On 5/19/2011 12:27 AM, K7ITM wrote:
On May 18, 3:42 pm, John wrote:
...
I'm not speaking for Wim, but I think we are both saying the following:

* You have a known load
* You have a transmission line with known characteristics
* Is is possible to use a Smith chart to get the impedance at the input
to the transmission line.
* We now know the load applied to the transmitter.

All we need to know we get from the chart. We admit that reflections are
responsible for the impedance transformation from load to line input.
But, we don't need to know anything about the reflection details, energy
content of the line, nor how light would like it.

So, we are saying that the load at the line input can be viewed as a
lumped circuit. So now we have a transmitter loaded with a lumped
circuit for further analysis.

That's all. It's simple.

John

Exactly so, John. Good summary. So long as the transmitter's
bandwidth is small enough that you are always operating practically at
steady-state conditions, the transmitter can't tell the difference
between whatever assembly of transmission lines and lumped loads
distributed along those lines you want, and a simple lumped circuit
that presents the same impedance as the steady-state value of the
jumble of transmission lines out there. (For very narrow-band loads,
you might want to use a lumped equivalent that presents sensibly the
same impedance as the load across the whole transmitted bandwidth, not
just at one point.)

It is NOT that anyone is assuming "faster than speed of light," it's
that we're recognizing that the (HF voice-bandwidth) transmitter is
slower than molasses relative to the propagation times involved in a
couple hundred feet of coax, or probably even a couple thousand feet.
The attenuation per foot of the lines we use is high enough that it's
just about impossible to deviate significantly from steady-state
conditions for the bandwidths we use.

That's certainly not true for pulsed radar signals, or for fast-scan
TV, or for other wideband signals. In those cases, you'll probably
find it pays to insure the line is matched to the load so there aren't
significant reflections, and you may want to arrange the source (PA/
transmitter) to have an output impedance close to the line impedance
so it absorbs any reflections that do happen at the load end of the
line. (If you want to get fancy, you might use a circulator to insure
dissipation of such returning signals.)

Cheers,
Tom


I understand, Tom, and thank your for your input. Of course, we are
discussing an ideal setup, so I did not emphasize those points. But, I
know you know that. Nevertheless, thank your for the disclaimers.

Cheers & 73,
John