## 2 Multibeam Bifocal Dual-Reflectarray Antennas in Ka-Band  
For many years, the bifocal technique has been applied to the design of dual-reflector systems to obtain an improved multibeam and beam scanning performance [21, 22, 23, 24]. Bifocal antennas provide better results for the extreme beams in terms of gain and side-lobe levels, resulting in reduced scan loss and lower interference levels than their monofocal counterparts [21, 25]. Moreover, the bifocal algorithm allows a certain degree of control in the angular separation between adjacent beams (through the election of the beam directions associated to the focal points), which has traditionally been used to increase the beam deviation factor with respect to the equivalent monofocal antenna for wide-angle beam scanning applications [22, 23, 24, 25]. Recently, the bifocal concept has been applied to DRAA systems formed by flat reflectarrays [20, 26]. A general 3D bifocal method has been presented in [20] for the design of offset DRAA configurations, with application to multibeam satellite antennas. The bifocal method has been validated by the fabrication and testing of the first offset bifocal DRAA prototype [20], which consists of a 57 cm \(\times\) 42 cm main reflectarray (main RA) and a 39 cm \(\times\) 35 cm sub-reflectarray (sub-RA) in an offset compact-range configuration, see Figure 1a. The DRAA demonstrator has been designed with reflectarray cells that enable independent phase control in each polarization, so the antenna is able to generate two adjacent beams in orthogonal polarization per feed at the Tx frequencies in the Ka-band (19.2-20.2 GHz). Figure 1b shows the simulated and measured radiation patterns in the offset plane for the 10 beamsproduced by 5 adjacent feeds (note that the beams alternate in polarization). A scaled version of this prototype with a 1.8 m main RA can be designed to produce multiple beams in the Ka-band from a GEO satellite [20], resulting in the feed and beam configurations shown in Figure 1c,d, respectively. The final separation between adjacent beams (in orthogonal polarization) in the same plane \"\(v=\) constant\" would be 0.56\({}^{\circ}\). However, the feed diameter imposes some restrictions in the beam spacing achieved in the orthogonal plane, so the antenna would be able to produce only one half of the multibeam coverage, and a second multibeam DRAA (designed with the same technique) should be used to produce the other half of the coverage. This paper presents a novel multibeam transmitting DRAA designed to generate a complete multiple spot beam coverage with a single main aperture, see Figure 2. In contrast to the previous bifocal antennas [21; 22; 23; 24; 25; 26], which are designed to provide a larger beam spacing (or similar beam spacing) than the equivalent monofocal antenna, the bifocal technique has been applied for the first time here to achieve a significant reduction of the beam deviation factor (around 50% reduction). This allows achieving 0.56\({}^{\circ}\) angular spacing in the offset plane between the beams produced in the same polarization by adjacent feeds. The bifocal antenna does not provide any beam spacing reduction in the orthogonal plane, so the interleaved beams required to complete the coverage will be generated in the orthogonal polarization by exploiting the dual-polarization capability of the reflectarray elements. Hence, each feed will produce two adjacent beams in orthogonal polarization, in a plane forming around 60\({}^{\circ}\) with the \"\(v=\) 0\" plane, see Figure 2b,c.