A New Fresnel Zone Antenna with beam Focused in the Fresnel Region Shaya Karimkashi and Ahmed A. Kishk Dept. of Electrical Engineering, University of Mississippi, University, MS, 38677, USA Phone: 662-380-7139 Fax: 662-915-7231 skarimka@olemiss.edu and ahmed@olemiss.edu Abstract A new Fresnel Zone (FZ) Antenna to focus the microwave power in the radiation near field region is presented. FZ Antennas, in comparison with array antennas or reflector antennas, conventional focused antennas, take the advantages of design simplicity as well as lower sidelobe levels (SLL) in both axial and transverse directions. Simulation results and comparisons made between the new structure and aperture antennas based on the quadratic phase distribution show a reduction of sidelobe levels in both axial and transverse directions. 1. Introduction Antennas are commonly characterized in terms of their far field radiation patterns. However, in some applications, antennas are specified to focus microwave power in their radiating near field regions. These antennas, called Focused Antennas, are of interest in medical, Microwave power transition and some other applications [1-3]. It was proved theoretically that in the focal plane of a focused aperture, near the axis, the electric field will have all the properties of the far field radiation pattern if a quadratic phase distribution is adjusted on the aperture of the antenna [4, 5]. Some attempts have been made using array antennas or conventional reflector antennas to generate quadratic phase distributions and focus power at nearby points [6-9]. However, the main problems arising in focused antennas with this kind of aperture distribution are high lobes in both axial (forlobes and aftlobes) and transverse (sidelobes) directions. Direct amplitude tapering on the aperture gives low sidelobes but high forelobes and aftlobes. On the other hand, inverse tapering (high at edge, low in center) gives low forlobes and aftlobes but high sidelobes [10]. In addition, a precise generating of a quadratic phase distribution using array antennas is complex, costly and limited due to the difficulties of implementing of beam forming networks. On the other hand, conventional reflector antennas can be used to generate a quadratic aperture distribution. However, the ability to represent a desired aperture distribution using a conventional reflector like parabolic reflector antenna is limited. Although the problem can be solved using shaped reflectors, their implementation is very costly. In addition, since usually a large aperture antenna is desired for this application, implementations will be more difficult and costly. In this paper, a new focused antenna using FZ concept is proposed. This antenna takes the advantages of low sidelobes, forelobes and aftlobes as well as design simplicity. In fact, the flatness aspect of the Fresnel zone antenna is its benefit in the manufacturing process. 2. Design Procedure A Fresnel zone consists of a set of radial concentric strips, arranged on a flat surface, which alternate between reflective and non-reflective surfaces (Fig. 1). If the zones be spaced so that radiating waves from all the zones arrive in phase at the desired focus point, there will be constructive inference. When these zone arrangements are illuminated by a source, like a horn antenna located at the focal point of the geometry and a /4 λ back plate is placed behind the FZ plate, all of the rays impinging on the reflector surface will add up in phase at the focus point after reflecting by the strip rings and the back plate [11, 12]. The geometrical optic (GO) method is used in the design in order to get constructive interference at the focus point. The design is performed in the plane (2 dim.) and then the shape is completed by revolving it around the axis of symmetry.