G. Valente

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.

Sardinia Radio Telescope wide-band spectral-polarimetric observations of the galaxy cluster 3C 129

We present new observations of the galaxy cluster 3C 129 obtained with the Sardinia Radio Telescope in the frequency range 6000-7200 MHz, with the aim to image the large-angular-scale emission at high-frequency of the radio sources located in this cluster of galaxies. The data were acquired using the recently-commissioned ROACH2-based backend to produce full-Stokes image cubes of an area of 1 deg x 1 deg centered on the radio source 3C 129. We modeled and deconvolved the telescope beam pattern from the data. We also measured the instrumental polarization beam patterns to correct the polarization images for off-axis instrumental polarization. Total intensity images at an angular resolution of 2.9 arcmin were obtained for the tailed radio galaxy 3C 129 and for 13 more sources in the field, including 3C 129.1 at the galaxy cluster center. These data were used, in combination with literature data at lower frequencies, to derive the variation of the synchrotron spectrum of 3C 129 along the tail of the radio source. If the magnetic field is at the equipartition value, we showed that the lifetimes of radiating electrons result in a radiative age for 3C 129 of t_syn = 267 +/- 26 Myrs. Assuming a linear projected length of 488 kpc for the tail, we deduced that 3C 129 is moving supersonically with a Mach number of M=v_gal/c_s=1.47. Linearly polarized emission was clearly detected for both 3C 129 and 3C 129.1. The linear polarization measured for 3C 129 reaches levels as high as 70% in the faintest region of the source where the magnetic field is aligned with the direction of the tail.

A Compact L-Band Orthomode Transducer for Radio Astronomical Receivers at Cryogenic Temperature

We describe the design, construction, and performance of a compact orthomode transducer (OMT) for the L-band receiver (1.3-1.8 GHz) of the Sardinia Radio Telescope (SRT). The complete OMT consists of a cylindrical orthomode junction (OMJ), which is presented in this paper, and two identical 180 ° hybrid power combiners in a double-ridged waveguide, which have been proposed in a previous work. The OMT operates at the cryogenic temperature of 20 K to reduce its thermal noise contribution to the noise of the receiving chain. Therefore, particular care has been taken in the design of the OMJ to minimize its dimension and insure a good thermalization. The proposed OMT has been designed and optimized by using CST Microwave Studio, and then manufactured, tested at room temperature, and installed on the L-band receiver of the SRT. The measured results fully comply with the design specifications. In particular, the isolation between the OMT output ports is more than 40 dB, and the cross polarization is less than -35 dB for both polarization channels.

Energy-Based Considerations for Ungrounded Wearable UHF Antenna Design

The robustness of wearable UHF-band ungrounded antennas with respect to body-coupling effects is addressed. Two different configurations of single-layer antennas, with different energy density distributions, are presented, and a design criterion to improve their performance with respect to the antenna-body separation is derived. Through an analysis of the antenna electric and magnetic energy density distributions, it is shown that the degradation of the antenna performance due to the proximity of the human body can be reduced if the electric energy density is confined in specific regions far from the antenna border. The proposed design criterion has been validated by numerical simulations and experimental measurements.

A compact L-band Ortho Mode Junction

We describe the design construction and performance of a L-band (1300-1800 MHz) Ortho Mode Junction for the L-P dual-band receiver to be installed on the 64 m Sardinia Radio Telescope (SRT), a new radio telescope which is being built in Sardinia, Italy. The Ortho Mode Junction (OMJ) separates two orthogonal linearly polarized signals propagating in a 172 mm diameter circular waveguide and couple them into four coaxial outputs. The OMJ is part of an OMT (Ortho Mode Transducer), which includes two 1800 hybrids allowing to recombine the out-of-phase signals from the balanced OMJ outputs. The OMJ consists of four probes arranged in symmetrical configuration across the circular waveguide. A shaped tuning stub with cylindrical profile is placed a quarter wavelength away from the probes to guarantee broadband operation with low reflection coefficient across L-band. The four identical probes have a cylindrical structure, each consisting of three concentric cylinders that attach to the central pin of standard 50 Ω 7/16-type coaxial connectors. The OMJ will be cooled at 80 K inside a compact dewar together with directional couplers and Low Noise Amplifiers. The two linearly polarized signals from an input 190 mm diameter room temperature L-band feed couple into the cryogenic dewar through a vacuum window located across the waveguide. Inside the dewar, the 190 mm diameter circular waveguide is tapered down to 172 mm using a conical transition (length 85 mm) filled with a Styrodur® foam that provides mechanical support for a 0.125 mm thick Kapton vacuum barrier. A 0.6 mm air gap across the 172 mm circular waveguide provides thermal decoupling between the ambient temperature and the 80 K OMJ, which is connected to the conical transition output.

A wideband quadruple-ridged horn antenna for the multifeed S-band receiver of the Sardinia radio telescope

Abstract A feed horn for the multifeed S-band receiver of the Sardinia Radio Telescope (SRT) has been designed. The antenna consists of a truncated circular waveguide with four ridges of variable height inside. The horn antenna has been designed and tested using Ansys HFSS and, then, the multifeed system has been coupled with the SRT using GRASP by TICRA. The return loss of the feed horn is larger than 20 dB over a 60% bandwidth (between 2.3 and 4.3 GHz), and the edge taper and the cross-polarization level of the multifeed system meet the specifications of the SRT S-band receiver.

A multifeed S-band cryogenic receiver for the Sardinia Radio Telescope primary focus

The noise temperature of existing radio telescope receivers has actually achieved very low values. In any case, there are other practical ways to increase the observational speed of a single dish antennas without using longer integration time: observe with multi-beam and large bandwidth receiver. In this paper we present the front end and the cryogenic dewar design of the 5 beams FPA double linear polarization receiver for the primary focus of the 64 m Sardinia Radio Telescope.

Imaging of SNR IC443 and W44 with the Sardinia Radio Telescope at 1.5 and 7 GHz

Observations of supernova remnants (SNRs) are a powerful tool for investigating the later stages of stellar evolution, the properties of the ambient interstellar medium, and the physics of particle acceleration and shocks. For a fraction of SNRs, multi-wavelength coverage from radio to ultra high-energies has been provided, constraining their contributions to the production of Galactic cosmic rays. Although radio emission is the most common identifier of SNRs and a prime probe for refining models, high-resolution images at frequencies above 5 GHz are surprisingly lacking, even for bright and well-known SNRs such as IC443 and W44. In the frameworks of the Astronomical Validation and Early Science Program with the 64-m single-dish Sardinia Radio Telescope, we provided, for the first time, single-dish deep imaging at 7 GHz of the IC443 and W44 complexes coupled with spatially-resolved spectra in the 1.5-7 GHz frequency range. Our images were obtained through on-the-fly mapping techniques, providing antenna beam oversampling and resulting in accurate continuum flux density measurements. The integrated flux densities associated with IC443 are S_1.5GHz = 134 +/- 4 Jy and S_7GHz = 67 +/- 3 Jy. For W44, we measured total flux densities of S_1.5GHz = 214 +/- 6 Jy and S_7GHz = 94 +/- 4 Jy. Spectral index maps provide evidence of a wide physical parameter scatter among different SNR regions: a flat spectrum is observed from the brightest SNR regions at the shock, while steeper spectral indices (up to 0.7) are observed in fainter cooling regions, disentangling in this way different populations and spectra of radio/gamma-ray-emitting electrons in these SNRs.

The 7-beam S-band cryogenic receiver for the SRT primary focus: project status

Existing radio receivers have a very low noise temperature. To further increase the observation speed, the new generation of radio receivers use a multi-beam focal plane array (FPA) together with wide bandwidth. In this article, we present the front-end and cryogenic design of the 7-beam FPA double linear polarization receiver for the 64-m primary focus of the Sardinia Radio Telescope. At the end of this article, we show the simulated performances of the front-end receiver and the measurements of the down-conversion section.

A Multiband Proximity-Coupled-Fed Flexible Microstrip Antenna for Wireless Systems

A multiband printed microstrip antenna for wireless communications is presented. The antenna is fed by a proximity-coupled microstrip line, and it is printed on a flexible substrate. The antenna has been designed using a general-purpose 3D computer-aided design software (CAD), CST Microwave Studio, and then realized. The comparison between simulated and measured results shows that the proposed antenna can be used for wireless communications for WLAN systems, covering both the WLAN S-band (2.45 GHz) and C-band (5.2 GHz), and the Wi-Max 3.5 GHz band, with satisfactory input matching and broadside radiation pattern. Moreover, it has a compact size, is very easy to realize, and presents a discrete out-of-band rejection, without requiring the use of stop-band filters. The proposed structure can be used also as a conformal antenna, and its frequency response and radiated field are satisfactory for curvatures up to 65°.