWA at different frequencies of operation. The measurements were done for frequencies equal to 4.4GHz, 4.9GHz, 5.3GHz, 6.2GHz, 6.7GHz, 8GHz. Apart from measuring raiation patterns, The reflection co-efficient (S11 ) was measured for the given frequency range. The experimental results show the main lobe scans electronically and continuously from 149 °to 28 °in H-Plane towards the endfire when operating frequency iccreases from 4.2 GHz to 8.9 GHz. Figure 3.3: The measured radiation patterns for 4.4 GHz, 4.9 GHz, 5.3 GHz 12 Figure 3.4: The measured radiation patterns for 6.2 GHz, 6.7 GHz, 8 GHz Figure 3.5: reflection co-efficient (S11 ) 13 3.1.3 Major Strengths Compared with the conventional half-width MLWA, this new design has the advantage of the main lobe scanning from backward to forward. The bandwidth of the antenna is wider than the conventional antenna, too. The construction is simpler than the right/lefthanded leaky wave antenna. 3.2 The Half-Width Microstrip Leaky Wave Antenna With the Periodic Short Circuis This study introduces a microstrip leaky wave antenna, where periodic short circuits are present. The backwards-to-forward scanning capability is achieved by the periodic construction of this antenna. The proposed antenna consists of a rectangular patch with short circuits that are placed with help of shorting pins between the antenna patch and the grounding plane. 3.2.1 Design Methodology As shown in the figure below, the microstrip antenna consists of a rectangular copper patch, mounted over a substrate of dielectric constant ϵr = 2.65 and loss tangent equal to 0.0015. The thickness of the substrate is h = 0.8mm. The patch was 11.6mm wide and 210mmlong. The transmission line’s width in the feeding circuit is 50Ω i.e s = 2.2mm and the width of the feeding line is 15 mm. The length of this antenna is just half of what was presented earlier in this chapter. There are several short circuits in the periodic architecture. The large rectangular patch appears to have short circuits on two different edges alternately. The ground plane and antenna patch are intermittently linked by shorting pins along the patch’s long edge. There shouldn’t be two shorting pins on two long radiating edges in the same place due to the power leakage. In each short circuit, there are only 7 shorting pins, with the exception of the short circuit at the patch’s feeding terminal, which has 8 shorting pins. The space between shorting pins a = 3mm and the radius of shorting pins is r = 0.5mm. 14 Figure 3.6: The 3D view of the half-width microstrip LWA with the periodic short circuits Figure 3.7: The layout of the proposed half-width microstrip LWA 3.2.2 Design Analysis To analyze the design in greater detail, we measured the radiation patterns of ye proposed periodic short circuits are measured in far-field conditions. The co-polarization and cross-polarization radiation patterns have been observed for the following frequencies 4.6GHz, 5GHz, 5.2GHz, 6.5GHz, 7.5GHz, 8.5GHz. With the increase in the frequency of operation, the main lobe elevation steers from the backward direction to the forward direction in the y-z plane. Apart from measuring radiation patterns, The reflection coefficient (S11 ) was measured for the given frequency range. Due to the similar radiation characteristics, the S-parameter agrees closely with the one from the periodic half-width microstrip LWA present in the design presented earlier. 15 Figure 3.8: Copolarization patterns the following frequencies 4.6GHz, 5GHz, 5.2GHz Figure 3.9: Copolarization 6.5GHz, 7.5GHz, 8.5GHz radiation 16 patterns the following frequencies Figure 3.10: Copolarization 4.6GHz, 5GHz, 5.2GHz Figure 3.11: Crosspolarization 6.5GHz, 7.5GHz, 8.5GHz radiation radiation 17 patterns patterns the following frequencies the following frequencies Figure 3.12: Reflection co-efficient (S11 ) 3.2.3 Major Strengths This new antenna design is simpler and cheaper than the conventional periodic antenna and the right/left-handed leaky wave antenna. This antenna design will be useful in manufacturing the automotive radar system or other consumer products. 18 Chapter 4 Implementation 4.1 Microstrip Leaky-Wave Antenna With I-Shaped Slots We have designed a microstrip leaky-wave antenna (LWA) for a single main beam. The suggested antenna with an array of I-shaped slots radiates just one primary radiation beam at the cross-sectional centre plane, unlike its typical second higher-order leakymode counterpart with two symmetric radiation beams. The cross-polarization of the antenna may be significantly improved by adding a transversally loaded slot array along the microstrip line, but a longitudinally loaded slot array disrupts the current flow and essentially lowers the two original radiation beams. The strip conductor is then etched with a series of I-shaped slots, comprising transverse and longitudinal slot segments, and its special properties are examined to produce a single primary radiation beam in the centre plane. 4.1.1 Design Methodology A three-way power dividing structure with pre-matched amplitudes and phases are built, and it is then used to excite the pure EH2-mode along the radiating strip conductor in order to validate the suggested LWA. The aforementioned feeding structure is erected and connected to a compatible load in order to efficiently absorb the residual power at the end of the LWA. LWA has slots in an I form. The table below gives the specific dimensional dimensions. 19 Parameter w0 wt lt w1 w2 Value (mm) 2.38 3.8 4.9 1.5 3 Parameter ls li p 20.2 10 16 W ws Value (mm) 36.4 1 The design Schematics are as follows Figure 4.1: EH2 -mode microstrip LWA with an array of I-shaped slots Figure 4.2: Feeding structure: Configuration of the EH0 -to-EH2 mode converter Figure 4.3: The 3-D view of Microstrip Leaky-Wave Antenna With I-Shaped Slots 20 Figure 4.4: Photograph of the design EH2 -mode LWA with I-shaped slots 4.1.2 Design Analysis The measured reflection coeffiefcient below 10 dB ranged from 4.59 to 5.10 GHz, which means that the obtained leakage band is consistent. The single primary radiation beam of the suggested LWA demonstrates an appealing beam-scanning capability from 20 to 55 at E-plane, which is often required, as the frequency fluctuates from 5.1 to 4.6 GHz in the leakage band. At 4.9 GHz, the greatest gain attained is around 14.6 dBi. Over the leakage band, the simulated and observed gain flatness are, respectively, 2.4 and 2.1 dB. The proposed LWA’s radiation patterns in the H-plane and E-plane are produced when the test antenna is positioned along the z-axis on the xz plane, as illustrated in Figure below. Figure 4.5: Photograph of the design EH2 -mode LWA with I-shaped slots 21 Figure 4.6: Photograph of the design EH2 -mode LWA with I-shaped slots Figure 4.7: Photograph of the design EH2 -mode LWA with I-shaped slots 22 Figure 4.8: Photograph of the design EH2 -mode LWA with I-shaped slots 23 Chapter 5 Conclusion In this work, we provide a mathematical model and HFSS simulations for evaluating the reflection coefficient of the designed EH2 mode LWA, Radiation pattern in E-plane for various θ. The I-shape slots are taken into account in the model. The I-shaped slots have finally been created up after the impacts of transversal and longitudinal slots along the strip conductor have been examined. It has been clearly demonstrated that the longitudinal slots significantly lower the radiation intensity of two θ-polarized beams, whereas the transversal slots primarily contribute to the amplification of ψ-polarized single main-beam radiation. Over the entire leakage band, the designed LWAs have exhibited an attractive beam-scanning capability along the E-plane as well 24 Chapter 6 Future Scope There is a scope of improvement in: 1. Size of antenna • In the structure proposed the size of the antenna is around 360 mm limiting the utility of antennas in communication devices and radar systems. The size can be reduced by reducing the no. of array elements or making each array element smaller. 2. Shape of slots • The shape of slots currently used is I. in I-shaped slots the longitudinal slots restrict the flow of current directly affecting the power output of the antenna, we should innovate new slotting patterns that don’t hurt the current flow in the antenna patch. 3. Feeding Structure • The main aim to use a power divider is to activate higher order modes of LWA, whereas it also results in power losses, when acting as a power divider, we can improve the efficiency of power divider to ensure maximum throughput 4. Fabrication Technology • The fabrication technology used limits the size of the antenna that can be manufactured to around 280 mm, making it impossible to produce antennas of larger size. 25