Electromagnetic radiation due to thermal agitation in a D-band cavity

At resonant frequencies in a microwave cavity, electromagnetic radiation is induced due to thermal agitation. We investigated the TM 010 mode of a D-band cavity at room and liquid nitrogen temperatures. We assumed that the power spectrum is Lorenzian and that the integral of the spectrum is equal to the expectation energy of a single degree of freedom. Using the coupling constants, measured with a network analyzer, and the Johnson-Nyquist noise as a reference power level, we evaluated the absolute value of the radiation at room temperature. The measured peak value was almost half the expected value. The disagreement can partly be explained in terms of an equivalent temperature which can be different from the physical temperature. This experiment was made possible through the adoption of a cooled high-electron-mobility-transistor (HEMT) amplifier and parallel band-pass filters.