Ian White, G3SEK wrote:
So now we have two cases, one where reflections happen and one where
they can't. The transmitter suffers exactly the same problem in both
cases, entirely because it sees the same wrong value of wrong load
impedance.

The transmitter possesses an IQ of zero. Hopefully, yours, mine, and
others exceeds that zero value. The goal is understanding. The goal
is (hopefully) NOT a reductio ad absurdum.

"Reflected power" simply doesn't enter into it.

If you don't care about the facts, reflected power doesn't enter into it.
But if you are trying to understand the physics, certainly the reflected
power enters into it. If you don't care about understanding physics, by
all means, go with your steady-state shortcuts. But please don't try to
talk all the people who are trying to understand the physics into just
accepting your steady-state religion on faith.
--
73, Cecil http://www.qsl.net/w5dxp



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Ian White, G3SEK wrote:

W5DXP wrote:
Even if the 1625's can't tell the difference, W5DXP can. :-)

That's the whole point - the *only* difference is a conceptual one that
exists inside your mind.

BS, Ian. My pet cockroach can tell the difference between a V/I
ratio resistance and a resistor. Why can't you?
--
73, Cecil http://www.qsl.net/w5dxp



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Ian White, G3SEK wrote:
Yes! That principle of impedance substitution is so simple, so
fundamental, some people never notice it's there at all.

And you would apparently like to pull the wool over the eyes of everyone
who notices that the definition of impedance has changed in the process.
Shame on you for that attempt at obfuscation!
--
73, Cecil http://www.qsl.net/w5dxp



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Dave Shrader wrote:
Now the question becomes: "Am I thinking like Cecil or is Cecil thinking
like me?"

Maybe we are both thinking like Ramo and Whinnery, two pretty smart individuals,
who taught me most of what I know about fields and waves.
--
73, Cecil http://www.qsl.net/w5dxp



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It's important not to confuse the sort of pulses or steps used in TDR
with transient sine wave conditions.

It's perfectly valid to derive the sinusoidal steady state conditions on
a transmission line by looking at the transient conditions that occur
from the time the source is first turned on. And because of the
transient nature of the signal, the most practical way to approach this
analysis is in the time domain.

TDR also (obviously) involves time domain analysis. But it's quite
different. Sinusoidal transient analysis assumes a sinusoidal source
that stays on once it's turned on. But TDR involves either a pulse type
source that's off when the pulse reflection returns, or a step type
source that provides a DC step to the transmission line. In this case,
the source voltage is a stable DC value from the time of the initial
step. In the case of the sinusoidal source, the source voltage continues
changing while the transients are propagating.

In both cases, the sum of all forward and reflected voltages or currents
have to sum to the correct values at all points, and this knowledge can
be used to derive various wave components. But the results and some of
the methods can be very different for the two cases. For example, when a
reactive load or impedance bump is present, a simple reflection
coefficient can be calculated for the sine wave, based on the reactance
at the sine wave's frequency. The reflected wave will be a simple
replica of the incident wave, altered only in phase and amplitude. You
can't do this with a pulse or step; a reactive load changes its shape,
defying a simply defined reflection coefficient. (Some confusion arises
because of the use in TDR of a reflection coefficient, usually denoted
rho. It's the same as the magnitude of the sine wave reflection
coefficient -- but only if the anomaly or load causing the reflection is
purely resistive and a constant value from DC or a low frequency up to
the equivalent maximum frequency contained in the TDR pulse and viewable
with the TDR system. With some TDR systems having equivalent bandwidths
of over 50 GHz, this can be an onerous requirement.) Another important
difference is what happens to a returning wave when it reaches the
source -- reaction to a source that's off, at a stable DC value, or at
some point in the cycle of a sinusoidal waveform is different.

TDR is a very valuable technique, providing important information and
illuminating insights about transmission line phenomena. But great care
has to be taken in extrapolating TDR observations to what happens in a
sinusoidal transient or steady state environment. As readers have seen,
I'm very wary of explanations of sinusoidal phenomena, either steady
state or transient, that depend on drawing parallels to TDR results. You
should be, too.

Roy Lewallen, W7EL

W5DXP wrote:
Tdonaly wrote:

I would like to know why Cecil, for instance, uses pulses, as in a
TDR, in
order to argue a steady state point.


Do steady-state signals obey one set of laws of physics and pulses
obey a different set of laws of physics? You seem to feel so but
I just don't have that much faith!

The useful steady-state shortcuts have developed into a religion that
has no place in science. I am not opposed to steady-state shortcuts.
I am opposed to the steady-state religion that has evolved based on
faith. "Have faith, there is no such thing as reflected waves."
"Have faith, photons can be exchanged between equivalent inductors
and capacitors in a transmission line so they move sideways at less
than the speed of light instead of lengthways at the speed of light."

Particle physicists would really be interested in any proof of that.

"Have faith, a V/I ratio is identical to a physical impedance because
a source, with an IQ of zero, cannot tell the difference." "Reflections
completely disappear the instant that steady-state conditions are reached."
There are many more faith-based characteristics of the steady-state model.
These are just the ones that come to mind.

Roy Lewallen wrote:
It's important not to confuse the sort of pulses or steps used in TDR
with transient sine wave conditions.

Why? Do they obey different laws of physics?

TDR also (obviously) involves time domain analysis. But it's quite
different.

Why? Does a TDR obey a different set of physics laws?

The reflected wave will be a simple
replica of the incident wave, altered only in phase and amplitude.

Not if it contains random noise and all waves contain random noise.

TDR is a very valuable technique, providing important information and
illuminating insights about transmission line phenomena. But great care
has to be taken in extrapolating TDR observations to what happens in a
sinusoidal transient or steady state environment.

Why? Does a TDR obey a different set of physics laws than sine waves?
--
73, Cecil http://www.qsl.net/w5dxp
"One thing I have learned in a long life: that all our science, measured
against reality, is primitive and childlike ..." Albert Einstein



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It might be of interest to note that old time video amplifiers with peaking
coils sometimes caused ghosts on the screen that were indistinguishable
from ghosts caused by reflections.

Tam/WB2TT
"Ian White, G3SEK" wrote in message
...
W5DXP wrote:
Think what would have happened if you had measured the impedance at
the TX end of your o/c transmission line (very high or very low,
depending on the length) and replaced it with a resistor and
inductor/capacitor giving the same value of R +/- jX.
There's no transmission line, so no traveling waves of anything, and
no reflections - just a transmitter with a very wrong value of load
impedance. The 1625s would have burned up just the same.

Yes they would, but in that case reflections are not the cause of the
impedance.

Good... hold that thought.

In the first case, reflections are the *CAUSE* of the
impedance that burned up the transmitter.

Correct; and the values of R and +/-jX that the transmitter sees are
calculated by considering the reflected voltage and current waves.

So now we have two cases, one where reflections happen and one where
they can't. The transmitter suffers exactly the same problem in both
cases, entirely because it sees the same wrong value of wrong load
impedance.

"Reflected power" simply doesn't enter into it.


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek

I think the steady state solution would be a lot more palatable if the
analysis started at T=0, when you turn the source on. The extra step of
seeing how you reach steady state makes the latter more "real". One could
almost be convinced that there are no reflections in the steady state;
problem is , there would be no other explanation for standing waves, and I
can measure them with unambiguous instruments.

Tam/WB2TT

Tarmo Tammaru wrote:
I think the steady state solution would be a lot more palatable if the
analysis started at T=0, when you turn the source on. The extra step of
seeing how you reach steady state makes the latter more "real". One could
almost be convinced that there are no reflections in the steady state;
problem is , there would be no other explanation for standing waves, and I
can measure them with unambiguous instruments.

I keep wondering what laws of physics get repealed just as the system
transitions to steady-state. Do photons really start moving from side
to side instead of end to end? Do the standing waves magically sustain
themselves without any reflected waves?
--
73, Cecil http://www.qsl.net/w5dxp



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Did you really read what I wrote?

Roy Lewallen, W7EL

W5DXP wrote:
Roy Lewallen wrote:

It's important not to confuse the sort of pulses or steps used in TDR
with transient sine wave conditions.


Why? Do they obey different laws of physics?

TDR also (obviously) involves time domain analysis. But it's quite
different.


Why? Does a TDR obey a different set of physics laws?

The reflected wave will be a simple replica of the incident wave,
altered only in phase and amplitude.


Not if it contains random noise and all waves contain random noise.

TDR is a very valuable technique, providing important information and
illuminating insights about transmission line phenomena. But great
care has to be taken in extrapolating TDR observations to what happens
in a sinusoidal transient or steady state environment.


Why? Does a TDR obey a different set of physics laws than sine waves?