Improvement of slotted wave array antenna to reduce sidelobe area and cross polarization

Master thesis

In the field of electrical engineering - telecommunications

Abstract

Improvement of slotted waveguide array antenna to reduce sidelobe area and cross polarization

Slotted wave antennas were introduced in 1943 at McGill University. The simple structure, high efficiency and linear polarization of these emitters have made them popular in many radar applications. For vertical polarization and large angular scanning, slits on the narrow body of the waveguide are used. A common form of these cracks is oblique cracks. Coupling between adjacent elements significantly affects the surface of the side lobe. An exact formulation is not available for the gaps on the thin body of the waveguide. Analytical analysis of these gaps is difficult due to the indentation of the length of the gaps into the wide body of the waveguide. In this case, measurement results are usually used, which is very expensive. In this work, using CST-Microwave Studio software, which is a full-wave numerical field analyzer, the conductivity and resonance lengths of the gaps are calculated by considering the coupling effect by applying periodic boundary conditions. A conductivity diagram for different elements is provided and used to design a slotted waveguide array. The tilting of the slits increases the cross-polarization. In this work, an unrotated slit pattern is proposed, which produces much less cross-polarization. The proposed structure is very easy to manufacture and has the advantage of being tunable with low sensitivity to manufacturing tolerance. In this case, a design diagram is provided for gap conduction that takes into account the effect of mutual coupling between gaps. The array designed with this example of slits is compared to a design with oblique slits to show the improvement in cross polarization.

Keywords: slotted antennas, sidelobe surface, cross polarization, traveling wave

Chapter 1.

Introduction

Radars are actually electromagnetic sensors that are used to locate and track various targets in space. Radars in different frequencies and powers are designed and used for very diverse careers. The characteristics that should be considered in radars are range and high accuracy. Most of the radars are made in the frequency range of VHF band to C band. In the frequencies of the VHF band, the radars have a long range and low accuracy, and as the frequencies move towards the C band, the range decreases and instead the accuracy increases. Therefore, the most attention in radars is related to the L and S bands. In these two bands, there is a compatibility between the two issues of accuracy and range. This means that the range of the radar is relatively acceptable and the radar has good accuracy. Among these two bands, the S band is also the most used around the world and most radars are designed and built in this band.

Radars actually radiate electromagnetic energy through an antenna in space [1]. A part of the radiated energy hits an object that is often called a target[2] and is reflected in various directions[3]. Part of this energy is reflected, propagated towards the radar and received by the antenna, and after that, signal amplification and processing [4] and It is done on it.

Therefore, an important part of radar systems is the antenna, which can have different specifications depending on the mission of the system. Today, the use of phased array radar technology [5] in which array antennas are used has found many applications. Array antennas have many advantages, including the ability to create directivity [6] or high gain and various beam shaping capabilities [7]. Frequency band, radar mission, desired bandwidth, amount of transmitted power, desired amount of gain and so on.. One of the determining factors is the type of element used in array antennas.

Slotted waveguide arrays[8] were invented in 1943 at McGill University in Montreal.]1[Simplicity of the geometry of the structure of slotted waveguide antennas, good efficiency, the ability to create waves with linear polarizations, the ability to send broadside beams, the ability to carry high power and so on. One of the important features of this type of antennas is that they are considered in radar applications. Especially in aerial applications, these types of antennas are a suitable option because they can be placed on the wings and body of the aircraft. Most of these types of antennas can be used in frequencies from 2 to 24 GHz.]2[

This type of antennas are widely used in the design and construction of phased array antennas due to their unique properties. These antennas are classified depending on the type of slot used and the type of structure used. In general, the structures of these antennas are divided into two categories: resonant[9] and waveguide[10]. Waveguide arrays are used in many applications due to their high frequency bandwidth.]1[

When vertical polarization is needed, oblique slots[11] are used on the narrow body of the waveguide. But reaching the level of the lower side lobe [12] in the case of these arrays has always been considered as a bottleneck. Also, oblique slits on the narrow body of the waveguide have very bad cross polarization [13]. Therefore, so far, many efforts have been made to reduce the cross polarization of this type of slits, all of which are very difficult in terms of construction. [10-3] In this research, a slotted waveguide array with side lobe surface and low cross polarization, which is practical in terms of construction, is designed and simulated in the S-band.

In the second chapter, definitions and necessary preparations for antenna design arrays and the basic parameters of these antennas will be stated and Taylor's basic theory, which deals with the design of an array antenna with a low sidelobe surface, will be examined and evaluated.

In the third chapter, some basic examples of slotted waveguide antennas, such as different types of slots on the waveguide bodies, as well as types of slotted waveguide arrays and how to design them, will be analyzed.

In the fourth chapter, a slotted homogebraic traveling wave array antenna with oblique slots on a narrow waveguide body with a low side lobe surface will be designed and simulated, and all issues related to design and simulation will be stated.

In the fifth chapter, a slotted traveling wave array antenna, which consists of a sample of non-rotating slots on a narrow waveguide body, will be examined as the output of this thesis and its specifications Radiation and return losses and its fine design points will be stated and finally the last chapter will provide suggestions for the continuation of this thesis by other students. traveling wave waveguide slot array antenna for reducing side lobe level and cross polarization Simple structure, high efficiency and linear polarization of these radiators make them popular for many radar applications. For vertical polarization and wide scan angle, the slots on the narrow side of the waveguides are used. A common form of these slots are inclined slots. coupling between the adjacent elements considerably affects the side lobe level. For the slots in the narrow side of the waveguide, an accurate formulation is not available since the extension of the slot lengths into the wide side part of the waveguide, makes the analytical analysis difficult. In this case results of measurements are usually used which are costly