Dr. Honeydew wrote:
Clearly the 40 ohm load dissipates more power than the 50 ohm load, so
we don't see how your answer and Mr. Harrison's posting can both be
correct.

It appears that the internal resistance of the
Thevenin equivalent circuit was chosen to be
~1 ohm. Maximum power transfer of ~13 watts
should occur when the load is ~1 ohm and the
current is ~3.6 amps with ~3.6 volts across
the 1 ohm load and across the ~1 ohm generator
impedance. How one gets 13 watts out of a source
rated for one watt is a non-linear physical
design problem, not a linear theory problem.
--
73, Cecil http://www.w5dxp.com

On Mar 23, 11:05 am, Dan Bloomquist wrote:
Your example assumes that the reflected power will see the 50 ohms of
the generator. And I had shown you a condition where it will see a short
no mater what the 'output' impedance of the generator.


Truly, we have found the root of the disconnect.

Kindly compute the reflection coefficient at the connection where
a 50 Ohm line is driving a 75 Ohm line.

For your convenience, recall that RC = (Z2-Z1)/(Z2+Z1).

Now redo the same, except that the 75 Ohm line is one-half wavelength
long terminated in a short.

If you get the same answer, then you will see why the reflected
voltage in the example does not encounter a discontinuity at the
entrance to the generator and is therefore not re-reflected.

If you get a different answer, then some study of reflection
coefficient
is in order.

....Keith

wrote:
I have followed this thread with interest There are some who seem to
dispute the existance of reflected power and its ability to do damage.
My answear is that reflected power can certainly do so any one want to
run a completly detuned antenna to prove me wrong may do so at their
expence and risk.

Reflected power doing damage is an overly simplistic
way of viewing things. The reflected wave is capable
of causing damage without giving up any of its energy.
It is not the energy in the reflected wave, per se,
that causes the damage. It is the interference pattern
established by superposition of the forward wave and
the reflected wave that causes the damage.

If the reflected voltage arrives back at the source
in phase with the forward voltage, that constructive
voltage interference can cause an over-voltage condition
that blows the finals. This over-voltage condition actually
reduces the dissipation in the finals because it is
accompanied by a destructive interference, reduced-current
condition. The finals are actually cooler than normal
when the over-voltage begins to fry them.

Of course, a Thevenin equivalent source doesn't suffer
from an over-voltage problem. Power consumption within
a Thevenin equivalent source actually decreases during
constructive interference between the forward and
reflected voltages.

If the reflected current arrives back at the source
in phase with the forward current, the result can
be an over-current condition which can indeed cause
over-heating and failure.

What happens in the finals depends upon the phase
of the reflected wave. Solid-state finals are
usually protected from both over-voltage and
over-current by detecting the SWR. A fold-back SWR
limit of 3:1 limits both the ratios of Vmax/Vmin
and Imax/Imin to 3:1.
--
73, Cecil http://www.w5dxp.com

Keith Dysart wrote:
Truly, we have found the root of the disconnect.

Truly, we have but it is not what you think. A
source doesn't obey the passive reflection rules.
The V/I ratio encountered within a source is
*active*, not passive. Active V/I ratios can and
do cause reflections.
--
73, Cecil http://www.w5dxp.com

Cecil Moore wrote:


Reflected power doing damage is an overly simplistic
way of viewing things. The reflected wave is capable
of causing damage without giving up any of its energy.
It is not the energy in the reflected wave, per se,
that causes the damage. It is the interference pattern
established by superposition of the forward wave and
the reflected wave that causes the damage.

If the reflected voltage arrives back at the source
in phase with the forward voltage, that constructive
voltage interference can cause an over-voltage condition
that blows the finals. This over-voltage condition actually
reduces the dissipation in the finals because it is
accompanied by a destructive interference, reduced-current
condition. The finals are actually cooler than normal
when the over-voltage begins to fry them.


Cecil,

Ya know, it is possible to simply add and subtract voltages. It is not
required to determine an "interference pattern". Solid state electronics
are damaged by high voltage or high power dissipation (or both). No
"interference patterns" required.

Perhaps the manufacturers could add exactly the correct length of
transmission line inside the transceivers so that the wrong kind of
interference could never occur at the finals.

8-)

73,
Gene
W4SZ

Cecil Moore wrote:

[snip]

Since the generator is not delivering any power and there is
a forward power and a reflected power, the reflected power is
supplying the forward power, i.e. being 100% re-reflected. The
re-reflection is associated with total destructive interference
toward the generator and total constructive interference
toward the load. Anything else violates the conservation of
energy principle.

Cecil,

You have highlighted a really useful mathematical aid.

Namely, include the desired answer as a basic condition for setting up
the problem, and the proof becomes easy.

8-)

73,
Gene
W4SZ

Gene Fuller wrote:
Ya know, it is possible to simply add and subtract voltages. It is not
required to determine an "interference pattern".

It is absolutely necessary to understand the interference
patterns if one wants to track the energy through the
system.
--
73, Cecil http://www.w5dxp.com

Gene Fuller wrote:
Namely, include the desired answer as a basic condition for setting up
the problem, and the proof becomes easy.

Yep, no need to make up new laws of physics to
explain things that obviously follow the old
laws of physics.
--
73, Cecil http://www.w5dxp.com

Cecil Moore wrote:

Keith Dysart wrote:

Truly, we have found the root of the disconnect.

Truly, we have but it is not what you think. A
source doesn't obey the passive reflection rules.
The V/I ratio encountered within a source is
*active*, not passive. Active V/I ratios can and
do cause reflections.

Well said.
Thanks, Dan.

On Thu, 22 Mar 2007 12:59:20 -0800, Richard Clark wrote:

On Thu, 22 Mar 2007 15:55:40 GMT, Walter Maxwell
wrote:

On Wed, 21 Mar 2007 08:18:14 -0500, "Richard Fry" wrote:

"Walter Maxwell" wrote
(RF): And if so, would that also mean that such a tx would not be prone
to producing r-f intermodulation components when external signals
are fed back into the tx from co-sited r-f systems?

This issue is irrelevant, because the signals arriving from a co-sited
system would not be coherent with the local source signals, while load-
reflected signals are coherent. The destructive and constructive
interference that occurs at the output of a correctly loaded and tuned
PA requires coherence of the source and reflected waves to achieve
the total re-reflection of the reflected waves back into the direction
toward the load.

Hi Walt,

It is not irrelevant, merely illustrative of the concept of reflection
that is consistent with a coherent source.

Your points of phase are the sine non quo to the discussion, but all
too often arguers only take the half of the 360 degrees available to
argue a total solution. Even more often, they take only one or two
degrees of the 360.

Richard, it's been my observation that many of those who argue are clueless concerning the phase relationships
required to obtain the destructive and constructive interference that achieves the re-reflection of the
reflected waves. A reflection resulting from a discontinuity in the path of a signal delivered by a souce is
guaranteed to be coherent with the source wave. If there is no coherence between the reflected wave and the
source wave there may be an interference, but none of the type that results in total destructive and
constructive interference relevant to impedance matching. I don't understand what you mean by 'taking only one
of two degrees of the 360.'

But even for coherent reflections, if the PA tank circuit has very low loss
for incident power (which it does), why does it not have ~ equally low loss
for load reflections of that power? Such would mean that load reflections
would pass through the tank to appear at the output element of the PA, where
they can add to its normal power dissipation.

The paragraph above seems to me to imply that RF doesn't understand the destructive and constructive
interference phenomena involved with re-reflection.

This is the symmetry of the illustration of external signals. You
used external signals yourself as part of your case study; hence the
relevance has been made by you.

Whoa, Richard! You'll have to point out where I've discussed external signals in any case study involving
phase relationships between forward and reflected waves. I've never done so knowingly.

Also, does not the result of combining the incident and reflected waves in
the tx depend in large part on the r-f phase of the reflection there
relative to the r-f phase of the incident wave? And the r-f phase of the
reflection is governed mostly by the number of electrical wavelengths of
transmission line between the load reflection and the plane of
interest/concern -- which is independent of how the tx has been
tuned/loaded.

And we return to the sine non quo for the discussion: phase.

That's true, but although RF apparently realizes that the phase relationship is relevant, he doesn't seem to
understand the details of the phase requirements that achieve the necessary interferences that accomplish the
impedance matching.

If the ham transmitter designs that your paper applies to produce a total
re-reflection of reverse power seen at their output tank circuits, then
there would be no particular need for "VSWR foldback" circuits to protect
them. Yet I believe these circuits are fairly common in ham transmitters,
aren't they? They certainly are universal in modern AM/FM/TV broadcast
transmitters, and are the result of early field experience where PA tubes,
tx output networks, and the transmission line between the tx and the antenna
could arc over and/or melt when reflected power was sufficiently high.

RF

Richard, your statement above begs the question, "Are you aware of the phase relationships between forward and
reflected voltages and between forward and reflected currrents that accomplish the impedance-matching effect
at matching points such as with stub matching and also with antenna tuners?

It seems he is on the face of it, doesn't it? Afterall, he is quite
explicit to this in the statement you are challenging.

No Richard, I don't believe he is. I don't see the 'explicitness' you seem to find. It's the complete lack of
the explicitness that makes me believe he doesn't quite get it.

When the matching is accomplished the phase relationship between the foward and reflected voltages can become
either 0° or 180°, resulting in a total re-reflection of the voltage. If the resultant voltage is 0°, then the
resultant current is 180°, thus voltage sees a virtual open circuit and the current sees a virtual short
circuit. The result is that the reflected voltage and current are totally re-reflected IN PHASE with the
source voltage and current. This is the reason the forward power in the line is greater than the source power
when the line is mismatched at the load, but where the matching device has re-reflected the reflected waves.

Nothing here contradicts anything either of you have to say.

True, but RF just hasn't said it all, because, as I said above, I don't believe he understands the details of
the phase requirements to achieve the match.

This phenomenon occurs in all tube transmitters in the ham world when the tank circuit is adjusted for
delivering all available power at a given drive level.

This introduces the two concepts of the "need for match" and the
"match obtained." They are related only through an action that spans
from one condition to the other. They do not describe the same
condition, otherwise no one would ever need to perform the match:

I don't comprehend your statements in the paragraph above.

When this condition occurs the adjustment of the
pi-network has caused the relationship between the forward and reflected voltages to be either 0° or 180° and
vice versa for currents, as explained above. When this condition occurs, destructive interference between the
forward and reflected voltages, as well as between the forward and reflected currents, causes the reflected
voltage and current to cancel. However, due to the conservation of energy, the reflected voltage and current
cannot just disappear, so the resulting constructive interference following immediately, causes the reflected
voltage and current to be reversed in direction, now going in the foward direction along with and in phase
with the forward voltage and current.

If a tree were to fall onto the antenna, a new mismatch would occur.
Would the transmitter faithfully meet the expectations of the Ham
unaware of the accident? No, reflected (0-179 degrees) energy would
undoubtedly offer a 50% chance of excitement in the shack. The
consequences of dissipation would be quite evident on that occasion.
For the other 180 (180-359) degrees of benign combination; then
perhaps not.

If a tree were to fall onto the antenna the new mismatch would surely detune the transmitter, causing unwanted
dissipation, of course, but only a lid would fail to retune the transmitter before removing the tree.

In transmitters with tubes and a pi-network output coupling circuit there is no 'fold back' circuitry to
protect the amp, because none is needed, due to the total re-reflection of the reflected power.

That would more probably be due to cost averse buying habits of the
Amateur community, and the explicit assumption of risk by them to
react appropriately in the face of mismatch. Tubes were far more
resilient to these incidents than transistors of yore.

It is only in
solid-state transmitters that have no circuitry to achieve destructive and constructive interference that
requires fold back to protect the output transistors.

They too have access to the services of a transmatch that is probably
more flexible than the tubes' final. If they didn't use a tuner, then
the foldback would render many opportunistic antennas as useless.
Again, as a cost item, this solution (fold-back) is dirt cheap and was
driven by the market economies of a more onerous and costly repair
through a lengthy bench time to replace the transistor (which has an
exceedingly high probability of a quicker failure for a poor job).

IMHO, Richard, the mfgrs of solid-state rigs with no means of matching the output to a load other than 50
ohms short changed the ham, thus requiring him to be satisfied with the power fold back, or buy an antenna
tuner.

Walt, W2DU