A REFLECTOR ANTENNA FEEDING SYSTEM BASED ON BI-DIMENSIONAL DOUBLE-FFT CIRCUITS D. Betancourt 1 , P. Angeletti 2 and C. del Rio 1 1 Public university of Navarra, Pamplona, Spain. 2 European Space Agency, ESA-ESTEC, Noordwijck, The Netherlands Abstract- The two-dimensional double fast-Fourier-transform Beam-forming-network (2D-Double-FFT- BFN) concept is introduced. This system based on Fourier Transform properties is studied to be applied in spacecraft applications. A feasibility study compares theoretical with physical models. Results shown how this system can be used to replace single –feed-per-beam architectures by compact ones based on this technology. INTRODUCTION Antenna systems based on reflectors have a broad field of applications over the telecommunications satellite industry. Several space applications have been developed in few past years based in this technology [1]. In the same way, this subject has been object of innumerable studies and research activity. Broadly speaking, a typical reflector based antenna system has three main components: a Beam Forming Network (BFN), a Feed Cluster and a Reflector. It is easy to establish that among them exist a high technological interaction, that is, depending on the antenna architecture selected to work with, is conditioned the BFN associated to it, and vice versa. A typical setup for a feed cluster is made, for instance, with Horn Antennas [2], these radiating elements have demonstrated a high performance for a huge number of applications. Nevertheless, in recent applications like multiple- beam based systems, Horn based feed clusters solutions present limitations in terms of compactness (i.e. number of required reflectors). This drawback, in the majority of cases, are due to the required radiating area of the horn antennas to achieve efficient spill-over performances that made it impossible to space out the radiating elements sufficiently close to obtain the desired beam lattice. One possible solution could be to change the type of primary feeds used to illuminate the reflector, so for instance, an array of electrically small radiating elements could have the same radiation properties of a larger Horn antenna and the single-feed-per-beam feed cluster could be replaced by an array of such radiating elements with each element participating to the formation of different beams. However, a system of these characteristics, termed Array Fed Reflector, brings with it several challenges related on how to feed the array without increasing the complexity of the system, and at same time, fulfil all the requirements of a set of single-feed-per-beam antennas (typically 3/4) in single reflector architecture. 1 2 M-1 M 1 2 M-1 M 2 1 N-1 N 2 1 N-1 N FFT MxM FFT NxN amplitude Phase Feed Points Useful Signals Aperture Plane Phase amplitude Reflector Antenna 1 2 M-1 M 1 2 M-1 M 2 1 N-1 N 2 1 N-1 N FFT MxM FFT NxN 1 2 M-1 M 1 2 M-1 M 2 1 N-1 N 2 1 N-1 N FFT MxM FFT NxN amplitude Phase Feed Points Useful Signals Aperture Plane Phase amplitude Reflector Antenna Figure 1.- Schematic representation of a 1-D Double FFT-BFN Figure 2.- Schematic representation of a 2-D Double FFT- BFN Henceforth, the problem is translated to the BFN field. In this context, there exist several technologies for Multiple- beam based systems, the most efficient ones are based on the Bultler Matrix concept and Fast Fourier Transform techniques [3-5]. These methods have been broadly studied, and therefore, we will use theirs well know results in the application proposed. The system that is introduced here is a two dimensional Beam Forming Network that uses the properties of the Fourier Transform to reduce its complexity. Giving, for instance, the possibility to replace a primary feed cluster, used in single-feed-per-beam reflector multiple-beam antenna system, by an array of electrically small radiating elements making possible the overlapping of the beams at aperture plane, thus, improving the whole radiation characteristics of the system. DOUBLE FFT-BFN The BFN based on the application of a double transformation over a unique input signal are designed to perform a physical movement of the spot beam defined at aperture plane. The main idea behind this system is to take advantage of