Twin-slot antennas and coplanar waveguides are a popular choice for coupling signals to state-of-the-art mixers and detectors at terahertz frequencies. Although these sensors show promising performance in terms of noise temperature, they usually also show a considerable downward shift in the center frequency of their spectral response, especially when compared with calculations obtained with commonly used simplified models. In this paper, we describe an accurate and complete electromagnetic model of these detectors, which represents a significant improvement over other published approaches. We present the procedure used to obtain a very good agreement between measurements and calculations at terahertz frequencies both in terms of center frequency and bandwidth. The wide variety of measured and calculated data presented in this paper also demonstrates the effectiveness and reliability of the electromagnetic model in all the investigated frequency bands.