The telegrapher's equations (or just telegraph equations) are a set of two coupled, linear equations that predict the voltage and current distributions on a linear electrical transmission line. The equations are important because they allow transmission lines to be analyzed using circuit theory. The equations and their solutions are applicable from 0 Hz (i.e. direct current) to frequencies at which the transmission line structure can support higher order non-TEM modes. The equations can be expressed in both the time domain and the frequency domain. In the time domain the independent variables are distance and time. The resulting time domain equations are partial differential equations of both time and distance. In the frequency domain the independent variables are distance x and either frequency, \omega, or complex frequency, s. The frequency domain variables can be taken as the Laplace transform or Fourier transform of the time domain variables or they can be taken to be phasors. The resulting frequency domain equations are ordinary differential equations of distance. An advantage of the frequency domain approach is that differential operators in the time domain become algebraic operations in frequency domain. The equations come from Oliver Heaviside who developed the transmission line model starting with an August 1876 paper, On the Extra Current. The model demonstrates that the electromagnetic waves can be reflected on the wire, and that wave patterns can form along the line. Originally developed to describe telegraph wires, the theory can also be applied to radio frequency conductors, audio frequency (such as telephone lines), low frequency (such as power lines), and pulses of direct current.

Distributed components

The telegrapher's equations, like all other equations describing electrical phenomena, result from Maxwell's equations. In a more practical approach, one assumes that the conductors are composed of an infinite series of two-port elementary components, each representing an infinitesimally short segment of the transmission line: The model consists of an infinite series of the infinitesimal elements shown in the figure, and that the values of the components are specified per unit length so the picture of the component can be misleading. An alternative notation is to use R', L', C', and G' to emphasize that the values are derivatives with respect to length, and that the units of measure combine correctly. These quantities can also be known as the primary line constants to distinguish from the secondary line constants derived from them, these being the characteristic impedance, the propagation constant, attenuation constant and phase constant. All these constants are constant with respect to time, voltage and current. They may be non-constant functions of frequency.

Role of different components

The role of the different components can be visualized based on the animation at right. All four parameters L, C, R, and G depend on the material used to build the cable or feedline. All four change with frequency: R, and G tend to increase for higher frequencies, and L and C tend to drop as the frequency goes up. The figure at right shows a lossless transmission line, where both R and G are zero, which is the simplest and by far most common form of the telegrapher's equations used, but slightly unrealistic (especially regarding R).

Values of primary parameters for telephone cable

Representative parameter data for 24-gauge telephone polyethylene insulated cable (PIC) at 70 °F (294 K) ! Frequency ! colspan="2" | R ! colspan="2" | L ! colspan="2" | G ! colspan="2" | C ! Hz ! Ω/km ! Ω/1000 ft ! μH/km ! μH/1000 ft ! μS/km ! μS/1000 ft ! nF/km ! nF/1000 ft This data is from Reeve (1995). The variation of R and L is mainly due to skin effect and proximity effect. The constancy of the capacitance is a consequence of intentional design. The variation of G can be inferred from Terman: "The power factor ... tends to be independent of frequency, since the fraction of energy lost during each cycle ... is substantially independent of the number of cycles per second over wide frequency ranges." A function of the form with close to 1.0 would fit Terman's statement. Chen gives an equation of similar form. Whereas G(·) is conductivity as a function of frequency, G_1, , f_1, and g_e are all real constants. Usually the resistive losses grow proportionately to f^{1/2} and dielectric losses grow proportionately to with so at a high enough frequency, dielectric losses will exceed resistive losses. In practice, before that point is reached, a transmission line with a better dielectric is used. In long distance rigid coaxial cable, to get very low dielectric losses, the solid dielectric may be replaced by air with plastic spacers at intervals to keep the center conductor on axis.

The equations

Time domain

The telegrapher's equations in the time domain are: They can be combined to get two partial differential equations, each with only one dependent variable, either V or I: Except for the dependent variable (V or I) the formulas are identical.

Frequency domain

The telegrapher's equations in the frequency domain are developed in similar forms in the following references: Kraus, Hayt, Marshall, Sadiku, Harrington, Karakash, Metzger. The first equation means that, the propagating voltage at point x, is decreased by the voltage loss produced by , the current at that point passing through the series impedance. The second equation means that, the propagating current at point x, is decreased by the current loss produced by , the voltage at that point appearing across the shunt admittance. The subscript ω indicates possible frequency dependence. and are phasors. These equations may be combined to produce two, single-variable partial differential equations. where \alpha is called the attenuation constant and \beta is called the phase constant.

Homogeneous solutions

Each of the preceding partial differential equations have two homogeneous solutions in an infinite transmission line. For the voltage equation For the current equation The negative sign in the previous equation indicates that the current in the reverse wave is traveling in the opposite direction. Note: where the following symbol definitions hold: ! Symbol !! Definition

Finite length

Johnson gives the following solution, where and x is the length of the transmission line. In the special case where all the impedances are equal, the solution reduces to.

Lossless transmission

When and, wire resistance and insulation conductance can be neglected, and the transmission line is considered as an ideal lossless structure. In this case, the model depends only on the L and C elements. The telegrapher's equations then describe the relationship between the voltage V and the current I along the transmission line, each of which is a function of position x and time t:

The equations for lossless transmission lines

The equations themselves consist of a pair of coupled, first-order, partial differential equations. The first equation shows that the induced voltage is related to the time rate-of-change of the current through the cable inductance, while the second shows, similarly, that the current drawn by the cable capacitance is related to the time rate-of-change of the voltage. These equations may be combined to form two exact wave equations, one for voltage V, the other for current I: where is the propagation speed of waves traveling through the transmission line. For transmission lines made of parallel perfect conductors with vacuum between them, this speed is equal to the speed of light.

Sinusoidal steady-state

In the case of sinusoidal steady-state (i.e., when a pure sinusoidal voltage is applied and transients have ceased), the voltage and current take the form of single-tone sine waves: where \omega is the angular frequency of the steady-state wave. In this case, the telegrapher's equations reduce to Likewise, the wave equations reduce to where k is the wave number: Each of these two equations is in the form of the one-dimensional Helmholtz equation. In the lossless case, it is possible to show that and where in this special case, k is a real quantity that may depend on frequency and is the characteristic impedance of the transmission line, which, for a lossless line is given by and V_1 and V_2 are arbitrary constants of integration, which are determined by the two boundary conditions (one for each end of the transmission line). This impedance does not change along the length of the line since L and C are constant at any point on the line, provided that the cross-sectional geometry of the line remains constant. The lossless line and distortionless line are discussed in and.

Loss-free case, general solution

In the loss-free case the most general solution of the wave equation for the voltage is the sum of a forward traveling wave and a backward traveling wave: where Here, f_1 represents the amplitude profile of a wave traveling from left to right – in a positive x direction – whilst f_2 represents the amplitude profile of a wave traveling from right to left. It can be seen that the instantaneous voltage at any point x on the line is the sum of the voltages due to both waves. Using the current I and voltage V relations given by the telegrapher's equations, we can write

Lossy transmission line

When the loss elements R and G are too substantial to ignore, the differential equations describing the elementary segment of line are By differentiating both equations with respect to x, and some algebra, we obtain a pair of hyperbolic partial differential equations each involving only one unknown: These equations resemble the homogeneous wave equation with extra terms in V and I and their first derivatives. These extra terms cause the signal to decay and spread out with time and distance. If the transmission line is only slightly lossy ( and ), signal strength will decay over distance as where.

Solutions of the telegrapher's equations as circuit components

[[File:Transmission Line, Unbalanced, Equivalent Circuit.png|thumb|360px|Equivalent circuit of an unbalanced transmission line (such as coaxial cable) where: 2/Zo is the trans-admittance of VCCS (Voltage Controlled Current Source), x is the length of transmission line, Z(s) ≡ Zo(s) is the characteristic impedance, T(s) is the propagation function, γ(s) is the propagation "constant", s ≡ j ω , and j2 ≡ −1 .]] The solutions of the telegrapher's equations can be inserted directly into a circuit as components. The circuit in the figure implements the solutions of the telegrapher's equations. The solution of the telegrapher's equations can be expressed as an ABCD two-port network with the following defining equations where and just as in the preceding sections. The line parameters Rω, Lω, Gω, and Cω are subscripted by ω to emphasize that they could be functions of frequency. The ABCD type two-port gives V_1 and I_1 as functions of V_2 and I_2 ,. The voltage and current relations are symmetrical: Both of the equations shown above, when solved for V_1 and I_1 as functions of V_2 and I_2 yield exactly the same relations, merely with subscripts "1" and "2" reversed, and the \sinh terms' signs made negative ("1"→"2" direction is reversed "1"←"2", hence the sign change). Every two-wire or balanced transmission line has an implicit (or in some cases explicit) third wire which is called the shield, sheath, common, earth, or ground. So every two-wire balanced transmission line has two modes which are nominally called the differential mode and common mode. The circuit shown in the bottom diagram only can model the differential mode. In the top circuit, the voltage doublers, the difference amplifiers, and impedances Zo(s) account for the interaction of the transmission line with the external circuit. This circuit is a useful equivalent for an unbalanced transmission line like a coaxial cable. These are not unique: Other equivalent circuits are possible.