Electromagnetic materials

Electromagnetic measurements on materials

Measurement of the dielectric permittivity and loss of low-, medium- and high-loss solids and liquids between 1 kHz and 100 GHz

Our facilities include:

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  • Open-ended coaxial probes in various sizes for measurement on high-loss liquids and malleable solid materials in the frequency range 100 MHz to 50 GHz. These are typically used for measuring tissue-equivalent phantoms used in SAR (Specific Absorption Rate) measurement, foodstuffs, and biomedical materials. NPL-developed Monte-Carlo modelling software is used for evaluation of uncertainty. Measurement at lower frequencies (down to 30 MHz) on liquids is possible with coaxial cells and a 44-mm coaxial sensor. Measurements on layered materials is also possible.
  • Formulation and supply of phantoms for use in SAR measurement.
  • 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. Coaxial cells based on 7 mm and 14 mm coaxial line are available. For waveguide measurement, spacers for a number of sizes (including X-band) are also available. For magnetic materials (e.g. ferrite RAM, ferroelectric composites) complex permittivity and complex permeability can both be determined.
  • Millimetre-wave open resonators for precise measurement on low-loss materials (ideally loss tangent < 0.003). Currently we have systems for approximately 36 GHz, 72 GHz and 94 GHz, and also a system for 8 GHz to 12 GHz.  
  • Dielectric resonator evaluation at microwave frequencies in a Courtney resonator or cylindrical cavity.
  • A selection of Split-Post Dielectric Resonators for measuring laminar materials at 1.8 GHz, 4 GHz, 8.2 GHz, 10 GHz and 14 GHz. Thin dielectric layers can be measured.
  • A number of microwave cavity resonator systems that allow measurements on rods and discs of material.
  • Re-entrant hybrid cavities that are used in the range 300 MHz to 1 GHz. These are typically used for measuring the loss of low-loss polymers and ceramics.
  • A Hartshorn and Ward resonant coil/capacitor system for measuring the loss of very low-loss materials with permittivity below 10 in the range 1 MHz to 70 MHz. The resolution for the measurement of loss tangent is approximately 0.00001.
  • Microwave by free-space quasi-optical methods at millimetre-wave frequencies.
  • Mapping of E-fields by using probing techniques, such as electrooptic sensors.

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