Detection of microwave radiation from a pseudo blackbody
Due to the randomness and weakness of the microwave signals, usually it is considered to be difficult to detect microwave radiation from a pseudo blackbody. In this dissertation, new methods of detecting these faint and random microwave signals are discovered. A Dicke type receiver has been designed and constructed in the effort to detect 22 GHz spontaneous emission from water vapor. The receiver compares the brightness temperatures of two waveguides, one containing gaseous H$\sb2$O at low pressure as the source channel, the other is evacuated as the reference. Each waveguide is terminated with a polished brass plate, which provides a low background brightness temperature, at one end and is connected to the input of the receiver at the other end. The system is capable of detecting a brightness temperature difference as small as 0.2$\sp\circ$K. In the experiment, the radiation from water vapor produces the brightness temperature difference between source and reference channels of 0.41$\sp\circ$K with H$\sb2$O pressure in the source chamber at 18 mmHg and at room temperature. The experimental results prove that the emission from water vapor is being detected. In this dissertation, the input noise to the receiver is analyzed by the radiative transfer equation model and lossy transmission line model. For the first time these models are found to be identical under matched impedance conditions. A theory to analyze the input noise of the receiver in an impedance mismatched system is also developed. A new technique named "threshold pulse counting" is developed during this study. The experimental results show that this technique improves the sensitivity of a heterodyne receiver up to 35 dB. With some modification, this technique is also used to detect the emission from water vapor. The experimental results agree with the results obtained by Dicke type receiver. The main advantage of the pulse counting technique over the Dicke type receiver is that one may choose the integration time to suit the needs of the experiment.
Shuming Tong Wang,
"Detection of microwave radiation from a pseudo blackbody"
(January 1, 1992).
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