Slot apertures in metallic ground planes are very attractive candidates for conformal antenna applications. However, a low-profile unidirectional antenna requires a backreflector close to the slot aperture and the resulting stripline feed causes strong excitation of parallel-plate modes. In this paper, we investigate unidirectional reflector-backed slot configurations with parallel-plate mode suppression by shorting pins. Starting from a parametric study with respect to shorting pin location and backreflector distance, we present a stripline-fed rectangular slot element with radiation efficiency of more than 80%, a bandwidth of about 5% at a center frequency of 10 GHz. For increased bandwidth applications, a bow-tie slot element with about 8% bandwidth and radiation efficiency of close to 80% is proposed.