On Jun 18, 7:55*pm, K1TTT wrote:
On Jun 18, 11:26 pm, Keith Dysart wrote:

On Jun 18, 7:04 am, K1TTT wrote:

On Jun 18, 12:00 am, Keith Dysart wrote:
except i can measure If and Ir separately and can see they are both
zero.

How would you measure them?

you said you had a directional wattmeter, so do i. *mine reads zero,
what does yours read?

That would most likely be because your wattmeter is AC coupled.
Working
with DC, you need a DC-coupled meter, in which case it would read as
I predicted.

mine is dc coupled... still nothing. *please provide the schematic or
model number for the one you use to get such a reading.

Oh my. Could you kindly provide the schematic or a reference? Before
posting I spent an hour searching my library and the web for the
circuit
for a dc-coupled directional wattmeter and was unsuccessful.

Nothing prevents one from being constructed. That one is not readily
available should not prevent you from using the equations that they
implement and then predicting the DC behaviour of the line.

A highly instructive exercise is to examine the schematic for your
directional wattmeter and work out how it implements the expressions
* Vf(t)=(V(t)+R0*I(t))/2
* Vr(t)=(V(t)-R0*I(t))/2

* Pf=avg(Vf(t)*Vf(t)/R0)
* Pr=avg(Vr(t)*Vr(t)/R0)
or some equivalent variant using If and Ir.

You will find a voltage sampler, a current sampler, some scaling,
and an addition (or subtraction). If your meter is analog, the
squaring function is usually implemented in the non-linear meter
scale. And the averaging function might be peak-reading or rely
on meter inertia.

You can, though, have an arbitrary current as an intermediate
result in solving a problem. With the two battery example, reduce
the resistor to 0 and an infinitely large current is computed in
both directions, though we know in reality that no current is
actually flowing.

no it isn't. *in the limit at R-0 *0V/R still looks like 0 to me..

When computing the intermediate results using superposition, you
have the full battery voltage in to 0 ohms. First the battery on
the left, then the one on the right, and then you add the two
currents to get the total current. 0 is always the total, but
the intermediate results can go to infinity if you connect
the two batteries directly in parallel.

there is no battery on the right, unless you have changed the case
again. *i thought we were doing the open circuit coax with the dc
source on one end.

No. I was referring to the simple DC circuit with two batteries and
a resistor. I was pretty sure that was your referent when you used
"R-0".

I understand that you have not found any flaws in my exposition,
except that you are still uncomfortable with the results. It is
possible to overcome this.

And I re-iterate the value of studying your directional wattmeter's
schematic, especially if it is DC-coupled.

....Keith