Fig, 1* The most basic physical characteristics of a transmission line are shown here. Assume that current is flowing from the source direction toward the load. The voltage from A to D must be the same as the sum of that from A to B, B to C, and C to D-but current flow through the wires means that the voltage drop from A to B and from C to D will be greater than zero, so the voltage between the two wires at A D is greater than that at B-C. Similarly, current at point E includes that flowing to charge capacitance FH, and so must be greater than that at point G„ These voltage and current relationships define the impedance and attenuation of any transmission line.
In practice, this is seldom true. The nature of the fields and waves which make up rf energy (or any ac energy for that matter) being what it is, a one-way travel of any set of waves will stay one-way only so long as that set of waves stays in the same medium. When any boundary is reached-that is, when anything gets in the way-some of the energy is reflected at the boundary, and at least some of the reflected energy comes back in the direction exactly opposite to its original direction.
It happens to be possible, from a mathematical point of view, to account for all the reflected energy in terms of just two sets of waves. One set is considered to be the original set, going "forward" or in the original direction, and the other set is considered to be going in the exact opposite direction. This is known as the "reverse" or "reflected" wave, while the original set is called the "forward" or "incident ' wave.
Note that this way of accounting for the energy does not claim that all the energy is actually contained in the forward and reverse waves; we actually do not know for sure exactly what does happen. It merely says that if we consider only the two sets of waves, we can account for what happens. If we can account for what happens, and our accounting method lets us predict what will happen in any normal set of circumstances, then it's a workable tool whether it's a correct one of not-and the concept of incident and reflected waves in a transmission line is just that, a workable tool. We don't need to know more than that in order to make use of it, any more than we need to know the atomic structure of steel in order to cut a hole through metal with a cold chiseL
ferman says the same thing in a different way on page 84 of his "Electronic and Radio Engineering" (fourth edition): "The voltage and current existing on a transmission line as given by equations 4-6 can be conveniently expressed as the sum of the voltages and currents of two waves/" Notice that his sole reason for doing so was convenience.
Now that we've established the incident and reflected waves on a transmission line as convenient tools-without going any deeper into whether they actually exist as such-let s see how we must de ine each of them in order to make it into a useful tool.
The incident wave starts at the source and goes toward the load. It is the wave we considered in Fig, 1, in which both the voltage and current get smaller as we go farther from the source. Since the wave takes a definite time to go any given distance down the line, the instantaneous phase at any point along the line must lag as we get farther from the source. That is, the part of the wave most distant from the source must represent a part which left the .source before any nearer part did.
There's nothing in the transmission line itself to alter the relationship between voltage and current in the incident wave, because the retarding effects of the distributed inductance along the line are cancelled by the advancing effects of the distributed capacitance and the net reactance is zero. Therefore the voltage and current at any point in the incident wave have the same phase relationship to each other that they had at the source.
All this means that we can describe the incident wave as a voltage accompanied by a current that is everywhere in phase with, and proportional to, the voltage. Because of the continual decrease ot both voltage and current we saw in Fig. 1, both the voltage and current decrease as we go away from the source, and drop back uniformly in phase.
This description corresponds to thai of a travelling wave, propagating away from a
Was this article helpful?