The results obtained in the frame of the ESA activity “
Over the last few years the public's reliance on computer networks including, but not limited to, Internet has increased exponentially. Many people today use Internet for business and entertainment. Aeronautical broadband services provisioning by satellites are becoming a reality moving from an experimental base to an operational condition.
Aircrafts use antennas for transmitting and receiving communication signals. Key aspects to consider in the design of these antennas are the aerodynamic drag, the beam pointing capability, and the receive/transmit functionality.
Concerning the aerodynamic aspects, the antenna has to exhibit the minimum impact on the aircraft in order to reduce the extra fuel consumption due to the drag of the antenna installed on the fuselage. As a rule of thumb, the height of antennas on board large aircrafts and inclusive of the radome should not exceed
Concerning the beam pointing characteristics, to be operative both at equatorial routes as well as at the extreme latitudes required by polar routes, the preferred antenna solution should exhibit wide-angle beam scanning capabilities.
The possible antenna architectures 0 are based on fully mechanical [
Complexity and cost of the antenna are other key factors to consider together with ability to operate simultaneously in reception and in transmission.
The principal target, in terms of antenna characteristics, is the operation in the complete Ku Band (
The minimum antenna requirements in terms of operative Field of View, G/T, EIRP, and cross polar discrimination are reported in Table
Antenna performances requirements.
Requirements | Antenna |
---|---|
Radiation gain pattern over RF band | In agreement with (*) |
Operative antenna field of view | 0° |
0° | |
G/T over RF band, in the entire field of view and for every selectable polarization | >8 dBK-1 |
EIRP over RF band, in the entire field of view | >43 dBW |
Number of beams | 1 |
Cross polarization discrimination (including pointing error) | >15 dB |
(*) Ref. [
Regulatory aspects [
ETSI regulation mask [
The proposed baseline antenna architecture is an evolution of the Hemi-Spherical Antenna Array shown in Figure
Hemi-spherical array.
This Antenna typology, due to its particular shape, once mounted on the aircraft is able to hold-on the appropriate link whatever the position of the airborne with respect to the satellite is.
In order to minimize the complexity of the antenna, the Hemi-Spherical continuous surface has been organized in flat facets. The icosahedron, a polyhedron with twenty equilateral triangles, has been selected to replace the sphere. Besides, a further division of each triangular face (obtained projecting the vertices of the triangles on the sphere) has been implemented. As a result of a dedicated optimization, the geodesic hemi-sphere with 40 facets shown in Figure
Faceted hemi-spherical antenna.
The final optimized antenna is combining on the same aperture of the receive-transmit capabilities, exhibiting 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 interspacing comparable to the wavelength at the Tx higher frequency.
Each triangular subarray, indicated as
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 equal-phase wave front perpendicular to the desired beam direction.
The antenna block diagram, presented in Figure
Hemi-spherical antenna block diagram.
The input/output ports are fed with the IF band signal, and they are converted in a Ku band signal by the up/down converter. Two dedicated beam forming networks
The antenna works simultaneously in reception and transmission on the entire Ku band (10.7 GHz/12.75 GHz in Rx; 14.0 GHz/14.5 GHz in Tx). A deep investigation, optimization, and trade-off have been implemented in order to identify the wide-band radiating element.
Two possible solutions are envisaged:
the first one is based on a wideband or dual band radiating element; in order to separate the receive signal from the transmit signal it is mandatory the use of a circulator or a diplexer;
the second one is based on a self-diplexing radiating element; this element presents four physical ports (two for Rx, two for Tx). The self-diplexing element has the advantage to minimize the ohmic losses, therefore improving both the G/T and the EIRP.
The selected solution is based on a self-diplexing radiating element. It offers better electrical performances and permits reducing the manufacturing complexity.
A prototype of the realized radiating element is shown in Figure
Radiating element prototype.
Measured radiating element return loss.
The radiation patterns of the radiating element at the center frequency in the receive and transmit band are shown in Figures
Simulated radiating pattern in receive.
Simulated radiating pattern in transmit.
Each radiating element presents an active circuitry directly connected to the output ports (Tx/Rx module). The Tx/Rx module is able to realign the linear polarization in receive and in transmit or, otherwise, it is able to generate a circular polarization [
Tx/Rx active module connected to the radiating element.
The manufactured modules are presented in Figure
Tx and Rx active device—control board view.
Their dimensions are 20.6
As mentioned, each Tx/Rx Active Module is directly connected to the passive radiating elements (see Figure
Tile of the hemi-spherical antenna.
Two dedicated equal-amplitude and equal-phase beam forming networks combine the transmit and the receive signals coming from the active module (Figure
Receive BFN layout.
Using the micro-coaxial cable, all the Rx and Tx ports behind the tiles are combined in two dedicated switching BFNs that are able to transmit and receive the signal of the selected tile. As mentioned, only a dedicated number of tiles, 14, are active instantaneously depending on the azimuth and elevation beam pointing. A micro-PC has controled all the active components taking into account the satellite position and the aircraft attitude. The beam forming network connecting the tiles is realized using commercial microwave components.
The tiles are positioned on the shell-shaped structure, shown in Figure
Antenna structure.
The result of the assembled hemi-spherical antenna is presented in Figure
Assembled Hemi-Spherical antenna.
Cable routing of a part of the antenna.
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 reduced degradations of the electrical performances, guaranteeing a minimum G/T of 8 dB/K-1 and an EIRP of 43 dBW.
The results are obtained using the simulated pattern of an embedded radiating element. Moreover, in the optimization of the excitation coefficients, the realistic functionality of the Rx/Tx active module (combining the amplitude and phase of the two orthogonal polarization for each radiating element) has been taken into account.
The G/T is shown in Figure
G/T versus elevation angle over the horizon.
Azimuth Rx Antenna Pattern at 11.7 GHz, considering a 30
In transmit all the active devices are able to work at the maximum power level, and the foreseen performance in term of EIRP is reported in Figure
EIRP versus elevation angle over the horizon.
The antenna is also able to control the transmit antenna patterns side-lobe in order to respect the applicable regulatory recommendations such as ETSI in the European countries [
The typical azimuth transmit antenna pattern is presented in Figure
Typical Azimuth Tx Antenna Pattern at 14.25 GHz, 30
Typical Elevation Tx Antenna Pattern at 14.25 GHz, 30
In the paper the results of the activities carried out in the frame of the ESA Contract “Advanced Antenna Concepts For Aircraft In Flight Entertainment” have been presented.
The designed Ku band active antenna exhibits a compact size and the possibility of full beam pointing coverage. The antenna radiating element module is providing also polarization alignment capabilities in transmission and reception.
The antenna exhibits really good technical performances. The next activities will be focused on reducing dimension, cost, and complexity of the entire antenna in order to meet the severe expectation of a market product.
The authors acknowledge the colleagues R. Pasta and L. Flamini for UTRI; A. Toresin for Aeronavali; E. Limiti for the University of Roma Tor Vergata; S. Selleri for the University of Florence for their support and contribution in the activity.