Attenuation of a horn antenna for thermal noise measurement

A cryogenic noise reference standard consists of an antenna, a cavity, an electromagnetic absorber, and a liquid nitrogen container. The horn antenna is installed at the upper part of the cavity to measure the thermal noise radiated from the absorber carefully placed at the bottom of the cavity. By filling liquid nitrogen to an appropriate level the electromagnetic absorber is maintained at approximately 77 K. In this paper, some design considerations are discussed to minimize the diffraction from the internal surface of the horn antenna. Using the resultant dimension of the horn, the attenuation of the overall horn antenna is evaluated to be 0.033 dB to 0.037 dB in the frequency range of 75 GHz to 110 GHz. The calculated attenuation with other relevant parameters is used to obtain the calculable output noise temperature of the cryogenic noise reference standard.