The ngVLA Front End Reference Design
Abstract
The ngVLA will provide near-continuous frequency coverage from 1.2 - 116 GHz in multiple bands, with a gap at the atmospheric absorption band between ~50 - 70 GHz. Maximizing sensitivity for each band, while also minimizing the overall operating cost are the primary design goals. Therefore, receivers and feeds are cryogenically cooled, with multiple bands integrated into common cryostats. The proposed ngVLA Front End design consists of six separate bands, each having an integrated feed. The upper five bands (2 - 6) are co-located within a single compact cryostat, while the lowest-frequency band (1) occupies a second cryostat of similar volume and mass. The Band 1 feed is cooled to 80K, while all other feeds are cooled to 20K. For optimum performance at higher frequencies, waveguide-bandwidth (~1.66:1) receivers are proposed for Band 3 (12.3 - 20.5 GHz), Band 4 (20.5 - 34 GHz), Band 5 (30.5 - 50.5 GHz) and Band 6 (70 - 116 GHz). Excellent low noise performance is achievable, without adding undue size or weight. An axially-corrugated conical feed horn design with wide opening angle (~55degree) is adopted. Aperture efficiency is comparable to traditional corrugated feed horns, but is simpler to fabricate, and very compact in size. Simulated and measured radiation patterns from a representative horn scaled for 4-8 GHz are presented. For continuous coverage between 1.2 - 12.3 GHz, waveguide or octave-bandwidth receivers are not cost-effective, given the > 10:1 frequency range. In Band 1 (1.2 - 3.5 GHz) and Band 2 (3.5 - 12.3 GHz), wideband LNAs mated to a quad-ridge feed horn (QRFH) are used. Aperture efficiency and receiver noise temperature are not as optimum compared to the other bands: on the other hand, very significant cost savings are realized, by effectively halving the number of receivers and cryostats required per antenna. The cryostats incorporate Gifford-McMahon (G-M) cryocoolers, run at variable speed to reduce both the average running cost and mechanical wear. The helium compressor supplying the cryocoolers will also be a modern variable-speed scroll-type unit for the same reasons. This combination substantially reduces the overall operating cost of the cryogenic system.
- Publication:
-
American Astronomical Society Meeting Abstracts #233
- Pub Date:
- January 2019
- Bibcode:
- 2019AAS...23336105S