FR4 is a common material used in PCBs, however, its dielectric properties are important in the microwave range.
In order to measure the dielectrics, Gregory F. Gusberti has devised three methods using ring resonators, a pair of microstrip stubs and a single microstrip, all attached to a microwave vector network analyser.
By calculating the permittivity effective to the whole environment, including air and any resist or silkscreen, the PCB design can be optimised for use at high frequencies.
