Jim Kelley wrote:
One should also carefully consider the more interesting variation of the
problem: an open transmission line. In the steady state we have 100
watts forward, 100 watts reflected, 200 Joules in the line, and 0 watts
being sourced by the generator. :-)

Expanding on my earlier response - For the first two seconds,
the source doesn't know it is looking into an open transmission
line so a 100 watt source would faithfully output 200 joules
into a one second long open circuit transmission line. That
200 joules cannot be destroyed. Is it mere coincidence that
the forward and reflected waves are 100 joules/sec*(one second),
exactly equal to the 200 joules supplied by the source?
--
73, Cecil http://www.qsl.net/w5dxp

is that line 1 second long, or 1/2 second long?? a 1 second long line would
take 2 seconds worth of energy from the generator before the reflection
returned to let it know the line is terminated. this is of course very
important if you are measuring lines with tdr's where you can really see
those returned waves, of course the time it takes them to get back to the
tdr is double the one-way travel time... mess it up and you are looking for
faults twice as far down the line as you calculate.


"Cecil Moore" wrote in message
. com...
Jim Kelley wrote:
One should also carefully consider the more interesting variation of the
problem: an open transmission line. In the steady state we have 100
watts forward, 100 watts reflected, 200 Joules in the line, and 0 watts
being sourced by the generator. :-)

Expanding on my earlier response - For the first two seconds,
the source doesn't know it is looking into an open transmission
line so a 100 watt source would faithfully output 200 joules
into a one second long open circuit transmission line. That
200 joules cannot be destroyed. Is it mere coincidence that
the forward and reflected waves are 100 joules/sec*(one second),
exactly equal to the 200 joules supplied by the source?
--
73, Cecil http://www.qsl.net/w5dxp

Dave wrote:
is that line 1 second long, or 1/2 second long?? a 1 second long line would
take 2 seconds worth of energy from the generator before the reflection
returned to let it know the line is terminated. this is of course very
important if you are measuring lines with tdr's where you can really see
those returned waves, of course the time it takes them to get back to the
tdr is double the one-way travel time... mess it up and you are looking for
faults twice as far down the line as you calculate.

The line is 1 second long. The source is generating 100 watts. So the
number of joules generated starting at t=0 is 100N where N is the
total number of seconds. At the end of 30 seconds, the generator will
have sourced 3000 joules which must be conserved.

I will generate an EXCEL spreadsheet at work today and post it to
my web page when I get home. It will cover the first 30 seconds
in 1 second increments. It will show that of the 3000 joules sourced
by the generator during that first 30 seconds, only 2700 joules have
reached the load. The other 300 joules are contained in the 200W
forward wave and 100W reflected wave.

(200 watts)(one second) = 200 joules in the forward wave

(100 watts)(one second) = 100 joules in the reflected wave

200 joules + 100 joules = 300 joules not delivered to the load
--
73, Cecil http://www.qsl.net/w5dxp

Cecil Moore wrote:

Jim Kelley wrote:

One should also carefully consider the more interesting variation of
the problem: an open transmission line. In the steady state we have
100 watts forward, 100 watts reflected, 200 Joules in the line, and 0
watts being sourced by the generator. :-)


Expanding on my earlier response - For the first two seconds,
the source doesn't know it is looking into an open transmission
line so a 100 watt source would faithfully output 200 joules
into a one second long open circuit transmission line. That
200 joules cannot be destroyed. Is it mere coincidence that
the forward and reflected waves are 100 joules/sec*(one second),
exactly equal to the 200 joules supplied by the source?

But you're missing, or trying to
circumvent, the most interesting aspect
of the problem. It's the one which
highlights the very core of our
disagreement. The energy stored in the
line, remains stored in the line as long
as steady state is maintained without a
single Joule of additional energy moving
into or out of the line. To me, this
illustrates clearly how the fields at
the impedance interfaces of a matching
transformer can be maintained without
requiring multiple rereflections of
energy. I'm hoping some day you'll see
it too.

73, ac6xg

In article ,
Jim Kelley wrote:

Expanding on my earlier response - For the first two seconds,
the source doesn't know it is looking into an open transmission
line so a 100 watt source would faithfully output 200 joules
into a one second long open circuit transmission line. That
200 joules cannot be destroyed. Is it mere coincidence that
the forward and reflected waves are 100 joules/sec*(one second),
exactly equal to the 200 joules supplied by the source?

But you're missing, or trying to
circumvent, the most interesting aspect
of the problem. It's the one which
highlights the very core of our
disagreement. The energy stored in the
line, remains stored in the line as long
as steady state is maintained without a
single Joule of additional energy moving
into or out of the line. To me, this
illustrates clearly how the fields at
the impedance interfaces of a matching
transformer can be maintained without
requiring multiple rereflections of
energy. I'm hoping some day you'll see
it too.

Jim,

How would your model view the case in which the source transmitted
into the T-line for 1.5 seconds (delivering 150 joules into the line)
and was then disconnected, leaving both ends of the T-line
open-circuited.

In this case, you'd continue to have 150 joules of total energy stored
in the line (modulo the amount of energy which does manage to radiate
out sideways). However, there would be periods (of 500 milliseconds,
one per two seconds) when the voltage near the source end of the
T-line, and the amount of current flowing through the T-line in this
area, were both zero. For the intervening 1.5 seconds out of each 2
seconds, there would be strong current flow through this portion of
the line (having a standing-wave characteristic for all but a very
short transition time on either end).

This state of affairs can, I don't doubt, be modeled purely as a
matter of interaction and interference between fields. The model
would appear to me to have to become extremely complex, in order to
produce the correct results at all points over the two-second
long-term periodicy of this system.

It can also be modeled as the effect of interference between forward
and reflected waves... and this is a somewhat simpler model to use to
describe systems such as this which do not exhibit a purely
steady-state behavior.

As far as I can see, *neither* of these models (fields, or reflected
waves) is fundamentally superior to the other. They are both equally
capable of producing an accurate description of the output of the
system at any point in time, given a set of inputs to the system.
Hence, by the usual standards of the validity of a scientific theory,
both models are equally valid.

Under certain circumstances, one model may be more "practically
useful" than the other. My impression is that your model of fields
may be more useful in looking at relatively local behavior (e.g.
within a wavelength or two) within a system that's at, or close to a
steady state. Cecil's preferred model of reflected power may be more
practical to use (i.e. simpler computations to produce a valid result)
when dealing with systems far from steady state.

In between those two extremes, it looks to me as if which model one
prefers is simply that - a personal preference.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

"Jim Kelley" wrote in message
...
But you're missing, or trying to circumvent, the most interesting aspect
of the problem. It's the one which highlights the very core of our
disagreement. The energy stored in the line, remains stored in the line
as long as steady state is maintained without a single Joule of
additional energy moving into or out of the line. To me, this
illustrates clearly how the fields at the impedance interfaces of a
matching transformer can be maintained without requiring multiple
rereflections of energy. I'm hoping some day you'll see it too.

You are, of course, talking about NET energy, and I agree with you about net
energy. But it is easy to prove we are NOT dealing with net energy by
observing
ghosting on a TV signal. It is easy to prove that multiple re-reflections
are
indeed actually occurring in reality.

Your argument is that if 5000 cars cross the bridge into the city during the
day
and 4990 cars cross the bridge out of the city during that day, that only 10
net cars have crossed the bridge in a day. I agree with your net figure but
note that the bridge cannot be replaced with a 1 car ferry boat operating
tens times per day which is akin to your above argument.
--
73, Cecil http://www.qsl.net/w5dxp