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

This is not contending nor contention and is content only for a non
sequitur. The line following a tuner exhibits considerable loss (poor
efficiency) that can only occur on the basis of power and mismatch.
You yourself offered in other correspondence that it exceeds cable
attenuation specifications found only in a matching condition. To

I am being picky, but "it *may* exceed cable attenuation specifications
found only in a matching condition, it may also be lower".

Hi Owen,

Lower? That is rather astonishing in light of responding to my
comment.

If I said it
as you stated, I made an error. The common statement (and I have no doubt
made it) that VSWR exacerbates line loss is actually wrong in the general
case. (Having Googled my own web site I see one statement along those
lines which needs further qualification!)

This is even more astonishing. Irrespective of you being the source,
why inject this confusing comment? SWR always exacerbates line loss!
Give me any normal line attenuation and SWR at the load, and I will
tell you exactly how much additional loss will occur. There's a
general solution for you.

Something tells me that your comments are based on a confusion between
the power loss of a cable, and its mismatch loss. They are not the
same thing although they are usually tightly twined in discussion. You
later exhibit a confusion between a conjugate match and an impedance
match. They are not the same thing either. The confusion on both
these points have abounded in this group in past "debates."

I meant the output at the PA terminals where an lumped constant load
would be attached for comparison.

This then removes the reflection from the argument, doesn't it? It
actually doesn't; but this unwarranted substitution is like Zen
Archery in that the line already demonstrates the validity of
reflected power as distinct from that "power" just being a
mathematical fiction.

Putting the lumped load at the PA terminal merely casts the proof back
into the box, it doesn't negate reflected power. As the proof is
already supported in the line, then removing it is not strictly a
valid counter argument. However, we will explore it further:

PAs can be designed to behave as an equivalent fixed voltage or current
source with fixed source impedance of Zo, but HF PAs are not usually
designed in that way.

OK so we are now in my sidebar of source resistance. Even so, it has
nothing to do with the concept of reflected power except insofar as
that resistance's ability to reveal that power's dissipation.

Other's should ponder how the reflected power has a caloric proof in
the line, and then question why it wouldn't prove out when it arrives
back in the box where the temperature rises on its return.

Same source, same power, same reflection, same loss. The only thing
that varies is the capacity of any point along this signal chain to
support that heat burden. Let's skip these as choices of design.

I know that there is a vein of thought that the process of adjusting a PA
for maximum output always, somewhat magically, creates a match condition
where the source impedance is the conjugate of the load at the PA
terminals, but it is contentious.

That contention arises out of mistaking Z0 Matches with Conjugate
Matches. This is a common affliction among "debaters" here. Let's
skip their prejudices.

What of broadband PA designs with no
such adjustment, are they source matched over a broad range of
frequencies?

Having had designed broadband amps, this is simply accomplished with
the proper feedback such that, yes, they are matched over a broad
range. The math is quite simple, the cost is another matter. Can you
afford one? Probably not. The lack of commercial examples available
to the Ham is not proof they do not exist. Let's pass on from issues
of economy.

On the other side of the aisle, I've worked with active loads that
will absorb as much power (up to a limit) at any frequency (up to a
limit) that you care to throw at it.

Observations are that experiments to discover the source
impedance by incrementally changing load current can produce a range of
values for the same PA on different frequencies, and at different power
levels.

This is called "Load Pulling," and is a classic technique to
demonstrate source Z. Thevenin first described it and Norton followed
suit. I cannot, for the life of me, recall any other intellectual
giants that have pulled these apart.

I have done this with my own gear. The variation from a source Z of
50 Ohms wandered the SWR range of 1.5:1 over all bands and most power
levels. Given this conformed to the manufacturer's specification, I
was not particularly surprised. Where it deviated the most, the rig
also operated the worst. What can we say about experience and
performance design converging?

Why do amplifiers with say tetrodes and triodes which exhibit
such different dynamic plate resistance but requiring the same load
impedance deliver the same equivalent source impedance?

A cable connector instead of binding posts? Let's dismiss this as
being obvious.

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.

Where is the rig specified to exhibit this condition?

Is your rig a VW that stalls trying to pull a trailer from a stop in
3rd gear? Or is it a Mack truck trying to park in the handicap zone
in an underground mall parking lot? Arguing other's incapabilities is
something I like doing, but with more flair. Let's skip these
Tritonic minnows.

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.

You have already said as much. I see nothing new so far.

The application of small signal analysis to amplifiers that sweep from
near cutoff to near saturation is suspect.

If it is near cutoff or saturation, it is suspect small signal
analysis. Certainly, anyone can conspire to fail gracelessly. Would
you care to elaborate the suspicion beyond the evidence of gross
negligence? Why don't we skip this minor excursion?

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.

Sound though it may be, if I were to line up another transmitter
boresight down the antenna connector of the first, light it up to
provide power with no equivocation of it being fictional; then yes,
all things may appear to be the same. ...and yet I have just
demonstrated reverse power arriving at the antenna terminal (where did
it go?). My having experience in doing just this (aka active load
already described above) fully conforms to your sound idea, and yet,
as for myself, it is not an idea I would rely on to deny the existence
of reverse power nor its capacity to fry the innards of a transmitter
(active loads are heavily heat-sinked and fan driven).

73's
Richard Clark, KB7QHC