A programmable microwave attenuator includes coax to slab line connectors which use rigid pin inner conductor captivation at an inner conductor interface surface within the confines of the coaxial outer conductor to improve mechanical reliability and reduce frequency dependent insertion losses. Narrow gaps between the edges of the slab line inner conductor and the coaxial outer conductor reduce unwanted modes of microwave energy transmission by concentrating the electric field at the edges of the slab line inner conductor before it emerges from the coaxial outer conductor into the slab line. A similar gap exists between the edges of the slab line inner conductor and the slab line outer conductor so that unwanted modes of microwave energy transfer are minimized after the outer conductor interface surface.