Shielded or unshielded twisted-pair for high speed data transmission?

Balanced transmission is the standard method for data transmission via twisted pair cables. A separate shield for the balanced twisted pair is used to improve the immunity or emission behavior of the transmission line. The different performance of shielded or unshielded twisted pairs is investigated by numerical calculation with CONCEPT, a program based on the method of moments. In addition, a two-dimensional model with length independent partial capacitances was used. The effect of a shield is discussed with respect to transmission characteristics and EMI-behavior. The additional shield has a less significant influence to data transmission aspects than attenuation and reflection. The model of partial capacitances shows that the shielded twisted pair provides a better symmetry of signal transmission and improves the suppression of crosstalk.