"Cecil Moore" wrote in message
...
Sal M. Onella wrote:
What did I say wrong?

You implied that reflected energy is always dissipated
in the transmitter.

Yeah -- well, I was trying to keep it simple, as I said to start the second
paragraph.

Tell me something, please: with a mismatched load, isn't overheating of the
transmitter sometimes a risk? (Assume no tuner, 50 feet of coax and an
assortment of attempted HF transmit freqs.)

How much of the reflected energy is dissipated in the
source depends upon the interference pattern at the
source. As w7el points out, dissipation in a voltage
source can be reduced to zero by the astute choice of
a special best case of complete constructive
interference in the direction of the load.

.... and I think I see an example of the "astute choice" illustrated by the
"ladder-line length selector for our all-band dipole" on your website. I
get that and I like it but it would never have occurred to me -- probably
because of lack of wave theory on my part. Am I right?

Thanks.

Think, this is the same thing that one "Kurt N.Sturba" says in
his columns, about swr: It Isn't LOST power
(Heating the final or , dissipated in the final),
but is redirected to the load (again). Tho might be sticking
my foot in mouth again, but the ONLY effect of SWR, would
be on the Transmission Line (because of the loss of THAT line)
First, because of the loss, to the load.
Second , because of the Reflected LOSS (again!)towards the Source.
And then The loss, as it is again redirected towards the
load! In short, SWR, doesn't effect the power-- the FEEDLINE does
and the higher the swr- the higher the loss the feedline presents to
the load! See World Radio magazine for his tomes! In short ,
MAXIMUM power transfer occurs when the impedence of the source= the
impedence of the load. But that doesn't mean that the power is lost
at the SOURCE as (heat, loss, ect). ANY loss must be in the (devices)
between the source , and the load! Jim NN7K

Roy Lewallen wrote:
Sal M. Onella wrote:

What did I say wrong?


As always, Ian has done a much better job than I could have, so there's
not much point in trying to repeat what he said.

But I did write up a lengthy essay several years ago, in response to the
same insistent rantings about reflecting waves of average power that's
still going on in this newsgroup, and it has some numerical examples
with a very simple circuit which illustrate the problems with what you
said. You can get it at http://eznec.com/misc/Food_for_thought.pdf. A
little past half way down is "Food for thought: Forward and reverse
power". If you're not interested in the math, scroll down a few
paragraphs from there to the table in Courier font. It and the text
below explain how it shows where the source dissipation is higher,
lower, and about the same when the load is matched, for three different
loads all having the same "reverse power". Another entry in the table is
an example where the "reverse power" equals the forward power (an
infinite SWR) yet the source dissipation is zero. People who believe
that "reflected power" ends up heating up the transmitter should take a
careful look at this, and see if they can explain it.

The established transmission line theory that's been well established
for over a century and that Ian, I, and countless others use daily for
solving real problems will, as shown in the example, tell us exactly how
much power is where and why. The "power is absorbed in the load" folks
can point to transmitters that get warm (sometimes) when working into
(some) mismatches. But they can never come up with a coherent reason for
the results shown in the essay table, or equations which will predict
just how much "reflected power" a transmitter will absorb and when. And
the reason is just as Ian said.

Unfortunately, some people, when presented clear evidence that the
concept is wrong, cling desperately to it nonetheless. For those,
explanations and evidence are a waste of time. But hopefully there are a
few readers out there who will see the problems in resolving their
theory with the evidence and redirect their thinking.

Roy Lewallen, W7EL

"Roy Lewallen" wrote in message
...
Sal M. Onella wrote:

What did I say wrong?


snip

Unfortunately, some people, when presented clear evidence that the
concept is wrong, cling desperately to it nonetheless. For those,
explanations and evidence are a waste of time. But hopefully there are a
few readers out there who will see the problems in resolving their
theory with the evidence and redirect their thinking.

Roy Lewallen, W7EL

Roy,

I don't cling to anything, desperately or otherwise. I keep an open
mind. I've learned a lot but I haven't learned everything.

Give me credit for having been the victim of conventional wisdom or
[insert your favorite disparaging term for misinformation]. It's that
simple.

Oh, and keep that "black cat" metaphor handy for any ol' time you want
to **** somebody off. It works.

"Sal"
[John Markham, KD6VKW
Amateur Extra
Commercial General
NARTE Certified EMC Engineer, retired]

Sal M. Onella wrote:
Tell me something, please: with a mismatched load, isn't overheating of the
transmitter sometimes a risk?

Yes, sometimes but not always. Reflected waves can
actually cool down the transmitter by reducing the
total current.

... and I think I see an example of the "astute choice" illustrated by the
"ladder-line length selector for our all-band dipole" on your website. I
get that and I like it but it would never have occurred to me -- probably
because of lack of wave theory on my part. Am I right?

Probably because of lack of Smith Chart experience?
--
73, Cecil http://www.w5dxp.com

"Sal M. Onella" wrote in
:

....
I don't cling to anything, desperately or otherwise. I keep an
open
mind. I've learned a lot but I haven't learned everything.


Sal / John,

I think Roy may have been referring to another 'helper' who lurks here.

Give me credit for having been the victim of conventional wisdom
or
[insert your favorite disparaging term for misinformation]. It's that
simple.

I think we were / are all vulnerable to learning from each other... it is
what amateur radio is about, the collective 'self training' that is part
of the ITU defintion of the amateur service.

Like yourself, I learned from the hobby elders, and some of it, a lot of
it was BS, and I freely admit that I am uncertain of some of what I
'know'.

We all need to test our own 'knowledge', especially the widely held stuff
that you just don't find in reputable text books. You have to ask have we
hams got an edge on the rest of the world, or is the proposition a myth.

A few years ago I worked on development of a handbook for our new six
hour hams. It was a considerable struggle to keep the 'reflected power is
dissipated in the PA and causes damage' myth out of the handbook that
would feed the hobby intake in this country.

Imagine a multiple choice question in the assessment:

Reflected power:
a) increases the power bill;
b) is a technique for operating 12V equipment in modern 24V vehicles;
c) reinforces your signal provided it is re-reflected by an ATU;
d) overheats transmitters causing serious damage;
e) is a fire hazard.


Owen

"Cecil Moore" wrote in message
...
Sal M. Onella wrote:
What did I say wrong?

You implied that reflected energy is always dissipated
in the transmitter.

How much of the reflected energy is dissipated in the
source depends upon the interference pattern at the
source. As w7el points out, dissipation in a voltage
source can be reduced to zero by the astute choice of
a special best case of complete constructive
interference in the direction of the load.
--
73, Cecil http://www.w5dxp.com

You haven't commented yet pro or con, but if I may, I want to expand on
the observation I made about your variable length twin-lead arrangement
being an "astute choice" to promote complete constructive interference in
the direction of the load.

If the line length will bring this about as you have demonstrated, does
this mean that a tuner appears to adjust the effective line length (from the
perspective of the source)? The only explanation of a tuner I have seen
describes it as creating a stub whose impedance, in parallel with the
impedance of the transmission line/antenna system, is seen as 50 ohms, a
presumptive good match. I don't know enough to accept or reject this
notion, so I shall do neither at the moment. However, I think of a stub as
shunting a transmission line, whereas the tuner is electrically in series,
so maybe something's fishy about that. I withhold judgment.

More to the point, when I was taught transmission lines, they were
diagrammed with lumped constants: series resistance and inductance + shunt
capacitance. The internals of a pi tuner are just more of the same and
would seem (by intuition, alone -- I am not claiming anything) to behave as
an adjustable length line, minus the resistive component. By extension,
Does a T tuner cancel some of the reactance of the line in which it is
inserted, thereby effectively shortening the line. (The phase "conjugate
impedance" is poking out from a fold of my brain.)

Am I onto something or have I simply launched myself into the eye of
another ****storm?

"Sal"

Sal M. Onella wrote:
"Roy Lewallen" wrote in message
...
Sal M. Onella wrote:
What did I say wrong?


snip

Unfortunately, some people, when presented clear evidence that the
concept is wrong, cling desperately to it nonetheless. For those,
explanations and evidence are a waste of time. But hopefully there are a
few readers out there who will see the problems in resolving their
theory with the evidence and redirect their thinking.

Roy Lewallen, W7EL

Roy,

I don't cling to anything, desperately or otherwise. I keep an open
mind. I've learned a lot but I haven't learned everything.

Give me credit for having been the victim of conventional wisdom or
[insert your favorite disparaging term for misinformation]. It's that
simple.

I'm sorry, what I said was poorly worded. I wasn't directing it at you,
but was venting a bit because this misconception keeps surfacing over
and over.. I'm very glad it doesn't apply to you!

Oh, and keep that "black cat" metaphor handy for any ol' time you want
to **** somebody off. It works.

And I'm sorry if it ****ed you off. But I hope it made my point:
Observing two things doesn't mean that one causes the other.

If what I posted played any part in helping you -- or other readers --
understand transmission lines even a little better, then it was
worthwhile doing. I know Ian could have done it without ****ing anyone
off, but I'm a lot clumsier with language.

Roy Lewallen, W7EL

On Mon, 5 Nov 2007 17:30:26 -0800, "Sal M. Onella"
wrote:

So, when you were on staff at USN ET "A" School, where we both taught, did
you know better than the "reflected power" legend/old-wives-tale/heresy?
Hell, that's where I first picked it up !!!!!!!!!!!!!!!

Hi John,

My path up through "A" school was through the Radar branch (I was the
only ETR2 in the Communications Branch when I taught there). The
Radar training gave us hands on experience with equipment that
presented both lumped (pulse forming network) AND line (Magic T and
such) designs. Also, reflected POWER was palpably lethal in both
applications. Melodramatic criticism of the fine points in
terminology carried little weight in lab exercises.

Luckily, I had the presence of mind as a student to hie myself into
Frisco to buy Terman's "Electronic and Radio Engineering." The Navy
course of instruction and training manuals so perfectly dove-tailed to
that book that the fit was a precision match. The "A" school syllabus
didn't go nearly to the depth of detail as offered by Terman (the
ET1&2 and ETC course work did), but it did heavily touch every chapter
found in that tome.

Insofar as heresy, the syllabus was rife with it. We taught that
antennas that were too short could be made to appear longer by the
addition of the "missing" wire in the form of a coil. Similarly, a
too long antenna could be reduced in length by inserting a capacitor.
The teaching aid was to think of the coil symbol as a spring that
could be stretched to lengthen the short antenna, and cap symbol as
providing space enough between the plates to collapse the extra length
of a too long antenna. Clearly the metaphors wheeze, but are
effective well beyond mathematical proofs that could only serve as
sleeping pills.

Before the purists roll their eye's and mutter "tut-tut" under their
beards - instruction was complete to point out these were short-cuts
as memory aides and did not serve as a complete discussion on the
topic. Reflected power certainly fell into that category as its
hazard was positively destructive and not a mental exercise subjected
to debate in a perfumed symposium.

Anyone with radar experience can fashion the plumbing to steer
reflected power to any load. -um- reflected energy be damned at that
point as any reflection was inherently returning to a source that was
long dormant (in terms of microseconds) and ready to dissipate
anything that came down the pipe. The whole point of the magnetron's
success in war was its robustness in the face of catastrophic
mismatches. The PFN might flame out or the thyratron burst, but the
magnetron would survive.

73's
Richard Clark, KB7QHC

Sal M. Onella wrote:
Does a T tuner cancel some of the reactance of the line in which it is
inserted, thereby effectively shortening the line.
Am I onto something or have I simply launched myself into the eye of
another ****storm?

A single series length of transmission line can only
transform impedances to other values located on the
SWR circle (or spiral) on a Smith Chart. There is
only one low resistance value on that SWR circle
and it is equal to Z0/SWR. For instance, if the
SWR is 9:1 on 450 ohm line, that value is 450/9 =
50 ohms. If the SWR is 4.5:1, that low resistive
value is 450/4.5 = 100 ohms. If the SWR is 18:1,
that low resistive value is 450/18 = 25 ohms. Only
one value of SWR (9:1) on Z0=450 ohm line will
transform the impedance to 50 ohms. Since my
transceiver is happy with any resistive impedance
between 25 ohms and 100 ohms, I am satisfied with
that maximum 50 ohm SWR of 2:1.

An antenna tuner not only can perform the equivalent
of the above transformation but it can also do the
equivalent of a transformation of the resistance
value to 50 ohms for any SWR within the range of
the tuner. So a tuner can transform 25 ohms or 100
ohms to 50 ohms. A tuner has one more dimension of
matching than does a single series transmission line.

Adding a tapped transformer to a variable length
transmission line system gives it that extra dimension.
A second/third section of transmission line in the
form of a stub will also add that extra dimension.

The main function of matching is to stop reflected
energy from reaching the transmitter leaving it no other
available path except back toward the antenna. That
is done by tuning for a Z0-match at the transmitter. A
tuner is only one way of achieving that Z0-match to
50 ohms.
--
73, Cecil http://www.w5dxp.com

The whole point of the magnetron's
success in war was its robustness in the face of catastrophic
mismatches. The PFN might flame out or the thyratron burst, but the
magnetron would survive.

A good thing too, or we would all be eating cold leftovers without
that magic 'radar range'...
Jeez, I still have the original Frigidaire that I bought in the late
60's when I worked for GM - got an employees special deal just in time
for xmas, the first family in our neighborhood to have a radar range,
the neighbors marveled... The kids whipped that puppy day and night...
It has outlived three houses... It has been relegated to my R&D shop
where it happily heats epoxy, paint, putty, and cups of tea with
aplomb... I cannot even begin to guess how many tens of thousands of
times it has been cycled... Dozens of times a day for a quarter
century when the kids were still home...

denny / k8do