Roy Lewallen wrote in
:


Adding a directional wattmeter to the mix raises an interesting issue.

Roy,

I suspect that the directional wattmeter is in a large part responsible
to the strong committment to the view the Pf and Pr are separate tangible
quantities, and that they can be dealt with independently.

You will recall a raging argument some years ago about the possible range
of rho, and insistence by some practical practitioners that rho could
never be greater than unity because it would mean that reflected power
was greater than forward power, obviously impossible, then citing
experience with a directional wattmeter (calibrated for V/I being purely
real) as evidence. Of course, telephony line designers know about this
effect and do not have directional wattmeters to confuse their thinking.

You example goes on to point out the absurdity of application of that
kind of thinking.

Miguel has asked why I keep quoting the term 'reflected power' etc, it is
because it is a deceptive label that misrepresents the quantity.

Roy has explained, and I have explained in the articles I referenced, the
assumptions that underly the concept that Pf=Vf*If, and its limitations.

There are many ham texts and articles that try to explain some concepts
and deliver to the user a language and model that encourages extension to
invalid application.

Miguel, in a steady state scenario with a source at one end of a TEM line
and a load at the other, the power (rate of flow of energy) at any point
is given by the product of instantaneous voltage and current. This is a
time varying quantity which can be integrated over time to obtain the
average power. At some points on the line, the instantaneous power may be
negative for some parts of the cycle, which indicates that over a cycle,
some energy is exchanged to and fro across that point, but the average
power will always be from source to load, and that quantity will decrease
from source to load as accounted for by line loss elements, though in the
general case, not purely exponentially as sometimes believed.

Owen