National Physical Laboratory

Dielectric Measurement Techniques

RF & Microwave Dielectric Research

Our current radio frequency (RF) and microwave dielectrics research at NPL focuses on three areas:

  • Small scale probing of dielectric properties
  • Measurement on functional thin films
  • 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.

The first two areas are being supported by the EURAMET project EMINDA which is being co-ordinated by NPL. EMINDA brings together five National Measurement Institutes (NMIs), two world-leading 'Top 10' research universities and a large equipment supplier to carry out research in dielectric metrology. For more information on EMINDA, please visit the website.

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 μm 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.

Schematic of NPL's Scanning Microwave Microscope (SMM)
Schematic of NPL's Scanning Microwave Microscope (SMM)

As part of the EMINDA project, NPL is currently researching the probing of dielectric properties on a scale of 10 µm - 100 µm in collaboration with Imperial College, though there is an intention to progress to even 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.

Measurement of functional thin films

As the telecommunications industries move to ever more sophisticated hand held devices there is a requirement to maximise the functionality of chips and it is no longer desirable to separate the microwave and computational electronics. One of the ways in which this will occur is by employing functional thin films.

To date, most of the research on functional thin films has been at DC or low frequency. NPL is performing research using Co-planar Waveguide (CPW) devices to characterise these materials at RF and microwave frequencies. CPW is the transmission medium of choice as it is the closest metrologically-sound geometry to that which will be employed in actual circuitry. In addition to this work NPL is modifying its scanning dielectric probes (NSMMs) to measure films under bias.

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 continues to expand enormously, and this leads to on-going public concern over health and; 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! Measurement of SAR is also important in medical diagnostics, (e.g. in Magnetic Resonance Imaging - MRI) and in RF and Microwave processing: in both cases significant RF fields can be generated, and one needs to guarantee safe working environments for employees.

Workshops and Measurement Clubs

NPL also runs workshops and club meetings to allow scientists, engineers and metrologists to present their work and share experiences. The EMMA club is the club associated with this area and through the EMINDA project it aims to become a pan-EU club.

Modelling of Electromagnetic Field Interactions with Materials and Metamaterials

Many measurement methods we use in the 21st century are highly dependent on the use of computer software. Software is used both to control automatic measurements and to relate measured quantities - the measurands - to parameters of interest. One example is network analyser measurements of dielectric properties of a specimen in a transmission line: the measurands are usually its scattering parameters (S-parameters), whereas the parameter of interest is generally its complex permittivity. The numerical modelling software that is applied to relate permittivity to the measured S-parameters in this case need not be too complex, but other measurement techniques require deeper analytical approaches to perform this task.

NPL aspires to making all of its measurements traceable to international standards and in methods that rely upon modelling software: this requires the validation of this software to ensure that it does what it is intended to do. There are many methods for validating measurement software, including checking against mathematically analytical cases, against previously validated software or against measurements on artefacts and specimens with known properties. Validation of this sort is essential if one is to have confidence in measurement results obtained with the aid of modelling software. NPL provides support for software validation through Mathematics & Modelling for Metrology.

CST microwave model
CST microwave model showing E-field patterns of a dielectric waveguide
contacting a dielectric specimen

More specifically, in the field of electromagnetic metrology and in particular in electromagnetic material measurement, a wide range of validated software is needed at NPL to cover the equally wide range of measurement applications. There is an on-going programme of software development to match our metrology R&D activities. Validated software is invaluable here because it is not only used in actual measurements, it can also be employed when designing instrumentation to perform sensitivity analysis for optimising performance and for estimating the effects of sources of uncertainty (e.g. from gaps around dielectric specimens in transmission lines). NPL's expertise in validated software for RF and microwave metrology includes:

  • Generic and flexible mode-matching software for a wide range of coaxial dielectric probes and resonator geometries, including TE-mode dielectric resonators and 'split-post' dielectric resonators for measuring complex permittivity on laminar specimens and thin films.
  • Gaussian Beam analyses of fields in open resonators, which are used for millimetre-wave dielectric measurement.
  • Monte Carlo modelling software for uncertainty analysis of dielectric measurements.
  • Software for near-field scanning of antenna patterns.

In addition to these areas, which support measurement activities for NPL customers, forward-looking research has been carried out at NPL on metamaterials. This has focused on numerical analysis of active and passive structured artificial dielectric surfaces.

Dielectric Measurement Services

NPL's capability here covers low loss, medium and high loss materials and both solid and liquid dielectrics. Services we can offer for suitable materials include:

  • Dielectric measurements (complex permittivity), 1 kHz to 110 GHz
  • Measurement of complex permeability, 100 MHz to 18 GHz
  • Provision of reference materials and traceability
  • Research collaborations on the electromagnetic properties of materials

Measurement Techniques

NPL specialises in the methods listed below. In a number of cases the methods mentioned were initiated or largely developed at NPL. Some of these techniques are used for fully traceable measurements for customers; others are used for research investigations. Many of the techniques listed can be used for both solids and liquids. NPL is always willing to develop new methods which are appropriate to your needs, so please contact us if you need help with electromagnetic materials measurements.

  • Liquid immersion cell for accurate, traceable 'static' permittivity measurements on solid dielectrics (typically operated at 1 MHz).
  • Admittance Cell
    Admittance Cell for liquid measurements,
    0.1-1.0 MHz
  • Admittance cells for medium loss (loss tangent 0.001 to 0.03) to high loss materials, relative permittivities in the range 1 to 30. Frequencies 1 kHz to 1 GHz (most cells operate up to 10 MHz) - planar specimens, or coaxial cells are used for liquids and particulate dielectrics.
  • Measurement of tissue equivalent liquids 30 MHz to 6 GHz, particularly those used in SAR (Specific Absorption Rate) measurements.
  • High temperature cells for, e.g., foodstuffs (up to 90 °C, 2.45 GHz), ceramics (up to 400 °C, 1 MHz) and reference liquids (from 10 °C to 50 °C, 10 kHz to 1 MHz).
  • Coaxial line and waveguide transmission line measurements from 100 MHz to 18 GHz for medium to high-loss dielectrics and for relative permittivities up to 100. Both 14 mm and 7 mm, and related reflection cells, are in use.
  • Coaxial and waveguide transmission line measurements 100 MHz to 18 GHz for magnetic materials (e.g. ferrite RAM, ferroelectric composites).
  • Coaxial and waveguide probes for minimally invasive measurements, ideally on malleable solids and liquids of medium to high loss, 50 MHz to 18 GHz (NPL-developed, with NPL numerical software, e.g. for layered and circular specimens).
  • Dielectric resonator evaluation, 100 MHz to 18 GHz in Courtney Resonator or Cavity specimen holders.
  • Resonators for low-loss microwave measurements: re-entrant (70 MHz to 700 MHz), split-post (1.8 GHz to 10 GHz), TM02 (approx. 500 MHz), TE01: (8 GHz to 12 GHz).
  • Millimetre-wave open resonators for low-loss materials, typically at 32 GHz to 110 GHz, also 8 GHz to 12 GHz.
  • Microwave Quasi-Optical Methods: including Gaussian Beam reflectometry and free-space reflectometry, with fields measured by an optically modulated scatterer (OMS) for measuring dielectric sheet and RAM reflections and surface waves.
  • Measurements on thin film and laminar dielectrics, typically 10 to 300 μm in thickness, in the frequency range 1 kHz to 110 GHz, by a number of techniques given above.

People

Andrew Gregory
Kevin Lees

Publications

  • Dielectric Metrology with Coaxial Sensors
    A. P. Gregory and R. N. Clarke
    Meas. Sci. Technol., 18, pp 1372-1386 (2007)
  • Tables of the Complex Permittivity of Dielectric Reference Liquids at Frequencies up to 5 GHz
    A. P. Gregory and R. N. Clarke
    NPL Report MAT 23, (2009); updated and re-issued 2012
  • Traceable measurement of dielectric reference liquids over the temperature interval 10-50 ◦C using coaxial-line methods
    A. P. Gregory, R. N. Clarke and M. G. Cox
    Meas. Sci. Technol. 20, page 19, 075106 (2009)
  • Traceable measurements of the static permittivity of dielectric reference liquids over the temperature Range 5 - 50 °C
    A. P. Gregory and R. N. Clarke
    Meas. Sci. Technol. 16, pp 1506-1516 (2005)
  • A Review of RF and Microwave Techniques for Dielectric Measurements of Polar Liquids
    A. P. Gregory and R. N. Clarke
    Trans IEEE Dielectrics & Elec. Insulation, DEIS-13, 4, pp 727-743 (2006)
  • A Guide to Characterisation of Dielectric Materials at RF and Microwave Frequencies
    R. N. Clarke (ed.)
    Published by the Institute of Measurement and Control and NPL (2003)
Last Updated: 8 Aug 2017
Created: 20 Mar 2012

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