method and apparatus for the nondestructive testing of metal - clad microwave circuit boards to determine dielectric constant variations within the dielectric sheets of such boards . the method involves using injected microwave energy to measure values related to the average dielectric constant along a plurality of paths in the plane of the sheet . the paths are divided into two intersecting groups , with each path being parallel to the members of its own group . resulting values are then compared in order to identify dielectric constant variations within the sheet . measurements may be made by means of phase delay or amplitude / resonance frequency techniques . a preferred apparatus for phase delay measurement is disclosed which includes means for making a simultaneous phase delay measurement along a reference air path .