On Monday, 20 June 2011 05:57:02 UTC+10, walt wrote:
....
The problem here Cecil, is that you've made the same error as Steve in
believing that the reflected voltage adds to the source voltage at the
source to establish the forward voltage. I've proven that it does not,

Best says at p5-6 of his first article "In all transmission line systems, the level of voltage or wave reflection at either end of the transmission line is determined from the reflection coefficient of the impedance terminating the transmission line. The “re-reflected” voltage, VRER, adds to the initial forward-traveling voltage, VI, and the process of reflection and “re-reflection” continues until equilibrium is reached."

In my view, his statement is incomplete, he does not address the response of the source to the new V/I ratio that exists at its terminals when the reflected wave arrives and is "re-reflected".

As I explained earlier in this thread, if Zs=Rs=Zo=Ro, (proof at http://vk1od.net/transmissionline/VSWR/Zs50.htm ) then the forward voltage component remains constant and so, reflected voltage can simply be added to the forward voltage to find the new terminal voltage.

Taking the case of a Thevenin source where Rs=0, terminal voltage V is constant and so Vf must change when the reflected wave arrives at the source, and so Vf is not independent of Vr and so we can say that Vf is not independent of Vr for all Zs, but for the special case Zs=Rs=Zo=Ro discussed in the previous par, it is.

When Best considered the case where Gamma at the source was not 0, then he needed to also consider the source response to the change V/I after the reflected wave arrived at the source. Best does discuss this response of the source to changing V/I in the second article where his SPICE model shows what is happening.

Discussing Thevenin sources driving transmission lines is quite interesting, but it is naive to think that typical RF power amplifiers response to terminal V/I is the same as a Thevinin source. In any event, this is an explanation of how the steady state is established rather than an analysis of the steady state.

Owen