Cecil Moore wrote:
Reg Edwards wrote:

Cecil, when there are several different power levels at different
places in
a circuit, it is entirely up to you how you reference one to another
in dB.

Now Reg, that just cannot be true.

It is true.

Otherwise, there would exist no
conventions.

Multiple conventions exist in all the technical disciplines. A well
versed practioneer is familiar with the various conventions commonly in
use and will choose the one most appropriate to the problem at hand.
General terms such as 'gain' and 'loss' always require the context be
carefully established before the term can be used with any precision.
For example, dissipative loss in a transmission line is different from
transducer loss, but both are often expressed in decibels. One relates
to an actual ratio of two physically meaningful power levels, and the
other is notional.

It's a simple question: When you tell me that the losses
in a transmission line with reflections are 2 dB, exactly what power
are you referencing those losses against?

Reasonable question. Without knowing the context the statement is
essentially meaningless.

Reflected volts - yes!
Reflected current - yes!
Reflected power - NO!

Reflected volts and reflected current existing without any associated
joules/sec????

Reg didn't say any such thing. The wave equation is usually expressed
in terms related to the two complementary field quantities associated
with energy storage. In the case of TEM transmission lines, the two
variables are voltage, and current. The method of reflections (images)
used to facilitate solution of the wave equation, utilizes the concept
of voltage and current reflection coefficient respectively. These
concepts are used to find the v(x,t) and i(x,t) without any reference to
to power or energy. Once v(x,t) and i(x,t) are known, the power, v(x,t)
* i(x,t) can easily be calculated at any point x and time t. The
energy storage density can be calculated as i(x,t)^2 * L/2 + v(x,t)^2 *
C/2. As you might expect, the v(x,t)*i(x,t) is the derivative of the
energy storage density.

I've heard of waves without any trace of energy before,
Reg, but I certainly didn't expect to hear miracle metaphysics from you.

'Solving' the wave equation for TEM transmission lines usually means
determining the values of the v(x,t) and i(x,t) as a function of place
and time. Energy distribution, and it's movement, is easily calculated
from the knowledge of v(x,t) and i(x,t).

Whatever happened to V*I*cos(theta) being power?

Hasn't changed; works for DC out to upper microwave frequencies.

The power companies
would be surprised to learn that they are not transferring any joules/sec
to their customers.

Indeed they would. Who do you know who believes they are not
transferring energy to their customers?

bart
wb6hqk