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At high frequencies, the performance of printed circuits depends not only on the mechanical, thermal, and hygroscopic characteristics of the base material, but also on its electrical data. This article deals with the electrical quality of the base, evaluated by means of measurements made on samples of the material when forming part of the dielectric in a transmission line. The results of the study can be given by means of four apparent constants; characteristic impedance, dielectric constant, attenuation constant, and power factor. These constants are measured in the frequency range of 100---1000 Mc by a simple method requiring two instruments, a signal generator and a receiver with an output indicator. Since the formulas are simple, a quality evaluation of a base material can be performed in the course of a few minutes. The accuracy of the measurement proves to be better than expected, since the effects of errors in the measurement are balanced out to a great degree in the final results. After the method is checked by measuring lines with known characteristics, samples with different dielectrics are tested. Finally, the loss measured at relatively low frequencies is separated into its different components.