Doug Henke

Minimizing RF Performance Spikes in a Cryogenic Orthomode Transducer (OMT)

The turnstile junction exhibits very low cross-polarization leakage and is suitable for low-noise millimeter-wave receivers. For use in a cryogenic receiver, it is best if the orthomode transducer (OMT) is implemented in waveguide, contains no additional assembly features, and may be directly machined. However, machined OMTs are prone to sharp signal drop-outs that are costly to overall performance since they show up directly as spikes in receiver noise. We explore the various factors contributing to this degradation and discuss how the current design mitigates each cause. Final performance is demonstrated at cryogenic temperatures.

Component development for ALMA Band 1 (31-45 GHz)

ALMA Band 1, covering 31-45 GHz, is the lowest signal frequency band of the ALMA telescope and development of the technology to be used for the front-end cartridge is currently in a research phase. We have made progress on various key components designed for use in the ALMA Band 1 cartridge, including the orthomode transducer (OMT), low-noise amplifier (LNA), lens, and down-converting mixer. Since the layout of the ALMA cartridges within the antenna is not optimized for the lowest band, a dielectric lens is required to avoid blocking other bands. Using a lens necessitates careful characterization of the dielectric properties controlling focal length and dielectric loss. It is also important to match the index of refraction of the lens to minimize reflection while still providing equal performance for both linear polarizations and not introducing any cross-polarization effects. Different anti-reflection techniques will be shown; for example, a hole array, as an anti-reflection layer, has been used for a vacuum window and measured results are compared with simulation. A test cryostat has been constructed by adding an extension to a commercial liquid helium cryostat. Initial sensitivity measurements of a simplified prototype receiver will be given, incorporating an HDPE window, commercial conical feedhorn, 3-stage LNA, and warm amplification stage. An overview of the system losses, receiver noise budget, and system alignment tolerances will also be shown. Furthermore, there is interest in either extending or shifting the existing frequency towards 50 GHz, and the impact on each component will be considered.

The Atacama Large Millimeter/sub-millimeter Array band-1 receiver

The Atacama Large Millimeter/submillimeter Array(ALMA) Band 1 receiver covers the 35-50 GHz frequency band. Development of prototype receivers, including the key components and subsystems has been completed and two sets of prototype receivers were fully tested. We will provide an overview of the ALMA Band 1 science goals, and its requirements and design for use on the ALMA. The receiver development status will also be discussed and the infrastructure, integration, evaluation of fully-assembled band 1 receiver system will be covered. Finally, a discussion of the technical and management challenges encountered will be presented.

CryoPAF4: a cryogenic phased array feed design

Phased array feed (PAF) receivers used on radio astronomy telescopes offer the promise of increased fields of view while maintaining the superlative performance attained with traditional single pixel feeds (SPFs). However, the much higher noise temperatures of room temperature PAFs compared to cryogenically-cooled SPFs have prevented their general adoption. Here we describe a conceptual design for a cryogenically cooled 2.8 – 5.18 GHz dual linear polarization PAF with estimated receiver temperature of 11 K. The cryogenic PAF receiver will comprise a 140 element Vivaldi antenna array and low-noise amplifiers housed in a 480 mm diameter cylindrical dewar covered with a RF transparent radome. A broadband two-section coaxial feed is integrated within each metal antenna element to withstand the cryogenic environment and to provide a 50 ohm impedance for connection to the rest of the receiver. The planned digital beamformer performs digitization, frequency band selection, beam forming and array covariance matrix calibration. Coupling to a 15 m offset Gregorian dual-reflector telescope, cryoPAF4 can expect to form 18 overlapping beams increasing the field of view by a factor of ~8x compared to a single pixel receiver of equal system temperature.

Coherent Multibeam Arrays Using a Cold Aperture Stop

To increase the mapping speed of a given area-of-sky, multibeam heterodyne arrays may be used. Since typical heterodyne arrays are spatially arranged sparsely at approximately 4·Nyquist sampling (i.e., two full-width-half-maximum beam widths), many pointings are required to sample fully the area of interest. A cold aperture stop may be used to increase the packing density of the detectors, which results in a denser instantaneous spatial sampling on-sky. Combining reimaging optics with the cold stop, good aperture efficiency can be obtained. As expected, however, a significant amount of power is truncated at the stop and the surrounding baffling. We analyze the consequence of this power truncation and explore the possibility of using this layout for coherent detection as a multibeam feed. We show that for a fixed area-of-sky, a “twice-Nyquist” spatial sampling arrangement may improve the normalized point source mapping speed when the system noise temperature is dominated by background or atmospheric contribution.

Development of band-1 receiver cartridge for Atacama Large Millimeter/submillimeter Array (ALMA)

The prototype cartridges for ALMA Band-1 receivers have been developed, based on the key components developed in ALMA Band-1 consortium laboratories. The prototype cartridges for each receiver consist of two parts, cold cartridge assembly and warm cartridge assembly. The cold cartridge assembly (CCA) consists of horn antenna, orthomode transducer and a pair of 35-52 GHz cold low-noise amplifiers, the amplified signals of both polarizations are transmitted to warm cartridge assembly by long waveguide sections. In warm cartridge assembly (WCA), two major modules incorporated, down-converter assembly including warm low-noise amplifier, high-pass filter, mixer and 4-12 GHz IF amplifier, and local oscillator based on a 31-40 GHz YIG-tunes oscillator. The frequency range is based on the upper sideband scheme. Based on the measured performance of the key components, the expected noise performance of the receiver will be 26-33K.

A low-cost directional log periodic log spiral antenna

The Square Kilometer Array (SKA) derives its name from its total aperture area, predominately achieved by thousands of parabolic dishes, each with an antenna feed situated at its focal point. The type of antenna feed selected for SKA operation in the 1 GHz to 10 GHz frequency range, referred to as a Wide Band Single Pixel Feed (WB+SPF) [1], has drawn increasing attention. This paper introduces a new WB+SPF to meet SKA scientific demands while also providing a cost-effective solution. The wide bandwidth of this new antenna is achieved by employing frequency independent (FI) design principles for the development of a log periodic (LP) antenna [2]. A basic adherence to FI principles can be seen in several of the antennas currently proposed as WB+SPF solutions for the SKA, including the Allen Telescope Array (ATA) feed [3], the Quasi-Self-Complementary (QSC) feed [4], and the Eleven feed [5]. These antennas efficiently illuminate the radio dish with a single main lobe, ruling out entirely planar FI antennas, which generate two radiation lobes orthogonal to the plane of the antenna. High sensitivity requirements prohibit the 3 dB power loss that would result from attenuating one of the lobes with an absorption cavity, as is often done for directional FI antennas. Scaling the LP elements into three-dimensional space, potentially over a ground plane [6], is the present method used for directional LP antennas. We instead propose confining these often complex LP elements into a single plane, while the ground “plane” takes on a three dimensional form. Planar LP patterns permit low-cost implementations, including PCB and water jet cutting technology, without requiring the introduction of a complicated scaffolding to support the LP elements in 3-D. Additionally, various technologies such as metal spinning, can cost-effectively shape the ground “plane” into a 3-D structure. This low-cost WB+SPF solution would scale well in the implementation of the thousands of antenna feeds that the SKA demands. We also introduce a previously unreported LP design: the log periodic log spiral (LPLS) antenna.

Performance of pre-production band 1 receiver for the Atacama Large Millimeter/submillimeter Array (ALMA)

The Atacama Large Millimeter/submillimeter Array (ALMA) Band 1 receiver covers the frequency range of 35-50 GHz. An extension of up to 52 GHz is on a best-effort basis. A total of 73 units have to be built in two phases: 8 preproduction and then 65 production units. This paper reports on the assembly, testing, and performance of the preproduction Band 1 receiver. The infrastructure, integration, and evaluation of the fully-assembled Band 1 receiver system will be covered. Finally, a discussion of the technical and managerial challenges encountered for this large number of receivers will be presented.

Q-band single pixel receiver development for the ngVLA and NRC

A single feed cryogenic Q-band (35 – 50 GHz) dual-linear polarization receiver is under development at the NRC, primarily to establish the antenna performance parameters of the Dish Verification Antenna 2 at its high-frequency limit and as a possible receiver system for the National Radio Astronomy Observatory’s Next Generation Very Large Array (ngVLA). The cryostat houses a corrugated feed horn cooled to 16 K with a wide opening half-angle of 55°. The linear orthomode transducer (OMT) was redesigned to incorporate noise injection couplers and the power dividing function thus reducing the amount of components, connections, and thermal mass. The low noise (TLNA = 12 K) amplifier (LNA) was also redesigned to replace coaxial ports with WR-22 waveguide ports. The specifications, receiver design, measured farfield feed horn beam patterns from a near-field planar scanner, simulated OMT results, and sub-20 K receiver noise analysis is presented, along with future plans for production and installation.

2SB and balanced receiver architectures using hole couplers

The ALMA telescope has now entered cycle 4 operations [1] and as part of a research study proposal [2], we are exploring possible improvements to the ALMA Band 3 receiver cartridge. The Band 3 receiver is capable of detecting both linear polarisations which are discriminated using a waveguide orthomode transducer (OMT). Within each polarisation, both the upper and lower sidebands are separated out using a sideband-separating (2SB) layout. We report on the progress of two new prototype designs that use an integrated OMT with hole couplers to give: (1) a dual-linear sideband-separating architecture, and, (2) a balanced, sideband-separating architecture.