The characterisation of the radiometric temperature of large area thermal sources by heterodyne and Fourier transform radiometry

In fusion plasma research using tokamak machines an important diagnostic of the state of the plasma is provided by measuring the temporal evolution of the spectral dependence of the electron cyclotron emission (ECE). However, this requires a calibration source with both a known radiometric temperature spectrum and a sufficiently large area that it can fill the field of view of the input antenna. The authors describe and compare two different methods of producing a radiometric calibration of such a source. The first method is based on conventional Dicke-type heterodyne radiometry for the frequency region from 125 to 153 GHz, while the second uses Fourier transform radiometry based upon two-beam interferometry for the frequency range from 70 to 600 GHz. The NPL large area thermal calibration source combines robustness, high radiometric temperature (700 K), large active area and portability