  The redshifted 21 cm line signal is a powerful probe of the cosmic dawn and
the epoch of reionization. The global spectrum can potentially be detected with
a single antenna and spectrometer. However, this measurement requires an
extremely accurate calibration of the instrument to facilitate the separation
of the 21 cm signal from the much brighter foregrounds and possible variations
in the instrument response. Understanding how the measurement errors propagate
in a realistic instrument system and affect system calibration is the focus of
this work. We simulate a 21 cm global spectrum observation based on the noise
wave calibration scheme. We focus on how measurement errors in reflection
coefficients affect the noise temperature and how typical errors impact the
recovery of the 21 cm signal, especially in the frequency domain. Results show
that for our example set up, a typical vector network analyzer (VNA)
measurement error in the magnitude of the reflection coefficients of the
antenna, receiver, and open cable, which are 0.001, 0.001, and 0.002 (linear),
respectively, would result in a 200 mK deviation on the detected signal, and a
typical measurement error of 0.48 degree, 0.78 degree, or 0.15 degree in the
respective phases would cause a 40 mK deviation. The VNA measurement error can
greatly affect the result of a 21 cm global spectrum experiment using this
calibration technique, and such a feature could be mistaken for or be combined
with the 21 cm signal
