Begging your pardon, but don't TDR's examine the transient response of
a system, rather the steady state response?

ac6xg


You're correct, of course, Jim, but I was intuitively assuming we'd not be continuing the use of the TDR with
the sine wave signal. I'm sure my intuition wasn't communiated, sorry.

Walt

Of course, real signals aren't just a single pulse but any CW signal can
be represented as a series of pulses. One VNA I used years ago (Wiltron)
allowed you to analyse a network by driving it with pulses ,
capturing the pulse or transient response and then doing a transform to
go from the time domain to the frequency domain, thus producing the
steady-state response. Very handy for analysing antennae in a confined
space. The time window was set to exclude reflections from walls etc. so
one didn't need an anechoic chamber.
Alan

On Apr 16, 2:30 am, Alan Peake
wrote:
Jim Kelley wrote:

I guess I may have been 'intuiting' too much, myself. Since virtual
shorts and opens only appear in the steady state, I wouldn't expect
pulses to reflect off of them. I don't expect sine waves to reflect
off of them in steady state either for that matter, but that remains a
point of contention apparently.

73, Jim AC6XG

Actually Jim, virtual shorts etc. act the same for pulse systems as for
CW systems. The classic case is the rotating joint in radar systems.
Alan

But isn't that a number of cycles of RF?
Enough to reach 'steady-state'?

When I read the word 'pulse', I think rising edge, flat top,
falling edge, and the virtual short is quite different than
a real short for this signal.

....Keith

Keith Dysart wrote:
So, out of curiosity, what do you think the outcome of my
experiment would be?

With an IC-706? I don't know. Others have tried it with
varying results.

Do 10 cent resistors ever work? Or is a circulator always needed
to prevent re-reflections?

Your 10 cent resistor can be thought of as a low dB
pad of sorts. It will attenuate but not eliminate
re-reflection. Again, let me remind you of Ramo &
Whinnery's warning not to attach importance to
what is calculated to happen inside an equivalent
source. There are models available for virtually
any amplifier you might choose but I don't know
how those models handle reflections.
--
73, Cecil http://www.w5dxp.com

Alan Peake wrote:
Of course, real signals aren't just a single pulse but any CW signal can
be represented as a series of pulses.

I think within the present context, we would say a CW dot
or dash has a transient response at the leading edge and
trailing edge and achieves steady-state in the middle.
I once calculated that it takes about ~30 cycles with
rho=0.707 to get very close to steady-state.
--
73, Cecil http://www.w5dxp.com

Alan Peake wrote:
But I still can't see that a virtual short would be
different to a real short.

A virtual short is (Vfor-Vref)/(Ifor+Iref) = 0
where |Vfor| = |Vref|

For the virtual short to exist, two equal magnitude
EM waves have to be *flowing through* the virtual
short. EM waves *cannot flow through* a real short.

For real shorts:
V/I=0 is a result caused by the real short.

For virtual shorts:
A virtual short is a result caused by V/I=0.
--
73, Cecil http://www.w5dxp.com

Actually Jim, virtual shorts etc. act the same for pulse systems as for
CW systems. The classic case is the rotating joint in radar systems.
Alan


But isn't that a number of cycles of RF?
Enough to reach 'steady-state'?

When I read the word 'pulse', I think rising edge, flat top,
falling edge, and the virtual short is quite different than
a real short for this signal.

...Keith

I see your point. But as a CW signal can be thought of as series of
rectangular pulses,
then the effect of a virtual short/open/whatever, should be the same.
For a such a series,
the resultant signal at any point in the TL is of course quite different
from a single pulse. But I still can't see that a virtual short would be
different to a real short.
Alan

Cecil Moore wrote:
Alan Peake wrote:
But I still can't see that a virtual short would be different to a
real short.

I forgot to say, the following applies only to a virtual
short in the absence of a physical impedance discontinuity.
A virtual short at a physical impedance discontinuity
involves interference between forward wave components and
reflected wave components.

A virtual short is (Vfor-Vref)/(Ifor+Iref) = 0
where |Vfor| = |Vref|

For the virtual short to exist, two equal magnitude
EM waves have to be *flowing through* the virtual
short. EM waves *cannot flow through* a real short.

For real shorts:
V/I=0 is a result caused by the real short.

For virtual shorts:
A virtual short is a result caused by V/I=0.
--
73, Cecil http://www.w5dxp.com

On Apr 15, 11:30 pm, Alan Peake
wrote:
Jim Kelley wrote:

I guess I may have been 'intuiting' too much, myself. Since virtual
shorts and opens only appear in the steady state, I wouldn't expect
pulses to reflect off of them. I don't expect sine waves to reflect
off of them in steady state either for that matter, but that remains a
point of contention apparently.

73, Jim AC6XG

Actually Jim, virtual shorts etc. act the same for pulse systems as for
CW systems.

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

Cecil Moore wrote:
Jim Kelley wrote:

Cecil Moore wrote:


Jim Kelley wrote:

Roy is absolutely right, Cecil. Interact is a very poor choice of
terms in this discussion.


Roy did NOT say "interact" was a poor choice of terms.


That's correct. I said that interact is a poor choice of terms.


But you implied that is what Roy said just above.

I observed that Roy is absolutely right, and, that 'interact' is a
very poor choice of terms in this discussion. I said it because waves
do not, according to the definition of the word, 'act upon one
another'. That of course does not mean there isn't a net effect when
they superpose. It simply means that waves do not effect other waves.
At this point I really don't expect you to understand that.

chose to use it as did Hecht. Hecht says waves interact.
Roy says they don't interact.


As I said, Roy is correct.


Roy is right and Hecht is wrong???

If Hecht actually weighed in on the subject, he would agree with Roy.
His use of the term caused you to infer something that he, I assure
you, did not intend to imply.

And the funny thing is, you say that even you know of instances in
which the net fields are zero, and yet the waves propagate beyond that
point.

Where do the reflected waves go that propagate beyond that
point and are measured as zero amplitude by a Bird wattmeter?

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. It doesn't matter which direction they're
traveling; in no instance do waves destroy or act upon other waves,
totally or partially. The result of their superposition may differ
from one case to the next, but the phenomenon itself does not. But
again, at this point I don't expect you to understand this.

Dr. Best said those zero energy canceled waves propagate right
into the source.

He might have a point. But since cancelled waves convey no energy, it
doesn't really matter one way or the other, as others here have noted.

Are
you making that same stupid assertion?

All I'm trying to do is point out when you make a stupid assertion.

I think you just like to argue.


No, I honestly think we would pinpoint our differences. But,
of course, you would never agree to such.

I've already made the differences as clear as I possibly can in every
way I can think of, Cecil. That is why at this point I really don't
expect you to understand. You could, but I think it's pretty apparent
that you have too much invested in your personal theories.

73, Jim AC6XG

Walter Maxwell wrote:

On 15 Apr 2007 15:10:11 -0700, "Jim Kelley" wrote:


On Apr 15, 12:50 pm, Walter Maxwell wrote:


Seems to me that the only disagreement with my original posting is whether the condition at the stub point can
be called a 'virtual' short circuit.

Hi Walt,

Most everyone has directly expressed complete agreement with that
idea.


Here's the recurring theme:


*******Virtual impedance discontinuities do not cause
reflections.********


73, Jim AC6XG


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. Looks like I've had it all wrong for lo these many years. I thought I've been reading
the same references as all the other posters.

Walt

Hi Walt,

Your entire treatise is brilliant and useful with the one exception
noted clearly above. Perhaps you could cite a single one of those
references (other than Reflections of course) which directly
contradicts my simple observation of an extremely well understood
fundamental of nature.

Obviously a revision of that one circumstantial claim would have
absolutely no impact on element spacing or how waves interfere, and it
would in my view perfect the book. Once you have the currents and
fields worked out properly, they look after themselves. You don't
need to help them by inventing another mechanism for them to do their
job. Faraday, JC Maxwell and others have already worked that out to
most everyone else's satisfaction.

I think the discussion of virtual impedances and reflection
coefficients is useful as an analytical tool. But it should also
follow that the behavior being attributed to virtual entities is
likewise, virtual i.e. it behaves as though....; that the actual cause
of reflections is the real physical boundaries. That is the more
reasonable approach, Walt. IMO.

73, Jim AC6XG