The Measurement of Thermal Radiation at Microwave Frequencies

TLDR

Johnson noise is linked to blackbody radiation through a simple thermodynamic model. The paper aims to describe a microwave radiometer and its operational theory. The radiometer is calibrated with a variable temperature resistive load, and its theory of operation is explained. Measured rms fluctuation of 0.4 °C matches the theoretical 0.46 °C, yielding a minimum detectable power of 10⁻¹⁶ W.

Abstract

The connection between Johnson noise and blackbody radiation is discussed, using a simple thermodynamic model. A microwave radiometer is described together with its theory of operation. The experimentally measured root mean square fluctuation of the output meter of a microwave radiometer (0.4°C) compares favorably with a theoretical value of 0.46°C. With an r-f band width of 16 mc/sec., the 0.4°C corresponds to a minimum detectable power of 10−16 watt. The method of calibrating using a variable temperature resistive load is described.