Four-element linear array of micros trip annular ring slot antenna at 9 GHz

Transcription

1 ndian Journal of Radio & Space Physics Vol. 27, August 1998, pp. 168-]72 Four-element linear array of micros trip annular ring slot antenna at 9 GHz Sandhya Gupta, Sunil K Khah & P K S Pourush Department of Physics, nstitute of Basic Sciences, Dr B R Ambedkar University, Khandari, Agra Received 24 November 1997; revised 31 March 1998; accepted 21 April 1998 The performance of a four-element linear array of microstrip annular ring slot antenna (MARSA) at 9 GHz is discussed. The far-zone field expressions for array geometry are derived using vector wave function technique and pattern multiplication approach. The total field patterns are computed and plotted for two different modes. The other important antenna parameters like direction of maximum radiation, half power beamwidth (HPBW) and radiation conductance are computed, and the results are compared with those of single element MARSA. t is observed that the array geometry is suitable for obtaining a directive beam. 1 ntroduction t is well known that the basic properties of a single microstrip antenna element are: low gain, low radiation efficiency and wide beamwidth. Particularly at high frequencies, the efficiency of even a single element diminishes to a large extent due to conductor and dielectric losses. So, array configurations must be employed in applications where reasonably high gain and narrow beamwidth are required. n recent years considerable research efforts have been made to the development of microstrip antenna arrays. These array antennas have many unique features which are very important for airborne or space-craft applications! 7. n this paper, an analysis of a four-element linear array of microstrip annular ring slot antenna (MARSA) is presented at 9 GHz of microwave frequency range. The far-zone field expressions of the array geometry are obtained using vector wave function technique and pattern multiplication approach.s The results of array geometry are computed, plotted and finally compared with those of a single element MARSA for the same input parameters. 2 Theory The geometry and co-ordinate system of array antenna under investigation is shown in Fig. 1" t consists of four identical circular slots in the ground plane of a dielectric substrate (Teflon) of thickness h and substrate pennittivity &, = The radius of each circular slot is a and are separated by a distance d. Each slot can be excited by a microstrip feedline connected at the edge. The slot antenna has an advantage over microstrip patches because of the scre ned feed which prevents spurious radiation.7 n such a geometry, TMnm mode with respect to z-axis are excited. The subscripts nand m are the mode numbers associated with x- and y-directions, respectively. An annular slot in a conducting plane of infinite extent may be viewed as an annular distribution of the magnetic surface current given by7 M (p, ~') = Ea (p, ~') x n... (1) where Ea is the aperture electric field, p the radius vector in cylindrical coordinate, ~' the cylindrical coordinate angle subtended by p on x-axis and n is the unit vector normal to the aperture. The total far-zone fields of the array geometry under investigation can be expressed by the fields of a single element positioned at the origin multiplied by a factor which is referred to as the array factor. This method is widely known as pattern multiplication approach8 Thus, using the expressions of single-element annular ring slot antenna7 and neglecting coupling between the elements9, the far-zone field expressions for four-element linear array of annular ring slot antenna are obtained as follows : ak E W -jkor Ee = jn 0 0 s. x _e cos n J~(aKosin 8) 2 r ~. " ''! " '1"""'/ '' ' ' 'f ' '~'l

2 GUPTA et al.: FOUR-ELEMENT LNEAR ARRAY OF MCROSTRP ANTENNA 169 z p x SUBSTRATE GROUND PLANE Fig. --Configuration and coordinate system of four-element linear array of microstrip annular ring slot antenna x [4 cos(kod cos8+ P) cos (Kod C~S8+P)] and E _ n neows e-jko" ;, - } 2 x -r- sin n Jcot8Jn(aKosin8) x [4COS(Kod cos8+ P) cos (Kod C~S8+P)] where, EOt' E", Components of total electric field vectors Eo Maximum field strength a Radius of the annular slot f3 Phase propagation constant r Magnitude of position vector of point P n Bessel's function of first kind J' n Derivative of Bessel's function of first kind Ko Propagation constant in free space Ws Width of the slot d Separation between slot elements which is a function of the geometry of the array, and the phase excitation between elements is the 2 array factor of the geometry. By varying the () separation d and phase propagation constant f3 between the elements, the characteristics of the array factor and the total field of the array antenna (3) t is obvious from the Eqs (2) and (3) that these expressions involve an additional term { cos(k od coso + fj) cos (Kod cos 2 0 +Pll J can be controlled. t can be shown that this additional term does not appear in the corresponding expressions of the single annular slot antenna given by Bhal and Bhartia Field patterns The values of Eo, and E;, are computed for a case taking source frequency Fr = 9 GHz, aka = 1 and 2, distance between two elements d = A/4, substrate permittivity 6;. = 2.33 and. phase difference P = 7r 12 for n=o and 1. The results are plotted for two different planes, i.e. J = 0 and 1l2 in free space and are shown in Figs 2-7. For the same input parameters, we have compared field patterns of the array geometry with the pattern of single-element microstrip annular ring slot antenna (MARSA). t is observed from the figures that the patterns of array antenna are directive in nature as compared to single element. t is also found that direction of maximum radiation is altered in case of antenna array (Table 1).

3 170 NDAN J RADO & SPACE PHYSCS, AUGUST 1998 Comparision of half power beam width (HPBW) of these two geometries is made and the values are given in Table Radiation conductance The expression for conductance is given as7 antenna radiation G= 2P v.2 o... (4) where, Vo is the edge voltage and P is the total power radiated by the antenna. which can be obtained by integrating the Poynting vector over a large sphere10 and expressed as " l~o'"l1,, POW R,dB o' -2 -, POW R,dB Fig. 2--Field pattern of annular slot and four-element array when n=o, ako = and = 0 and tr2 plane Fig. 5-Field pattern of annular slot and four-element array when n=,ako = and = tr/2 plane,ed' ~ 4 ~ ~ ~ -s ~ ~ ~ ~ POWER POWER,d Fig. 3-Field pattern of annular slot and four-element array when n=o, ako = 2 and = 0 and ti/2 plane Fig. 6-Field pattern of annular slot and four-elemel\t array when n=,ako = 2 and = 0 plane,.& -12,e -'0 POWER,dB Fig. 4-Field pattern of annular slot and four-element array when n=l, ako = and = 0 plane Fig. 7-Field pattern of annular slot and four-element array when n=, ako = 2 and = tr/2 plane '" " '" "l" ~'" '1'1 '! " ' '

4 GUPTA et oj.: FOUR-ELEMENT LNEAR ARRAY OF MCROSTRP ANTENNA 171 Table -Position of maximum intensity Pattern Major lobe Minor lobe ako= Sing~Array element 90' 30', SO' 74',106' Case n=o ako=2 SingkArray element 90' 30', SO' 70',110' Single element Array +=0 +=t/2 +=0 +=t/2 sotropic 0', SO' 0', SO' 0', SO' SO' 70', 6S', 112' 110' Case n=1 Single element ako=2 Array +=0 +=t/2 +=0 +=](/2 0',90' 0', SO' 0', SO' 0', SO' so', 100' 6S', 110' Radiation S5 Single Single Table Array Single 39" -element element 2-Array Values ako=2 of ako=2 HPBW and radiation Case n=o conductance Case n=1 element ~o 36.2xlO-2 7So 7.6x xlO x x xlO So X 10-2J.34x for single - element and four - element linear array antenna. conductance + [(G) =0 inplane mho) [ Eo E; Jo Jo' ]r2 sin8d8d; o... (5) The values of radiation conductance have been p = _1_ f21r r calculated for the array geometry and the single element using above expressions for two modes, i.e. n=o and 1 and for aka = 1 and 2 taking same input parameters. The integral involved in Eq. (5) has been solved using numerical methodll The calculated values are given in Table 2. t is observed from Table 2 that the values of G are considerably high for array geometry in both the cases. 3 Discussion and conclusions The radiation characteristics of four-element linear array of microstrip annular ring slot antenna at 9 GHz have been studied. The results of the array geometry are compared with those of singleelement MARSA. t is found that there is a significant change in the radiation characteristics of array geometry. Figures 2-1 illustrate the radiation field patterns of the antenna geometry under investigation for two principal plane, i.e. ~ = o and 1t 12 in free space. t is obvious from Figs 2-7 that the shape of the field pattern has been changed to a great extent and it redistributes the field intensity. t is also observed that in case of array the beam is split into major and minor lobes and correspondingly the position of maximum radiation is shifted. A comparision of this effect is given in Table 1. Obviously the radiation patterns of a single element antenna contain only one major lobe of considerably wide beamwidth, while the array geometry produces a directive beam with a narrow beamwidth. A comparison of values of HPBW for antenna geometry un4er investigation is given in Table 2 which also shows the calculated values of radiation conductance for two cases, i.e. n=o and n= 1. lt can be seen from Table 2 that values of G are considerably higher for array geometry. t is further observed that for ~ = 2 the corresponding values of G for n=o and 1 for array are quite high. Finally, it is concluded that the MARSA array has unique radiation characteristics and can be employed in applications where a directive beam of narrow beamwidth is required. A possible application for such an antenna geometry is for a vehicular mobile satellite system as an alternative to microstrip patch antennas. Acknowledgement The authors are grateful of Prof. Jai Shanker, Head, Department of Physics, Faculty of Science, Dr. B.R. Ambedkar University, Agra, for

5 172 NDAN J RADO & SPACE PHYSCS, AUGUST 1998 providing necessary facilities, encouragement and helpful discussion. References 1 Batchelor J C & Langley R J, Electron Let! (UK), 32 (1996) Bhattacharyya A K, EEE Trans Antennas & Propag (USA), 44 (1996) Hall P S & Haskins P M, Electron Lett (UK), 28 (1992) Motta Cruz E & Daniel JP, Electron Let! (UK), 29 (1993) 88. constant 5 Bhartia P, Rao K V S & Tomar RS, Millimeter-wave microstrip and printed circuit antenna (Artech House, Bostan, London), Pourush P K S & Singh B, ndian J Radio & Space Phys, 25 (1996) Bahl J & Bhartia P, Microstrip antenna (Artech House, Bostan London) Balanis C A, Antenna theory and design (Harper and Row Publisher nc, New York), Krowne C M, EEE Trans, Antennas & Propag (USA), AP-21 (1983) Gupta R K, ndian J Radio & Space Phys, 1 (1972) Jain M K, yengar S R K & Jain R K, Numerical methods for scientific and engineering computation (Wiley Eastern Ltd), "',.' ' 1"!l"'"' '!! '