On Apr 14, 6:01 pm, Roy Lewallen wrote:
Walter Maxwell wrote:

Consider my two explanations, or definitions of what I consider a virtual short--perhaps it should have a
different name, because of course 'virtual' implies non-existence. The short circuit evident at the input of
the two line examples I presented---do you agree that short circuits appear at the input of the two lines? If
so, what would you call them?

I'd call them "virtual shorts". If they were short circuits, we should
be able to connect a wire across the transmission line at that point
with no change in transmission line operation. But we can't. While
things will look the same on the generator side, they won't be the same
beyond the real short. So they aren't short circuits.

I want to bring up another reason to be very careful to even call them
"virtual shorts." They are virtual shorts only at certain
frequencies. To me, that is a very important distinction. Keeping
that frequency dependence in mind helps me be ever aware that they are
not anything like a real short.

Cheers,
Tom

Roy Lewallen, W7EL wrote:
"If you`ll read what I`ve written, you`ll hopefully see that my only
point of contention is with your claim that waves reflect from a
"virtual short". They do not."

Seems to me they do.

If you are lucky enough to have a copy of Terman`s 1955 opus, we can
reason together.
On page 91 is found Fig. 4-3 Vector (phasor) diagrams showing manner in
which incident and reflected waves combined to produce a voltage
distribution on the transmission line.

At an open circuit, the voltage phasors are in-phase.

E2, the reflected phasor, rotates clockwise as it travels back toward
the source.

E1, the incident phasor, rotates counter-clockwise as we look back
toward the source.

Looking 1/4-wavelength back from the open-circuit, E2 and E1, each
having rotated 90-degrees, but in opposite directions, are now
180-degrees out-of-phase.

On page 92, Fig. 4-4 shows the current, which summed to zero at the open
circuit, has risen to its maximum value at 1/4-wavelength back from the
open-circuit while the voltage dropped to its minimum, nearly zero,
maybe close enough to declare a "virtual short-circuit", 1/4-wavelength
back from the open-circuit.

What`s a short-circuit? Little voltage and much current.

What`s the difference between a physical short and the virtual short?
Nothing except the shunting conductor.

Is there current flowing at the open-circuit end of the 1/4-wave line
segment? No, the open-circuit won`t support current.

If a high-impedance generator of the same frequency were connected to
the virtual short point on the line, would it also be shorted? Yes.
Where? At the virtual short, not the open-circuit at the end of the
line.

Best regards, Richard Harrison, KB5WZI

K7ITM wrote:

I want to bring up another reason to be very careful to even call them
"virtual shorts." They are virtual shorts only at certain
frequencies. To me, that is a very important distinction. Keeping
that frequency dependence in mind helps me be ever aware that they are
not anything like a real short.

And only in steady state. And only in one direction. Yes, care in needed.

Roy Lewallen, W7EL

Roy Lewallen wrote in news:1323ikbgpa8cfb6
@corp.supernews.com:

K7ITM wrote:

I want to bring up another reason to be very careful to even call them
"virtual shorts." They are virtual shorts only at certain
frequencies. To me, that is a very important distinction. Keeping
that frequency dependence in mind helps me be ever aware that they are
not anything like a real short.

And only in steady state. And only in one direction. Yes, care in
needed.

It is simple inadequate models that lead to the thinking:

In the presence of mismatch, there is a reflected wave.

The anode glows red under mismatch, it obviously is caused by the power
reflected from the antenna mismatch. (The observation is only made when
the anode is red, so since the anode being red is always associated with
a mismatch, then it is believed that mismatch always causes the anode to
glow red, even though that is not a logical conclusion. The element of
danger to equipment reinforces this, and elevates it to the status of a
law.)

One solution is to insert an ATU near the transmitter, it works by re-
reflecting the power in the reflected wave so it is all goes to antenna
and totally radiated, thats what it is all about, getting all the
transmitter power up the stick, how else could it work, the reflected
power doesn't reach the transmitter any more. The anodes run cooler,
clear proof that the explanation is sound.

ATU is really a misnomer, it doesn't tune the antenna at all (we all knew
that), it is really a total-re-reflector when you have the true insight.

We have to remember that in the absence of good models of transmission
line behaviour (eg quantitative models), people will try to fit models
that they can understand, good or bad. If the path from mismatch to red
anodes is too complicated, simplify it, leave all the intermediate
explanation and conditions out, cut to the chase, what is the outcome,
make it a rule.

I agree with you Roy. I think that inventing explanations that are based
on things that aren't or don't happen is satisfying the learner's quest
for knowledge with potentially false information that must be unlearned
to move forward. Worse is that these kearners seize upon these inadequate
models and propagate them, the new experts of ham radio.

One of the risks to ham radio of the new six-hour hams is our feeding
them with inappropriate and inadeqate dumbed down models. I suppose it is
not new, this is probably the root of most of the myths of ham radio (eg
resonant antennas always work markedly better... make it resonant and it
will improve out of sight).

Owen

I wrote:
"Where? At the virtual short, not the open-circuit at the end of the
line."

If the virtual short were replaced with a real short, would anything
change? Not a thing except the line voltage distribution diagram would
lose its final 1/4-wavelength.

Best regards, Richard Harrison, KB5WZI

Richard Harrison wrote:
I wrote:
"Where? At the virtual short, not the open-circuit at the end of the
line."

If the virtual short were replaced with a real short, would anything
change? Not a thing except the line voltage distribution diagram would
lose its final 1/4-wavelength.

Don't you consider it a significant difference that no voltage, current,
or power would reach the load?

Roy Lewallen, W7EL

Owen Duffy wrote:

One of the risks to ham radio of the new six-hour hams is our feeding
them with inappropriate and inadeqate dumbed down models. I suppose it
is not new, this is probably the root of most of the myths of ham radio

Not new at all... There's a huge amount of new stuff for beginners to
learn, so they need simplified ideas to get them started. But it
shouldn't ever have to be about unlearning. We shouldn't be feeding them
false ideas that they will need to throw away completely.

Our local radio club does a lot of teaching, and at all levels we try to
say: "Learn this to check the right box in the exam, but remember
something else about it: it isn't a hard fact. It's actually an onion."

At the next level, we peel away a few more of the skins. The aim is
always to show them how last year's simplified information fits into a
bigger and deeper picture. We don't want them to throw the old
information away; at the next level we want them to keep it, understand
what was right about it, and also see its limitations.

At least, that's what we are aiming for. The challenge for the teacher
to live up to it.



--

73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

Ian White GM3SEK wrote in news:xG9HBmG1ReIGFAV3
@ifwtech.co.uk:

Owen Duffy wrote:

One of the risks to ham radio of the new six-hour hams is our feeding
them with inappropriate and inadeqate dumbed down models. I suppose it
is not new, this is probably the root of most of the myths of ham radio

Not new at all... There's a huge amount of new stuff for beginners
to
learn, so they need simplified ideas to get them started. But it
shouldn't ever have to be about unlearning. We shouldn't be feeding
them
false ideas that they will need to throw away completely.

Our local radio club does a lot of teaching, and at all levels we try
to
say: "Learn this to check the right box in the exam, but remember
something else about it: it isn't a hard fact. It's actually an onion."

At the next level, we peel away a few more of the skins. The aim is
always to show them how last year's simplified information fits into a
bigger and deeper picture. We don't want them to throw the old
information away; at the next level we want them to keep it, understand
what was right about it, and also see its limitations.

At least, that's what we are aiming for. The challenge for the teacher
to live up to it.

Ian,

That sounds a good approach, and it doesn't betray the trust that
learners should have in their trainers.

When ham radio is being reduced to a "communicator" hobby, it is worth
emphasising that there is great opportunity for personal development and
satisfaction in working through those layers. Some of us think that is
what ham radio is about, the ITU does, ITU-R RR Article 1 says "1.56
Amateur service: A radiocommunication service for the purpose of self-
training, intercommunication and technical investigations carried out by
amateurs, that is, by duly authorised persons interested in radio
technique solely with a personal aim and without pecuniary interest".

Owen

K7ITM wrote:
. . .
The analogy may not be prefect, but I think it's a lot like the
usefulness of the idea of a "virtual ground" at the inverting input of
an op amp. But it's a virtual ground only under specific conditions:
strong negative feedback is active, and the non-inverting input is at
(AC, at least) ground potential. For it to be a useful concept
without too many pitfalls, the person using it has to be aware that
the conditions that make it a good approximation don't always hold.
Similarly for a "virtual short" on a line.
. . .

Let me relate a story. . .

Years ago at Tektronix, I transferred to a different group. Across the
aisle was a very bright engineer, fresh from school with a Masters or
PhD degree -- I don't recall which. I recall that his advanced education
had specialized in nonlinear control systems, very much a mathematically
complex and challenging field. His entirely academic background had been
very different from mine, so I was often enthralled by his attempts to
reconcile reality with the idealized world he had, until very recently,
occupied.

One day I found him muttering, trying this and that, until he asked for
some help with his test setup. He was driving an inverting op amp
circuit with a square wave, and he was seeing sharp pulses at the
"virtual ground" summing junction with his 'scope. He had tried moving
his probe grounds, replacing the op amp, bypassing, and everything else
he could think of, to rid his display of this obvious erroneous
artifact. The voltage at the summing junction, he explained, should
always be zero, since it's a virtual ground. Those spikes shouldn't be
there.

I tried to explain to him how a "virtual ground" was created: An input
signal initially generates a voltage at the op amp input. The op amp
responds by sending an inverted signal back to the summing junction
which adds to the initial voltage to produce very nearly zero at the
input. I explained that it can never be zero, but at best is the op amp
output voltage divided by its open loop gain. But more to the point, the
op amp isn't infinitely fast, so it takes some time for it to respond to
that initial voltage or any changes. And during that lag, the summing
junction voltage can move a great distance from zero. So the spikes are
occurring during the time it takes the op amp to respond to changes in
the input stimulus.

Well, he didn't get it. He just couldn't make the transition from the
idealized, infinitely fast and infinite gain op amps of his academic
models to the real things he had to work with. And looking back on it, I
think the basic problem was that he never really fully understood just
how that virtual ground came about even in an idealized world.

After a number more frustrating and unresolved collisions with reality,
he wisely quit and got a teaching job. I'm sure he did well in the
academic world.

Those many models we use daily to keep calculations, concepts, and
analyses manageable are just that -- models. It's imperative to
constantly be aware of the range over which those models are valid, and
alert to any situation which might make the model invalid. People
solving routine problems can, unfortunately, often get along for a long
time without realizing the limitations of their models, and can be
lulled into a belief that they're not models at all but reality. But in
the environment where I've spent most of my time, this carelessness
leads you very quickly into places which can be very difficult to get
out of.

Roy Lewallen, W7EL

Walter Maxwell wrote:
..
..


We have thus proved that the virtual short circuit established at the stub point is actually performing as a
real short circuit.
..
..

Walt, W2DU

It is interesting to look at a single short pulse propagating along the
TL. At the stub point, the pulse must encounter a discontinuity in
impedance and therefore there will be a reflection. This can been seen
on a TDR. So there is a real reflection from a stub regardless of
whether or not it is a virtual short.
Alan
VK2ADB