Richard seems to have a great deal of respect for Terman. So I suggest
that he read Terman's explanation of directional coupler operation in
_Radio Engineering_. In the Fourth Edition, at least, he does so without
a single mention of power, let alone "directional" power or "power waves".

Roy Lewallen, W7EL

Ian White, G3SEK wrote:
Richard Harrison wrote:

Keith wrote:
"I agree completely when the volts and amps are the measurable resultant
voolts and amps."

There is a big problem with resultant volts and amps. It is the
resultant variation in amplitude which is position dependent.

The only average variation in forward and reflected powers is a decline
with distance caused by line loss. Power flows at a constant average
rate into, through, and out of a transmission line. Line loss causes
decline in power along a lossy line.

The convenient way to get useful numbers is to separate energy by its
direction of travel and to measure these. A directional coupler is
needed and the Bird among others does this.


I'm sorry to keep picking away at this one, but it seems to be necessary...

The statement that a directional coupler can "separate energy by its
direction of travel" involves some unaware assumptions involving
transmission-line theory.

If we're trying to get that theory right, we have to avoid using it
unawarely in order to prove itself... because that way would let us
"prove" just about anything.

A directional coupler only senses the current (directionally) at a
particular location on the line, and the voltage between the two
conductors at that same location. The directional coupler tells us
NOTHING else. We have to be very literal-minded about that.

We cannot determine the reflection coefficient, the SWR, or what is
happening to the energy, without applying some flavor of
transmission-line theory. When the whole discussion is about getting
that theory right, we have to be very careful to avoid unawarely arguing
in circles.