Recent developments in RF and microwave dielectric measurements at the UK National Physical Laboratory

Microwave dielectric measurements commenced at NPL in the late 1960s. Instruments developed under that programme-TE/sub 01/-mode cavities and open resonators-remain in use today because they provide the most sensitive means of measuring very low dielectric losses in the frequency range 10-144 GHz. More recently, in the mid-1980s, a programme of very high loss materials measurements commenced which was originally concerned with biomedical applications but which has been extended to the characterization of foodstuffs, various polar dielectric liquids and radio absorbing material (RAM). A major concern in all of this work has been the requirement to ensure that measurements are demonstrably traceable to national standards: usually impedance, attenuation or dimensional standards are the most relevant. Another important aspect of the work is the estimation and reduction of measurement uncertainties. In pursuit of these aims the laboratory has always sought to take part in measurement intercomparisons both at local and international levels. NPL is the pilot laboratory for a BIPM intercomparison of complex permittivities of polar reference liquids. A third aspect of the work is to extend existing techniques to enable them to perform measurements upon new types of sample or to cover different ranges of complex permittivity. This can be a very cost-effective way of expending measurement capability since it usually does not involve gross expenditure on new instrumentation. These aspects of the work are illustrated in this review by reference to three recent dielectrics projects undertaken at NPL.<>