Radio913 wrote:
I will try the experiment when i get the chance.

Excellent. There is nothing better than seeing it with your own
eyes.

I don't need to see it with my eyes,
when i know you can't get a larger RMS voltage reflected from a capacitor, than
the RMS that charges it.

Scientists are usually interested in producing models which will
allow them to predict the behaviour of the real world. To do this
it is necessary to check the predictions of the models against
the real world. If you find it unnecessary to do this, then you
are not interested in producing models which can predict real
world outcomes and your models can be anything you want.

But such models won't have much utility.

....Keith

Cecil Moore wrote:

wrote:
Can you expand on why you think a TDR causes me difficulties?
I can't think of any reasons.

There's more than just voltage in those returned pulses.

Yes, there is actually energy in them; easily computed at any point
on the line by using the voltage and current waveform at that point.

As for standing waves, I have no difficulties with forward
and reflected voltage waves. They work perfectly fine.

That takes care of the E-field. But do you think a wave can exist
without an H-field? If not, the wave possesses energy, by definition.
Energy flowing past a point is power.

Your voltage-only waves violate the conservation of energy
principle and the accepted laws of physics for EM waves.

I somewhat sympathize with your dilemma. You have latched on to
these forward and backwards waves for so long that you have started
to believe that they are real and are therefore ascribing to
them all the properties one would expect of a real EM wave such
as current and power.

Some of the authors you quote are not so convinced of their reality.
Consider this quote from sometime back....

"Johnson continues: "We can regard the first term in this
expression as the power associated with the forward-traveling wave,
and the second term as the reflected power (associated with the
rearward-traveling wave)""

As Peter points out in

http://groups.google.ca/groups?q=reg...ing.net&rnum=5

the use of the word "regard" is critical to this passage. Mr Johnson
seems well regarded and it seems likely that if he had intended the
passage to mean "is" he would have used "is" rather than "regard".

Just for a brief moment attempt to relax your hold on forward and
reverse waves and think of the real voltages and currents on the
line. These can be read with real voltmeters and real current meters.
These are the voltages and currents that have to satisfy Es and Hs
and energy flows and power. (And before Richard H pipes up about
directional voltmeters, I take this opportunity to remind him that
all a directional voltmeter actually detects is the real voltage
and current at the point of insertion in the line; all the rest
is computation based on the real voltage and current).

Forward and reflected voltage and current waves are convenient ways
of describing the real voltage and current distributions on the line,
but that does not make them real. And the fact that they correctly
predict the voltages and currents but sometimes fail to predict the
power (when Z0 is not real, for example, but there are many other
examples in simple circuit theory) is a strong indicator that they
are a convenience and not a reality.

Consider a mundane example in the physical world. You have a post
supporting two clotheslines each leaving the post at 90 degrees.
At 135 degrees from the clotheslines is a single guy wire to keep
the post from bending. While for the purposes of analysis you can
pretend that there are two guy wires and this will assist you in
discovering the forces involved, never forget that there is really
only one for otherwise you may be sorely surprised.

Much like this example, the superposition of voltages is a useful
analytical technique, but one must always be aware of its
limitations or one will be lead quite astray.

When one starts believing that the intermediate results represent
reality, trouble begins. It is for this reason that quality authors
use mushy words like "regard" when describing these intermediate
results and not solid words like "is".

....Keith

Cecil Moore wrote:

wrote:
Forget that forward and reverse stuff. That's not what is radiated.

Uh Keith, power delivered to the antenna equals forward power minus
reflected power. I notice you have not provided a way for standing
waves to develop without the existence of reflected waves. Do you
also believe that standing waves don't exist?

I think you have the tail wagging the dog a bit here.

It is the real voltages and currents on the line which deliver the
power to the antenna. These real voltages and currents sometimes
are in a standing wave pattern on the line. This standing wave
pattern can be described by postulating and using forward and
reflected voltage and current waves. Superposition lets you
compute the real voltages and currents but this does not make
the postulated forward and reflected voltages and currents
real.

And this model works so well (unfortunately) that, even though
in general you can not use superposition with power, in the
special case of lines with non-complex Z0, it even works for
power. This fluke, of course, serves to reinforce in the minds
of many the 'reality' of these forward and reflected powers.

When 'the many' encounter the general case, they are quite
perturbed that superposition no longer works for power and
strive mightily to modify the flawed model to account for
the discrepancies rather than just letting go of the model.

The reality is that forward and reflected voltage and current
waves are a convenience that assist in solving problems but
only real voltages and currents move energy.

....Keith

Cecil Moore wrote:

wrote:
When you use a pulse, p(t) = v(t) * i(t) shows the pulse going
by and coming back too, if it reflects. But for that, you have to
look at things in the time domain, something a number of readers
here refuse to do for continuous sinusoidal excitation. When you
look at continuous sinusoidal excitation in the time domain all
the information you need about power is provided without having
to resort to Pfwd and Prev.

Yes, but those steady-state shortcuts often lead to a distorted
view of reality. Exactly what magic happens at the instant when a
system goes from the transient state to steady-state?

I am unsure where you construed "steady-state" in the original
passage. It is, of course, in the time domain that one can explore
the non-steady state mature of the system.

....Keith

Richard Harrison wrote:

Keith wrote:
"---it will turn out that there is no value in forward and reverse power
on a line with non-real Zo."

The Bird wattmeter works well enough on coax lines used between the
usual transmitter and antenna. Loss produces reactance in coax. We don`t
seek lossy coax. If it becomes lossy, the line is likely to be replaced.

You, of course, are correct. I overstate the case slightly in
an attempt to get readers to let go of their model. Once they
are free of forward and reverse power as being an accurate
model of reality, they are then free to understand what a
Bird really indicates.

And it is useful for lines with near real impedances (as many
RF lines are) as a TLI. And in cases where reflected power is
near zero, it will even give a hint as to how much power is
being delivered to the load (within 5% of full scale).

The question is, does the reader understand how it works,
why it is useful and its limitation; or does the reader
believe the markings on the scale and think that it is
actually MEASURING the power in a forward and reverse
wave.

....Keith

I don't need to see it with my eyes,
when i know you can't get a larger RMS voltage reflected from a capacitor,
than
the RMS that charges it.

Scientists are usually interested in producing models which will
allow them to predict the behaviour of the real world. To do this
it is necessary to check the predictions of the models against
the real world. If you find it unnecessary to do this, then you
are not interested in producing models which can predict real
world outcomes and your models can be anything you want.

But such models won't have much utility.

...Keith



"Scientists" are also human too, and tend to want to disregard or not even
try to measure data that may contradict their models. This makes them feel
comfortable that they are "right".


Slick

wrote:
This standing wave
pattern can be described by postulating and using forward and
reflected voltage and current waves. Superposition lets you
compute the real voltages and currents but this does not make
the postulated forward and reflected voltages and currents
real.

Can you think of any other way for standing waves to exist
except for the superposition of a forward wave and a reverse
wave? Until you can describe how standing waves can even exist
without a forward wave and a reverse wave, it doesn't do any
good to deny their existence.
--
73, Cecil http://www.qsl.net/w5dxp



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Cecil Moore wrote:

wrote:
I somewhat sympathize with your dilemma. You have latched on to
these forward and backwards waves for so long that you have started
to believe that they are real and are therefore ascribing to
them all the properties one would expect of a real EM wave such
as current and power.

So what causes standing waves if there are no forward and backwards
waves?

When you solve the differential equations describing the circuit
with distributed capacitance, inductance, resistance and conductance,
given the excitation and boundary conditions, you discover that
the voltage and current distributions along the line form a
standing wave. No need for reflections at all.

It is a whole lot more convenient using the reflection model
though.

Just for a brief moment attempt to relax your hold on forward and
reverse waves and think of the real voltages and currents on the
line.

I will just as soon as you demonstrate how to generate standing
waves without the existence of a forward wave and reverse wave.

OK. Go for it.

....Keith

Radio913 wrote:

I don't need to see it with my eyes,
when i know you can't get a larger RMS voltage reflected from a capacitor,
than
the RMS that charges it.

Scientists are usually interested in producing models which will
allow them to predict the behaviour of the real world. To do this
it is necessary to check the predictions of the models against
the real world. If you find it unnecessary to do this, then you
are not interested in producing models which can predict real
world outcomes and your models can be anything you want.

But such models won't have much utility.

"Scientists" are also human too, and tend to want to disregard or not even
try to measure data that may contradict their models. This makes them feel
comfortable that they are "right".

I completely agree.

So can you overcome your "tend[ancy] to want to disregard or
not even try to measure data that may contradict their models."?

....Keith

| wrote:
| ...
| As has been aptly demonstrated in another thread, it does
| not work for lines with complex Z0.
| ...

Dear Keith,

Could you tell me please
in which thread?

Sincerely,

pez
SV7BAX