Improvement of the WR-5 Calorimeter at NMIJ Using a Novel Adiabatic Waveguide

In recent years, waveguide-type calorimeters have been developed for higher frequency ranges. The National Metrology Institute of Japan (NMIJ) has also developed a waveguide calorimeter capable of operating up to 330 GHz. As stated in previous work [1], it is crucial to take into account the effect of heat generation caused by transmission losses in adiabatic waveguides (AW) on measurement precision. Moreover, it should be noted that the isothermal control method at room temperature has certain limitations that have resulted in a worst-case relative expanded uncertainty of 12.7 % (k = 2). The purpose of this paper is to enhance the measurement performance of calorimeters by improving AW transmission loss. To achieve this goal, the newly designed AW structure, transmission characteristics, and adiabatic performance analysis will be evaluated. The low frequency region demonstrates improvements in the uncertainty of the WR-5 (from 140 GHz to 220 GHz) band calorimeter.