Design is an iterative process and initial RF design is based on frequency-independent characteristics. For a transmission line, quantities such as the characteristic impedance, effective permittivity, phase and group velocities, and the RLGC parameters are taken as fixed. The RLGC parameters are the basis of the simplest lumped-element circuit model of a transmission line. Each of the components of this model will vary with frequency. The resistance per unit length, $$R$$, increases with frequency as charges concentrate on the surface of conductors and charges bunch (i.e., become less spread out). Both of these effects are due to the finite time it takes to transmit the EM signal that rearranges charges to support an alternating wave.