On Tue, 27 Feb 2007 21:14:38 GMT, Owen Duffy wrote:

I am also aware that supporters of the inherent source match position
assert that you must be selective in choosing tests for source impedance.
It is all rather unconvincing when only some of the implications of a
particular source impedance are effective.

It is my view that modelling the PA as a fixed voltage or current source
with fixed source impedance of Zo, and where reflected waves on a
transmission line are absorbed by the matched source is not a good
general model for HF PAs.

Hi Owen,

This quote gives me no confidence in what you have offered to me
recently:

On Wed, 28 Feb 2007 08:55:24 GMT, Owen Duffy wrote:

You have not yet actually offered any treatment that denies the bone
of contention that lies in two subject lines:
1. Reverse power is manifest;
2. The source will absorb and dissipate it.

Richard, if you go back over my postings in this thread, I have not denied
either of these things.

As to point 1 (or 2 it is difficult to determine what you are
responding to specifically), explicitly stated by me, you have
expressed your self in relation to "supporters of the inherent source
match position" without actually identifying if you stand
1. With them;
2. Against them;
3. Indifferent to them.

As to point 2, explicitly stated by me, you have again described
yourself in a negative relation by discussing a model that does not
work.

Perhaps it is this style of ambivalence that clouded my appreciation
of your statement:
I believe that it is sound (in the steady state) to resolve the forward
and reflected wave voltages and currents at the source end of the
transmission line, calculate the complex impedance, and predict the
effects of that impedance as a PA load using the same techniques that
were used to design the PA.
where you do allow 1 and 2.

However, I could be mistaken again because you don't actually
acknowledge return power impinges upon the final stage, you transform
it into another solution. Note that I accept such a transformation of
the problem. It is common alternative explanation and perfectly
valid. However, that transformation, in and of itself, does not speak
to the issue of reflected power as a physical fact and a separable
entity. In fact, the development of a lumped equivalent doesn't need
to acknowledge SWR either.

73's
Richard Clark, KB7QHC

On Wed, 28 Feb 2007 08:11:30 GMT, Owen Duffy wrote:

Now, are you prepared to post your solution?

Hi Owen,


Your quick computation of 3.3 dB is suitably close to my reference's
first pass solution (3.27 dB), but it neglects the contribution of the
source's resistance.

The solution is 4.9 dB.

If we were to revisit your 1 meter long cable used in the 80M band and
force the transmitter to be a voltage source through the common
mechanism of adding a substantial resistor, and mismatch the other end
of that 1 meter long cable to the same degree (each end seeing 10K Ohm
for the purpose of this statistical curiosity); then that same cable
will heat up with its contribution of at least 3dB of ADDITIONAL loss.

The 10K Ohm specification is a forced one, but it responds in kind to
the original forced solution too. In fact, it comes close to the
source resistance found in a tube amplifier (a common voltage source)
driving a halfwave element (a common application for such a source)
and demonstrates the common futility of using coax (that I have
already expressed) to accomplish this.

However, we don't have voltage sources to conveniently solve either of
these statistical curiosities. Both the tube transmitter, and the
solid state transmitter employ impedance matching to either draw down,
or pull up the native source resistance to a level suitable for
applying to a transmission line.

I would again point out that reverse power suitably accounts for the
1.6 dB difference between your answer and the solution, it also
accounts for the 3 dB difference between your short cable's example,
and my twist in its application.

All such differences have been described and used in design for quite
a few decades, and they have been couched in exactly the terms I've
used here.

If anyone wants to challenge the 4.9 dB solution, they can impeach my
reference "Reference Data for Radio Engineers," (various editions). I
can supply other references that have been named in this group too,
but I would suggest with tackling one authority at a time.

73's
Richard Clark, KB7QHC

On Wed, 28 Feb 2007 07:15:38 -0800, Jim Kelley
wrote:

Not that I dispute anything here necessarily, but I would like to know
how you went about measuring the reflected power dissipated within a
source. Also, how the power being dissipated?

Hi Jim,

Dissipation is caloric, however it can arrive catastrophically by one
of two mechanisms; and they reflect, no pun here, the two types of
phase sense offered by the random opportunity (being phase adding or
subtracting for current or voltage as the occasion demands).

One caloric method is simple in measuring the heat load expressed by
airflow temperature measurements in a confined volume. When I
designed the Flight Recorder, the FAA mandated a heat budget for its
acceptance. This is certainly far afield from the immediate topic,
but it responds to the attention offered in design to this issue. The
point of this sidebar is that efficiency translated immediately into
temperature and this was rigorously anticipated and tested. The same
design philosophy is mandated in RF final design and considerable
attention has been devoted to it in the trade papers.

Returning to our concerns, for certain phase combinations that caloric
solution can arrive suddenly in the form of an arc. Most operators
will immediately act to correct that situation and the heat build up
may not be great, but the damage may still be irreversible. This
harkens back to my discussion of a kitchen table laser cracking a
window pane. Average power may be unspectacular, but instantaneous
power, localized, can be very dramatic and destructive beyond
expectation (it certainly surprised my friend).

For other phase combinations that caloric solution can arrive
gradually (heat soaking); and catastrophe arrives through thermal
runaway. Operators rarely observe this until it is too late.

I hope that the readers can differentiate between these two, and how
certain designs (eg. solid state, and tube design) respond in these
cases and correlate to experience each to their own characteristic
failure mechanism.

73's
Richard Clark, KB7QHC

Expanding generously (gusting on):

When I
designed the Flight Recorder, the FAA mandated a heat budget for its
acceptance.

Aircraft electronics lives with a common airduct. Your design must
not load the cooling air such that it becomes a flame thrower into the
next instrument in the stack. I won't go into issues of crash
survivability.

Returning to our concerns, for certain phase combinations that caloric
solution can arrive suddenly in the form of an arc.

I'm sure most readers who run tube rigs will recognize this situation
immediately. However, there is more than one combination of phases
and currents/voltages. I have also seen heat soaking arrive at a tube
to watch the plates glow cheerily. This, too, is probably an
experience borne by several tube rig operators. In fact, it can be
tolerated far more than a solid state amplifier, and tubes are noted
for their resilience. However, I have also seen the glass envelopes
turned into a taffy consistincy and the vacuum draw them like
heatshrink around the internal structure. Surprisingly, I have also
witnessed that these tubes still worked!

For other phase combinations that caloric solution can arrive
gradually (heat soaking); and catastrophe arrives through thermal
runaway. Operators rarely observe this until it is too late.

The latest generation of solid state components have survivability
design into them such that they are specified to operate into an
infinite mismatch (or some such similar claim). This is suitably
taken care of by being able to withstand more voltage. Other issues
of current crowding, the original thermal disaster for transistors,
has been long solved. That solution revealed how the problem was in
heat confined to a small volume.



Finally, my measurements were never pushed to the point of failure.
All may well anticipate that this sudden arrival would preclude any
accuracy in the heat determination to demonstrate a quid-pro-quo of
returned power. Further, once the failure occured, heat is usually
removed by the very failure it brought - it usually removes the source
too. ;-)

73's
Richard Clark, KB7QHC

On Wed, 28 Feb 2007 09:41:56 -0800, Richard Clark
wrote:

I would again point out that reverse power suitably accounts for the
1.6 dB difference between your answer and the solution, it also
accounts for the 3 dB difference between your short cable's example,
and my twist in its application.

Lest there be any doubt about there being concurrent explanations,
this loss is also expressed in lumped equivalency and circulating
currents. It can be correlated to a very common issue with literal
lumped circuit antenna tuners. It can also be described in terms of Q
and cavities. It can also be correlated to short radiators, radiation
resistance, and Ohmic loss.

Each description is accurate, and as varied as the authors each
offering their interpretations, but no explanation denies the validity
of the other, and reflected power in a line is no exception.

73's
Richard Clark, KB7QHC

Richard Clark wrote:
On Wed, 28 Feb 2007 07:15:38 -0800, Jim Kelley
wrote:


Not that I dispute anything here necessarily, but I would like to know
how you went about measuring the reflected power dissipated within a
source. Also, how the power being dissipated?


Hi Jim,

Dissipation is caloric, however it can arrive catastrophically by one
of two mechanisms; and they reflect, no pun here, the two types of
phase sense offered by the random opportunity (being phase adding or
subtracting for current or voltage as the occasion demands).

One caloric method is simple in measuring the heat load expressed by
airflow temperature measurements in a confined volume. When I
designed the Flight Recorder, the FAA mandated a heat budget for its
acceptance. This is certainly far afield from the immediate topic,
but it responds to the attention offered in design to this issue. The
point of this sidebar is that efficiency translated immediately into
temperature and this was rigorously anticipated and tested. The same
design philosophy is mandated in RF final design and considerable
attention has been devoted to it in the trade papers.

What I meant was, in what way were you able to attribute and apportion
this heat to its various sources? What evidence were you able to
obtain to show reflected energy re-entering the source output? What
component in the system in fact dissipated the reflected energy? How
were you able to determine the exact source and amount of energy at
any given location within the source? Or did you just presume that
you understood the underlying mechanisms?

Thanks in advance,

Jim AC6XG

Cecil Moore wrote:

On Feb 27, 2:53 am, "Jeff" wrote:

Adding a circulator to a system will not change "the load line" (if a
transmission line or circulator can have such a thing), but it will cause
the power in the reflected wave to be separated so that it can be monitored
and measured. Surprisingly power monitored in this way ties up with the
notion that power is reflected at a mis-matched load.


Yes, and a little modulation added to the source signal will prove
that the
signal being dissipated by the circulator resistor has made a round
trip
to the load and back. That's hard to explain if reflected energy
doesn't
actually exist.
--
73, Cecil, w5dxp.com

Your example is the same as putting a load resistor on an open
transmission line, measuring the dissipated power, and then claiming
the same thing happens without the load resistor there.

ac6xg

On Wed, 28 Feb 2007 13:55:47 -0800, Jim Kelley
wrote:

What I meant was, in what way were you able to attribute and apportion
this heat to its various sources? What evidence were you able to
obtain to show reflected energy re-entering the source output? What
component in the system in fact dissipated the reflected energy? How
were you able to determine the exact source and amount of energy at
any given location within the source? Or did you just presume that
you understood the underlying mechanisms?

Hi Jim,

This knowledge arrived by many avenues.

For one, in a heavily heatsinked design, mapping of temperatures
generally reveal a very diffuse origin. That, of course, is the
purpose of the heatsink. So, in that regard the assignment of where
dissipation occurs is done by induction. You can eliminate a lot
circuitry as being incapable of supporting this dissipation, as it is
both remote from the signal path, and remote physically. The
literature of design reveals much of what is discovered in the field.

That literature reveals the dissipation occurs in the
emitter/collector junction of the finals' transistors. Failures have
been confirmed through post-mortem examination by microscope (no, I
have not done this).

Experience with new designs and frequency of failure (those activities
that I have participated in) lead to the same conclusion. In one
particular case it was a manufacturing/assembly problem of mounting
the transistor to the heatsink. A bur was found in many such mounts
that interfered with a complete mating of surfaces. This raised the
thermal resistance in the path from that same junction to the mating
surface, to the heatsink, to the environment. Knowing each thermal
resistance in that path makes it rather simple to forecast the
junction temperature at the time of failure (or rather, to say failure
which occurred was guaranteed a fatal temperature) when you know the
power consumed by the component. All such "resistance" conform to the
simple math of Ohm's law (once you substitute the necessary units for
heat).

When we return to the design guidelines and this junction, almost
every manufacturer of power transistors specifies a junction
resistance value at rated power. Casting this value through the chain
of transformations and to the antenna connector reveals a value very
nearly 50 Ohms. There are newer power amplification designs today,
and yet the market for Ham gear is dominated by the Class AB design
which is exhibits this property nicely.

Inductive logic leads us to this junction as the principle target of
reflected power (the signal path is symmetric, after all). Experience
has supported this logic. Failures are attributable to design flaw
(or assembly flaw), or poor application (driving a mismatch), or both.

As for tubes, I've already testified to the obvious location for
dissipation. It is far easier to see.

73's
Richard Clark, KB7QHC

Jim Kelley wrote:

Cecil Moore wrote:
The joules/sec are real quantities but whether joules/sec
is power depends upon the definition of "power".

In our case here on the internet, it depends on whether or not you
choose to equate 'units of power' with the definition of power.

Most engineers equate the units of power to power, i.e.
joules/sec = watts and so does the IEEE dictionary. But I
am content to assert that the joules in the joules per
second of a reflected wave is real energy. Do you disagree?
--
73, Cecil http://www.w5dxp.com

Richard Clark wrote:
Any who complain about their transmitter having:
1. No source resistance;
2. Not this much resistance:
3. Not this little resistance;
4. None of the above (the usual response).
can take heart that if you simply substitute a tuner ...

Yep, the great majority of amateur radio antenna systems
are matched by a tuner. That act of matching prohibits
reflected load energy from reaching the PA. Except for
overall efficiency, when an antenna system is matched,
the PA impedance doesn't matter. A 5 ohm PA, a 50 ohm PA,
and a 500 ohm PA all output the same power if the output
voltage is the same into the same load.
--
73, Cecil http://www.w5dxp.com