Roger wrote:

Why is it the same equation? I understand your P = V(t) * I(t) to be V
and I as functions of time, but Keith to be using what ever he reads
from his voltmeter and ammeter.

If you'll look back through Keith's postings you'll see that he was
referring to the functions of time. It looks like he left off the
explicit (t) at some places which would lead to confusion.

But I hope you realize that you can also find the average power just
fine by calculating Pavg = Vrms * Irms * cos(theta) at any point along
the line, where Vrms and Irms are the total voltage and current at the
point, and theta is the angle between the two. This is true regardless
of the SWR. Also, you can calculate power as Irms^2 * Rser or Vrms^2 /
Rpar where Rser and Rpar are the series and parallel equivalent
resistive parts of the impedance at any point. Like v(t) and i(t), Vrms
and Irms can be found if desired by summing the forward and reflected
waves to find the total value at the point of interest; superposition
applies. It doesn't apply to power, so always do the summation of
voltages and currents before calculating power.

One property of the P(t) = V(t) * I(t) equation is that it also applies
to non-sinusoidal and even non-periodic waveforms -- it can *always* be
used. And you can always find the average power by integrating it then
dividing by the integration period. The average value calculation
reduces to Vrms * Irms * cos(theta) for pure sine waves but not other
waveforms.

Roy Lewallen, W7EL