An analysis is presented of a finite-length cylindrical antenna which extends through the center of a circular aperture in a perfectly conducting screen separating two dissimilar half-spaces. The screen is planar and of infinite extent while the half-spaces are homogeneous, of semi-infinite extent, and possibly lossy. The screen and the cylinder are perpendicular. The cylinder is excited by the traditional slice generator. Coupled integral equations for the cylinder current and aperture electric field are derived and solved numerically. The effect of the PEC screen on the antenna current distribution is investigated as is the effect of the dissimilar properties of the two half-spaces. It is also pointed out that, for a sufficiently large aperture, the cylinder current is a good approximation of that on the ground stake antenna. Data are presented for cases of interest and are compared with special-case results available in the literature.