The Square Kilometer Array (SKA) represents the future of radioastronomy. It will be composed of several subsystems operating in various frequency bands from 50 MHz to 10 GHz. The lower portion of this spectrum will be covered by a sparse array of antennas with more than 250000 elements which will provide incredibly high sensitivity and resolution performance.
Italy is very active in the SKA design. A long-term collaboration between the Istituto di Radio Astronomia (IRA) of the Istituto Nazionale di Astrofisica (INAF) and the Istituto di Elettronica ed Ingegneria dell'Informazione e delle Telecomunicazioni (IEIIT) of the CNR produced several interesting results.
First of all, a Vivaldi-type dual-polarization array element has been designed in the frequency band 70-450 MHz. It exhibits a very low cross polarization on the principal axes, a sky coverage of 45° and a good matching at its two unbalanced 50 Ohm outputs. The metal-only antenna structure, which does not require a ground plane, has been already manufactured by the Italian companies Cospal Composites S.r.L. and Sguinzi S.p.A. with several techniques to obtain a low-cost prototype which is suitable for mass-production.
The characterization of low-frequency antennas directly placed on the ground and in the array configuration is a very challenging activity as far as the radiation pattern measurement is concerned. A very good solution has been found using an Unmanned Aerial Vehicle (UAV) equipped with a radio-frequency continuous wave transmitter as the probe antenna of an outdoor far-field test range. The know-how on the UAV technologies has been provided by the Geomatics research group of Politecnico di Torino - Dipartimento di Ingegneria dell'Ambiente, del Territorio e delle Infrastrutture (DIATI). Their measurement techniques allowed to track the position of the flying UAV with a centimetric accuracy. In this way, a very accurate characterization of both reference antennas and the designed dual-polarization Vivaldi in the array configuration have been obtained. Recently, the UAV source has been used to perform a digital beam forming calibration at the Medicina Station (BO) with good results. In the near future, this system will be also used to characterize other SKA antenna candidates around the world.
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