On Feb 22, 5:37 am, Keith Dysart wrote:
On Feb 21, 9:53 pm, Cecil Moore wrote:



Keith Dysart wrote:

Hey Keith, how about this one?

Rs Pfor=50w--
+----/\/\/-----+----------------------+
| 50 ohm --Pref |
| |
Vs 45 degrees RLoad+j0
100v RMS 50 ohm line |
| |
| |
+--------------+----------------------+

The dissipation in the source resistor is:

P(Rs) = 50w + Pref

How can anyone possibly argue that reflected power
is *never* dissipated in the source resistor? :-)
--
73, Cecil http://www.w5dxp.com

Two Saturdays ago I was on a road trip and used
9 litres/100km. How can anyone argue that the fuel
consumption is never equal to the day of the month?
Numerical coincidences can be much fun.

But you will like the generator below even better.
The power dissipated in the generator resistors is
always equal to 50 + Pref, regardless of the load
and line length, thereby always accounting for Pref.

Pfor=50w--
+--/\/\/---+----------------+-------------------+
| 100 ohm | --Pref |
| | 100 ohm |
| +--/\/\/--+ any length |
| ^ | 50 ohm line any load
Vs Is | |
100v RMS 1A RMS | |
| | | |
+----------+---------+------+-------------------+
The generator output impedance is 50 ohms.
Dissipation in the generator resistors is always
50 Watts plus Pref.
With a shorted or open load, the power dissipation
in the generator is 100 W.

Numerical coincidence as proof that Pref is always
dissipated in the generator. :-)

On a more serious note, how would you analyze this
generator using reflected power and constructive
and destructive interference?

...Keith

It's easy to lose sight of what's important when you get bogged down
in numerical coincidences and the like. To me, some things are
clearly important with respect to analyzing such systems:

1. If a generator is linear and matched to a line (Zgen = Zline, not
Zgen* = Zline), then no matter where a "reverse" signal comes from,
that signal does not reflect at the source:line junction. The
"reverse" signal can come from a reflection at a load, from another
generator at the other end of the line, from something feed in through
a coupler, from an electric eel biting the line--it doesn't matter.
There is no need for an analysis involving "constructive" or
"destructive" interference.

1a. Just because a "reverse" signal on the line does not reflect at
the generator:line junction, that does NOT mean that additional power
is dissipated inside the source.

2. You MUST have an accurate model of the inside of the source to
know how it will respond to some particular load and to signals that
impinge on its output port. With respect to figuring out what goes on
inside the source and what power may or may not be dissipate there,
there is NO advantage to knowing how the load or signals got there.

3. To correctly analyze conditions on a line that's fed only from one
end, with a load on the other end, there is NO NEED OR ADVANTAGE to
know what goes on inside the generator (beyond knowing the power it
delivers to that effective load, perhaps).

3a. There may be some advantage in knowing the source impedance of a
generator (or transmitter) in calculating the power delivered to a
load at the source's output port, but there is no advantage to knowing
it if you want to determine the standing wave ratio or reflection
coefficient on the line, or what net impedance that line+load presents
to the source; that is all determined solely by the line and the load.

The stuff about constructive/destructive interference with respect to
figuring out what happens inside a source is, to me, just so much
dancing on the head of pins. Welcome to dance if you so wish, but I'd
just as soon sit that one out.

Cheers,
Tom