On Sun, 23 Mar 2008 19:57:24 GMT
Cecil Moore wrote:
Roger Sparks wrote:
(the reflected voltage should ADD to the source voltage)
If you graph the two voltages you will find that half the time
the reflected voltage adds to the source voltage and half the
time the reflected voltage subtracts from the source voltage.
Both are true half the time. You can point out either case on
the graph. That's why the average interference term is zero
for this special case and therefore why 100% of the average
reflected power is dissipated in the source resistor for
this special case.
You might wonder why I would consider this alternative.
If the destructive interference included a 90 degree delay,
how would I know whether the 30v was the delayed voltage or
exactly in phase with the source?
By looking at the graphs?
Where is the power stored for 90 degrees?
In the equivalent reactance of the transmission line.
That's what reactances do in AC circuits. They store
energy and deliver it back to the system some time
later.
--
73, Cecil http://www.w5dxp.com
I did not use a graph, but created a spreadsheet that calculated Vrs for the short circuit, 45 degree long line. It shows the 90 degree transfer of power that you described. I posted the spreadsheet in PDF format at
http://www.fairpoint.net/~rsparks/Reflect_short.pdf.
To me, this shows that my traveling wave analysis on an instant basis is not correct because the energy can not be located precisely on a degree-by-degree scale. Yes, it is correct on the average over 360 degrees, but not instantaneously. We are missing something.
Central to traveling waves is the assumption that the wave is not compressable. The energy is assumed to flow in a consistantly predictable mannor that is linear and described by a sine wave. That assumption is violated when energy is delayed for reasons other than distance of travel, which is demonstrated in this example.
I am not ready to suggest a cure for my traveling wave analysis. I only see that it does not work to my expectations.
Thanks for providing the examples and comments.
--
73, Roger, W7WKB