We present a modal analysis of guiding structures patterned in a two-dimensional metallic photonic crystal. Measurements are carried out on a T-stub structure with various dimensions in the 22–40 GHz frequency band. The pronounced resonances extracted from measured transmissivity spectra are interpreted in terms of multimode propagation phenomena in the T-stub region. Experimental data are assessed by the solution of a suitable form of the 2D Helmholtz equation established for metallic propagation media. ©1998 American Institute of Physics.