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This method describes the nondestructive measurement of the relative permittivity and loss tangent of unclad dielectric substrates at microwave frequencies using a splitcylinder resonator (see Figure 1).  This test method is directly applicable for measuring the in-plane (the plane parallel to the surface of the specimen) permittivity of the specimen because the electric field is in-plane. The permittivity of isotropic dielectrics can also be measured with this method. Note: This measurement method does not measure the outof- plane (direction normal to the surface of the specimen) permittivity of the specimen. However, for most printed boards the measurement uncertainties associated with this method are typically less than the difference between in-plane and out-of-plane permittivity values. Furthermore, comparison with methods measuring the out-of-plane permittivity is difficult because those methods typically do not provide measurement confidence intervals.