Accuracy of Bolometric Power Measurements

When the RF power distribution along a bolometer wire differs from the low-frequency power distribution, the substitution method of measurement may give an error unless certain special conditions are satisfied. These conditions are most closely fulfilled, in practice, by a convectively cooled wire whose length to diameter ratio is very large. The possible error for the case of a Wollaston wire mounted in air at atmospheric pressure is analyzed and compared with that obtained with wires mounted in vacuo. It is shown that the air-mounted Wollaston wire is subject to a smaller error than are the evacuated units and that this advantage increases as the wire length becomes an appreciable fraction of a wavelength. It is concluded that Wollaston wire bolometers, when properly designed and mounted, can be used to measure cw power over a frequency range extending into the millimeter wavelength region with an accuracy approaching that of low-frequency measurements.