Tracking System using Fixed Beamwidth Electronics Scanning

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International Journal of Engineering & Technology IJET-IJENS Vol:14 No:01 166

146901-7575-IJET-IJENS © February 2014 IJENS

Electronics Research Institute, El-Tahreer St. Dokki, Giza, Egypt Corresponding author: haythm_eri@yahoo.com This work was supported by Science and Technology Development

Fund (STDF)), Ministry of scientific research, Egypt.

Tracking System using Fixed Beamwidth Electronics

Scanning

Haythem H. Abdullah, Hala A. Elsadek, and Hesham Eldeeb

Abstract

—

In several applications for tracking systems, the scanning rate is considered a critical factor. The mechanical steering appears to be impractical for its slow rate and space consumption so the need for electronics scanning appears clearly. But unfortunately, the electronic scanning suffer from beamwidth broadening and gain variations with steering angles. In this paper, a fixed beamwidth electronic scanning algorithm is proposed. The proposed algorithm is based on synthesizing sets of excitation coefficients to direct the main beam at some scanning angles. The synthesis takes into consideration the fixation of the beamwidth at these angles. The main concept is to enlarge the antenna array aperture size to a predefined optimum value to get a fixed beam in all the scanning angles. The synthesis of the excitation coefficients is done using a scheme based on the method of moments due to its accuracy in solving such problems. The optimum spacing between elements is determined using the genetic algorithm. One of the main advantages of the proposed algorithm is the applicability of synthesizing multibeam antenna array of fixed beamwidths using the superposition principle. The system is already built and tested with conventional microstrip antenna array elements where good tracking results are observed.

Index Term— Genatic alagorithm, Method of Moments, Electronic scanning.

I. INTRODUCTION

The synthesis of fixed beamwidth scanned antenna arrays with minimum number of antenna elements is of main concern in many applications such as Radar systems and Tracking systems. Some of applications have mechanical steering systems, where a directive antenna array of a specific beam is mounted on a mechanical rotating system that directs the array to any direction keeping its beam unchanged such as rolling radar [1]. But the rate of direction change of the mechanical system is slow. The electronic scanning is used when it is necessary to vary the direction, or rate of change of direction of the array beam faster than is possible by mechanical movement of the aperture [2]. But electronic scanned radars suffer from the beamwidth variations with steering the main beam direction. The variations in the antenna array beamwidth as a function of the steering angle results in corresponding gain variations, and broadening of the main beam beamwidth which may cause signals from different transmitters to interfere with the desired transmitter signal. In this paper, it is introduced a new algorithm based on a combination between the method of moments (MoM) [3] and the genetic algorithm (GA) [4-7]. The proposed algorithm is used for the synthesis of scanned linear

antenna arrays, and multi-beam antenna arrays with fixed minimum beamwidth at any direction with minimum number of equispaced antenna elements. In this paper, a tracking system based on fixing the beamwidth when scanning targets is presented. The paper is organized as follows: section II explains the problem formulation. In section III, the results and discussions are introduced. Finally, Section IV presents the conclusions for this research.

II. PROBLEM FORMULATION

The array factor of a linear antenna array consisting of

isotropic antenna elements positioned symmetrically along the z-axis as shown in Figure 1 with uniform element spacing is given by [1];

( ) ∑ ( ( ) ) (1)

Where is the excitation coefficient of the element, is the element spacing, and ⁄ is the free space wave number. The moment method and the genetic algorithm (MoM/GA) [9] are utilized to reconstruct new element locations and excitations that fulfill the required characteristics in the desired pattern. This synthesis method has shown its ability of reducing the number of elements for linear arrays compared to pencil-beam patterns and shaped-beam patterns [12]. The algorithm is based on solving a system of linear equations which is written in a matrix form as

(2) where is the excitation coefficients vector to be determined. The elements of the

matrix take the form

∫ ( ) ( ) (3) The main focus is to introduce a scheme for filling the matrix

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A. Synthesis of Fixed Beamwidth Scanned Antenna Arrays

To get a fixed beamwidth using the phase scanned array, the array effective length should be retain constant at any direction and equals the array length as illustrated in Figure 2.

( ) (4) In Eq. (4), keeping the element spacing unchanged, a fixed

array effective length is obtained by increasing the number of array elements from N to M such that Eq. (4) is rewritten as follows

( ) ( ) (5) where is the number of antenna elements required to synthesize a radiation pattern with a fixed beamwidth within a specific radiating sector keeping the element spacing

unchanged. By solving Eq. (5) for , is calculated as

( ) ₍ ₎ ₍( ) ₎ ₍( )₎ (6) It should be noted that, for a fixed beamwidth scanning within a sector of width around the broadside direction, the minimum number of array elements is calculated according to Eq. (6). Also, it should be noted that, may be less than or greater than depending on the sector width .

As an example, consider a phase scanned ⁄ Tschebyscheff array consists of antenna elements with

, and broadside [32]. Figure 3 shows the variations in the HPBW with changing the steering direction. It is calculated by steering the ordinary

⁄ Tschebyscheff pattern [10] via phase shifting and then measuring the . According to Eq. (6), Figure 4 shows the number of array elements required to synthesize a fixed beamwidth, at all directions. It is noticed that, the required number of elements increases rabidly as the main beam approaches the end fire direction. The proposed

scheme provides scanning and multibeam pattern synthesis with fixed beamwidth by optimizing the number of elements M, element spacing d, and the excitation coefficients . The aforementioned analyses are coded by the team using Matlab software.

B. Tracking System Block Diagram and Implementations.

The implemented security system is described in the block diagram of Figure 5. The system consists of a transceiver system that transmits a wave in a predefined direction and then

Fig. 2. The geometry of the linear antenna array Fig. 1. Geometry of a linear array of M elements positioned

along the z-axis [9].

Fig. 3 The variations in the HPBW with the steering directions for the 20-elements phase scanned λ ⁄ Tschebyscheff pattern

with

Fig. 4. The number of antenna elements required to synthesize a phase scanned λ ⁄ Tschebyscheff pattern with fixed beamwidth at all directions with

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receives the backscattered signal from that direction. The angle of transmission and reception is specified in such a way that the system scan the surrounding space within a sector of right angle. The sector angle is divided into 18 scanning angles for testing. By analyzing the backscattered signal, the appearance and the disappearance of the monitored objects will be determined by the system.

The block diagram starts by the design of two similar antenna arrays each of 8 elements. The designed antenna element is the conventional microstrip antenna [10].

As illustrated in Figure 5, the system block diagram is divided into two parts; the transmitter part and the receiver part. The transmitter ends with an antenna array of 8 elements as an example. Each element is excited via a voltage controlled phase shifter (VCPS) and a voltage controlled attenuator (VCA). The operational flow of the system is depicted in Figure 6. The system starts by initializing the scanning direction by setting the scanning angle with the starting angle Ө0 which is -450 in

our case. The next step is to generate a wave of frequency f0

which is set in this experiment to 2.4 GHz. Before transmission, the direction of the transmitting and the receiving beams must be adjusted to be oriented to the pre-assigned scanning angle. The adjustment is done via controlling the voltages of the VCPSs and the VCAs. The transmitter and the receiver must have the same maximum radiation to ensure maximum receptions of the object back scattered waves. At the receiver part, a power detector is utilized to capture the backscattered wave and transform it into a DC signal that could be manipulated within the signal processing unit. The captured signal is stored in a storage memory of a lab top for further data processing. The scanning processing continues until the whole scanning sector is completely scanned then the process is repeated again until the system is off manually.

The RF front end is the main part of the system that controls the system scanning process.

The phase and amplitude coordinates are stored in the processing unit. The processing unit is based on the Atmel 8051 microcontroller that can be connected to the PC computer via the RS232 protocol. In order to input the magnitude and phase data to the antenna, a digital to analog Converter (DAC) is required. The IC DAC0808 is a suitable one for our application. Since we will have a large number of DC signals for the voltage controlled phase shifters (VCPS) (JSPHS-2484) and the voltage controlled attenuators (VCA) (ZX73-2500+), it is required to have an analog multiplexer with N output DC channels connected to the phase shifter and variable attenuator of each antenna element. The RF signal is input to an 8 port equal power divider. Each port connected to the variable attenuator which is connected to the phase shifter of each antenna element. In case of increasing the number of the array elements to perform more precise accuracy of the point coordinates or increasing the scanning area, the number of

analog multiplexer channels will be increased according to the number of microstrip transmitted antenna array elements. The system starting with the RF source that is connected to a power divider to branch the RF signal into N RF signals, one for each array element.

The last step is the control of the amplitude and the phase of the input power to each array element. This control will be done via both the VCA and the VCPS.

In the receiving part, instead of the PA, low noise amplifiers are applied to the received signals from each antenna array element. The received signals are combined using a power combiner that is connected to a power detector to convert the received RF power into DC signals that can be treated easily.

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III. RESULTS AND DISCUSSIONS

In this work, the scanning sector is chosen such that ( ), where the main beam at is required to have a beam width close to the broadside main beam at . Using Eq. (6), the minimum number of elements required for array synthesis is found to be

with optimum element spacing . The synthesized pattern has the same beamwidth as the broadside beamwidth and acceptable side lobe level as shown in Figure 8 in the broadside direction while it approaches -10.2 dB at an angle of as illustrated in Figure 9.

Start

Generate signal at

frequency f

0

Direct the beam of

the transmitter

toward Direction Ө

N

Direct the beam of

the receiver toward

Direction Ө

N

Detect the signal

level at direction Ө

N

Store the signal level

at direction Ө

N

Ө

N

=Ө

N-1

+ 1

Initialize scanning

direction

Ө

N

<

Ө

END

Get peaks locations

(Target direction)

Fig. 6. Overall system operational flow

Fig. 7. The experimental setup

Fig. 8. The synthesized radiation pattern with 8 elements antenna array with broadside radiation

Fig. 9. The synthesized radiation pattern with 8 elements antenna array with maximum radiation at Ө𝑜 with

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System Integration and Testing.

The integrated system is shown in Figure 7, where the two antenna arrays are fabricated over FR4 substrates of thickness 1.6 mm. the RF front ends of the transmitter and the receiver are integrated on separate boards. The connection between boards is done via coaxial cables of bandwidth up to 18 GHz. The two RF boards are controlled via two control boxes where each box adjusts the feeding voltages of the attenuators and the phase shifters. The whole system is controlled via a lab top through the USB port. Before starting the system, the output attenuation factor and the phase shifts of each of the RF front end branches are calibrated by adjusting the control voltage of each component.

After calibrating the system which is done once at the fixation process, a metal slab object with dimensions of m2 is positionedat a distance 2 m from the antenna for testing. The back scattered signal at each scanning angle is captured using the power detector. The object is positioned at different angles from the detecting system. Figure 10 illustrates the detected voltage level in the receiver side when the object is positioned at an angle zero from the normal axis of the detecting system. It is noticed that, the detected power approaches its peak value at an angle -10 where the object is already exist. The process is

repeated when the object is positioned at an angle -350. Figures 11 illustrate the detected voltages at each scattering angle when the object is positioned at -350. As in the first case, the system detects the object at its right position.

IV. CONCLUSION

The theory of fixing the radiating beam width of a linear antenna array with the change of the scanning angle using the MoM/GA algorithm is introduced. The MoM is used to estimate the excitation coefficients of the antenna array while the GA is used to estimate the optimum element spacing. The array synthesis with fixed beamwidth is performed with the minimum number of antenna elements which is much lower than the required number of elements in case of the ordinary phased scanning arrays. The proposed algorithm has efficiently overcome the beamwidth broadening and gain variations with steering angles in the phase scanned arrays. The proposed technique can be used when it is necessary to vary the direction, or rate of change of direction of the array beam faster than that is possible by the mechanical movement of the stationary beam arrays. This paper introduces the details of building a whole security tracking system. The details of each of the RF components are presented. The system is tested over a metal object placed at different angles with respect to the system. The system succeeds in detecting the position of the target precisely.

REFERENCES

[1] B. W. Tietjen, \The rolling radar," 2005 IEEE International Radar Conference, 16-21, May 9-12, 2005.

[2] M. F. Radford, \Electronically scanned antenna systems," Proceedings of the Institution of Electrical Engineers, Vol. 125, No. 11, 1100-1112, Nov. 1978.

[3] R. F. Harrington, Field Computation by Moment Methods, IEEE Press, 1993.

[4] Y. Rahmat-Samii, and E. Michielssen, Electromagnetic Optimization by Genetic Algorithms, John Wiley & Sons, Inc., 1999.

[5] H Houck, C. R., J. A. Joines, and M. G. Kay, \A genetic algorithm for function optimization: A matlab implementation," http://www.ie.ncsu.edu:80/mirage/GAToolBox/gaot/.

[6] C. K. Ting, \On the mean convergence time of multi-parent genetic algorithms without selection," Advances in Artificial Life, 403-412, 2005, ISBN 978-3-540-28848-0.

[7] R. L. Haupt, and D. H. Werner, Genetic Algorithms in Electromagnetics, IEEE Press Wiley Interscience, 2007.

[8] R. C. Hansen, Phased Array Antennas, Wiley & Sons, 1998.

[9] A. H. Hussein; Abdullah, H. H.; Salem, A. M.; Khamis, S.; Nasr, M.; "Optimum Design of Linear Antenna Arrays Using a Hybrid MoM/GA Algorithm," IEEE, Antennas and Wireless Propagation Letters, vol.10, no., pp.1232-1235, October 2011.

[10] C. A. Balanis, Antenna Theory: Analysis and Design, 3rd Edition, Wiley, New York, 2005.

[11] S. J. Orfanidis, Electromagnetic Waves and Antenna, 2004, Available: http://www.ece.rutgers.edu/»orfanidi/ewa/.

[12] Y. Liu, Q. H. Liu, and Z. Nie, “Reducing the number of elements in the synthesis of shaped-beam patterns by the forward-backward matrix pencil method,” IEEE Trans. Antennas Propag., vol. 58, no. 2, pp. 604–608, Feb. 2010.

Fig. 10. The power detector output voltages at each scattering angles when the Object at angle -100

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International Journal of Engineering & Technology IJET-IJENS Vol:14 No:01 171 Dr. Haythem H. Abdullah received a BSc. degree in

Electronics and communication engineering from the University of Benha, Egypt in 1998 and received his M.Sc. and Ph.D. degrees from Cairo University in 2003 and 2010, respectively. His M.Sc. is dedicated in the simulation of the dispersive materials in the Finite Difference Time Domain numerical technique and its application to the SAR calculations within the Human head. The Ph.D. is dedicated on the radar target identification. He now employed as a researcher at the electronics research institute (ERI), Giza, Egypt. His current research interests are design and optimization of microstrip antenna arrays and their applications.

Dr. Hala Elsadek. is a professor at Microstrip Department, Electronics Research Institute. She acted as department head from 2008-2012. Dr. Elsadek graduated from Faculty of Engineering, Ain Shams University, Cairo, Egypt 1991. Dr. Elsadek had her master degree from University of Gunma, Japan in 1996 while her PhD was through a channel between Cairo University, Egypt and University of California, Irvine, USA in 2002. Her research interests are in the field of RF wireless communications, electromagnetic engineering and microstrip antenna systems. Dr. Elsadek published three books, lead guest editors in one international journal in antennas and propagation and two patents in wireless communications and antenna systems. She acts as a single author and as a co-author in more than 90 research papers in highly cited international journals and in proceedings of international conferences in her field, as IEEE Transactions on Antenna and Propagation, IEEE Antenna and Wireless Propagation Letters, Microwave and Optical Technology Letters, etc. She is a supervisor on master and PhD theses in different universities in Egypt and abroad (Japan and USA). Dr Elsadek participates in more than 12 research projects at the national and international levels as Egypt- NSF-USA joint fund program, Egypt STDF-France IRD joint fund program and the European Committee programs of FP6 and FP7. Her role is from C-PI to PI. She is an IEEE senior member from 2011. She is awarded the Egyptian Government Encouragement Prize for Young scientists in Engineering Science and Technology at 2006. She is included in several biographical indexes all over the world: American Biographical Institute Inc, Cambridge, England as one of the leading scientists of in 2008 and "Marquis Who's Who" Encyclopedia from 2008- to 2010 as one of the worldwide scientists who demonstrate outstanding achievements in her field. Dr. Elsadek is also a reviewer in many international societies in her field as IEEE Antenna and Propagation Society.