Measurement of the dielectric permittivity and loss of solids and liquids
Dielectric properties are critical design parameters for electronics applications in aerospace, automotive, food and medical industries which need accurate measurements to incorporate the most suitable material into its intended application. Dielectric measurements determine the complex permittivity of materials where the real part of permittivity of a material is the degree to which it can store charge. The loss tangent characterises the dissipation of an electromagnetic wave propagating through the material as a result of dielectric loss mechanisms.
We have comprehensive facilities for measurement of the dielectric permittivity and loss of materials ranging from composites and ceramics to liquids and natural materials, between 1 kHz and 100 GHz. We can determine the most appropriate measurement technique for a particular sample based on the measurement frequency, the loss of the material and the texture. As an NMI, we can ensure traceability and intercompare our techniques.
'A guide to the characterisation of dielectric materials at RF and microwave frequencies' describes the interpretation of dielectric data, measurement techniques and good practice in dielectric metrology. This publication is jointly published by NPL and the Institute of Measurement and Control, please contact us to receive a copy via email.
Our measurement techniques include:
- Measurement of capacitance and dissipation with admittance methods that use LCR meters (electronic test equipment used to measure the inductance (L), capacitance (C) and resistance (R) of an electronic component).
Three-terminal cells are used to give best accuracy. Measurements are made typically at 1 MHz, but are possible in the range 1 kHz to 10 MHz. We have a liquid cell, and a parallel plate cell for planar specimens at least 50 mm across.
Measurements with the parallel plate cell are made by the Lynch method
- Measurement on high-loss liquids and malleable solid materials in the frequency range 100 MHz to 50 GHz.
Open-ended coaxial probes in various sizes 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.
- Coaxial line and waveguide transmission line measurements from 100 MHz to 18 GHz for medium to high-loss dielectrics.
Coaxial cells based on 7 mm and 14 mm coaxial line are available. For waveguide measurement, spacers for several sizes (including X-band) are also available.
- Determination of complex permittivity and complex permeability of magnetic materials (such as ferrite RAM and ferroelectric composites).
- Precise measurement on low-loss materials using millimetre-wave open resonators with a loss tangent < 0.003. Systems are available for 36 GHz, 72 GHz, 94 GHz and 144 GHz, and also a system for 8 GHz to 12 GHz.
- Measurement of laminar materials at 1.8 GHz, 2.45 GHz, 4 GHz, 8.2 GHz, 10 GHz and 14 GHz. A selection of split-post dielectric resonators. Thin dielectric coatings can also be measured.
- Measurements on rods and discs of material using microwave cavity resonator systems.
- Measuring the loss of low-loss polymers and ceramics using re-entrant hybrid cavities in the range 300 MHz to 1 GHz.
- Measuring the loss of very low-loss materials with permittivity below 10 in the range 1 MHz to 70 MHz using a Hartshorn and Ward resonant coil/capacitor system. The resolution for the measurement of loss tangent is approximately 0.00001.
- Expertise in free-space quasi-optical methods for measurement on dielectrics at millimetre-wave frequencies.
- Map of E-fields in or around dielectric media (such as phantoms) by using probing techniques, such as electrooptic sensors.
NPL can also:
- Develop and manufacture instrumentation for measuring dielectric materials
- Formulate and supply of phantoms for use in SAR measurement
- Provide technical consultancy and advice
- Measure the RF sheet resistance of thin films
Examples of recent work include:
- Measurement of dielectric loss for very low-loss materials used as RF windows used in high power applications.
- Assessment of 5G signals through building materials.
- Measurement of polymers at millimetre-wave frequencies by resonant and quasioptical transmission methods.
- Measurement of permittivity and conductivity of liquid phantoms used for assessment of induced heating near passive implants (including stents and artificial joints) during magnetic resonance imaging.
We carry out measurements on:
- Organic liquids
- Aqueous liquids
- Liquid and solid phantoms used in SAR measurements
- Radiation absorbent materials
- Fruit, vegetables and foodstuffs
- Biomedical materials
- Ceramics such as alumina
- Composite materials
- Soil and wood
- Building materials
- Materials for 5G and 77 GHz automotive radar systems
- 3D printed materials
- Metamaterials and artificial dielectrics
Don’t see what you are looking for? Our diverse skill set enables us to provide bespoke solutions. Please contact us to discuss your requirements.