Sergio Mariotti

Status of the Sardinia Radio Telescope project

We present the status of the Sardinia Radio Telescope (SRT) project, a new general purpose, fully steerable 64 m diameter parabolic radiotelescope capable to operate with high efficiency in the 0.3-116 GHz frequency range. The instrument is the result of a scientific and technical collaboration among three Structures of the Italian National Institute for Astrophysics (INAF): the Institute of Radio Astronomy of Bologna, the Cagliari Astronomy Observatory (in Sardinia,) and the Arcetri Astrophysical Observatory in Florence. Funding agencies are the Italian Ministry of Education and Scientific Research, the Sardinia Regional Government, and the Italian Space Agency (ASI,) that has recently rejoined the project. The telescope site is about 35 km North of Cagliari. The radio telescope has a shaped Gregorian optical configuration with a 7.9 m diameter secondary mirror and supplementary Beam-WaveGuide (BWG) mirrors. With four possible focal positions (primary, Gregorian, and two BWGs), SRT will be able to allocate up to 20 remotely controllable receivers. One of the most advanced technical features of the SRT is the active surface: the primary mirror will be composed by 1008 panels supported by electromechanical actuators digitally controlled to compensate for gravitational deformations. With the completion of the foundation on spring 2006 the SRT project entered its final construction phase. This paper reports on the latest advances on the SRT project.

The dual-band LP feed system for the Sardinia Radio Telescope prime focus

We present the design of the passive feed system of the dual-band receiver for the prime focus of the Sardinia Radio Telescope (SRT), a new 64 m diameter radio telescope which is being built in Sardinia, Italy. The feed system operates simultaneously in P-band (305-410 MHz) and L-band (1300-1800 MHz). The room temperature illuminators are arranged in coaxial configuration with an inner circular waveguide for L-band (diameter of 19 cm) and an outer coaxial waveguide for P-band (diameter of 65 cm). Choke flanges are used outside the coaxial section to improve the crosspolarization performance and the back scattering of the P-band feed. The geometry was optimized for compactness and high antenna efficiency in both bands using commercial electromagnetic simulators. Four probes arranged in symmetrical configuration are used in both the P and the L-band feeds to extract dual-linearly polarized signals and to combine them, through phased-matched coaxial cables, into 180 deg hybrid couplers. A vacuum vessel encloses the two P-band hybrids and the two L-band hybrids which are cooled, respectively at 15 K and 77 K. For the P-Band, four low loss coaxial feedthroughs are used to cross the vacuum vessel, while for the L-Band a very low loss large window is employed. The P-band hybrids are based on a microstrip rat-race design with fractal geometry. The L-band hybrids are based on an innovative double-ridged waveguide design that also integrates a band-pass filter for Radio Frequency Interference (RFI) mitigation.

InP MMIC amplifiers for cryogenic radioastronomical applications

Radioastronomical observation development pushes technology research to develop equipment working up to 100-150 GHz. This trend makes the noise contribution of the low noise amplifiers (LNA) more serious. A significant improvement comes from the use of indium phosphide (InP) technology, as it gives the best noise performance in this range. Moreover, as radioastronomy is moving towards the array receiver configuration, this implies high integration and repeatability. In the frame of the FARADAY Project, we have been involved in designing several kinds of MMIC amplifiers in the 18-26 GHz range in order to approach the InP technology and the MMIC design methodology.

Comparison of cryogenic W band low noise amplifier based on different III-V HEMT foundry process and technologies

We present the results of a development activity for cryogenic Low Noise Amplifiers based on HEMT technology for ground based and space-borne application. We have developed and realized two LNA design in W band, based on m-HEMT technology. MMIC chips have been manufactured by European laboratories and companies and assembled in test modules by our team. We compare performances with other technologies and manufacturers. LNA RF properties (noise figures, S-parameters) have been measured at room and cryogenic temperature and test results are reported in this paper. Performance are compared with those of state-of-the-art devices, as available in the literature. Strengths and improvements of this project are also discussed.

A HIGH TEMPERATURE SUPERCONDUCTOR MICROWAVE FILTER WORKING IN C-BAND FOR THE SARDINIA RADIO TELESCOPE

A planar band-pass filter based on High Temperature Superconductor (HTS) has been designed for possible implementation in the cryogenic front-end of the C-band receiver for the Sardinia Radio Telescope. The band-pass filter is designed to operate at relatively high frequencies: center frequency 6.7 GHz with 30% bandwidth. Seven nominally identical filters have been fabricated to test different carrier materials and connector types aimed to keep the fabrication of the HTS filter simpler and more cost competitive. In addition to the conventional approach, silver plating, copper carriers and SMA connectors have been used. Cryogenic scattering parameter measurements show a good agreement with numerical results: the average of the transmission losses turns out to be in the range 0.15–0.25 dB depending on the prototypes, whereas the reflection coefficient is below -16 dB. The insertion loss has been also measured by using a radiometric approach based on the cold attenuator method showing consistent results with those given by the Vector Network Analyzer. Multiple cool-down measurements have been performed successfully proving the data repeatability both in short- and medium-term. Concerning alternative technical solutions, the SMA connectors and silver plating appear to be valid options whereas the copper carriers are inclined to destroy the circuit. Finally, numerical simulations and experimental measurements on a traditional copper filter operating at 20 K show that the HTS filter improves the losses of about 0.2 dB with respect to the copper one.

UAV-based method for the sensitivity measurement on low-frequency receiving systems

One of the most important requirements for the astronomical Low-Frequency Aperture Arrays (LFAAs) is the high sensitivity, defined as the ratio between the effective area and the system noise temperature. This figure of merit is strictly related to the single element sensitivity, which is difficult to measure using astronomical sources. On the other hand, the characterization of a low-frequency antenna inside an anechoic chamber is not straightforward down to 50 MHz. In this contribution a novel method to measure the sensitivity of a single LFAA receiving chain is presented. The proposed technique utilizes a micro Unmanned Aerial Vehicle equipped with a RF test source. A preliminary error budget analysis demonstrated that this technique is able to measure the sensitivity of a LFAA chain with high accuracy.

Status of the radio receiver system of the Sardina Radio Telescope

In this article, we present the design and performances of the radio receiver system installed at the Sardinia Radio Telescope (SRT). The three radio receivers planned for the first light of the Sardinian Telescope have been installed in three of the four possible focus positions. A dual linear polarization coaxial receiver that covers two frequency bands, the P-band (305-410 MHz) and the L-band (1.3-1.8 GHz) is installed at the primary focus. A mono-feed that covers the High C-band (5.7-7.7 GHz) is installed at the beam waveguide foci. A multi-beam (seven beams) K-band receiver (18- 26.5 GHz) is installed at the Gregorian focus. Finally, we give an overview about the radio receivers, which under test and under construction and which are needed for expanding the telescope observing capabilities.

New RF data on ECCOSORB CR/MF absorber

ECCOSORBTM CR/MF is a widely used absorber at radio and millimeter wavelengths. It is used both at room and at cryogenic temperature to realize loads and calibrators both for laboratory and for space-borne instruments. Data on its RF properties are available from the data sheet at room temperature. But it is also widely used outside the design wavelength range and at cryogenic temperature, where specific measurement of electromagnetic and thermal properties are needed. Scarce information is available in the literature and inconsistencies are frequent. We report here new RF data in Ka and W-band at room temperature obtained with waveguide measurements with different setups.

European low-noise MMIC technologies for cryogenic millimetre wave radio astronomical applications

The Low Noise technology has a paramount relevance on radiotelescopes and radiometers performances. Its influence on sensitivity and temporal stability has a deep impact on obtainable scientific results. As well known, front end active part of scientific instruments are cryocooled in order to drastically reduce the intrinsic thermal noise generated by its electronic parts and consequently increase the sensitivity. In this paper we will describe the obtained results by an Italian Space Agency funded activity. The aim is to validate European MMIC Low Noise technologies and designs for cryogenic environments in the range of millimetre wave. As active device, HEMT (High Electron Mobility Transistor) are considered the best device for high frequency and low noise cryo applications. But not all the semiconductor foundry process are suitable for applications in such environment. Two European Foundries has been selected and two different HEMT based Low Noise Amplifiers have been designed and produced. The main goal of this activity is identify an European technology basement for space and ground based low noise cryogenic applications. Designs, layout, architectures, foundry processes and results will be compared.

From an MMIC chip to a working cryogenic low-noise amplifier: a detailed study on packaging

Cryogenic Low Noise Amplifiers, based on MMIC HEMT technology, require a careful packaging to reach optimal performance. Differences between modeled and measured performance can often be related to chip mounting details. In the framework of the development of new cryogenic LNAs, described in a companion paper, we have developed a specific packaging to host W-band cryogenic MMIC LNAs. We present here some of the main factors analyzed in the design and chip integration activities. In particular, mechanical and thermal modeling, LNA chip gluing and adhesive properties, sensitivity to components integration accuracy (i.e. deviation from the ideal orientation). Preliminary test results are also reported.