These extend the useable line lengths by flattening the frequency response of the voice band in the 2 to 3 KHz regions. However, beyond this range the frequency response will roll-off quite dramatically due to the addition of these loading coils. Therefore, loading coils are normally removed by the telephone administrations when operation is beyond the voice band. 2.0 Theory of Operation 2.1 Equivalent Circuit A wideband equivalent circuit of a two-wire transmission line is shown in Figure 2. The circuit consists of series inductance and resistance along with shunt capacitance and conductance.
G represent the resistive losses of the transmission line.
The inductance, L, and capacitance, C, are the reactive components of the line. These elements are not lumped, but are distributed over the entire length of the line. Therefore, a theoretical equivalent circuit would consist of an infinite number of sections. In addition, these values are not fixed but are frequency dependent. For example, the component G represents the dielectric losses in the insulation of the wire and these losses increase with frequency.