We report on terahertz electromagnetic field transmission through metallic slot antennas supported on various dielectric substrates. The substrate effect strongly modifies the transmission resonance compared to the free-standing case, while the resonance can be determined systematically by introducing an effective refractive index. The relative contribution of the substrate index to the effective index has been measured for various substrates, and shows a good agreement with theoretical calculations based on a coupled mode method. In addition, the slot antenna arrays embedded in a flexible substrate show a transmission resonance governed by a substrate refractive index. (C) 2012 Elsevier B. V. All rights reserved.