Richard Clark wrote in
:

On Thu, 01 Mar 2007 19:21:43 GMT, Owen Duffy wrote:

If you are asserting that VSWR on a real or even theoretical line
varies sinudoidally with displacement, it is time to go back to
basics. You need some time with a reputable text book.

Hi Owen,

Under the right circumstances (and they have been presumed in some
discussion here), then the power terms (as expressed by a power meter
inserted into the line) will vary sinusoidally with displacement, even
if the SWR does not.

Richard,

I am not sure of what you mean by the "right circumstances".

Firstly, except in the very special case of an almost purely inductive
load on a lossy cable at low frequencies, the VSWR calculated from Vf and
Vr at a point on a line, decreases smoothly from the load end to the
source end. (Vf and Vr each decay exponentially in magnitude from source
to load, and the forward and reflected power values calculated from those
samples will vary as the square of the exponentially decaying Vf, Vr from
load to source.)

If you use an instrument that is calibrated for an impedance other than
the line under test, your measurement does not indicate VSWR on the line
under test, and the instrument readings will be different than I outlined
in the previous paragraph. Fig 3 in my article at
http://www.vk1od.net/VSWR/VSWRMeter.htm shows a line labelled "VSWR(50)"
that indicates the values that would be indicated / calculated using a 50
ohm instrument in a 75 ohm cable with a 1.5:1 VSWR.

Owen

On Thu, 1 Mar 2007 20:13:32 +0000, Ian White GM3SEK
wrote:

I ordered the book from the other side of the world because I wanted to
be very sure of my answers next time around.

Hi Ian,

It will contain much of interest. For instance, it relates to Owen's
moribund thread "the power explanation." Page 205, third paragraph
from the bottom conforms to one of my recent posts the
"Although the power delivered by the source to the line is thus
shown to be reduced by the amount of the reflected power returning
to the input terminals ... the implication of the latter reasoning
that the reflected wave power is entirely absorbed in the source
impedance without affecting the total output of the signal source
generator, is incorrect."

Contrary to that teaching, is discussion on page 203, last paragraph.
It relates to figure 9-26, clearly illustrating a mismatched line fed
by a source with a source resistance. This may be upsetting to many:
"At the signal source end of the line ... none of the power
reflected by the terminal load impedance is re-reflected on
returning to the input end of the line."
The ellipsis reveals that the source Z matches the line Z.

To begin at the beginning of multiple reflection coverage, go to the
same named section (8.8) on page 174. It is not his complete say on
the topic, but it starts here formally. To add insult to someone's
injury, his math includes source Z. However, by the same token
Chipman explicitly states:
"... the shape of the standing wave pattern ... is in no way
affected by the quantities Vs, Zs and Rho-s at the source."

I would also note the irony in that Chipman expresses reflections in
lines in terms of power. To subdue that irony, I would also admit he
is quick to shift to energy when the usage of power is to lead to
problematic solutions (so, using power as an expression in this
context is allowable by precedent as being informal).

Of course, Chipman must be accepted as an authority for any of these
issues to be considered valid.

73's
Richard Clark, KB7QHC

On Thu, 01 Mar 2007 21:04:36 GMT, Owen Duffy wrote:

I am not sure of what you mean by the "right circumstances".

Hi Owen,

It should be under "all circumstances." However, to reveal it
requires the "right circumstances." This again returns us to the
discussion of source resistance/impedance. A matching source driving
a mismatched load through a line of indeterminate length can exhibit
this variation, but it will require considerable skill to see it. If
you force the problem by mismatching the source as well (this then
means that the line is mismatched at both ends, much like your
halfwave model); then you can observe a variation in power readings
along its length that vary sinusoidally.

If you use an instrument that is calibrated for an impedance other than
the line under test,

That is not the case, although there are occasions where power has to
be determined in a heavily mismatched situation - this is done with
considerable error if the line lengths are unknown. If they are, then
corrections can be made.

your measurement does not indicate VSWR on the line

I have restricted myself to the cyclic display of powers.

under test, and the instrument readings will be different than I outlined
in the previous paragraph. Fig 3 in my article at
http://www.vk1od.net/VSWR/VSWRMeter.htm shows a line labelled "VSWR(50)"
that indicates the values that would be indicated / calculated using a 50
ohm instrument in a 75 ohm cable with a 1.5:1 VSWR.

Well, at a quick glance and noting no tabular form of data, what is
presented wouldn't reveal cyclic variation anyway for two reasons:
1. It lacks resolution (not enough places);
2. It lacks sufficient mismatch.
This is not a complaint, merely an observation because the evidence
for your example is hidden deep in the decimal places.

The solution is simple conceptually and mathematically. Reference any
discussion of Two Beam Interference as treated in Optics. For the
special case the math devolves to:
I = 2 · I1 · (1 + cos(theta2 - theta1 - delta))

Conceptually, it is only the combination of phase and amplitude from
two sources (each reflecting interface on the ends of the line).

73's
Richard Clark, KB7QHC

Richard Clark wrote:

Is power/energy separable from its source?

What other point is there to attaching an antenna to a transmitter?

73, Jim AC6XG

On Thu, 01 Mar 2007 14:28:58 -0800, Jim Kelley
wrote:

Richard Clark wrote:

Is power/energy separable from its source?

What other point is there to attaching an antenna to a transmitter?

Hi Jim,

I will take that as an affirmative.

When it is generally accepted that our sources do not exhibit 50 Ohms
source resistance/impedance, what resistance/impedance do they
exhibit?

73's
Richard Clark, KB7QHC

Richard Clark wrote:
When it is generally accepted that our sources do not exhibit 50 Ohms
source resistance/impedance, what resistance/impedance do they
exhibit?

It doesn't matter. The net power supplied by the source
is *always* the difference between the forward power
and the incident reflected power *by definition*.

The definition seems to assume that 100% of the
incident reflected power is re-reflected *as if*
a 100% re-reflection condition exists.
--
73, Cecil http://www.w5dxp.com

Richard Clark wrote in
:

I have restricted myself to the cyclic display of powers.
....
Well, at a quick glance and noting no tabular form of data, what is
presented wouldn't reveal cyclic variation anyway for two reasons:
1. It lacks resolution (not enough places);
2. It lacks sufficient mismatch.
This is not a complaint, merely an observation because the evidence
for your example is hidden deep in the decimal places.

The graph at http://www.vk1od.net/lost/RG58sol.gif has not been labelled
for presentation, so you will need to make some allowance in reading it.

The case that is plotted is an extreme mismatch, ou you might argue
impractical, but it is extreme enought to show the effects clearly on a
graph. The x axis is displacement from the load (-ve towards the
generator).

The red line is the so called "forward power" (Real(Vf^2/Zo)) that would
be indicated by a correctly calibrated sampler like a Bird 43, but
correctly calibrated means for the actual Zo, not the nominal 50+j0.

The dashed purple line is the so called "reflected power" (Real(Vr^2/Zo))
under the same conditions.

The power at a point is shown by the cyan line.

The dashed olive line is Real(Vf^2/Zo))-Real(Vf^2/Zo) (so called forward
power - reflected power). It is not the same as as the power because it
ignores two terms of the power expansion. A Bird 43 callibrated for the
actual Zo would lead you to this line.

This is a very detailed RLGC model, and it reveals from the P(x) line
that attenuation per unit length is not constant, a result of the loss
being higher in the region of a current maximum. Nevertheless, a sampler
that responds to Vf or Vr will not expose the true power curve (due to
the two missing terms).

I have thought at times of writing an article that explains the effects
on a true practical transmission line, and what practical instruments
calibrated for 50+j0 would indicate. I doubt that it would have appeal,
people like the "reflected power is dissipated in the transmitter and may
overheat it" explanation... it is easier to swallow.

Owen

Owen Duffy wrote in
:


Hmmm, replying to my own postings again!

This is a very detailed RLGC model, and it reveals from the P(x) line
that attenuation per unit length is not constant, a result of the loss
being higher in the region of a current maximum. Nevertheless, a
sampler that responds to Vf or Vr will not expose the true power curve
(due to the two missing terms).

The two "missing" terms are the second and third lines in the legend, the
crossproducts in the power equation expansion. In the case where Zo is
purely real, these terms are equal and opposite in phase and cancel out.

Owen

Cecil Moore wrote:
Dave wrote:
Cecil, as an engineer you should stick with standard vocabulary.

Just trying to appease the physicists, Dave. They are
arguing that it is not power until work is done. They
say that since reflected energy is not doing any work,
it cannot be reflected power. Therefore, reflected
power doesn't exist. It's purely semantics. The very
essence of an EM wave is its energy content.

So the real question is: Since standing waves obviously
exist and just as obviously cannot exist without two
coherent waves traveling in opposite directions, does
reflected energy exist? (That question seems to cause their
skivvies to get all bunched up.)

I will just be happy when they admit that reflected
EM waves possess a certain amount of energy that cannot
stand still and according to the theory of relativity
must necessarily travel at the speed of light.


Cecil,

Physicists know when power is equal to work and when it represents
undissipated flow of energy. No need to keep beating that dead horse.

I must have missed class the day they talked about obviousness. Why is
it "obvious" that standing waves cannot exist without coherent traveling
waves? Do you believe that traveling waves are somehow more pure or more
fundamental than standing waves?

Have you ever tried working out the mathematical details of the wave
equation when loaded with a standing wave trial solution? Did it explode
or otherwise fail? (Hint, the answer should be "no".)

The question of standing waves or traveling waves is purely one of
mathematical convenience. The physical phenomena are identical
regardless of your choice. Indeed, this is the point that seems to
always trip you. There is no added information from manipulating the
form of the equations. That is the sort of thing, if done carelessly,
that leads to adding power waves and other nonsense.

73,
Gene
W4SZ

Owen Duffy wrote:
people like the "reflected power is dissipated in the transmitter and may
overheat it" explanation... it is easier to swallow.

If the forward current and reflected current are in phase
at the source, it may indeed become overheated due to an
over-current condition.

But just as likely (with random feedline lengths) is that
the forward voltage and reflected voltage may be in phase
at the source and blow out the finals due to over-voltage
conditions.

The two above events never occur at the same time. Over-
current conditions occur at low voltages. Over-voltage
conditions occur at low currents.
--
73, Cecil http://www.w5dxp.com