Design of Circularly Polarized Waveguide Crossed Slotted Array Antenna at Ka Band Sayan Chatterjee
Arijit Majumder
Dept. of Electronics & Telecommunication Engineering Jadavpur University Kolkata, India
[email protected]
Circuit & System Division SAMEER Kolkata Centre Kolkata, India
[email protected]
Abstract—Present paper highlights an improvement over the use of conventional cross slot radiator in waveguide broadwall for circular polarization by using a combination of cross slot and inclined slot pair. Accordingly, a circularly polarized travelling wave slotted array antenna at Ka band (38.6GHz) of 20dB gain has been designed and verified in a nonstandard waveguide using the modified cross slot radiator.
I.
INTRODUCTION
Slotted array antenna has been widely used in the application of monpulse RADAR and space astronomy. Conventionally linear slotted array antenna are designed to be linearly polarized, whereas, in some of the application of Secondary Surveillance RADAR (SSR), polarization of antenna is needed to in symmetry with received signal polarization. Traditionally the well established technique is to use planar polarizer, which converts a linearly polarized signal to circular polarization. Among them, meander line polarizer is an attractive cost effective solution [1]. In other way, different types of perturbation has been introduced in patch antenna, where the edge chamfering contributes circular polarization[2]. The problem of those microstrip patch antenna arrays are of low power and bandwidth. Circular polarization can be achieved in waveguide by placing a cross slot at the broadwall of waveguide. An Analytical expressions and theoretical discussions about the power budgeting of a circularly polarized crossed-slot antenna placed on the broad-wall of a rectangular waveguide has been provided by Simmons[3]. The article exhibits a closed form expressions of the axial ratio and the fraction of field reflected from each leg of the crossed-slot. Circular polarization has been achieved by the placement of cross slot on broadwall of the waveguide such that the longitudinal and transverse magnetic field symmetry at the slot center has been maintained. In [4] a circularly polarized linear array antenna using dielectric rod as a feeder element has been proposed. The concept of single cross slot in waveguide has been further extended in a design of an array by Perrotti [5]. The array exhibits a combination of left and right handed circular polarization for transmission and reception purpose. Center cross slot has been aligned in such a way that 1800 phase difference has been achieved. Hirokawa in [6] has provided an analysis process for cross slotted waveguide array antennas, in which both kind of electrical and magnetic Green’s functions has been employed
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for various kinds of waveguide loadings. In [7] an asymmetric cross slot has been used as a matching element for a large cross slotted waveguide antenna array. It exhibits a phase correction along the travelling wave direction by asymmetric longitudinal and transverse field coupling. In [8] design of a S band circularly polarized antenna for Surveyor spacecraft has been studied, in which two complex pair of compound slot has been positioned with cross slot. Use of such compound slot provides a better phase correction, exhibiting an in-phase broadside radiation. In present paper, the idea of asymmetric cross slot has been introduced with complex slot pairs for improvement of axial ratio and in-phase radiation in a travelling wave antenna array at 38.6GHz. II.
DESIGN OF UNIT CELL
Traditionally, circularly polarized antennas are designed by placing cross slot on the broadwall of the waveguide at an inter-element spacing of full guide wavelength. As a result grating lobe generation is a well established phenomenon. Moreover as the cross slot are typically placed in a travelling wave waveguide structure, hence progressive phase shift along slots are such that, the in-phase radiation cannot be obtained. The results exhibit a poor radiation efficiency and a mismatch along field components. Using the two offset inclined slots with cross slot improves the field matching in such a way that the in-phase radiation has been improved. Fig.1 shows the comparative layout of two different types of unit cell. Further, the design has been extended by introducing asymmetric leg cross slot structure, which contributes a better phase correction along the array, such that the overall realized gain and axial ratio can be improved. Fig.2 shows the asymmetric cross slot pair. Improvement of axial ratio has been observed from Fig.3a in which best possible result has been observed for inclined slot pair with cross slot. It has been observed from Fig.3b that the input S11 has also been improved. III.
DESIGN OF ARRAY ANTENNA
Design of an end fed cross slotted array antenna of 20dB gain at 38.6GHz has been carried out, in which unit cell contains asymmetric cross slot and a pair of inclined slot as shown in Fig.4. The design contains 26x2 cross slot elements positioned in two waveguide, placed side by side and excited by 1800 out of phase signal. To avoid the elevation plane grating lobe, it
has been observed that the interelement slot distance in elevation plane should be kept at a distacnce of 0.75λ0. Accrodingly to accommodate the waveguide broawall and wall thickness, nonstandard waveguide of 6.5mm x 3.25mm has been selected at design frequency.
20.66dB and first side lobe level of 22.89dB has been observed in azimuth plane. The pattern exhibits a 10 of beam tilt.
Fig. 5. Fabricated prototype Fig. 1. layout of unit cell (a) single cross slot, (b) with inclined slot pair (c) Efield pattern in single cross, (d) E-field pattern in cross slot with inclined slot
Fig. 2. layour of Asymmetric cross slot with inclined slot pair
Fig. 6. Comparative study between measurement and simulation (a) S11, (b) axial ratio
Fig. 7. Comparative study for radiation pattern (a) Azimuth plane, (b) Elevation plane Fig. 3. Comparative study between different slot combination (a) axial ratio, (b) S11
REFERENCES [1] [2]
[3] [4]
[5] Fig.4 Layout of array using cross slot and inclined slot pair (a) 3D structure, (b) magnetic field in waveguide, (c) E field plot during in phase radiation
IV.
RESULTS AND DISCUSSION
A prototype has been fabricated and shown in Fig. 5. Fig. 6a shows the comparative study of simulated and measured result of input S11,wheras Fig. 6b provides the axial ratio measurement plot. It has been observed that measured results are in symmetry with simulated results. Measured radiation pattern for azimuth and elevation plane at 38.6GHz has been complied with simulated results in Fig.7. Realized gain of
[6]
[7]
[8]
Leo Young, Lloyd A. Robinson and Colin A. Hacking, “Meander-Line Polarizer”, IEEE Transac AP, Vol. 21, issue 3, pp. 376-378, May 1973. Z.N.Chen, X.Qing, H.L.Chung, "A Universal UHF RFID Reader Antenna," IEEE Transac MTT, Vol- MTT-57, No. 5, pp. 1275-1281, May, 2009. Alan J. Simmons, “Circularly Polarized Slot Radiators”, IRE Trans. Antennas Prop, Vol. 5, Issue. 1, pp. 31-36, Jan-1957. T.Hori and T. Itanami," Circularly Polarized Linear Array Antenna Using Dielectric Image Line," IEEE Transac MTT, Vol- MTT-29, No. 9, pp. 967-970, September, 1981. E.J.Perrotti," Circularly Polarized Crossed Slot Waveguide Antenna Array," US Patent : 4266228, May 5 1981. Jiro Hirokawa, “A Study of Slotted Waveguide Array Antennas”, Doctoral Dissertation, Department of Electrical and Electronic Engineering, Tokyo Institute of Technology, Nov. 1993. T. Hirano, J.Hirokawa, M.Ando, " Waveguide matching crossed slot," IEE Proc. Microw. Antennas Propag, Vol-150, No.3, pp. 143-146, Jun 2003. A.F.Seaton, G.A.Carnegis," A novel circularly polarized planar array for surveyor," 1958 IRE International Convention Record. pp. 2-9, 21-25th March , 1966.
