NPL:
Contact for measurement enquiries: Dr Kevin Lees: kevin.lees@npl.co.uk,
Tel. +44-20-8943-6269.
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NPL's established capability covers low loss, medium and high loss materials and both solid and liquid dielectrics. Services offered for suitable materials include:
Dielectric measurements (complex permittivity), 1 kHz to 110 GHz and at THz frequencies (refractive index)
Measurement of complex permeability, 100 MHz to 18 GHz
Provision of reference materials and traceability
Research collaborations on the electromagnetic properties of materials and metamaterials
Measurement Techniques
Some of these techniques are used for fully traceable measurements for customers; others are used in research investigations and collaborations with academic and industrial partners. Many of the techniques listed can be used for both solids and liquids.
Liquid immersion cell for accurate, traceable 'static' permittivity measurements on solid dielectrics (typically operated at 1 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 (and solids) 10 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 or waveguide transmission line measurements 100 MHz to 18 GHz for medium to high loss dielectrics and for relative permittivities up to 100. Both 14 mm and 7 mm coaxial, and related reflection cells, are in use. Waveguide cells can be used for measuring anisotropy in dielectrics.
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). Work on coaxial probes is now being extended to high temperatures for measurements above 1000 ºC.
Dielectric resonator measurements of permittivity and loss, 100 MHz to 18 GHz in Courtney Resonator or Cavity specimen holders, the latter can be used at elevated temperatures up to 200 ºC.
Resonators for low loss microwave measurements on laminar specimens:
Re-entrant cavity: 300 MHz to 700 MHz for laminar specimens.
Split-post resonators: 1.8 GHz to 14 GHz for laminar specimens and thin films TM02-mode cavity, approx. 500 MHz for rod or tubular specimens and liquids pssing through the cavity via tube.
TE01-mode cavity, 8 GHz to 12 GHz for low loss laminar specimens.
Millimetre-wave open resonators for low loss materials, typically at 32 GHz to 110 GHz, also 8 GHz to 12 GHz for laminar specimens.
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 and using Coplanar Waveguide (CPW). This capability is subject to further development in the EMINDA project and it is expected that it will be expanded during the course of the project.
Terahertz: quasi-optical measurements on materials properties are carried out at THz frequencies by using THz Pulse Imaging (TPI) methods.
Measurement of extrinsic properties of materials. All of the above methods are concerned with measurement of intrinsic materials properties such as complex permittivity, complex permeability and conductivity. NPL also has capabilities for measuring such extrinsic properties as transmission and reflection coefficients and screening coefficients (e.g. for carbon fibre based composites). Such measurements are carried out in NPL's anechoic chambers, which are also used for characterising Frequency Selective Surfaces (FSSs). Other facilities that can be used for free-field materials characterisation include TEM-cells and mode-stirred chambers.
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.
Near-field Scanning Microwave Microscope (NSMM): This technique is subject to ongoing research in the EMINDA project to broaden its capability, but NPL already has a capability for scanning polished dielectric surfaces on to 10-micron scale for microwave dielectric properties and is keen to work with collaborators or customers who may have an interest in this capability.
Measurement capability is often more flexible than is implied in the above list. NPL is willing to develop new methods which are appropriate to customers' needs, so do please contact NPL if you need help with electromagnetic materials measurements.
For further information see:
http://www.npl.co.uk/science-technology/functional-materials/research/dielectric-measurement-techniques and
http://www.npl.co.uk/electromagnetics/electromagnetic-materials/
Good Practice Guide: Many of the lessons learnt from RF and Microwave dielectric measurements over many years at NPL have been compiled into a good practice guide which is available from the NPL web-site. It is concerned with practical measurement and how to improve their accuracy:
'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. Available from http://www.npl.co.uk/publications/a-guide-to-the-characterisation-of-dielectric-materials-at-rf-and-microwave-frequencies
