This work explores the bandwidth limits of connected slot arrays, focusing on their low-frequency bound. We show how the ground plane, present in this structure to ensure unidirectional radiation, places a limit on their operation in terms of active reflection coefficient at lower frequencies. In particular, a low-frequency approximation of the active reflection coefficient of a representative connected slot array is derived based on a Green’s function formulation and an equivalent circuit model of the array. The design parameters of the array unit-cell are taken into account and their influence on the proposed limit is analyzed. Finally, the low-frequency approximation reveals the impact of surface waves when scanning over the E-plane of the array. A solution to overcome such an issue is proposed. It considers the introduction of vertical metallic walls within the substrate supporting the slots of the array. The proposed low-frequency bound is modified accordingly to further highlight the benefits of this solution.