Instrumentation for faint microwave line emission measurement

It is difficult to detect faint microwave line emission from a low-temperature source. In this paper, cost-effective methods of detecting these emissions are presented. A Dicke-type receiver (DTR) with an analog lock-in amplifier has been constructed to detect 22 GHz spontaneous emission from water vapor. The receiver compares the brightness temperatures of two waveguide chambers, one containing water vapor and the other empty. The system is capable of detecting a brightness temperature difference as small as 0.2 K. The experimental results prove that the emission from water vapor is detected for the first time with water vapor at 18 mm Hg and room temperature. The observed brightness temperature difference is 0.41 K. A "sampling digital" integration technique (SDIT) is also used to detect the emission from water vapor. The experimental results agree with the results obtained with the analog lock-in amplifier. The main advantage of SDIT over the DTR is the flexibility on the integration time and the range. In both methods, cost-effective brass plates are used for the low-temperature backgrounds for the first time.