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A new vacuum melting extraction and casting (v.m.e.c.) process for polyolefins has resulted in exceptionally low dielectric losses in the range 10ߝ3000 GHz. At 12.5 GHz, losses in polypropylene have been found to be usually below 30 μ rad and in a few cases as low as 5 μ rad. Measuring techniques have been refined to cope with these low values. Loss angles at 13 and 28 GHz can be measured in waveguide bridges to a precision of ± 2 μ rad. Submillimetre spectra can be obtained by Fourier-transform spectroscopy with a reproducibility of better than 0.2% in the range 2ߝ200 cm-1 (60ߝ6000 GHz).