Er of a substat

If you have the raw unetched board, you measure the capacitance between the metal plates and the physical dimensions. From this you can back out the dielectric constant.

sorry what's the raw unetched board :(

This is the condition of the material as it comes from the factory. The PC material is in standard rectangular sizes with copper layers covering both surfaces.

my problem I pose it in diagram. one can regarded a substrate as a line of transmission.
f = frequency, L (length of substrate) ,T and R (experimental), they are known.

I want to know er