Near-Field Scanning Characteristics of Focused Reflectarray Antennas Payam Nayeri 1, Atef Z. Elsherbeni 1, Randy L. Haupt 1, and Fan Yang 2 1 EECS Department, Colorado School of Mines, Golden, CO 80401, USA [email protected], [email protected], [email protected] 2 Electronic Engineering Department Tsinghua University, Beijing 100084, China [email protected] Abstract ņ Focused reflectarray antennas are designed with the aim of investigating the scanning capabilities of these systems. Near-field scanning with a 30° scan coverage is demonstrated with a Ku-band reflectarray system. Index Terms - Focused antenna, reflectarray. I. INTRODUCTION Electromagnetic power can be concentrated into a small spot by focusing the antenna . Focused antennas arrays have numerous applications in remote sensing, radar, medicine, and imaging systems, and therefore have received a great deal of attention over the years. Reflectarray antennas on the other hand, combine the many favorable features of reflectors and arrays, and have now emerged as the new generation of high-gain antennas . The spatial feed system of the reflectarray, combined with the individual phase tunability of its elements, provides a myriad of design capabilities for applications requiring high-gain antennas. In this paper we present computed far field patterns from near-field scanned reflectarray antennas. Focusing characteristics of reflectarrays are investigated and a Ku-band focused reflectarray system achieving 30° scan is demonstrated. It is shown that a focused reflectarray can be a good candidate for limited field-of-view (FOV) applications. II. NEAR-FIELD ANALYSIS OF REFLECTARRAYS Parabolic-phase reflectarray antennas are designed for highly directional beams in the farfield zone . While different analysis approaches are available for reflectarrays , accurate studies of the antenna near-field characteristics typically requires a full-wave analysis method. In this study we use the commercial electromagnetic software FEKO , and investigate the near-field performance of Ku-band reflectarrays designed for the operating frequency of 14.25 GHz. (a) (b) Fig. 1. Electric fields of the reflectarray in the xzplane: (a) |Ex|, phase of Ex. A circular aperture with a diameter of 360 mm is selected for the reflectarray. An offset system with a feed offset angle of 15 degrees is considered. The reflectarray aperture is placed in the xy-plane and the feed is pointing to the center of the coordinate system. The phase center of the feed antenna is at X = -91.88 mm, Y = 0, Z = 342.9 mm. The feed is an A-INFO LB-62-15 pyramidal horn antenna. Variable size patch elements are used for the reflectarray phasing elements , and the dimensions of the patches are selected to scan the far-field beam to ș = 15°, ĳ = 0°. The electric fields of this reflectarray are shown in Fig. 1, where the formation of the plane wave is clearly visible in the phase plot. Note that however, the radiated power is not focused in the near-zone of the antenna, thus this design will not yield good performance for near-field applications. III. NEAR-FIELD FOCUSED SCANNING REFLECTARRAYS To focus the radiated power of the reflectarray antenna, the radiated waves from all elements of the array should add up in phase at the desired focusing point. The required phase shift on the aperture which focuses the beam at the desired point is computed as given in . Here we study near-field scanning capabilities of 3 reflectarrays which focus the radiated power at ș = 0°, 15°, and 30°, in the xz-plane. The focal distance is set to 355 mm from the center of the aperture. The masks of these reflectarrays are given in Fig. 2. Note that frozen-state designs are considered in the current study, however several designs of reconfigurable reflectarray elements are available that can be used for dynamic operation . (a) (b) (c) (d) (e) (f) Fig. 3. Electric fields of scanned reflectarray antennas in the xz-plane. (a) |Ex| (ș = 0°), (b) phase of Ex (ș = 0°), (c) |Ex| (ș = 15°), (d) phase of Ex (ș = 15°), (e) |Ex| (ș = 30°), (f) phase of Ex (ș = 30°). (a) (b) (c) Fig. 2. Masks of the focused reflectarray antennas. The electric fields of these focused reflectarrays are shown in Fig. 3. It can be seen that all three designs focus the electromagnetic power at the desired point, and a stable scan performance is achieved over a 30° scan range. This can be seen in an almost constant peak field magnitude at the desired focal point. In comparison between the three designs, the array designed for ș = 15° shows the best performance, which is similar to the case for far-field operation. As such, very wide-angle scanning with reflectarray may not be practical. Nonetheless, for limited FOV systems, near-field focused reflectarrays can be a suitable candidate. IV. CONCLUSION The feasibility of near-field scanning with reflectarray antennas is investigated. A Ku-band focused reflectarray system achieving 30° scan is demonstrated. ACKNOWLEDGMENT The authors acknowledge the contributions of Altair Inc. to Colorado School of Mines. REFERENCES  J. W. Sherman, “Properties of focused aperture in the Fresnel region,” IRE Trans. Antennas Propag., vol. 10, no. 4, pp. 399–408, Jul. 1962.  J. Huang and J. A. Encinar, Reflectarray Antennas. New York, NY, USA: Wiley-IEEE, 2008.  P. Nayeri, A. Z. Elsherbeni, and F. 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