MEDICINA OBSERVATORY

In the following some activities at the station laboratories are summarized about the microwave field and related matters to antennas.

32 GHz SCIENTIFIC WORKBENCH for SPOrt MISSION ACTIVITIES

SPOrt is the acronym for Sky Polarisation Observatory, a payload to be mounted on an express pallet on the International Space Station. It will be composed of four polarimetric radiometers at the frequencies of 22,32,60,90 GHz and it is intended to measure the polarisation contribution coming from the sky and from CMB. For complete information link to http://sport.tesre.bo.cnr.it.

Each radiometer has a block diagram as the following and it is a direct detection receiver with 10% bandwidth:

At the Medicina laboratories the bench version of the 32GHz radiometer has been assembled in order to get experience on polarimetric radiometers showing high sensitivity, high stability, very low spurious polarisation. IRA people of the Medicina observatory, as part of scientific team, designed the electronics and assembled the whole 32GHz SWB radiometer:

The core of the radiometer is the 5 level Hybrid Phase Discriminator (HPD). It is an RF analog correlator giving at the outputs the Stokes parameters I,Q,U. The rejection of this device with respect to spurious polarisation, together with a lock-in scheme is intended to reject the spurious polarisation at a level of -60dB and to be as insensible as possible at the 1/f gain fluctuations. Moreover a high performance OMT (isolation at a level of -50÷-60dB) is also needed.

HPD, waveguide filters and OMT are designed by Dr. Tascone group, as part of SPOrt scientific team,

CNR-IRITI Corso Duca degli Abruzzi 24, 10129 Torino, Italy
THE ACTIVE SURFACE for THE NOTO RADIOTELESCOPE

In order to overcome the antenna efficiency degradation due to gravity deformation on the structure supporting the 32m primary mirror of the Noto radiotelescope (Sicily, Italy) a concept of active surface has been installed. The complete system has been designed at the Medicina laboratories, together with actuators and panels alignment tools used during the installation. The installation phase started on August 2001 and lasted until January 2002. Together with new panels mounted (0.1mm surface rms) and the foreseen upgrade of the subreflector surface, the active surface can provide an increase by three times of the maximum operating frequency, allowing an antenna originally designed for centimetric wavelengths to operate up to 86GHz.

The system is composed of 244 electromechanical actuators mounted under each panels four corners. The whole primary mirror surface can be moved to recover a perfect parabolic shape at every elevation so flattening the antenna gain curve.

The actuators are compact electronics/mechanics devices with onboard microcontroller/driver component to do the movement once the actuator is addressed.

The actuators network is managed by a master computer that stores the matrix of deformation vs elevation. It receives the antenna elevation from the host computer and manages the parabolic surface recovering.

Preliminary measurements at 22GHz were done on DR21 on an not yet optimized surface correction. The comparison between AS disabled with respect to AS enabled are showed in the antenna gain diagrams below.
Monitoring panel
Antenna measurement

SRT, Sardinia RadioTelescope, A NEW 64m ANTENNA IN ITALY

Sardinia RadioTelescope will be a shaped 64m dish intended to operate from 300MHz to 100GHz, using primary, gregorian and beamwaveguide foci. The SRT project is going on from 1998, the year when funding was available from Ministry of Scientific Research. At the beginning of 2001 the whole design was completed and since that foundation design and the preparation of a call for tender for the construction phase started. At the moment the foundation construction is starting and the design of the buildings (offices, laboratories, etc.) is finished. To get information on antenna specification link to the following site: http://www.ca.astro.it/srt/

MICROWAVE LABORATORY

The microwave laboratory of the Medicina station provides the design and maintenance of receivers used on the antenna. The Medicina antenna receivers span from 1.4 to 22GHz. A 43 GHz prototype receiver was also used in the past. The facilities of the laboratory provide a cryogenic dewar for testing amplifiers, an automatic system to measure Noise Figure, Noise Temperature and Gain of microwave devices (MAMBO, see figure), a bonding machine (see figure) and a milling machine to make circuit boards. Instruments and tools are available in order to make vector microwave measurements up to 40GHz and up to 75GHz for scalar ones.
MAMBO: microwave amplifier measurement board
The bonding machine

A 30GHz FEED for PLANCK MISSION

A feed design was performed in the frame of the Planck LFI experiment by Renzo Nesti, now with IRA, sezione di Arcetri (Firenze). The specification was as the following

Center Frequecy:30 GHz
Band: 20% (27-33 GHz)
Return loss: >25 dB
Cross-polar level: < -25 dB
Edge Taper: 30dB@22°

The design aimed to obtain a compact feed with phase center as near as possible to the horn mouth, with variation in the band kept at the minimum.