The first objective of this paper is to present a technique and a particular application to calculate the dielectric profile, the elements of the matrix, and its derivatives of the dielectric profile in the cases of circular and periodic circular profiles in the cross section of the rectangular straight waveguide. The second objective is to investigate the influence of the circular and periodic circular profiles on the output fields. The proposed technique relates to the method that is based on the Laplace and Fourier transforms, the inverse Laplace and Fourier transforms. This model is useful to predict the structure of the output fields for circular and periodic circular profiles in a rectangular metallic waveguide. The application is useful for straight waveguides in the microwave and the millimeter-wave regimes.