RF & Microwave Dielectric Research
Our current RF and microwave dielectrics research at NPL focuses on three areas:
- Small scale probing of dielectric properties
- Measurements on materials to support RF and microwave industrial processing
- Measurements on liquids to support RF exposure health and safety research and Specific Absorption Rate (SAR) standards
In all three areas, NPL is working closely with scientific collaborators to ensure that maximum benefit can be obtained from combined experience and expertise.
Small scale probing of dielectric properties
In many fields of science and technology, for reasons of speed, lightness and compactness, technological developments are taking place on ever-smaller scales. There are therefore clear requirements for dielectric metrology to pursue techniques for small-scale determination of dielectric properties – this is an important requirement for the development of devices based upon functional materials. ‘Small scale’ here refers to techniques with spatial resolutions from 100 mm down to the nano-scale. Applications outside the electronics sector for measurements at these scales have also been identified in mineral extraction and in RF microdosimetry for health and safety studies on the effects of RF exposure.
NPL is currently undertaking a project in this field, in collaboration with Imperial College and Nottingham University, which is concerned with the probing of dielectric properties on a scale of 10 µm – 100 µm, though there is an intention to take this approach down to much smaller scales. A number of researchers have developed resonant microwave probes – they are referred to typically as Near-Field Scanning Microwave Microscopes (NSMMs), or Evanescent Microwave Probes (EMPs) – and it is this class of probe that the project is focusing on. Its three aims are first to develop techniques for making EMPs fully quantitative and traceable, secondly, to perform sensitivity analyses that will allow their performance to be optimised and thirdly, to delimit the measurement ranges (e.g. of permittivity and loss) that the technique is capable of. The intention is that the measurement of microwave dielectric properties will contribute to the developing range of physical parameters that NPL is capable of probing down to the nano-scale.
Measurements to support RF and microwave industrial processing
RF processing and microwave processing are growing industrial fields. They have long been employed for food preparation, but are increasingly being used or investigated for use in other fields: for example: microwave sintering of ceramics, mineral extraction (see below) and pharmaceutical production via the technique of microwave chemistry. The efficiency of these methods can be greatly improved by employing on-line measurement of key parameters in a feedback system to control the process, for example, by varying the heating rate. In most cases where RF or microwave heating are in use, it is the dielectric properties of the processed materials – most specifically their dielectric losses – that are the key parameters that govern the power absorption from the electromagnetic field and therefore the rate of heating. In virtually all materials dielectric loss is strongly dependent on temperature, therefore on-line monitoring of this parameter is needed if processing is to be optimised and wherever undesirable outcomes such as thermal runaway are likely to occur.
NPL is currently collaborating with the UK National Centre for Industrial Microwave Processing (NCIMP) at Nottingham University to develop dielectric measurement tools for such processing. A major challenge is that many of the processes of interest involve extreme measurement conditions – both high temperatures and pressures. The collaboration combines NPL’s experience in metrology with Nottingham’s processing experience to achieve new solutions for process control. One example of this is the use of small-scale dielectric probing (see above) to characterize multiphase minerals in order to improve the efficiency of microwave mineral extraction.
Measurements to Support RF Exposure Research and Specific Absorption Rate (SAR) standards
The use of human-based mobile communications systems (e.g. mobile phones) and home-based systems like Wi-Fi is expanding enormously and this has led to widespread public concern over health & safety implications of ever increasing human exposure to RF and microwave electromagnetic fields. A parameter of major importance in the quantification of human exposure is the Specific Absorption Rate (SAR) of power from electromagnetic (EM) signals into human tissues – a quantity measured in watts per kilogram. Traceable dielectric measurements are an essential part of SAR metrology as it is effectively focused on the interaction of EM fields with biological dielectrics, i.e. people! Standard methods for measuring SAR from broadcasting devices like mobile phones make use of tissue-equivalent liquids, which have similar dielectric properties to human tissues. NPL already has established national standards for SAR in the 300 MHz – 6 GHz frequency range, and these are based on our capability for determining the complex permittivity of these liquids. Recent collaborative research has covered the development of low toxicity liquids for these purposes, but research is now focusing on development of SAR standards and suitable liquids and measurement techniques down to 10 MHz. This will permit traceable RF exposure metrology for RF processing and Magnetic Resonance Imaging (MRI) to be established.
In recent years, NPL has played a major role in this field by supporting the UK government’s main research programme on Mobile Telecommunications Health Research (MTHR), working with many UK research to ensure that the RF and microwave measurements undertaken in their laboratories could be effective and traceable.
Contact
Customer Service tel: +44 20 8943 8681
E-mail: materials_enquiries@npl.co.uk