精密宇宙学中反射系数测量的新技术
A novel technique for reflection coefficient measurement in precision cosmology
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中文总结 AI 辅助
本研究开发了新型现场矢量网络分析仪，采用噪声源校准与互相关频谱仪技术实现天线复反射系数的高精度测量，验证其性能并证明可用于精密21厘米宇宙学观测。
中文摘要 AI 辅助
从宇宙黎明和再电离时期探测全球21厘米信号仍是全球实验面临的挑战。该早期宇宙信号在静止帧频率1420.405MHz处发射，经红移后变为40-200MHz，最大亮温度为几百mK。探测这种天球平均信号的实验包括成形天线测量背景射电频谱（SARAS）和利用氢信号探测宇宙再电离（PRATUSH）等。探测该微弱信号需要对天线进行精确校准，包括其反射系数的高精度测量。由于天线特性随环境显著变化，该测量必须在观测现场（in situ）进行。天基辐射计PRATUSH还面临热循环引起的结构畸变这一额外挑战，因此需要多次测量反射系数。本研究重点开发了一种现场矢量网络分析仪，该分析仪采用新型噪声源校准方案和互相关频谱仪，以测量天线复反射系数的幅度和相位。此外，我们利用精密网络分析仪的独立测量值和被测器件的反射系数建模来验证所设计网络分析仪的性能。研究发现，对于21厘米信号探测需要严格控制的非平滑校准系统误差水平约为10^-5。最后，我们研究了反射系数校正对天空测量的影响，突出了其用于精密21厘米观测的可用性。
英文摘要
The detection of the global 21-cm signal from the Cosmic Dawn and Epoch of Reionisation remains a challenge to experiments worldwide. Emitted at a rest-frame frequency of 1420.405~MHz, this signal from the early Universe is redshifted to 40-200~MHz with a maximum brightness temperature of a few 100~mK. Efforts to detect this sky-averaged signal include experiments such as the Shaped Antenna measurement of the background RAdio Spectrum (SARAS) and Probing ReionizATion of the Universe using Signal from Hydrogen (PRATUSH). Detecting this faint signal requires precise calibration of the antenna, which includes a high-precision measurement of its reflection coefficient. This measurement must be performed \textit{in situ} at the observation site, as the antenna characteristics vary significantly with the environment. PRATUSH, a space-based radiometer, faces the additional challenge of structural distortions induced by thermal cycling, necessitating multiple measurements of the reflection coefficient. This work highlights the development of an \textit{in situ} Vector Network Analyser, which utilises a novel noise source-based calibration scheme and a cross-correlation spectrometer to perform magnitude and phase measurements of the complex reflection coefficient of the antenna. Further, we demonstrate the performance of the designed network analyser using independent measurements from a precision network analyser and reflection coefficient modelling of the device under test. We find the level of non-smooth calibration systematics, which need critical control for 21-cm signal detection, to be about $10^{-5}$. Finally, we study the impact of reflection coefficient correction on sky measurements, highlighting its usability for precision 21-cm observations.
