  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.
