The objective of the ESA-funded RESKUE project is to design and implement a prototype of a novel antenna for a portable satellite user terminal for provision of high data rates via a GEO satellite in Ku-band. The satellite terminal has to be easy to transport without a vehicle and easy to deploy. The antenna design is novel in that it comprises fine electronic pointing capability by means the reconfigurability of PIN-diode controlled elementary cells on the main reflector of a reflectarray. To keep the design cost efficient a reconfigurable elementary cell with 1-bit phase resolution is adopted.
Bidirectional high data rate ad-hoc satellite links are needed in several security and emergency scenarios to exchange various data information in remote locations where other telecommunication infrastructures may be either not available or inadequate.
Antenna apertures based on flat active reflectarray panels may allow the realization of large high gain deployable antennas with beam agility at this frequency band. By employing simple deployment mechanisms, reflectarray antennas composed of flat panels can be folded in a compact way and rapidly installed in a given location.
A certain level of beam agility eases the pointing of the antenna, allowing the user to only roughly align the antenna towards the satellite and then leaving the reflectarray antenna to perform the fine pointing in an automatic way.
The objective of the activity is to design, manufacture and test a portable and deployable transmit/receive Ku-band antenna composed of small flat active reflectarray panels. The antenna shall include a limited beam steering functionality and support self pointing once integrated with the transceiver.
When compared to existing systems, designed reflectarray antenna shall exhibit a reduced cost, a limited folded volume and an easy deployment while providing comparable radiofrequency performances.
The ResKue project reflectarray shows a lot of challenging aspects. In order to obtain a full electronic reconfigurability of a reflectarray of these dimensions is necessary to find state-of-the art solutions able to allow at the same time an easy control of the reconfigurable devices and a low power consumption.
The specifications of the project moreover impose a very large relative bandwidth (more than 40%) in order to cover both the TX and RX bandwidth necessary for a bidirectional satellite link. The antenna must be operative also for both the possible linear polarizations. Considering all these aspect together the ResKue project represent a potenital brakthrough for the advances of the satellite systems.
A number of transportable products for satellite communications in Ku-band are already available on the market. All of these systems are based on foldable parabolic reflectors usually realized in carbon fibre to diminish its weight with a mechanical movement obtained by means of three-axis motors. The GPS, the processing algorithms and power measurement devices, automatically locate and track the satellite in elevation and azimuth and then adjust the polarization by rotating the feed. These systems have directivity ranges from 36 to 44dBi, depending on the size of the dish.
One of the main drawbacks of these antennas is the heavy weight (20-40Kg), due to the necessary motors for automatic satellite tracking. The proposed antenna based on the reflectarray principle, shall avoid the need of motors in fact after a rough pointing made by the operator, the electronic steering will provide a fine pointing, making the antenna much simpler and lighter.
The reflectarray surface is intrinsically foldable, yet with the unique advantage of being completely flat. This will undoubtedly ease the folding mechanism and the fabrication of the support. Another advantage is that the radiating surface can be fabricated in light-weight printed board, this also keeping low the fabrication costs.
The adopted antenna architecture is a single offset configuration using a 1-bit reconfigurable reflectarray and have been properly dimensioned in order to accomplish the requirements on Gain and on EIRP over RF transmit band (ETSI EN 301.428 and ITU-R S.728.1).
The selected solution provide the use of a reconfigurable main reflector in horizontal position, and the use of a sub-reflector in a fixed configuration.
This architecture allows to increase the focal distance and to reduce the overall profile of the antenna. The problem of portability and big size of the antenna remain a critical issue. For these reasons an antenna folding, which allows size reduction has been proposed.
The antenna in deployed configuration is a pentagon while in stowed position is made up by a rectangle (50cmx60cm) and three triangles folded upon it.
A subreflector, illuminated with a dual polarized elliptical feed, has been designed considering 38x48 elements. The elements are based on either two or three layers of varying-sized rectangular patches and are dimensioned in order to obtain the desired field distribution on the main reflector. In order to obtain acceptable performance the main reflector surface will necessarily employ thousands of reflecting elements.
The architecture of the elementary cell has therefore a strong impact in the overall cost and complexity, and the minimization of the number of electronic devices to be used is mandatory. To this purpose the most effective approach is the use of 1-bit elementary cells. The cell is a stacked structure made up of multiple coupled resonators in different technologies (microstrip and slotline) that allows to cover the whole operative bandwidth imposed by the specifications.
Reconfigurability of cells is achieved employing PIN diodes and providing a proper biasing voltage the elementary cell can be switched between two different configurations with a relative phase difference of 180°.
The project plan starts with a review of the state of the art technologies related to the field of transportable transmit/receive antennas and with an identification of the most suitable architectures.
Two of them were selected for an RF preliminary design and optimization. Proposed reflectarray architectures were then compared in a review meeting in order to select the most suited to the project objectives.
According to the selected structure (shown in the previous sections) the RF design of the antenna has been carried out as well as the control circuit and the mechanical design. The RF design involve the project of PIN-diode based reconfigurable elementary cells, the sub-reflector, the feed and the dimensioning of the geometry of the antenna necessary to accomplish the requirements. The control circuit has been designed in order to drive all the elementary cells on the main reflector with a binary stream generated by a micro-controller integrated in the structure. The mechanical design focused on the need of a deployable and light structure easy to transport.
The results of all these activities converged in the prototype shown in the following figure (antenna during the measurements).
Results of the measurements validated the principle of operation of the antenna. The antenna properly reconfigured is able to point the radiation pattern in the desired direction and scan the beam both in azimuth and elevation in a wide scanning range as shown by the next figure.
The project is currently closed. A wideband dual-polarization TX-RX reconfigurable reflectarray for emergency satellite communications in Ku-band has been developed within RESKUE project. The results of all the activities carried out resulted in a prototype of a full-reconfigurable reflectarray whose performance has been experimentally evaluated in anechoic chamber.
The results obtained validate the principle of operation of the design adopted: the antenna is able to completely reconfigure its main reflector in order to scan the main lobe in the desired direction with a very good accuracy. The results show that the state-of-the-art antenna proposed could open the road for systems able to innovate modern satellite communications.