This paper is concerned with the diffraction of an electromagnetic wave by a unidirectionally conducting circular disk. First, the behavior of a time-harmonic electromagnetic field near the edge of an arbitrary plane unidirectionally conducting screen is determined from the condition that the energy density must be integrable over any finite domain. Secondly, the problem of the diffraction of an arbitrary time-harmonic electromagnetic wave by a plane unidirectionally conducting circular disk is treated for the low-frequency case, i.e., the product $ka$ of the wave number $k$ and the disk radius $a$ is small. Expansions in powers of $ka$ are derived for the far field, the scattered energy and the field on the disk. Some special results for the case of plane-wave excitation are presented.