On Tue, 17 Apr 2007 10:35:11 +1000, Alan Peake
wrote:

So how does one prove or disprove the existence of a real reflection
from a virtual discontinuity?

Mac already covered that in the space of four sentences:
On Sun, 15 Apr 2007 18:30:59 -0500, "J. Mc Laughlin" wrote:

One characteristic of a "virtual short" is that its presence or location is
dependent on frequency. Another characteristic is that signals are expected
to exist on both sides of a "virtual short." One characteristic of a
"physical short" is that it does not depend on frequency. Another
characteristic of a "physical short" is that signals exist on only one side
of the "physical short's" location.

73's
Richard Clark, KB7QHC

Hi Alan -

Reflections measured by a TDR are caused by physical impedance
discontinuities. Virtual impedances are defined by the superposition
of forward and reflected voltages in the steady state. Pulsed systems
offer the ability to study the transient effects of a system by
viewing reflections caused only by changes in the characteristic
impedance of the transmission line. Since TDR doesn't use CW (not to
be confused with Morse Code) it does not operate under steady state
conditions and can therefore neither prove nor disprove the claim for
reflections from virtual impedances.

73, Jim AC6XG

Yes, I had a bit of LNBF (Late Night Brain Fade) when I threw in the
rotary joint example.
What I was trying underline was that there will be a real reflection at
the point where a stub is attached - simply because it becomes a
discontinuity in the TL. A TDR can show this but of course, I agree that
a single pulse won't see the same discontinuity as a CW.
So how does one prove or disprove the existence of a real reflection
from a virtual discontinuity?
Alan

Hi Alan -

Reflections measured by a TDR are caused by physical impedance
discontinuities. Virtual impedances are defined by the superposition
of forward and reflected voltages in the steady state. Pulsed systems
offer the ability to study the transient effects of a system by
viewing reflections caused only by changes in the characteristic
impedance of the transmission line. Since TDR doesn't use CW (not to
be confused with Morse Code) it does not operate under steady state
conditions and can therefore neither prove nor disprove the claim for
reflections from virtual impedances.

73, Jim AC6XG

Hi Jim, not sure if my previous reply got through.
Yes, I have to admit to LNBF (Late Night Brain Fade) when I threw in the
rotary joint example. Of course it is CW in the pulse. I was trying to
underline that a stub puts a physical discontinuity on the TL which will
give a real reflection. But as you point out, this reflection is not the
same as the reflection from a virtual discontinuity for CW. However, if
the CW can be thought of as a series of pulses, then does that not mean
that real reflections occur and that the sum of the reflections for each
pulse looks like they have come from a virtual discontinuity?
If not, how would one go about proving or disproving the idea of
reflections from virtual discontinuities?
Alan

Alan Peake wrote:

However, if
the CW can be thought of as a series of pulses, then does that not mean
that real reflections occur and that the sum of the reflections for each
pulse looks like they have come from a virtual discontinuity?

Hi Alan -

The point of using pulses is that their width is short, and the time
between pulses is long compared to the delay times in the system. In
the case of of CW there will be standing waves all throughout the
system obscuring any possible measurement of transient response.
These pulses only reflect from physical discontinuities in the surge
impedance of the transmission line. Otherwise, TDR would be a
complete wild goose chase; a real cluster _blank_, in the vernacular
of the trade.

If not, how would one go about proving or disproving the idea of
reflections from virtual discontinuities?
Alan

Disproving the idea of reflections from virtual discontinuities would
be done, for instance, and has been suggested, by measuring the
presence of fields beyond the virtual short in a 1/4 wave stub.
Finding waves reflecting instead from the open end sure would not lend
support to the notion. The fact that the idea is inconsistent with
Maxwell's equations doesn't help either.

I don't think there is a way to prove the idea of reflections from
virtual discontinuities. But with certain specific exceptions, a
system could in other ways appear to behave as though reflections are
originating at virtual impedance discontinuities (+/- n half wavelengths).

73, Jim AC6XG

Jim Kelley wrote:
I said it because waves do
not, according to the definition of the word, 'act upon one another'.

But they can act upon one another, Jim. The Florida State web
page says so. The Melles-Groit web page says so. It says their
energy components are redistributed. How can their energy
components be redistributed if they have no effect on each
other? You really need to join me in the s-parameter analysis.

b1 = s11(a1) + s12(a2)

That's phasor math proving that components of waves a1 and
a2 have an effect on b1 and therefore on each other. Every
time two coherent waves are collinear in the same direction
in a transmission line, they have an effect on each other.
It's called interference, either constructive or destructive.

If Hecht actually weighed in on the subject, he would agree with Roy.

Good grief, Jim, now you are mind-fornicating Hecht. Hecht
would certainly not agree with your obviously false assertions.

His use of the term caused you to infer something that he, I assure
you, did not intend to imply.

Your assurance and three bucks will get me a cup of Starbucks.

Take a look at the interference pattern created in space by two,
separated, coherent, point sources of light. The light waves
propagating from each point sources have absolutely no effect on each
other as they pass through one another, alternately interfering
destructively and constructively as they continue to propagate totally
unaffected by the process.

Yes, because they are not collinear. If they don't intersect,
they also don't interfere. You can find billions of cases where
they don't interfere. That doesn't mean they don't ever interfere.

Just as illustrated on the Florida State web page, when coherent
waves are also collinear, as they are in a transmission line, they
merge into the total wave and cease to exist as separate wave
components.

b1 = s11(a1) + s12(a2)

s11(a1) and s12(a2) lose their identities and merge into b1.
If your statements were true, an s-parameter analysis wouldn't
be valid but it is. Therefore, your statements are false. That's
why you need to wade through an s-parameter analysis because
you don't understand what happens or comprehend the physics
behind it.

It doesn't matter which direction they're traveling;

On the contrary, coherent waves traveling in the same direction
in a transmission line are *collinear*. They merge and permanently
interfere with each other thus proving your strange assertions to
be false.

I've already made the differences as clear as I possibly can in every
way I can think of, Cecil.

But you are uttering assertions that are patently false. Given
two coherent waves traveling in the same direction in a Z0
transmission line, with equal magnitudes, V, and equal phases,
0 deg, what is the total magnitude? Do you even know how to do
phasor math?

V at 0 deg + V at 0 deg = ____________________________

If you need help, ask your supervisor what the answer is.
--
73, Cecil http://www.w5dxp.com

Alan Peake wrote:
So how does one prove or disprove the existence of a real reflection
from a virtual discontinuity?

One sure way would be to demonstrate a reflection from
a virtual impedance where no physical impedance discontinuity
exists. Good luck on that one.
--
73, Cecil http://www.w5dxp.com

Walter, W2DU wrote:
"OK, Jim, if that`s so, then I`ve got to figure out a new way to explain
how antenna radiation patterns are modified by changing the relative
phase of the signals fed to multiple radiators, and by changing the
spacing between the radiators."

Walter`s systen isn`t broken so it shouldn`t be fixed. Signal strength
at a point in space depends on the vector totals of its constituents.
Walter`s totals are determined by positions of the radiators and phases
of the currents in those radiators.

Obviously, where vectors are in-phase they add and where they are
out-of-phase they subtract.

The system works, that`s why the FCC endorses it.

Best regards, Richard Harrison, KB5WZI

Cecil Moore wrote:

Jim Kelley wrote:

I said it because waves do not, according to the definition of the
word, 'act upon one another'.


But they can act upon one another, Jim. The Florida State web
page says so. The Melles-Groit web page says so.

No they don't. If the waves themselves changed, then their resultant
superposition would also change. It's a completely unfounded notion,
Cecil.

It says their
energy components are redistributed.

Which is not the same as saying waves have an effect on other waves.
I said I didn't expect you to understand, and clearly you don't.

How can their energy
components be redistributed if they have no effect on each
other?

I don't know what exactly an "energy component" is, but I would assert
that it would be redistributed in the same way completely
independently of however you or I might happen to feel about it.

His use of the term caused you to infer something that he, I assure
you, did not intend to imply.

Your assurance and three bucks will get me a cup of Starbucks.

Not to mention a more realistic viewpoint.

Take a look at the interference pattern created in space by two,
separated, coherent, point sources of light. The light waves
propagating from each point sources have absolutely no effect on each
other as they pass through one another, alternately interfering
destructively and constructively as they continue to propagate totally
unaffected by the process.


Yes, because they are not collinear. If they don't intersect,
they also don't interfere. You can find billions of cases where
they don't interfere. That doesn't mean they don't ever interfere.

As I said, I don't expect you to understand, and clearly here you don't.

Just as illustrated on the Florida State web page, when coherent
waves are also collinear, as they are in a transmission line, they
merge into the total wave and cease to exist as separate wave
components.

Yes, it very effectively shows how 1 + -1 = 0. Very profound, Cecil.

ac6xg

Alan Peake wrote:

Hi Jim, not sure if my previous reply got through.
Yes, I have to admit to LNBF (Late Night Brain Fade) when I threw in the
rotary joint example. Of course it is CW in the pulse. I was trying to
underline that a stub puts a physical discontinuity on the TL which will
give a real reflection. But as you point out, this reflection is not the
same as the reflection from a virtual discontinuity for CW. However, if
the CW can be thought of as a series of pulses, then does that not mean
that real reflections occur and that the sum of the reflections for each
pulse looks like they have come from a virtual discontinuity?
If not, how would one go about proving or disproving the idea of
reflections from virtual discontinuities?
Alan

If you think of CW as a series of pulses, a "virtual short" occurs only
when an inverted reflected pulse arrives at the same point at the same
time as a non-inverted non-reflected pulse, causing the two to add to
zero. (Or, of course, more complex combinations of multiple pulses
arriving at the same point.) It isn't the same pulse which appears twice
to interfere with itself; it's different pulses of the pulse string,
sent at different times but arriving at the same point simultaneously
due to one being delayed by reflection and the other not. So you see,
the interval between those pulses is critical; if it changes, then the
location of the "virtual short" changes. This is analogous to the steady
state CW situation where the location of the "virtual short" changes
with frequency.

In theory, you could prove that reflection isn't occurring from a
"virtual discontinuity" by making an abrupt change in the excitation,
for example abruptly changing its level, then noting that the effect of
the change isn't seen back at the input until it propagates through the
"virtual discontinuity", on to physical discontinuities where reflection
actually takes place, and back. This might be difficult to do in
practice, though, except with some fairly sophisticated equipment or
very long lines because of the time intervals involved.

But let's suppose that you did somehow prove that a "virtual
discontinuity" reflects waves. Then you have to explain the mechanism by
which waves alter each other in a linear medium. Since you won't find
any such mechanism described or explained in any reputable text, you'll
have to come up with some pretty creative alternative physical laws or
derivations on your own. They would have to explain such interesting
phenomena as the diode-like nature of a "virtual discontinuity" -- that
is, why the fields interact going one way and not the other. Also,
you'll need to come up with equations which take into account the
infinite number of reflections from the "virtual discontinuities" which
occur at nearly every point along any line not terminated in its
characteristic impedance. At the end of the day, the results of those
equations have to be the same as those which assume no reflections from
"virtual discontinuities", because equations assuming no such reflection
have been in use for over a century and have so far not been found to be
in error.

Any of the proponents of "virtual discontinuity" reflections up to it?

Roy Lewallen, W7EL

Richard Harrison wrote:
Obviously, where vectors are in-phase they add and where they are
out-of-phase they subtract.

In fact, Jim Kelley's assertion that there is no interaction
between waves would result in isotropic radiation in the
far field of every antenna if one went out far enough to
measure the waves after they are propagating free of each
other. I wonder if NASA knows that?
--
73, Cecil http://www.w5dxp.com