Van Atta retro‐directive slot‐antenna array on cylindrical laminate

Abstract

A Van Atta, retro-directive, slot-antenna array on a cylindrical laminate is designed for use at 3.25 GHz. The array comprises four slots in a line, engraved on the front side of the laminate; the interconnecting microstrip feed lines are on the backside. Radiation patterns [6,7]. This paper explores the possibility of retro-directive operation by non-planar arrays. A passive Van Atta microstripline-fed slotantenna line array on a cylindrically shaped laminate has been designed for use at 3.25 GHz and the radiation characteristics of that array have been determined by calculations and measurements. DESIGN AND FABRICATION OF THE SLOT-ANTENNA ARRAYThe abovementioned array comprises four slots engraved on the front (copper) surface of a cylindrical laminate (Fig. 1). The slots are azimuthal and they are coupled to 50⍀ microstrip feed lines, which are laid on the backside of the laminate; the feed lines are along cylinder generators (Fig. 2).The radiation pattern of individual slots is similar to that of an electric dipole, albeit with E and H planes interchanged; the H-plane radiation pattern is not symmetrical with respect to the xOy plane (Fig. 2) because of the laminate curvature [8]. The wider the half-power beamwidth (HPBW) of individual radiating elements, the wider the angular range over which retro-directivity may be achieved by the array [4]. Slots have been chosen as radiating elements because their HPBW in the yOz plane, where retro-directivity is sought, is almost 100°.The slot antenna and the four-slot array have been designed using the finite-difference time-domain (FDTD) method. The input impedance Z in of any slot, along with the accompanying quarterwavelength stub ( Fig. 2), is purely resistive (Z in ϭ 255⍀) at 3.25 GHz. The distance between adjacent slots is equal to 0.65 0 , where 0 is the free-space wavelength at the aforesaid frequency. Thus, coupling between adjacent slots is reduced and the grating lobes are suppressed.Four slots in a line constitute an inner pair and an outer pair. Retro-directivity requires that the microstrip line connecting the slots in any pair be electrically as long as in every other pair [5]. The connecting line in the outer pair of slots is physically 5 g longer than the connecting line in the inner pair, where g ϭ 6.68 cm is the guided wavelength of the microstrip lines.The line array of slots has been fabricated on Taconic laminate TLY-5 with r ϭ 2.2 and tan␦ ϭ 0.001. The dielectric thickness is h ϭ 31.5 mil and the copper layer thickness on either side of the dielectric slab is t ϭ 1.4 mil. Copper has been removed by use of a T-Tech milling machine. The laminate was initially 26.76-cm long and 11.87-cm wide; it has been shaped into a cylindrical surface of (inner) radius equal to 14 cm. CALCULATIONS, MEASUREMENTS, AND DISCUSSIONFDTD calculations of the bistatic radar cross-section (RCS) of the slot-antenna array are shown in Figure 3; the incidence direction î corresponds to ϭ 130°, ϭ 270°. The bistatic RCS of an all-copper surface, having the same size a...