Advanced Techniques for VNA Characterization of Millimetre-Wave Orthomode Transducers (OMTs)

We report on advanced techniques for the characterization of millimeter-wave orthomode transducers (OMTs). These techniques include standard mm-wave frequency-domain VNA (Vector Network Analyser) measurement and time-domain methods, which can be applied to remove the effects of the waveguide transitions necessary to access the OMT ports and excite the desired modes. First, we present the main parameters of the OMT by defining its general S-matrix and discuss the different methods that allow characterizing the insertion loss, the reflection, the cross-polarization and the isolation of the device. Advantages and disadvantages of each method are presented and compared between them. We provide a list of waveguide components necessary in the various OMT test setups (adapters, loads, quarter-wave and longer waveguide sections, feed-horn, etc.). Different OMT configurations, with distinct orientation of the waveguide input and outputs are discussed. Alternative characterization methods of the OMT parameters are presented and the associated setups discussed. Although the presented techniques refer to the characterization of a specific configuration of a W-band (3 mm wavelength) OMT, the described method can be applied to other OMT configurations and frequency ranges (from microwave to THz frequencies), therefore having a general validity

Double Ridged 180° Hybrid Power Divider With Integrated Band Pass Filter

We describe the design, construction and performance of a 180 hybrid power divider with integrated band pass filter for L-band (1.3–1.8 GHz). The hybrid is based on a cavity and two novel broadband coaxial-to-double ridged waveguide transitions. The design was optimized using a commercial 3-D electromagnetic simulator. The device was tested at room and cryogenic temperature. At the physical temperature of 77Kthe measured input reflection was less than -17 dB, the maximum deviation from the ideal -3dB coupling and 180° phase difference were respectively +/- 0.25 dB and +/- 0.9°. The experimental results are in good agreement with simulation

A waveguide cavity 180° hybrid coupler with coaxial ports

We describe the design, construction, and performance of a 180° hybrid coupler for the frequency band 1.3-1.8 GHz (L-band). The hybrid utilizes a WR650 rectangular waveguide cavity and has three 50 Ω, coaxial ports. The signals are coupled in and out of the cavity through broadband coaxial probes attached to 7/16-type connectors. The measured performances are in excellent agreement with the electromagnetic simulations. © 2009 Wiley Periodicals, Inc

A simple K-band waveguide-to-microstrip probe transition

We describe the design, construction, and test of a simple K-band waveguide-to-microstrip transition based on a radial-shaped probe patterned on a quartz substrate. From 16 to 28 GHz, the measured reflection coefficient at the coaxial SMA input, used for test purposes of the adapter, was less than -14 dB, and the dissipative loss was about 0.30 dB. The estimated loss of the waveguide to microstrip transition only, with coaxial connector effects removed, is of about 0.08 dB

Progetto di sistema per ricevitori e back-end a srt

Si descrive l'architettura di gestione dei ricevitori, attuali e in costruzione, del radio telescopio della Sardegna (SRT) e il loro interfacciamento con tutti i back-end attuali e futuri. Vengono discusse le problematiche insite in questa nuova architettura e le possibili configurazioni, indicando anche quella scelta e i motivi della decisione

Electronically Tuned Local Oscillators for the NOEMA Interferometer

We present an overview of the electronically tuned local oscillator (LO) system developed at the Institut de RadioAstronomie millimetrique (IRAM) for the superconductor-insulator-superconductor (SIS) receivers of the NOrthern Extended Millimeter Array interferometer (NOEMA). We modified the frequency bands and extended the bandwidths of the LO designs developed by the National Radio Astronomy Observatory (NRAO) for the Atacama Large Millimeter Array (ALMA) project to cover the four NOEMA LO frequency ranges 82-108.3 GHz (Band 1), 138.6-171.3 GHz (Band 2), 207.7-264.4 GHz (Band 3), and 283-365 GHz (Band 4). The NOEMA LO system employs commercially available MMICs and GaAs millimeter MMICs from NRAO which are micro-assembled into active multiplied chain (AMC) and power amplifier (PA) modules. We discuss the problem of the LO spurious harmonics and of the LO signal directly multiplied by the SIS mixers that add extra noise and lead to detections of unwanted spectral lines from higher order sidebands. A waveguide filter in the LO path is used to reduce the higher order harmonics level of the LO at the output of the final frequency multiplier, thus mitigating the undesired effects and improving the system noise temperature

Optical design of S-band multifeed for the Sardinia Radio Telescope primary focus

We present the optical design of an S-band seven feed cryogenic radio astronomy receiver for illuminating the 64-m diameter Sardinia Radio Telescope (SRT) dish from the primary focus. The feeds are arranged in a compact hexagonal configuration with a central one and are cryogenically cooled at 20 K inside a cryostat. Each feed accepts two linear polarizations and use a circular waveguide with a single outer corrugated section to achieve a nearly constant beamwidth and low cross polarization across the 3.0-4.5 GHz band. The simulated radiation pattern of the SRT telescope is obtained by coupling the array of feed-horn beam patterns (optimized with the electromagnetic software CST) with the 64-m parabolic dish (through a physical optics analysis carried out with GRASP). We compare the simulated beam pattern of an isolated feed with those of the same feed embedded in the dense array and analyze the effects of an absorber located inside the cryostat around the cryogenic feeds. We found that the absorber improves the overall system performance by decreasing the cross-coupling effects between the feeds while adding negligible noise to the system

Estimates of system noise temperature in W-band at SRT and effects of beam truncation due to the Gregorian radome

We report on system noise temperature estimates from the SRT site in W-band (70-116 GHz) based on recorded atmospheric data at four specific days, three-months apart, in the period 15 Oct. 2019-15 July 2020. The estimates are based on the atmospheric model described in [1] and on the feed-horn beams model and receiver noise specification of the W-band multibeam receiver being built for the Gregorian focus of the SRT. We used different values for the antenna forward efficiency ηf to estimate the impact of such parameter on overall system noise Tsys. The beam truncation due to the 1 m diameter SRT Gregorian focus radome, which protects the receiver cabin from the atmospheric agents, is evaluated assuming that the cabin surrounding the radome contributes with a 293 K thermal noise (greater than the sky noise). The beams from the W-band receiver will be slightly truncated by the radome, especially at the lowest frequencies, where the beams are larger. We conclude that the effects of truncation on Tsys are negligible even in the scenario of lowest thermal emission from the sky, expected during winter season. The estimated seasonal variations of the atmospheric conditions at the SRT show that, for the four specific days for which the system noise temperature was calculated, the Tsys have broad minima near the 3 mm band atmospheric window, around 85-105 GHz and achieves values of order 100 K or lower during winter. The system noise increases towards the W-band receiver band edges at 70 GHz and 116 GHz; the highest Tsys is obtained at the highest frequency of the band, 116 GHz, and reaches values beyond 200 K the 15th of October, 2019 and the 15th of July, 2020

Design of Active Waweguide OMT for Radio Astronomy Receiver Array in the 3 MM Band

We describe the design of an integrated cryogenic receiver module based on an “active” waveguide Orthomode Transducer (OMT) for dual-polarization radio astronomy observations across 75-116 GHz (3-mm band). The receiver module consists of passive and active sections that can be incorporated in a very compact mechanical assembly suitable for integration in a focal plane array. The passive section of the receiver module employs a broadband backward-coupler waveguide OMT while the active section consists of ultra-low noise MMIC (Monolithic Microwave Integrated Circuit) amplifiers

Misure di temperatura di rumore con diversi materiali, sulla copertura del Gregoriano di SRT

Lo scopo di questo lavoro è quello di valutare l’incremento della temperatura di rumore nei ricevitori banda K (18 - 26 GHz) e banda C (5.5 – 7.7 GHz), posizionati, rispettivamente, nel fuoco gregoriano e nella Beam waveguide del Sardinia Radio Telescope (SRT), in seguito all’inserimento di una nuova protezione come copertura della Vertex room. La necessità di tale studio è dovuta alle criticità presentate dall’attuale materiale usato come protezione della stanza del fuoco gregoriano del radio telescopio. Il materiale di base attualmente in uso è lo Styrodur 3035CS, un polistirene espanso estruso che ha un bassissima attenuazione, come dimostrato dal suo notevole utilizzo nelle finestre dei diversi ricevitori sviluppati in questi anni all’INAF, ma con l’inconveniente di peggiorare le sue prestazioni col tempo a causa dei danni procurati dagli agenti atmosferici (quali raggi UV, pioggia, ghiaccio). Al fine di ridurre l’usura dello Styrodur, abbiamo testato l’utilizzo di diversi elementi protettivi superficiali come vernici, prodotti idrorepellenti e teli di Goretex e polietilene, stimando l’effetto di queste protezioni sulla temperatura di rumore del sistema. Per tale verifica è stata misurata la variazione della temperatura di sistema Tsys del radiotelescopio al variare del tipo di protezione utilizzato