VSWR doesn't matter?
 

On Mar 13, 2:48 pm, Gene Fuller wrote:
No one denies the existence of reflections.

But some people deny that there is any energy in the reflections -
see below. :-)


But a big argument is about the
round-trip travel of energy in the steady-state. Some people seem to
believe that energy continues to flow back and forth from one end of a
(mismatched) transmission line to the other under steady-state
conditions, even simultaneously traveling in both directions. (Passing
like ships in the night?) Since energy is a scalar quantity, and any
given joule is not distinguishable from another, it is not clear how the
proponents keep track of the bookkeeping, but they muddle through somehow.

Again Gene, to be able to prove your point, you need to present an
example of a standing wave that exists without a forward traveling
energy wave and a reverse traveling energy wave. You keep implying
that is possible, but have presented no proof.

Water is also a scalar. If you had one gallon per minute flowing into
a barrel and two gallons per minute flowing out of the barrel, would
you argue that there is no water flowing into the barrel and only one
gallon of water flowing out of the barrel? Or would you say the *net*
water flow is one barrel per minute out of the barrel?
--
73, Cecil, w5dxp.com

VSWR doesn't matter?
 

On Mar 13, 9:27 pm, Walter Maxwell wrote:
That was over 40 years ago, but I seem to remember that any discontinuity that
resulted in a VSWR greater than 1.005:1 produced a ghost that could not be
tolerated in the transmitted picture.

And of course, the same thing can happen in a simple system consisting
of
a TV source, a transmission line, and a mismatched load. TV ghosting
can
prove that the reflected traveling waves make multiple round trips to
the load
and back to the source, yet many continue to deny that proof. The best
(copout) argument that I have heard against it is that, "That is not
steady-state",
and therefore doesn't count. :-)
--
73, Cecil, w5dxp.com

VSWR doesn't matter?
 

On Mar 14, 6:53 am, Roy Lewallen wrote:
So any explanation of the effects (such as the red plates
of the mismatched transmitter posed earlier) has to be made without
resorting to the bouncing energy.

That's simply not true. When the load is connected directly to the
source, incident power is often still rejected, it just doesn't
have very far to "bounce". And since it is internal to the source, the
"bouncing" is difficult if not impossible to quantitize.

If you hang a purly reactive load on a source output, it rejects all
the the incident power just like it does at the end of a one-
wavelength long transmission line. If we leave the source output
terminals open, i.e. an infinite impedance, all of the source power is
rejected at the source output terminal, i.e. there is a standing wave
on the internal wire (often coax) connected to the source connector.

In the same way that a source doesn't know whether it is connected to
a transmission line or a lumped circuit, a purely reactive load
doesn't know whether it is connected to a source or to a transmission
line. Either way, it does an immediate rejection of incident power.
Whether the load is connected to a transmission line or directly to a
source, the reflection at the load is a same-cycle reflection. Since
it happens at the load with a transmission line, why are you surprised
that it happens at the load with a source?
--
73, Cecil, w5dxp.com

VSWR doesn't matter?
 

On Wed, 14 Mar 2007 03:51:01 GMT, Owen Duffy wrote:

Walter Maxwell wrote in
:

On Tue, 13 Mar 2007 22:28:02 GMT, Owen Duffy wrote:

Walter Maxwell wrote in
:

On 13 Mar 2007 14:19:12 -0700, "Cecil Moore"
wrote:

Did you guys on this thread know that it's been proven statistically
that five out of four people have trouble with fractions?

That's vulgar!

The stat ement or the fractions?

5/4

Yer right, that's what I thought you mean't.

Walt

VSWR doesn't matter?
 

Cecil Moore wrote:
On Mar 13, 2:48 pm, Gene Fuller wrote:
No one denies the existence of reflections.

But some people deny that there is any energy in the reflections -
see below. :-)


But a big argument is about the
round-trip travel of energy in the steady-state. Some people seem to
believe that energy continues to flow back and forth from one end of a
(mismatched) transmission line to the other under steady-state
conditions, even simultaneously traveling in both directions. (Passing
like ships in the night?) Since energy is a scalar quantity, and any
given joule is not distinguishable from another, it is not clear how the
proponents keep track of the bookkeeping, but they muddle through somehow.

Again Gene, to be able to prove your point, you need to present an
example of a standing wave that exists without a forward traveling
energy wave and a reverse traveling energy wave. You keep implying
that is possible, but have presented no proof.

Au contraire, mon frere. You continue to claim that a standing wave MUST
be made up of two traveling waves, but without proof.

My contention is that this distinction is merely a matter of
mathematical preference. When standing waves occur, there is absolutely
no physical difference between the standing wave and its traveling wave
constituents. If you find some physically significant difference due to
considering traveling wave constituents rather than the standing wave,
then you have made a mistake in your calculations.

Water is also a scalar. If you had one gallon per minute flowing into
a barrel and two gallons per minute flowing out of the barrel, would
you argue that there is no water flowing into the barrel and only one
gallon of water flowing out of the barrel? Or would you say the *net*
water flow is one barrel per minute out of the barrel?

This is totally irrelevant to the issue at hand. Try to keep on task.

73,
Gene
W4SZ

VSWR doesn't matter?
 

billcalley wrote:

snip

I've been reading the posts on this. One poster said this has been going
on for twenty years! (For the other groups, this thread has life on
rec.radio.amateur.antenna) It doesn't need to be so.

First, there should be no doubt that reflected power on a transmission
line is real. Sure, you can replace the line with a lump but that
doesn't clear up the question for others.

For the next two examples, see page 179:
http://cp.literature.agilent.com/lit...4753-97015.pdf
All examples assume the same impedance for source and line.

First example, step into an open line with a Thevenin source. The energy
is divided between the source and the line. Half the energy is moving
down the line and when it returns changes the impedance the source sees
to an open circuit. The energy does not flow back into the source, so,
where did it go? It is stored in the capacitance of the line.

Second example, step into a shorted line. When the energy returns the
source now sees a short. The energy does not flow back into the source,
so, where did it go? It is stored in the inductance of the line.

So here are two examples where the energy sent down the line do not
return to the source.

Third example. Send a pulse down the line. The Thevenin voltage source
will go to short, as it should, when the pulse falls. The pulse is
reflected from either an open or a short at the end of the line. All the
energy is dissipated in the source impedance when this pulse returns.
That is where the energy goes. And it is obviously the _same_ energy
created at the source.

Sure, non of the cases above represent steady state AC. But they do show
that energy may or may not be returned to the real component of the source.

With the above in mind, it can be shown, (in some part II), that a real
accounting of energy from source to load and back is possible.
Equivalent circuits are just that, the trading of line for lump. But,
and this is really important, the only reason the effective impedance at
the input of a 50 ohm line is not 50 ohms is because of reflected energy.

Best, Dan.

VSWR doesn't matter?
 

"Richard Fry" wrote in :

In analog TV transmit systems with a typical 500+ foot length
transmission line from the tx to the antenna, a 5% reflection from a
far-end mismatch can be quite visible, showing as a "ghost" image that
is offset from the main image as related to the round-trip propagation
time of the transmission line.

Richard, read this carefully, I do NOT disagree with what you have said
in the paragraph above.

For a ghost to be visible to viewers getting signal off air, there is
more at play that a 5% reflection from a far end mismatch, the reflected
wave is heading towards the transmitter, and needed to encounter another
at least another reflection point with sufficiently high reflection
coefficient that a sufficiently large, time delayed copy of the original
modulated wave reaches the load (even if in turn part is reflected
again).

This second point of reflection could be a transmission line
discontinuity, but it is most likely that it was that the tx end of the
transmission line was not matched, that is that the tranmitter did not
terminate the line with a near perfect match.

It leaves the questions "are transmitters matched to Zo in real life, is
matching an unavoidable consequence of optimising power output?".

For avoidance of doubt, the discussion in this post is about the
transient behaviour of the tranmission line and waves, it is relevant to
long transmission lines with analogue TV modulation, but that doesn't
mean it is necessarily important to other applications.

Owen

VSWR doesn't matter?
 

"Owen Duffy" wrote
This second point of reflection could be a transmission line
discontinuity, but it is most likely that it was that the tx end of the
transmission line was not matched, that is that the tranmitter did not
terminate the line with a near perfect match.

It leaves the questions "are transmitters matched to Zo in real life, is
matching an unavoidable consequence of optimising power output?"
___________

Field experience in my tenure with RCA Broadcast when measuring and
minimizing the "ghosting" propensities of commercial analog broadcast TV
transmission systems showed/shows that a measured 5% or greater voltage
reflection from the transmit antenna/input elbow complex, when sufficiently
displaced in time from the main image, will result in an objectionable ghost
image seen on an off-air TV set tuned to that station.

That would not be true if such a nominal 5% far-end reflection from the
antenna system essentially was absorbed by the TV transmitter (whether or
not that turned the tube PA plates red, or caused the failure or other
compromise of a solid-state PA).

Maximizing the output power and efficiency of a broadcast r-f amplifier
dictates that its effective output Z must be greatly different than the load
Z it is expected to drive. In the case of broadcast transmitters, that
source impedance is low (a few ohms), compared to the typical 50 or 75 ohm
Zo of the load it is driving.

And this it the reason that much of the voltage reflected from an
antenna/far-end mismatch returns from the tx back to the antenna to be
radiated, and so to produce the TV ghost image seen under those conditions.

An equivalent effect is a reality in FM broadcasting, where a poor Z match
of the antenna system across the FM channel bandwidth produces synchronous
AM, and adds to stereo and SCA crosstalk.

RF

VSWR doesn't matter?
 

On Mar 14, 12:24 pm, Gene Fuller wrote:
Au contraire, mon frere. You continue to claim that a standing wave MUST
be made up of two traveling waves, but without proof.

On the contrary, I have presented at least three references as proof.
If I remember correctly, it was Ramo, Whinnery, Hecht, and Balanis.
You, OTOH, have presented none.

My contention is that this distinction is merely a matter of
mathematical preference. When standing waves occur, there is absolutely
no physical difference between the standing wave and its traveling wave
constituents.

Obviously false as proven by the different equations for the two types
of waves. We laid that one to rest long ago. In fact, it was you who
pointed out that standing wave phase is completely different from
traveling wave phase and cannot be used to measure phase shift through
a coil. If I remember correctly, it was the difference between
cos(x*wt) and cos(x)*cos(wt), i.e. *very* different.

Water is also a scalar. If you had one gallon per minute flowing into
a barrel and two gallons per minute flowing out of the barrel, would
you argue that there is no water flowing into the barrel and only one
gallon of water flowing out of the barrel? Or would you say the *net*
water flow is one barrel per minute out of the barrel?

This is totally irrelevant to the issue at hand. Try to keep on task.

No, it is virtually identical to your argument. Saying it is "totally
irrevelent" doesn't change anything. You are arguing that net energy
transfer is primary and the underlying energy components are
irrelevant if nonexistant.
--
73, Cecil, w5dxp.com

VSWR doesn't matter?
 

On Mar 14, 5:36 pm, "Richard Fry" wrote:
And this it the reason that much of the voltage reflected from an
antenna/far-end mismatch returns from the tx back to the antenna to be
radiated, and so to produce the TV ghost image seen under those conditions.

For whatever reason, ghosting of this sort proves that the reflected
energy
makes a round trip from the load to the source and back. The ghosting
delay is exactly what a speed-of-light EM traveling wave would
experience.
In fact, multiple ghosting is caused by multiple reflections.
--
73, Cecil, w5dxp.com