Reflections And Standing Waves On The Tianlai Cylinder Array

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 similar to 142 ns is most clearly identified which is produced in the 15-meter feed cable. We also identify 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.