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Old June 16th 10, 04:21 AM posted to rec.radio.amateur.antenna
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First recorded activity by RadioBanter: Aug 2006
Posts: 572
Default Where does it go? (mismatched power)

On Jun 15, 1:36*pm, Roy Lewallen wrote:
For all the fluff about
photons, optics, non-dissipative sources, and the like, I have yet to
see an equation that relates the dissipation in the resistance in one of
those painfully simple circuits to the "reflected power" in the
transmission line it's connected to.


The power density equation containing an interference term is what you
need to use and I seriously doubt that, after 5+ years, you are
ignorant of that equation. In your food-for-thought, forward/reflected
power example, all you have to do is figure out the power in the
forward wave and the power in the reflected wave at the source
resistor and plug them into the following equation:

Ptot = P1 + P2 + 2*SQRT(P1*P2)*cos(A)

where 'A' is the angle between the forward voltage and reflected
voltage. For instance, if the reflected voltage arrives back at the
source resistor in phase with the forward voltage, cos(A) = cos(0) = 1
and there is constructive interference which increases the dissipation
in the source resistor. If the reflected voltage arrives back at the
source resistor 180 degrees out of phase with the forward voltage,
cos(A) = cos(180) = -1 and there is destructive interference which
decreases the dissipation in the source resistor. If the reflected
voltage arrives back at the source resistor 90 degrees out of phase,
cos(A) = cos(90) = 0, and there is no interference and all of the
reflected power is dissipated in the source resistor. If you had ever
read my energy article, published many years ago, you would know what
effect superposition accompanied by interference can have on the
redistribution of energy. But you instead said, "Gobbleygook" (sic)
and plonked me. Time to pull your head out of the sand.

The above power density equation not only agrees with all of your
power calculations, it tells anyone who desires to acquire the
knowledge, exactly where the reflected energy goes and why it is not
always dissipated in the source resistor.
--
73, Cecil, w5dxp.com
 
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