Gregory, A P; Hodgetts, T E; Clarke, R N; Hill, G J (2023) Calculations of the Fringing Capacitance of Cylindrical Electrodes. NPL Report. TQE 28

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Abstract

The accuracy of measurements of dielectric permittivity made with admittance cells and similar instruments is often limited by the effects of fringing-capacitance. These effects can be reduced by a physical approach that requires an additional electrode that is known as a guard ring, or by numerical corrections derived from calculations of the fringing capacitance. This report studies electrostatic calculations of fringing capacitance for a two-electrode capacitor consisting of a rod or disc that is enclosed by a longer surrounding cylinder or reentrant cylinder. The space between the ends of the electrodes is filled by air and an optional sheet of a uniform dielectric. Comparisons are made between calculations with several types software, including finite-element modelling, boundary-element modelling and mode matching. Corrections for the effects of fringing capacitance on measurements of permittivity by the Lynch method are derived for two radio-frequency instruments. For a Hartshorn and Ward apparatus resonant at 6 MHz, the corrections are found to improve measurement accuracy, but for a reentrant cavity resonant at 300 MHz they are not consistent with observations.

Item Type:	Report/Guide (NPL Report)
NPL Report No.:	TQE 28
Keywords:	Fringing capacitance, edge capacitance, Lynch method, electrostatics, dielectric measurement, simulation, FastCap, FEMM, reentrant cavity
Subjects:	Electromagnetics > Electromagnetic Materials
Divisions:	Electromagnetic & Electrochemical Technologies
Identification number/DOI:	10.47120/npl.TQE28
Last Modified:	29 Jan 2024 10:44
URI:	https://eprintspublications.npl.co.uk/id/eprint/9910