[Paper Review] Design and implementation of a noise temperature measurement system for the Hydrogen Intensity and Real-time Analysis eXperiment (HIRAX)

Emily Kuhn, B. R. Saliwanchik+25 more|arXiv (Cornell University)|Jan 16, 2021

Radio Astronomy Observations and Technology19 references4 citations

TL;DR

This paper presents a Y-factor-based noise temperature measurement system using two temperature-controlled RF absorber-filled cavities (300 K and 77 K) to characterize the noise performance of HIRAX's embedded low-noise amplifier feeds. The system enables precise lab measurements of feed noise temperature critical for detecting faint 21 cm cosmological signals, with verification showing systematic errors manageable through shielding and upgraded data acquisition.

ABSTRACT

This paper describes the design, implementation, and verification of a test-bed for determining the noise temperature of radio antennas operating between 400-800MHz. The requirements for this test-bed were driven by the HIRAX experiment, which uses antennas with embedded amplification, making system noise characterization difficult in the laboratory. The test-bed consists of two large cylindrical cavities, each containing radio-frequency (RF) absorber held at different temperatures (300K and 77K), allowing a measurement of system noise temperature through the well-known 'Y-factor' method. The apparatus has been constructed at Yale, and over the course of the past year has undergone detailed verification measurements. To date, three preliminary noise temperature measurement sets have been conducted using the system, putting us on track to make the first noise temperature measurements of the HIRAX feed and perform the first analysis of feed repeatability.

Motivation & Objective

• To develop a laboratory system capable of measuring the noise temperature of HIRAX's embedded low-noise amplifier (LNA) feeds, which cannot be measured using conventional methods due to the amplifier's integration into the feed.
• To verify the system's accuracy and stability through controlled measurements using passive HIRAX feeds and commercial amplifiers with known noise temperatures.
• To identify and mitigate systematic errors—particularly from RFI and backlobe coupling—that could bias noise temperature measurements.
• To establish a repeatable, traceable method for verifying feed noise performance prior to deployment in the HIRAX array.
• To enable future characterization of production feeds and next-generation feed designs for consistency and compliance with the <50 K system noise target.

Proposed method

• The system uses two large cylindrical cavities lined with RF absorber, one maintained at 300 K (room temperature) and the other at 77 K (liquid nitrogen temperature), to enable Y-factor measurements.
• The Y-factor method is applied by measuring the system's output power with the input connected to the cold and hot cavity, respectively, to calculate the system noise temperature.
• The apparatus was designed in CST Microwave Studio and constructed at Yale, including a cryogenic cavity capable of safely holding over 550 L of liquid nitrogen.
• Verification measurements used both passive HIRAX feeds and commercial amplifiers to assess return loss, spectral response, and systematic errors in the measurement chain.
• To address RFI contamination, the system is being upgraded with a Faraday cage and integration with the HIRAX ICE board for high-speed, low-noise data acquisition.
• Backlobe contributions are modeled and corrected for, with simulations indicating that backlobes >1.5% of the total beam can add >5 K to the noise temperature, requiring < -17 dB backlobe suppression.

Experimental results

Research questions

• RQ1 Can a Y-factor measurement system with cryogenically controlled cavities accurately measure the noise temperature of HIRAX's embedded LNA feeds in the lab?
• RQ2 What are the dominant sources of systematic error in the measurement system, and can they be identified and mitigated?
• RQ3 How does RFI contamination affect the noise temperature measurement, and what mitigation strategies are effective?
• RQ4 To what extent do backlobe responses contribute to the measured noise temperature, and how can this be corrected?
• RQ5 Can the system reliably verify the noise temperature of commercial amplifiers with known specifications, thereby validating its accuracy?

Key findings

• The measurement system has been successfully constructed and verified, with initial tests showing it operates within required tolerances for noise temperature measurement.
• Verification measurements identified RFI as a significant contaminant, with rapid spikes potentially biasing results if not captured by high-bandwidth acquisition.
• The use of the HIRAX ICE board for data acquisition is expected to improve detection of transient RFI and reduce noise floor compared to handheld spectrum analyzers.
• Backlobe contributions were found to be a critical source of error, with simulations indicating that feeds with backlobes >1.5% of the main beam can add >5 K to the measured noise temperature.
• The system is expected to measure the HIRAX feed noise temperature with an accuracy sufficient to verify compliance with the <50 K system noise target, including a predicted 10–20 K contribution from feed loss.
• The system will be used to verify the noise temperature of all 256 feeds in the initial HIRAX deployment, enabling quality control and consistency checks.