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 distributed resistance of the conductors is represented by a series resistor (expressed in ohms per unit length). In practical conductors, at higher frequencies, increases approximately proportional to the square root of frequency due to the skin effect.
The conductance of the dielectric material separating the two conductors is represented by a shunt resistor between the signal wire and the return wire (siemens per unit length). This resistor in the model has a resistance of ohms. accounts for both bulk conductivity of the dielectric and dielectric loss. If the dielectric is an ideal vacuum, then .
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 , , , and 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 componentsEdit
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.
The inductance makes it look like the current has inertia – i.e. with a large inductance, it is difficult to increase or decrease the current flow at any given point. Large inductance L makes the wave move more slowly, just as waves travel more slowly down a heavy rope than a light string. Large inductance also increases the line's surge impedance (more voltage needed to push the same AC current through the line).
The capacitance controls how much the bunched-up electrons within each conductor repel, attract, or divert the electrons in the other conductor. By deflecting some of these bunched up electrons, the speed of the wave and its strength (voltage) are both reduced. With a larger capacitance, C, there is less repulsion, because the other line (which always has the opposite charge) partly cancels out these repulsive forces within each conductor. Larger capacitance equals weaker restoring forces, making the wave move slightly slower, and also gives the transmission line a lowersurge impedance (less voltage needed to push the same AC current through the line).
Resistance corresponds to resistance interior to the two lines, combined. That resistance R dissipates a little of the voltage along the line as heat deposited into the conductor, leaving the current unchanged. Generally, the line resistance is very low, compared to inductive reactanceωL at radio frequencies, and for simplicity is treated as if it were zero, with any voltage dissipation or wire heating accounted for as an afterthought, with slight corrections to the "lossless line" calculation deducted later, or just ignored.
Conductance between the lines represents how well current can "leak" from one line to the other, and higher G dissipates more current as heat, deposited in whatever serves as insulation between the two conductors. Generally, wire insulation (including air) is quite good, and the conductance is almost nothing compared to the capacitive susceptanceωC, and for simplicity is treated as if it were zero; the caveat is that materials that are good insulation at low frequencies are often "leaky" at very high frequencies.
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 cableEdit
Representative parameter data for 24-gauge telephone polyethylene insulated cable (PIC) at 70 °F (294 K)
More extensive tables and tables for other wire gauges, operating temperatures, and insulation are available in Reeve (1995). Chen (2004) gives the same data in a parameterized form, that he reports is usable up to 50 MHz.
The constancy of the capacitance is a consequence of intentional, careful 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, and are all real constants.
G(·) in this table can be modeled well with
Usually the resistive losses grow proportionately to 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 telegrapher's equations are:
They can be combined to get two partial differential equations, each with only one dependent variable, either or :
Except for the dependent variable ( or ) the formulas are identical.
General solution for terminated lines of finite lengthEdit
be the Fourier transform of the input voltage then the general solutions for voltage and current are
the shunt admittance per unit length (reciprocal of the shunt impedance). The parameter represents the total length of the line, and locates an arbitrary intermediate position along the line. is the impedance of the electrical termination.
With no termination (broken line), is infinite, and the terms vanish from the numerator and denominator of the transfer function, If the end is perfectly grounded (shorted line), is zero and the terms vanish.
Remarks on notation
As in other sections of this article, the formulas can be made somewhat more compact by using the secondary parameters
replacing the product and ratio square root factors written out explicitly in the definition of
The Fourier transforms used above are the symmetric versions, with the same factor of multiplying the integrals of both the forward and inverse transforms. This is not essential; other versions of the Fourier transform can be used, with appropriate juggling of the coefficients that ensures their product remains
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 linesEdit
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.
The telegrapher's equations are developed in similar forms in the following references:
Kraus (1989),: 380–419 Hayt (1989),: 382–392 Marshall (1987),: 359–378 Sadiku (1989),: 497–505 Harrington (1961),: 61–65 Karakash (1950): 5–14 and
Metzger & Vabre (1969).: 1–10
These equations may be combined to form two exact wave equations, one for voltage V, the other for current I:
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.
In the case of sinusoidalsteady-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 is the angular frequency of the steady-state wave. In this case, the telegrapher's equations reduce to
f1 represents the amplitude profile of a wave traveling from left to right in a positive x direction whilst
f2 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 lineEdit
In the presence of losses the solution of the telegrapher's equation has both damping and dispersion, as visible when compared with the solution of a lossless wave equation.
When the loss elements and are too substantial to neglect, the differential equations describing the elementary segment of line are
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 : 130
Signal pattern examplesEdit
Changes of the signal level distribution along the single dimensional transmission medium. 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 medium 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.
In the first approximation, the current in a thin antenna is distributed exactly as in a transmission line. — Schelkunoff & Friis (1952)(p 217 (§8.4))
Because the conductor of an antenna element closely approximates a single-conductor transmission line, the telegrapher's equations can be used to analyze antenna currents, as was commonly done in the first half of the 20th century, before the common availability of computing equipment.
Solutions of the telegrapher's equations as circuit componentsEdit
Equivalent circuit of an unbalanced transmission line (such as coaxial cable) where: 2/Zo = trans-admittance of VCCS (Voltage Controlled Current Source), x ≡ length of transmission line, Z(s) ≡ Zo(s) ≡ characteristic impedance, T(s) ≡ propagation function, γ(s) ≡ propagation "constant", s ≡ j ω ,j2 ≡ −1 .
Equivalent circuit of a balanced transmission line (such as twin-lead) where: 2/Zo = trans-admittance of VCCS (Voltage Controlled Current Source), x ≡ length of transmission line, Z(s) ≡ Zo(s) ≡ characteristic impedance, T(s) ≡ propagation function, γ(s) ≡ propagation "constant", s ≡ j ω ,j2 ≡ −1 .
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.
The bottom circuit is derived from the top circuit by source transformations. 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: 44
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 and as functions of and The voltage and current relations are symmetrical: Both of the equations shown above, when solved for and as functions of and yield exactly the same relations, merely with subscripts "1" and "2" reversed, and the terms' signs made negative ("1"→"2" direction is reversed "1"←"2", hence the sign change).
In the bottom circuit, all voltages except the port voltages are with respect to ground and the differential amplifiers have connections to ground not shown. An example of a transmission line modeled by this circuit would be a balanced transmission line such as a telephone line. The impedance Zo(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 blocks marked "T(s)" carries the forward wave and the other carries the backward wave. The depicted circuit is fully symmetric, although it is not drawn that way. The circuit depicted is equivalent to a transmission line connected from to in the sense that , , and would be same whether this circuit or an actual transmission line was connected between and 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 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, 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 microstrip line.
These are not unique: Other equivalent circuits are possible.
Karakash, John J. (1950). Transmission Lines and Filter Networks (1st ed.). Macmillan. pp. 5–14, 44.
Metzger, Georges; Vabre, Jean-Paul (1969). Transmission Lines with Pulse Excitation. Academic Press. pp. 1–10.
Miano, Giovanni; Maffucci, Antonio (2001). Transmission Lines and Lumped Circuits. Academic Press. p. 130. ISBN 0-12-189710-9.
The book uses the symbol μ instead of α.
McCammon, Roy (June 2010). SPICE Simulation of Transmission Lines by the Telegrapher's Method(PDF). cmpnet.com. RF Design Line. Retrieved 2010-10-22; also "Part 1 of 3". SPICE simulation of transmission lines by the telegrapher's method. Microwave & RF design. E.E. Times.
Hayt, William H. (1971). Engineering Circuit Analysis (2nd ed.). New York, NY, USA: McGraw-Hill. pp. 73–77. ISBN 0-07027382-0.