Leakage due to wet or imperfect insulation between the signal conductors in long cables creates a per-unit-length amount of shunt conductance G . If present, G dissipates power, attenuating and distorting signals in a manner similar to series resistance. Because the shunt conductance G is practically zero at DC for the types of insulators used in most modern digital transmission applications, the shunt conductance term G is rarely used.
For low-voltage digital applications involving pcbs or long UTP, STP-A, or coaxial cables of all types you may safely assume the DC value of shunt conductance G is zero.
Dielectric loss models for high-frequency applications generally incorporate AC dielectric losses into the definition of complex permittivity. This technique imbues the capacitance term C with both real and imaginary parts . When the imaginary part of C is multiplied by j w in the equations, it turns real, generating a high-frequency conductance term “ w Im( C ). Such a term has the same practical effect as the use of a G term that varies in proportion to frequency (see Section 2.12, "Dielectric Effects").
POINTS TO REMEMBER
Transmission Line Parameters
Pcb (printed-circuit board) Traces
Generic Building-Cabling Standards
100-Ohm Balanced Twisted-Pair Cabling
150-Ohm STP-A Cabling
Time-Domain Simulation Tools and Methods
Points to Remember
Appendix A. Building a Signal Integrity Department
Appendix B. Calculation of Loss Slope
Appendix C. Two-Port Analysis
Appendix D. Accuracy of Pi Model
Appendix E. erf( )