Shenzhe Xu

The 21 cm global spectrum provides an excellent window to observe the evolution of the early Universe. A high impedance receiver can be used in the global spectrum experiment, which offers a nearly uniform response over a large relative bandwidth, thus breaking the limitation imposed by the impedance matching between the antenna and receiver. This paper analyzes the measurement precision of the high-impedance receiver in the global spectrum experiment. We consider the main systematic errors: the vector network analyzer measurement error, the temperature measurement error, and the gain stability error. We use a Monte Carlo simulation to generate 10,000 sets of error combinations, and assess their impacts on observational results. For our experiment setup, we find that the level of systematic error is about 35 mK. This result is applicable to the Hongmeng Project (also known as the Discovering Sky at the Longest wavelength or DSL project), which will deploy an array of satellites on the cislunar orbit by a single rocket launch, to make low frequency imaging and global spectrum measurements. One of the satellites is dedicated to the 21 cm global spectrum observation in the 30–120 MHz frequency range, which employs a multi-receiver design scheme, to minimize systematic effects by cross-comparison of different receiving channels. The high-impedance channel is one to be used. Thus, within an acceptable range of systematic errors, the high-impedance receiver can provide a good measurement of the 21 cm signal, even if the 21 cm signal feature spans a broad frequency range.

The Tianlai Cylinder Pathfinder Array consists of three adjacent cylindrical reflectors fixed on the ground, each 40 m long and 15 m wide, with the cylinder axis oriented along the North–South (N–S) direction. Dual linear polarization feeds are distributed along the focus line, parallel to the cylinder axis. Measurement of the primary beam profile of these cylindrical reflectors is difficult, as they are too large to be placed in an anechoic chamber. While the beam profile along the East–West (E–W) direction can be measured with the transit observations of bright astronomical radio sources, the beam profile along the N–S direction remains very uncertain. We present a preliminary measurement of the beam profile of the Tianlai cylindrical antenna along both the N–S direction and E–W direction in the frequency range of 700–800 MHz, using a calibrator source carried by an unmanned aerial vehicle (UAV) flying in the far field. The beam profile of the Tianlai cylindrical antenna is determined from the analysis of the auto-correlation signals from the cylinder array correlator, taking into account the emitter antenna beam profile, itself measured with a dipole antenna on the ground. The accuracy of the UAV-based determination of the cylinder beam profiles is validated by comparing the results with the one derived from bright astronomical source transits, and with simulated beams.