A Wideband Widebeam Tapered Slot Array Antenna for Active Electronically Scanned Array Antenna

Transcription

1 Doi: /ijc A Wideband Widebeam Tapered Slot Array Antenna for Active Electronically Scanned Array Antenna Babu Saraswathi K.Lekshmi *1, I.Jacob Raglend 2 *1 Research scholar, Electrical and Electronics Engineering, Noorul Islam University, Tamilnadu, India 2 Professor, Electrical and Electronics Engineering, Noorul Islam University, Tamilnadu, India *1 kavitha20012@gmail.com; 2 jacobraglend@rediffmail.com Abstract Phased arrays when mounted on aircraft and missiles will be capable of allowing only the desired signals to be received and simultaneously suppress all other unwanted ones incident from hostile sources. This in turn will make them invisible for the radars at the enemy base or on their aircraft. This paper discusses about the design of Phased array antenna for military airborne radar applications. A milllimeter wave active phased array antenna has been developed that is capable of wide scanning angle in both E- and H-planes. In order to achieve wide angle scanning over X-band frequency, a linear tapered slot antenna (LTSA) element has been designed and the resulting element is capable of scanning out to 60 o from broadside in all scan planes for a bandwidth of 11-14GHz and an active reflection coefficient less than -10 db. Beam scanning angles of above ±45 degrees in the E-plane and ±60 degrees in the H-plane were obtained for an isolated element. An end fire array pattern with half power beamwidth of 14 degrees in E-plane is achieved over 11-14GHz frequency. This design was implemented on linear array antenna consisting of 7-elements and planar array antenna consisting of 49 (7x7) elements. The predicted performance of the antenna was verified by simulation of element patterns, array radiation patterns and S-parameter plot using commercially available electromagnetic simulator HFSS. Since the radar antenna is intended for applications where stealth is important. The antenna aperture consists many radiating elements used for beam-forming and hence through the adjustment of elemental phases steer the beam efficiently. Moreover, they are capable of rejecting all the unwanted components of the received signals while maintaining at the same time sufficient pattern gain in the desired directions. These characteristics can be implemented for stealth applications. Keywords Radar; Stealth; Tapered Slot Antenna; Phased Array Antenna; Scan Angle; Beam Width; Radiation Pattern Introduction A phased array is a directive antenna made up of a number of individual antennas or radiating elements. Its radiation pattern is determined by the amplitude and phase of the current at each of its elements [Balanis]. The phased array antenna has the advantage of being able to have its beam electronically steered in angle by changing the phase of the current at each element. The beam of a large fixed phased array antenna therefore can be rapidly steered from one direction to another without the need for mechanically positioning a large and heavy antenna [Skolnik]. A typical phased array antenna for microwave radar might have several thousand individual radiating elements that allow the beam to be switched from one direction to another in several microseconds or less [Babu et al]. Phased array elements that are considered mainly as broadband elements may be suitable for wide angle scanning if the bandwidth is reduced; the tapered-slot element is such an element. The average reflected power over the scan range normalized to unit input power was a good measure to evaluate wide-angle scanning performance [Ellgardt et al]. This paper discuss about Tapered slot antennas, Doubly layered linear tapered slot antenna, Vivaldi notch elements and a design of array antenna. Tapered slot antennas has a slotline flare from a small gap (50Ω) to a large opening, matching to free space's wave impedance of 377Ω and is usually parasitically fed by a "hockey-stick" balun at the base of the element. A tapered slot antenna uses a slot line etched on a dielectric material, which is widening through its 10

2 International Journal on Communications (IJC) Volume 4, length to produce an endfire radiation [Gibson]. An electromagnetic wave propagates through the surface of the antenna substrate with a velocity less than the speed of light which makes TSAs gain slow wave antenna properties. The EM wave moves along the increasingly separated metallization tapers until the separation is such that the wave detaches from the antenna structure and radiates into the free space from the substrate end. The E-plane of the antenna is the plane containing the electric field vectors of the radiated electromagnetic (EM) waves. For TSAs, this is parallel to the substrate since the electric field is established between two conductors that are separated by the tapered slot. The H-plane containing the magnetic component of the radiated EM wave runs perpendicular to the substrate [Tan et al]. TSAs have moderately high directivity (on the order of 10-17dB) and narrow beamwidth because of the traveling wave properties and almost symmetric E- plane and H-plane radiation patterns over a wide frequency band as long as antenna parameters like shape, total length, dielectric thickness and dielectric constant are chosen properly. Other important advantages of TSAs are that they exhibit broadband operation, low sidelobes, planar footprints and ease of fabrication [Khabat et al]. A TSA can have large bandwidth if it exhibits a good match both at the input side (transition from the feed line to slot line) and the radiation side (transition from the antenna to free space) of the antenna. The gain of a TSA is proportional to the length of the antenna in terms of wavelength. Tapered slot antennas are also suitable to be used at high operating frequencies (greater than 10 GHz), where a long electrical length corresponds to a considerably short geometrical length. The main disadvantage of the TSA is that only linear polarization can be obtained with conventional geometries [Yao et al]. The modern shared aperture radar concept explored the development of multi function wideband arrays capable of simultaneous and time interleaving, electronic warfare, and communications functions [Hemmi et al]. This necessitated the need of frequency independent wide band antennas. With the term frequency independent it is meant that the antenna pattern and impedance remain constant over a relatively wide frequency bandwidth. The stripline-fed tapered slot antenna (TSA) array was introduced by Lewis in 1974 and its potential for wideband and widescan arrays makes it a prime candidate for highperformance phased-array systems [Lewis et al]. Phased array antennas are cumbersome to analyze directly due to their large size in terms of wavelengths. An additional problem is that the arrays often are densely packed, which yields strong coupling between the elements. As a consequence of the strong coupling the active reflection coefficient changes with excitation and therefore multiple calculations with different excitations are always required to fully characterize an antenna. To design a phased array antenna one is usually required to either disregard the coupling between the elements or to assume that the antenna elements behave as if they were positioned in an infinite array. The infinite array approximation is usually the best choice for analyzing large dense arrays, for which the approximation is good for the central elements [Ellgardt]. Design Parameters of Linear Tapered Slot Antenna The Linear tapered slot antenna model illustrates two substrates back to back consisting of radiating flare geometries on the two opposite faces. One of the substrate is etched completely on the opposite side of the flare and the other substrate consists of stripline feed being printed and sandwiched between the two substrates containing the flares. Fig. 1 shows the schematic of the three layers forming the complete antenna assembly viz., namely; bottom layer, top layer and the middle layer (stripline feed). Bottom layer Top layer Stripline feed Dielectric substrate 1 Dielectric substrate 2 FIG. 1 EXPLODED VIEW OF STRIPLINE-FEED LINEAR TAPERED SLOT ANTENNA The design parameters of a double layered tapered slot antenna is defined in Fig. 2. They can be classified into two categories: substrate parameters (relative dielectric constant εr, and thickness t) and antenna element parameters, which can be subdivided into the stripline 11

3 /slotline transition, the tapered slot, and the stripline stub and slotline cavity. The RT-Duroid dielectric substrate used for this design with εr = 2.2 and t = 1.27 mm. The flares act as an impedance transformation network between free space and the stripline feed. Radiation from the antenna occurs when the slotline impedance is matched to the impedance of free space.the stripline/slotline transition is specified WST (stripline width) and WSL (slotline width). The exponential taper profile is defined by the opening rate R and two points P1(z1,y1) and P2 (z2,y2) where, d c 2 = L y = c 1 e Rz +c 2 (1) c 1 = W ST P 1 P 2 FIG. 2 DEFINITION OF PARAMETERS OF LINEARLY TAPERED SLOT ANTENNA WITH SQUARE CAVITY LINEARLY TAPERED SLOT AND RADIAL STUB STRIPLINE FEED. The taper length L is z2-z1 and the aperture height H is 2(y2-y1) +WSL. In the limiting case, where opening rate R approaches zero, the exponential taper results in a socalled linearly tapered slot antenna for which the taper slope is constant and given by s0= (y2-y1)/(z2-z1). For the exponential taper defined by (1), the taper slope s changes continuously from s1 to s2, where s1 and s2 are the taper slope at z=z1 and z=z2, respectively, and s1<s<s2 for R>0. The taper flare angle is defined by α = b H D SL R R r W SL A R L G tan -1 s. The flare angles, however, are interrelated with other defined parameters, i.e H, L, R and WSL. In this study, the distance from the transition to the taper (LTA) and the distance from the transition to the slotline cavity (LTC) are taken as zero (otherwise LTA and LTC should be large enough to accommodate the stubs of the transition). The Radius of radial stripline stub (Rr = 2.02 mm) and square slotline cavity (DSL = 2.6 mm x 2.6 mm) are investigated in this parametric study. The Table 1 depicts the antenna parameters considered for the Linear tapered slot antenna (LTSA). The bandwidth of the antenna was improved with these non-uniform stubs and also noted that radial stub was more advantageous regarding the overlapping between stripline and slotline stubs. The stripline feeding increased the antenna bandwidth compared with the microstrip feeding [Shin et al]. TABLE 1 LTSA DESIGN PARAMETERS Parameters Specifications (in mm) Aperture Height (H) 8.5 Antenna breadth (b) 14.2 Taper length (L) 14.2 Taper depth (d) 25.7 Radius of radial stripline stub (Rr) 2.02 Angle of radial stripline stub (AR) 130 o Square slotline cavity (DSL) 2.6x2.6 Slotline width (WSL) 0.2 Stripline width (WST) 0.5 Length of slotline cavity from ground plane(lg) 8.9 Results, Discussions and Performance The unit cell model of linear tapered slot antenna design is shown in Fig. 3. The Fig. 4(A) illustrates an array of 7-elements having inter-element spacings dy = 16mm. The Fig. 4(B) illustrates an array of 7x7 elements having inter-element spacings dy = 16 mm and dx = mm has been chosen in this work. The designed LTSA has been optimized in the array environment in HFSS, for the desired specifications mentioned previously. Array performance strongly depends on the coupling in a wide bandwidth and wide scanning arrays. So the mutual coupling study is extremely important in the design process of an active phased array. The linear array antenna is assumed to be consisting of K- identical elements, displaced a distance d, with respect to one another and being matched to a voltage source. Since the radiators are match-connected to their sources, the voltage waves traveling in negative 12

4 International Journal on Communications (IJC) Volume 4, directions are only due to mutual coupling from surroundings. To obtain the scan element pattern (SEP), only one of the K elements will be excited, the other elements will be disconnected from their sources and will be terminated in matched (reflection less) loads. of a singly excited element in its array environment where all other elements are terminated into matched loads. SEP will provide the phased array antenna gain at the position of the scanned beam as a function of scan angle. For a large phased array antenna, all SEP will be nearly identical and hence, the phased array antenna performance may be approximated by applying pattern multiplication-scan element pattern is multiplied with the array factor. The S-parameter plot (Fig. 5) shows the return loss (RL), which is less than -10dB for an isolated LTSA element over the frequency of 11GHz-14GHz and the antenna resonates at 12.4 GHz frequency with the maximum return loss of -26 db. FIG. 3 UNIT CELL MODEL OF LTSA AS MODELED IN HFSS. FIG. 4(A) 1-DIMENSION E-PLANE LTSA ARRAY MODELED IN HFSS The Fig. 6 shows the simulated radiation patterns of an isolated linear tapered slot antenna. The results of radiation patterns for the H-plane cuts (Φ = 0 o ) illustrating the achieved 3-dB beamwidth (HPBW - Half Power Beam Width) greater than 120 o and E-plane cuts (Φ = 90 o ) illustrating the achieved 3-dB beamwidth greater than 90 o. The Fig. 7 shows the simulated 3-D plots and polar plots of radiation patterns of an array with the gain of db at 12.5GHz. The Fig. 8 shows the Linear array radiation patterns in H-plane (Φ = 0 o ) and E-plane (Φ = 90 o ) cuts illustrating the achieved 3-dB beamwidth is approximately 14 o. The Fig. 9 shows the active reflection coefficient S(4,4) of center element of linear array including mutual coupling effects of all other array elements, which satisfies the requirement of return loss, which is less than -10 db over the operating frequency 10.8GHz-13.6GHz. These studies have clearly established the suitability of the designed linear tapered slot antenna as a candidate for wide scanning active phased array. Conclusions FIG. 4(B) 2-DIMENSIONAL LTSA ARRAY MODELED IN HFSS The Scan element pattern (SEP) is the radiation pattern A wide band linearly tapered slot antenna array is designed with gain of approximately 10.6dB. The simulated S-parameter result for an isolated tapered slot antenna element and array achieved is less than -10dB over 11GHz-14GHz of operating frequency. The wide scan angle performance is achieved in H-plane (above 120 o ) and E-plane (above 90 o ) over the X-band operating frequency for the isolated linear slot antenna element. Thus, the linearly tapered slot antenna is a promising candidate for the airborne active phased array radars and other similar applications. 13

5 m S Parameter db(s(1,1)) Setup1 : Sweep db(s(1,1)) m Freq [GHz] m m m FIG. 5 SIMULATED S-PARAMETER PLOT OF AN ISOLATED LINEAR TAPERED SLOT ANTENNA. m4 XY Plot 21 m3 m5 Freq='12.5GHz' Phi='0deg' Freq='12.5GHz' Phi='90deg' Theta [deg] FIG. 6 RADIATION PATTERNS OF LTSA IN E-PLANE AND H-PLANE Radiation Pattern Freq='12.5GHz' Phi='0deg' Freq='12.5GHz' Phi='90deg' FIG. 7 THREE DIMENSIONAL AND POLAR PLOTS OF LINEAR ARRAY RADIATION PATTERNS WITH GAIN OF 10.52DB

6 International Journal on Communications (IJC) Volume 4, m XY Plot 21 m3 Freq='12.5GHz' Phi='0deg' Freq='12.5GHz' Phi='90deg' Radiation pattern in E-plane and H-plane HPBW=14 degrees Gain = 10.5 db Theta [deg] FIG. 8 RADIATION PATTERNS OF LINEAR TAPERED SLOT ANTENNA ARRAY IN E-PLANE AND H-PLANE XY Plot 28 db(s(4,4)) Setup1 : Sweep db(s(4,4)) Freq [GHz] FIG. 9 SIMULATED S-PARAMETER PLOT OF LINEAR TAPERED SLOT ANTENNA ARRAY WITH THE RETURN LOSS IS LESS THAN -10 db OVER 10.8 GHZ-13.6 GHZ OPERATING FREQUENCY. REFERENCES A. Ellgardt, A Scan Blindness Model for Single-Polarized Tapered-Slot Arrays in Triangular Grids, IEEE Transactions on Antennas and Propagation, Vol. 56, No. 9, pp , September Babu Saraswathi Kavitha L., Jacob Rglend. I, " A wide-scan phased array antenna for a small AESA antenna A Review," Proceedings of IEEE international conference on circuit, Power and Computing technologies, ICCPCT 2013, art.no , pp Balanis, C. A., Modern Antenna Handbook, John Wiley and Sons, C. Hemmi, R.T.Dover, F.German and A.Vespa, 15

7 "Multifunction wideband array design," IEEE Trans. Antenna Propagation, Vol.47, Mar.1999 J. Shin and D.H. Schaubert," A parameter study of striplinefed vivaldi notch antenna arrays," IEEE Trans. Antennas Propagation, Vol. 47, , Khabat Ebnabbasi, Student Member, IEEE, Dan Busuioc, Ralf Birken, and Ming Wang, "Taper Design of Vivaldi and Co-Planar Tapered Slot Antenna (TSA) by Chebyshev Transformer," IEEE Transactions on Antennas and Propagation, vol. 60, no. 5, May L.R. Lewis, M. Fasset, and J.Hunt, A broadband stripline array element, in IEEE international Symposium Antennas Propagation dig., 1974, pp M. A. Ellgardt and H. Steyskal, Antenna elements for wideangle phased arrays, Division of Electromagnetic Theory, Royal Institute of Technology, Tech. Rep., P. J. Gibson, The Vivaldi Aerial, Proc. 9th European Microwave Conf. Brighton, U.K, pp , Skolnik, M. I., Introduction to Radar Systems, McGraw Hill Book Company, Tan-HuatChio and Daniel H. Schaubert, "Parameter Study and Design of Wide-Band Wide-scan Dual-Polarized Tapered Slot Antenna Arrays," IEEE Transactions on antennas and propagation, vol. 48, no. 6, June Y. Yao M. Liu W. Chen Z. Feng, "Analysis and design of wideband widescan planar tapered slot antenna array," IET Microw. Antennas Propagation, 2010, Vol. 4, Iss. 10, pp Babu Saraswathi K. Lekshmi was born in India in She received the Bachelors of Electrical and Electronics Engineering degree from Manonmaniam Sundaranar University, India, in She received the Master of Engineering degree in the field of Power Electronics and Drives from Anna University, India, in Currently, she is a research scholor doing Ph.D in Noorul Islam Uniersity in the area of Phased Array Antennas for airborne radar apllications. Jacob Raglend. I was born in India and received his Bachelors degree in Electrical Engineering from Manonmaniam Sundaranar University and the Masters degree in Power Systems Engineering from Annamalai University with first class in 2000 and 2001 respectively. He has done his Ph.D. degree in the Department of Electrical and Electronics Engineering, Indian Institute of Technology, Roorkee, India in the year Then he has joined the School of Electrical Sciences, Vellore Institute of Technology (VIT) India, as a Senior Lecturer and Assistant Professor during 2007 and presently working as a Professor in the Department of Electrical Engineering, Noorul Islam University, India. He taught course in Basic Electrical Engineering, Power Systems, Artificial Intelligence and Soft Computing Techniques. His field of interest is Unit Commitment, Economic Dispatch, Power System Restructuring and Deregulation, Artificial Intelligence Applications to Power System and FACTS. 16