Gonna try this with math notation… here goes.
But first… I haven’t seen the book, but at the point where you are in the discussion, the author is introducing forward and reflected power. You probably get this, but just in case there are other readers out there…. There are a lot of different ways to wrap your head around this. It all depends on how you’re wired. Here’s one of the ways.
Assume that the voltage everywhere was 0 at the start. When a voltage of any sort is introduced from Vs down the line it will travel down the transmission line as a change from whatever the voltage was, to whatever it is right now — think of a wave advancing down the line. The *forward* voltage and *forward* current along the line are related strictly to the line impedance and the voltage and current introduced at the start. It has to be this way, because otherwise the source end of the line would know what was at the load end of the line. That can’t happen because information can’t travel faster than light. So the source won’t know what is at the far end until one round trip.
But for now, we’re only concerned with the forward trip.
On the forward trip, the source only knows the impedance looking in to the line — Z0. So Vf (V1 at the beginning of time) is the result of a voltage divider between Zs and Z0. hence
\$V_f = \frac{V_s Z_0}{Z_0 + Z_s}\$
But what is the current? It must be the same through Zs as it is into the transmission line. So
\$I_f = V_f / Z_0\$
So now we’ve got the forward current and the forward voltage
\$P_f = V_f I_f = V_f V_f / Z_0 = \frac{V_s Z_0}{Z_0 + Z_s} \frac{V_s Z0}{Z_0 + Z_s} \frac{1}{Z_0}\$
Cancelling out a Z0 and consolidating
\$P_f = \frac{V_s^2 Z_0}{(Z_0 + Z_s)^2}\$
Where the units (volts squared over impedance) makes me feel good about the units .
Now lets set Z0 to be Z_s. We get
\$P_f = \frac{V_s^2 Z0} {(2 Z_0)^2} = \frac{V_s^2}{4 Z_0}\$
Which feels pretty good.
Soon after all this, the author should introduce the backward wave Vr and Ir. I’m not sure how he’ll introduce it, but it will follow some of the same lines. In all the developments, it becomes important to remember superposition and Kirchoff’s rules, especially the current-into-a-node rule. And I’ve found it helpful to work through all this assuming a step function at Vs. This helps reasoning about it from intuition. Once you are convinced of that case, application to sinusoids is a little matter of mathemagic using Mssr. LaPlace.
Apologies if there was a post prior to this covering the same thing — I had to step away in the middle of this.
And, in case I screwed up the math… Here’s all that stuff de-latexized.
Vf = (Vs Z0) / (Z0 + Zs)
If = Vf / Z0
Pf = Vf If
= Vf Vf / Z0
= (Vs Z0)/(Z0 + Zs) * (Vs Z0)/(Z0 + Zs) / Z0
Pf = (Vs^2 Z0) / (Z0+Z0)^2
if Z0 = Zs
Pf = (Vs^2 Z0) / (2 Z0)^2
= Vs^2 / (4 Z0^2)