Characterization of a Thermal Isolation Section of a Waveguide Microcalorimeter

The aim of this study is to examine the characterization of a thermal isolation section (TIS) for a waveguide microcalorimeter, used to characterize the effective efficiency of a thermistor power sensor (TPS). The power loss in the TIS has been analyzed for both the dielectric and conductor losses. Its effect on the thermopile output has been assessed using a foil short method through analysis of the heating ratio. This method involves a one-off measurement of the microcalorimeter system with the foil short before the unknown power sensor measurement and does not require additional S-parameters measurements of the isolation section. The estimated value of the heating ratio effect has been obtained between 1 for a fully reflected signal from the input of the unknown power sensor and 2 for a perfectly matched power sensor. The full analytical model and an estimated model for the heating ratios have been calculated for the National Physical Laboratory (NPL)'s WG25 (WR15) microcalorimeter and a commercial TPS. The analytical model has been applied to an effective efficiency measurement, and good agreement has been obtained when compared with the existing methodology used at NPL. This model can be applied to any metallic waveguide-type TIS in other bands. A rigorous uncertainty analysis of the analytical model for the heating ratio is also presented and shows an expanded uncertainty between 0.008 and 0.023 ( k = 2) for this microcalorimeter.