Radiation from an open-ended coaxial transmission line into an N-layer dielectric medium is studied in application to nondestructive evaluation of materials. Explicit formulations for two cases of layered media, one terminated into an infinite half-space and the other into a conducting sheet are addressed in general form. In the theoretical derivations it is assumed that only the fundamental TEM mode propagates inside the coaxial line. The terminating admittance of the line is then formulated using the continuity of the power flow across the aperture. The admittance expressions for specific cases of two-layer dielectric composite with generally lossy dielectric properties, and a two-layer composite backed by a conducting sheet are presented and inspected explicitly. The numerical results of the aperture admittance formulation are discussed and compared with the available infinite half-space model which had been experimentally verified
