# Development of Enabling Technologies for Ku-Band Airborne SATCOM Phased-Arrays
## 3 Antenna Design  
The hemi-spherical antenna array when mounted on the aircraft is able to guarantee the proper link, whatever the position of the airborne with respect to the satellite is, thanks to its shape [8]. In order to minimize the complexity of the antenna, the hemi-spherical continuous surface has been approximated with flat facets. To replace the sphere, the icosahedrons, a polyhedron with twenty equilateral triangles, has been selected as starting geometry. Then a new solid of 80 facets has been generated; where, to the original vertexes, the points of middle edge of each triangle have been added after the projection on the sphere surface. Half portion of the new solid is covered by sub-arrays and, after further refinements, the position of the tile is adjusted to avoid the intersection of the extruded part which contains the RF components and the control logic. As a result of the optimization, the geodesic hemi-sphere with 40 facets shown in Figure 2 has been selected as antenna aperture. Each triangular face is composed by the same sub-array, i.e., the same number of radiating elements arranged in a triangular lattice. The final optimized antenna is able to combine on the same aperture the receiver and transmitter capabilities, has a diameter of 84 cm, a height of 39 cm and a weight in the order of 90 Kg. The number of radiating elements is 1440 with an inter-spacing of 22 mm, which corresponds to one wavelength in the transmit higher frequency. Each triangular sub-array, indicated as tile, presents 36 radiating elements. For a fixed beam direction, only a sector composed by 14 tiles is active while all the other elements are turned off. All the radiating elements in the active sector are excited in order to generate an equi-phase wave front perpendicular to the desired beam direction. \begin{table}
\begin{tabular}{l c} \hline \hline
**ITEM** & **Value** \\ \hline Number of beams & 1 \\ \hline Cross polarization discrimination & \textgreater{}15 dB \\ \hline \hline \end{tabular}
\end{table} 
Table 2: _Cont._  
Figure 1: European Telecommunications Standards Institute (ETSI) regulation mask [7]. It is important to note that when projecting the conformal aperture in an arbitrary pointing direction, the triangles are deformed and become irregular; this concept is clear watching at Figure 2. The resulting sparse array, thanks to this irregular lattice, exhibits a pattern without grating lobes. Moreover, the optimization of the excitation coefficients in amplitude and phase, or in phase only, permits to maintain the side lobes within the spectral density mask imposed by the regulatory aspects as demonstrated in the paper. Concerning the antenna electrical architecture, the input/output ports are fed with the IF band signal, and converted in a Ku band signal by the up/down converter. Each tile is composed by the radiating elements and the transmit/receive modules, directly connected on the back. Two dedicated beam forming networks 1:40 for the receiver and the transmitter mode are connected to the tiles and the CPU controls a set of switches in order to involve the selected sub-arrays to form the antenna beam. The CPU controls the beam steering and the polarization alignment. The tracking capability of the antenna permits to point the beam electronically in a hemi-spherical field of view (i.e., 360 degrees in azimuth and 90 degrees in elevation) with negligible degradations of the electrical performance, guaranteeing a minimum G/T of 8 dB/K-1 and an EIRP of 43 dBW. The results obtained using the simulated pattern of an embedded radiating element are reported in the following. The excitation coefficients have been optimized considering the realistic functionality of the Receive(Rx)/Transmit(Tx) active module (combining the amplitude and phase of the two orthogonal polarization for each radiating element). The G/T result is shown in Figure 3a. The performance matches the required 8 dB/K in the entire field of view.