Telegrapher's equations

The telegrapher's equations (or just telegraph equations) are a pair of coupled, linear differential equations that describe the voltage and current on an electrical transmission line with distance and time. The equations come from Oliver Heaviside who in the 1880s developed the transmission line model, which is described in this article. The model demonstrates that the electromagnetic waves can be reflected on the wire, and that wave patterns can appear along the line. The theory applies to transmission lines of all frequencies including high-frequency transmission lines (such as telegraph wires and radio frequency conductors), audio frequency (such as telephone lines), low frequency (such as power lines) and direct current.

Distributed components

Schematic representation of the elementary components of a transmission line.

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. 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

Schematic showing a wave flowing rightward down a lossless transmission line. Black dots represent electrons, and the arrows show the electric field.

The role of the different components can be visualized based on the animation at right.

Values of primary parameters for telephone cable

Representative parameter data for 24 gauge telephone polyethylene insulated cable (PIC) at 70°F (294 K)

Frequency R L G C
Hz Ω/km Ω/kft mH/km mH/kft µS/km µS/kft nF/km nF/kft
1 172.24 52.50 0.6129 0.1868 0.000 0.000 51.57 15.72
1k 172.28 52.51 0.6125 0.1867 0.072 0.022 51.57 15.72
10k 172.70 52.64 0.6099 0.1859 0.531 0.162 51.57 15.72
100k 191.63 58.41 0.5807 0.1770 3.327 1.197 51.57 15.72
1M 463.59 141.30 0.5062 0.1543 29.111 8.873 51.57 15.72
2M 643.14 196.03 0.4862 0.1482 53.205 16.217 51.57 15.72
5M 999.41 304.62 0.4675 0.1425 118.074 35.989 51.57 15.72

More extensive tables and tables for other gauges, temperatures and types are available in Reeve.[1] Chen[2] gives the same data in a parameterized form that he states is usable up to 50 MHz.

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[3] "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 G(f) = G_1 \left( \frac {f}{f_1}\right)^{ge} with ge close to 1.0 would fit the statement from Terman. Chen [2] gives an equation of similar form.

G in this table can be modeled well with

f_1 = 1\;\mathrm{MHz}
G_1 = 29.11\;\mathrm{\mu S/km} = 8.873\;\mathrm{\mu S/kft}
ge = 0.87

Usually the resistive losses grow proportionately to  f^{0.5} \, and dielectric losses grow proportionately to  f^{ge} \, with ge > 0.5 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. The dielectric can be reduced down to air with an occasional plastic spacer.

Lossless transmission

When the elements R and G are very small, their effects 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:

 V = V(x,t)
 I = I(x,t)

The equations

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.


\frac{\partial V}{\partial x}  =
-L \frac{\partial I}{\partial t}

\frac{\partial I}{\partial x}  =
-C \frac{\partial V}{\partial t}

The Telegrapher's Equations are developed in similar forms in the following references: Kraus,[4] Hayt,[5] Marshall,[6] Sadiku,[7] Harrington,[8] Karakash,[9] and Metzger.[10]

These equations may be combined to form two exact wave equations, one for voltage V, the other for current I:


\frac{\partial^2 V}{{\partial t}^2}  -
u^2 \frac{\partial^2 V}{{\partial x}^2}  = 0

\frac{\partial^2 I}{{\partial t}^2} -
u^2 \frac{\partial^2 I}{{\partial x}^2}  = 0

where

u = \frac{1}{\sqrt{LC}}

is the propagation speed of waves traveling through the transmission line.

Sinusoidal steady-state

In the case of sinusoidal steady-state, the voltage and current take the form of single-tone sine waves:

 V(x,t) = \mathrm{Re} \{ V(x) \cdot e^{ j\omega t  } \}
 I(x,t) = \mathrm{Re} \{ I(x) \cdot e^{ j\omega t  } \} ,

where \omega is the angular frequency of the steady-state wave. In this case, the Telegrapher's equations reduce to


\frac{d V}{d x}   =   -j \omega L I

\frac{ d I}{d x}   =    -j \omega C V

Likewise, the wave equations reduce to

\frac{d^2 V}{d x^2}+ k^2  V = 0
\frac{d^2 I}{d x^2} + k^2 I= 0

where k is the wave number:


k = \omega \sqrt{LC} =  {  \omega \over u } .

Each of these two equations is in the form of the one-dimensional Helmholtz equation.

For all transmission lines made of parallel perfect conductors with vacuum between them, this speed is equal to the speed of light.

In this case, it is possible to show that

V(x) = V_1 e^{-jkx} + V_2 e^{+jkx}

and

I(x) = { V_1 \over Z_0 } e^{-jkx} - { V_2 \over Z_0 }e^{+jkx}

where Z_0 is the characteristic impedance of the transmission line, which, for a lossless line is given by

Z_0 =  \sqrt { {L \over C}}

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 Sadiku,[11] and Marshall,[12]

General solution

The general solution of the wave equation for the voltage is the sum of a forward traveling wave and a backward traveling wave:

V(x,t) \ = \ f_1(x - ut) + f_2(x + ut)

where

f1 represents a wave traveling from left to right in a positive x direction whilst f2 represents 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.

Since the current I is related to the voltage V by the telegrapher's equations, we can write

I(x,t) \ = \ \frac{f_1(x - ut)}{Z_0} - \frac{f_2(x + ut)}{Z_0}

Lossy transmission line

When the loss elements R and G are not negligible, the original differential equations describing the elementary segment of line become


\frac{\partial}{\partial x} V(x,t) =
-L \frac{\partial}{\partial t} I(x,t) - R I(x,t)

\frac{\partial}{\partial x} I(x,t) =
-C \frac{\partial}{\partial t} V(x,t) - G V(x,t)

By differentiating both equations with respect to x, and some algebraic manipulation, we obtain a pair of hyperbolic partial differential equations each involving only one unknown:


\frac{\partial^2}{{\partial x}^2} V =
L C \frac{\partial^2}{{\partial t}^2} V +
(R C + G L) \frac{\partial}{\partial t} V + G R V

\frac{\partial^2}{{\partial x}^2} I =
L C \frac{\partial^2}{{\partial t}^2} I +
(R C + G L) \frac{\partial}{\partial t} I + G R I

Note that 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 (small R and G = 0), signal strength will decay over distance as ex, where α = R/2Z0

Signal pattern examples

Changes of the signal level distribution along the single dimensional transmission media. Depending on the parameters of the telegraph equation, this equation can reproduce all four patterns.

Depending on the parameters of the telegraph equation, the changes of the signal level distribution along the length of the single-dimensional transmission media may take the shape of the simple wave, wave with decrement, or the diffusion-like pattern of the telegraph equation. The shape of the diffusion-like pattern is caused by the effect of the shunt capacitance.

Solutions of the telegrapher's equations as circuit components

Equivalent circuit of an unbalanced transmission line (such as twin-lead). where: 2/Z = trans-admittance of VCCS (Voltage Controlled Current Source), X = length of transmission line, Z(s) = characteristic impedance, T(s) = propagation function, γ(s) = propagation “constant”, s = jω, j²=-1. Note: Rω, Lω, Gω and Cω may be functions of frequency.
Equivalent Circuit of an Balanced Transmission Line (such as coaxial cable). where: 2/Z = trans-admittance of VCCS (Voltage Controlled Current Source), X = length of transmission line, Z(s) = characteristic impedance, T(s) = propagation function, γ(s) = propagation “constant”, s = jω, j²=-1. Note: Rω, Lω, Gω and Cω may be functions of frequency.

The solutions of the telegrapher's equations can be inserted directly into a circuit as components. The circuit in the top figure implements the solutions of the telegrapher's equations.[13]

The bottom circuit is derived from the top circuit by source transformations.[14] It also implements the solutions of the telegrapher's equations.

The solution of the telegrapher's equations can be expressed as an ABCD type two-port network with the following defining equations[15]

V_1 = V_2 \cosh ( \gamma  x) + I_2 Z \sinh (\gamma x) \,
I_1 = V_2 \frac{1}{Z} \sinh (\gamma x) + I_2 \cosh(\gamma x). \,
The symbols: E_s, E_L, I_s, I_L, l  \, in the source book have been replaced by the symbols : V_1, V_2, I_1, I_2, x  \, in the preceding two equations.

The ABCD type two-port gives V_1 \, and I_1 \, as functions of V_2 \, and I_2 \, . Both of the circuits above, when solved for V_1 \, and I_1 \, as functions of V_2 \, and I_2 \, yield exactly the same equations.

In the bottom circuit, all voltages except the port voltages are with respect to ground and the differential amplifiers have unshown connections to ground. An example of a transmission line modeled by this circuit would be a balanced transmission line such as a telephone line. The impedances Z(s), the voltage dependent current sources (VDCSs) and the difference amplifiers (the triangle with the number "1") account for the interaction of the transmission line with the external circuit. The T(s) blocks account for delay, attenuation, dispersion and whatever happens to the signal in transit. One of the T(s) blocks carries the forward wave and the other carries the backward wave. The circuit, as depicted, is fully symmetric, although it is not drawn that way. The circuit depicted is equivalent to a transmission line connected from V_1 \, to V_2 \, in the sense that V_1 \, , V_2 \, , I_1 \, and I_2 \, would be same whether this circuit or an actual transmission line was connected between V_1 \, and V_2 \, . There is no implication that there are actually amplifiers inside the transmission line.

Every two-wire or balanced transmission line has an implicit (or in some cases explicit) third wire which may be called shield, sheath, common, Earth or ground. So every two-wire balanced transmission line has two modes which are nominally called the differential and common modes. The circuit shown on the bottom only models the differential mode.

In the top circuit, the voltage doublers, the difference amplifiers and impedances Z(s) account for the interaction of the transmission line with the external circuit. This circuit, as depicted, is also fully symmetric, and also not drawn that way. This circuit is a useful equivalent for an unbalanced transmission line like a coaxial cable or a micro strip line.

These are not the only possible equivalent circuits.

See also

Notes

  1. Reeve 1995, p. 558
  2. 1 2 Chen 2004, p. 26
  3. Terman 1943, p. 112
  4. Kraus 1989, pp. 380–419
  5. Hayt 1989, pp. 382–392
  6. Marshall 1987, pp. 359–378
  7. Sadiku 1989, pp. 497–505
  8. Harrington 1961, pp. 61–65
  9. Karakash 1950, pp. 5–14
  10. Metzger 1969, pp. 1–10
  11. Sadiku 1989, pp. 501–503
  12. Marshall 1987, pp. 369–372
  13. McCammon 2010
  14. Hayt 1971, pp. 7377
  15. Karakash 1950, p. 44

References

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