Short Paper ACEEE Int. J. on Signal and Image Processing , Vol. 5, No. 1, January 2014
Rotman Lens Performance Analysis Shruti Vashist1, Dr. M.K Soni2 ,Dr.P.K.Singhal3 1
Research Scholar, Department Of Electronics & Communication Engineering, Faculty of Engineering and Technology, Manav Rachna International University Faridabad-121004, Haryana, India 1 shruti.fet@mriu.edu.in, 2ed.fet@mriu.edu.in, 3pks_65@yahoo.com quite difficult to integrate rotman lens realization in compact transceiver designs. This paper presents the analysis of the effects of variation of antenna element spacing and focal ratio on the performance of rotman lens. Simulations were carried out using RLD1.7 designer software and various important parameters such as array factor, insertion loss, s-parameters, beam to array port phase error, beam to array port coupling magnitude, beam to sidewall coupling magnitude and SWR are analysed to know the effects on the performance of the lens. This paper is organized as follows: Section II presents the basic principle of the rotman lens in which trifocal lens design equations and its important parameters are discussed. Section III presents the analysis approach of the designed rotman lens and also gives the technical specifications of the designed lenses. In Section IV simulation results are presented and finally in section V conclusions are drawn.
Abstract—This paper presents a trifocal Rotman Lens Design approach. The effects of focal ratio and element spacing on the performance of Rotman Lens are described. A three beam prototype feeding 4 element antenna array working in L-band has been simulated using RLD v1.7 software. Simulated results show that the simulated lens has a return loss of – 12.4dB at 1.8GHz. Beam to array port phase error variation with change in the focal ratio and element spacing has also been investigated. Keywords— Rotman Lens, Array Factor, Return Loss, Phase Error, Beam Steering.
I. INTRODUCTION Microwave lens has emerged as a beam forming network and is now currently used in many cutting edge applications such as radars, remote satellite sensors, automobile collision avoidance systems and ECM. Different active and passive beam forming networks (BFN’s) are known from literature [1], [4]. BFN’s which are based on buttler matrix are easy to construct and are also implementable on printed circuit boards, but the major drawback of these BFN’s is that the produced beams are dependent on frequency and the beam shift occurs as frequency varies, which is not desirable in most communication links. Hardware complexity of these BFN’s grows exponentially with the number of elements and hence it becomes difficult to realize such kind of BFN’s for large number of radiation beams. Rotman lens provides an effective solution for beam forming networks when a large number of radiating elements are required. Rotman lens is a true time delay beamformer which provides linear phase shifts at the output ports by utilizing different paths within the lens structure. Rotman lens is a type of microwave lens and is capable of operating in high microwave frequency ranges with wide angle scanning capabilities. It is an Electronically Scanning Antenna (ESA) which has the ability to position the antenna beam instantaneously to any position and can also be implemented in the microstrip configuration. W.Rotman and R.Turner first proposed the rotman lens which consist of airfilled parallel conducting plates fed by co-axial probes [1]. D. Archer[2] gave a modified design of rotman lens in which a dielectric material is filled between parallel conducting plates fed by microstrip lines. Both type of the above mentioned rotman lens have been implemented mostly in the microwave band. At higher microwave frequencies the lens loss increases and there physical structure becomes more difficult to build [3]. Conductor loss is a dominant factor in the overall lens loss and this is due to the increase in the surface resistance at high frequency. At low microwave frequencies it becomes © 2014 ACEEE DOI: 01.IJSIP.5.1.1534
II. PRINCIPLE OF ROTMAN LENS Fig.1 shows a basic diagram of the rotman lens. It consists of a set of input and output ports arranged along an arc. The lens structure between both sets of ports functions as an ideal transmission line between the individual input and output ports. The signal applied to the input port is picked up by the output port. The different electrical lengths between a specific input and all output ports, generates a linear progressive phase shifts across the output ports of the lens. Dummy ports are also an integral part of the rotman lens and serve as an absorber for the spill over of the lens and thus it reduces multiple reflections and standing waves which deteriorates the lens performance[7-9]. A. Lens Design Equation Fig.2 shows a schematic diagram of a trifocal Rotman lens[12-13]. Input ports lie on contour C1 and the output ports lie on contour C2. C1 is known as beam contour and C2 is known as array contour. There are three focal points namely F1, F2 and F3. F1 is located on the central axis while F2 and F3 are symmetrically located on the array contour at an angle of +α and – α respectively. It is quite clear from Fig.1 that the coordinates of two off-axis focal points F2, F3 and one on axis focal point F1 are (-f2cosα, f2sinα), (-f2cosα, -f2sinα ) and (-f1, 0) respectively[10][14]. where
f1 -On axis focal length f 2 -Off axis focal length -Off center focal angle 101