The definition of terms related to dielectric constant can be shown in the following picture.
However due to stray capacitance or edge capacitance formed on the edges of the electrodes, the measured capacitance is larger than the capacitance of the dielectric material.
Therefore, guarded electrode concept was introduced to reduce this edge effect.
A straightforward measurement is developed as “contacting electrode method”, since this method derives permittivity by measuring the capacitance of the electrodes CONTACTING the MUT (sample) directly
Dielectric constant and loss factor can be calculated by the above equation. However, no matter how flat and parallel both sides of the MUT are fabricated, an air gap cannot be completely avoid. A significant measurement error can occur if this air gap effects are not considered.
A thin film electrode can be applied to illuminate this air gap issue. However extra step will be taken in order to achieve more accurate results. Another way is to introduce a non-contact method.
Assuming the electrode gap (tg) is only a little bit larger than tm, the thickness of the MUT, or tg – tm<<tm , Permittivity can be calculated by the following equation:
Sample Preparation: typically circular shape with diameter of 50mm; thickness of 25um to 8mm. The surface needs to be smooth and flat.
Testing Instrument can be Keysight’s 16451B dielectric test fixture, which has capabilities to measure solid materials up to 30 MHz or 16453A dielectric material test fixture, which has capabilities to measure solid materials up to 1 GHz. Agilent has similar testing instrument as well. The following are the images of testing instrument and MUT fixture from Intertek PTL, as an example.
Part of the above info come from ASTM D150 standard, Keysight website and InterTek PTL.