Unique properties of unstable ring resonators are sometimes useful. A collimated beam in the gain medium may be desirable. Spatial hole burning is eliminated. Beam rotation may be helpful. There is a drawback, however. As usually constructed, a ring resonator has half as many passes through the gain medium as can be achieved with a standing-wave resonator. We have performed a geometrical and a wave-optics numerical simulation of a type of ring resonator that allows counter-propagating collinear passes through the gain medium, while there is also a section with a unidirectional beam. The resonator includes a polarizing beam splitter. The linear polarization is transformed to the orthogonal state by optical elements at the two ends of the region with counter-propagating beams. The wave-optics simulation treats a UR90, for which the output beam is unobscured.