WR-5 Antenna Design for the Horn-type Noise Standard

The horn-type noise standard is an important kind of the thermal noise source. The noise signal generated by the electromagnetic (EM) absorber maintained at constant temperature is picked up by a horn antenna. Usually the absorber has good absorption characteristics in a wide frequency range, so this kind of noise source can be applied to different frequency bands by replacing the pickup antenna. The antenna design of the WR-5 standard cryogenic noise source whose absorber immersed in liquid nitrogen is discussed here. Through the optimized design, the return loss of the antenna is better than 30 dB in full band. The attenuation of the antenna is estimated to be 0.081 dB to 0.091 dB between 140 GHz and 220 GHz, resulting in an excess noise temperature of 4.1 K to 4.6 K. The output noise temperature and uncertainty of the WR-5 standard cryogenic noise source were evaluated based on the antenna efficiency. The uncertainty of the output noise temperature of this noise source is about 1.5% over the full band.