## 1 Introduction  
The development of high throughput satellites (HTS) in the Ka-band has originated an increased interest in novel multiple beam antenna architectures [1; 2; 3]. Current HTS systems provide a cellular coverage formed by a large number of closely-spaced spot beams, according to a multi-color reuse scheme [1]. Reflector antennas in single feed per beam (SFPB) configuration provide high gain and bandwidth, but they cannot produce all the beams satisfying the required conditions in terms of spillover, limited side-lobe level and sufficient isolation. The inter-feed spacing imposed by the small beam separation (a typical value is 0.56\({}^{\circ}\)) is not compatible with the feed diameter required for the efficient illumination of the reflector [2]. To circumvent this limitation, four reflectors operating in transmission (Tx) and reception (Rx) in a SFPB scheme are usually adopted, so that the adjacent beams are produced by different reflectors [2; 3]. One of the main drawbacks of this multibeam antenna configuration is associated to the volume required to accommodate four large reflectors (between 2 m and 2.5 m in diameter) on the satellite. Different solutions have been investigated to produce a complete multibeam coverage using a single main aperture [4]. The use of a single reflector to produce all the beams in a SFPB scheme results in a highly oversized reflector, around 4 m in diameter, with a large focal distance [5]. The area of the oversized reflector is comparable to the total area of the standard four-reflector system, so it exhibitssimilar accommodation problems. Moreover, the oversized reflector has to be significantly shaped in order to produce a larger beamwidth, which implies higher manufacturing costs [5]. Another type of multibeam antenna is represented by the so-called multiple feed per beam (MFPB) architecture, where the beams are generated by subarrays overlapped in the focal region. MFPB systems permit reducing the number of antennas on board the satellite [6, 7], but they present some drawbacks associated to the complex and expensive beamforming networks that feed the antennas. In practice, this class of antenna has been used, up to now, to generate a limited number of beams with slightly inferior electrical performance as compared to SFPB systems [3, 6]. Multibeam antennas based on direct radiating arrays [8, 9] and active lenses [10] have also been investigated, but they are typically highly complex and costly. Another promising antenna architecture, which exhibits less mechanical complexity, is based on the use of quasi-optical beamformers that feed a multibeam lens or reflector system [11, 12, 13]. Reflectarray antennas combine their capability to generate independent beams in different polarizations [14, 15] or frequencies [16, 17] with the hardware simplicity of SFPB systems. In fact, the generation of two beams per feed (associated to two polarizations [18] or two frequencies [19]) can lead to a reduction in the number of apertures required to provide a complete multibeam coverage. Recently, a novel bifocal design technique has been developed for dual-reflectarray antennas (DRAAs) in offset configurations [20], which allows improving the multibeam performance of the antenna for Ka-band satellite applications. In this paper, the authors present the design of a bifocal DRAA to generate a complete multiple spot beam coverage with a single main aperture, for transmission from a geostationary (GEO) satellite operating in the Ka-band. The bifocal technique has been applied for the first time to obtain a significant reduction of the beam deviation factor in the offset plane (i.e., a reduction by 50%) with respect to the equivalent monofocal antenna.