# Development of Enabling Technologies for Ku-Band Airborne SATCOM Phased-Arrays
## 1 Introduction  
Broadband connectivity has become essential in everyday life both for business and for entertainment. This, together with the increasing demand of mobile connectivity in locations where terrestrial networks are not available or less efficient, has promoted the development of antennas to be installed on aircrafts, trains, boats and vans to provide interconnection via satellite.  
Moving from experimental verification to operational conditions, aeronautical broadband services provided by satellites is today a reality; but to be commercially attractive, the services offered via satellite should have a quality comparable to those available via terrestrial networks, should allow the same type of applications and should be offered at an affordable cost.  
Together with the technical issues to be addressed, a main driver is the cost, which comprises the cost of ground segment, which is related to materials, installation (mainly related to the antenna system) and maintenance of the equipment, the cost of the satellite segment and the overall operations of the system.  
Focusing on the aeronautical platforms, satellite communication is the only means to provide continuous connectivity over the oceans. Airborne RF communication system applications pose challenging problems to current antenna technology:  
* Interactions with the aircraft aerodynamics impacting drag, lift and controllability;
* Influence of the aircraft structure on the antenna performance;
* Electromagnetic Compatibility (EMC)/Electromagnetic Interference (EMI) with other RF and/or electronic systems on board the aircraft;
* Very wide-angle scanning performance, so as to operate at the extreme latitudes required by polar routes;* Compatibility with other satellite systems and with terrestrial communication services (involving beamwidths, beam pointing accuracy and sidelobes performance);
* Keyhole problem typical of narrow beam antenna systems employing two axes for tracking a satellite. The keyhole is caused either by mechanical constraints or position slew rate limitations of the antenna mount [1].  
The ideal antenna should actually be high gain, broadband, low profile, highly reliable and low cost. Different choices of the antenna architecture [2] could bring to a fully mechanical [3], fully electronic [4] or \"hybrid\" (mechanical-electronic) solutions [5; 6].  
The fully mechanical and hybrid solutions are based on antennas with rectangular or highly elliptical apertures in order to minimize the antenna thickness. They create highly elliptical beams and, because they rely usually only on two motors, it is not always possible to project the most directive cut of the pattern along the equatorial plane. The addition of a third axis permits limiting this keyhole problem. Solutions based on fully electronic steering solve the keyhole issue and permit to maintain a transmit-receive link also when flying near the Equator where respecting the regulatory aspects is particularly challenging. Moreover, the fully electronic antenna, thanks to the graceful degradation (typical of active arrays) exhibits lower costs in terms of maintenance as compared to mechanically steering antennas. Electronically scanning antennas can be flat or conformal. In this development conformal antenna are considered in order to guarantee equalized performances in an entire half sphere. It is important to mention that by limiting the scanning directions with respect to the zenith within a cone with an angular opening of 60 degrees instead of 90 degrees, the antenna thickness could be reduced by a factor of two.