Reflections and Standing Waves on the Tianlai Cylinder Array

TL;DR

This paper analyzes the standing waves and reflections in the Tianlai Cylinder Array's bandpass, revealing their impact on 21cm intensity mapping and proposing methods to mitigate their effects for better calibration.

Contribution

It identifies specific sources of standing waves in the Tianlai array and demonstrates how to partially recover a smoother frequency response by removing reflection-induced modulations.

Findings

Identified a 142 ns delay standing wave from the feed cable.

Detected a shorter delay peak likely from reflector-feed and IF cable reflections.

Showed that removing reflection modulations can improve frequency response smoothness.

Abstract

In 21~cm intensity mapping, the spectral smoothness of the foreground is exploited to separate it from the much weaker 21~cm signal. However, the non-smooth frequency response of the instrument complicates this process. Reflections and standing waves generate modulations on the frequency response. Here we report the analysis of the standing waves in the bandpass of the signal channels of the Tianlai Cylinder Array. By Fourier transforming the bandpass into the delay time domain, we find various standing waves generated on the telescope. A standing wave with time delay at about 142 ns is most clearly identified which is produced in the 15 meter feed cable. We also find a strong peak at a shorter delay of $\tau < 50 \ns$, which may be a mix of the standing wave between the reflector and feed, and the standing wave on the 4 m intermediate frequency (IF) cable. We also show that a smoother frequency response could be partially recovered by removing the reflection-inducted modulations. However, the standing wave on the antenna is direction-dependent, which poses a more difficult challenge for high precision calibration.