Investigating the effects of coupling in an array of wire antennas for use in passive radar

Master's thesis in the field of electrical-telecommunications engineering (field)

Abstract

Investigation of coupling effects in arrays of wire antennas for use in passive radar

The detection of targets that are immune to visual vision has always been of interest. Electromagnetic waves can be used to identify targets. The first step to receiving waves is a suitable antenna array. After reaching the antenna, the waves cause a current flow on its surface. The amount of flux is a function of various parameters such as the type of antenna, the effective cross section of the antenna, the location of the antenna, the amplitude and angle of the wave front and so on.

The work that has been done in this thesis is to calculate the size and phase of the induced current on wire antennas due to electromagnetic wavefront stimulation. For this purpose, with the help of the moment numerical method, a software has been prepared that calculates the current caused by the impact of the wave front on the antenna on the antenna wires. This software solves one of the solutions of Maxwell's integral-differential equation called Hallen's equation. In addition, in this thesis, the impact of different basis functions on the accuracy of problem solving in the moment method has been investigated. In Hallen's equation, pulse, triangular and sinusoidal functions are used as the basic functions of the moment method, and the accuracy and speed of convergence are compared. Also, graphs of the effect of the wavefront entrance angle on the size and phase of the terminal voltage of arrays of different wire antennas are provided. Also, a matrix called calibration matrix [1] has been introduced to compensate for the mutual coupling between the voltage measured from the antenna terminal and using the obtained uncoupled voltage, the angle of arrival of the wavefront is calculated using the music algorithm [2]. The presented method for calculating the calibration matrix as well as how to use it in orientation results in a relative improvement in orientation accuracy compared to previous methods. In order to understand the capability and efficiency of the designed antennas, they had to be built, as a result, analyzing the efficiency of the designed antennas required a lot of costs, and sometimes, to make the smallest change in the design and check its results, the entire manufacturing process had to be repeated. With the advancement of computer science and technology, researchers and engineers of antenna and wave propagation were able to simulate and solve complex problems with the help of electromagnetic numerical methods with acceptable accuracy.

Along with the advancement of technology and the production of more powerful computers, numerical methods were expanded and more complex and accurate algorithms were presented. In this thesis, we have used electromagnetic numerical methods to accurately solve a practical problem in the field of passive radar, and we have examined the effectiveness of several numerical techniques. Also, we have investigated how to improve and a method to remove destructive effects of cross-coupling in Yagi-Oda antenna arrays.  

In this treatise, first a brief explanation about passive data radar and how the receiver antenna works in this radar has been examined. After learning about Green's functions and their properties, as well as basis and weight functions; The method of moment and how to use this method to solve electromagnetic equations have been investigated. Then the wire antennas are introduced and the current on the wires of these antennas is obtained under the influence of the external field. For this purpose, Hallen's equation is solved by the moment method. In addition, the process of implementing the moment method to solve the desired equations and also the concerns in this implementation are also mentioned. At the end, the results are shown in the form of tables and graphs.

In the second chapter, a brief description of the two-base radar [1], the review of the receiver antennas and the problems in the design of the receiver antenna are stated. In the third chapter, in order to obtain the induced current on the surface of wire antennas, an integral equation of thin wire is calculated from Maxwell's electromagnetic equations and how to solve it is shown using the moment method.In the fourth chapter, the induced current on the wires of arrays of four dipoles is calculated due to the impact of a wave front. Also, in this chapter, the effect of adding resistance to the terminal of the dipole antenna [2] and bending the dipole antenna [3] is investigated. In the fifth chapter, the software designed to calculate the current on the wires and calculate the voltage on the antenna terminals and the methods used to integrate and solve the equations are discussed. It also mentions how to use the software and its capabilities. In the sixth chapter, the results obtained from the thesis are shown along with graphs and tables.

Getting to know passive radar

2-1- Introduction to passive radar

The idea of ??building two-base radars [1], that is, radars that do not have an illuminator [2] and reveal the target by means of waves diffracted from their surface, is not a new idea. The first experiments were carried out in 1935 by Robert Watson Watt [3] in England. He was able to detect a bomber by short waves at 12 km. Bi-base radars are radars whose receiver and transmitter are located in two different places. Early radars were all dipole because the technology had not advanced enough to enable the antenna to switch from transmitting to receiving mode. Until pulse waveforms and transmitter/receiver switches were designed for radar applications. With the passage of time, the amount of attention paid to two-base radars has always been changing. But nowadays, these radars have received a lot of attention.

Types of transmitters used in bipod radars:

Transmitters that are specially designed for use in a bipod radar.

Abstract

Study of Coupling Effect between an Array of YAGI-UDA Antennas in Passive Radar Application

By

Raoof Tooghi

In this thesis, the magnitude and phase of the induced current on the elements of an array of wire antennas resulting from excitation by an electromagnetic wave front is calculated. A software, using the method of moments (MOM) which solves Hallen's integral equation, is developed for this purpose. The effect of different basis functions on the accuracy of the MOM is investigated. Also a method for decoupling antenna elements is devised. For this purpose, the impedance matrix of the array in the receiving mode is calculated through which a calibration matrix is ??obtained. The calibration matrix is ??then used to decouple the measured terminal voltages of the array. In addition, an algorithm for DOA estimation, using the pre-calculated calibration matrices is introduced. The algorithm takes advantage of the MUSIC algorithm to calculate the DOA of arriving signals, using the decoupled terminal voltages. The decoupling methods of this thesis are shown to perform more efficiently than the previous methodologies in the related literature.