question about wire antenna and tuner
 

"Sal M. Onella" wrote in news:ptyXi.1068
:

What did I say wrong?

Hi Sal,

A recurring theme here is the myth that all the energy in reflected waves
on a transmission line from a mismatched antenna is carried all the way
back to the transmitter and necessarily dissipated as heat which is likely
to damage the PA.

It is no doubt an appealling explanation of why a PA may run hotter under
some circumstances, but it does not explain why for instance under some
circumstances, a PA may run cooler on a mismatched load.

Being appealing does not imply correctness of the explanation.

I will sit down now, and await the inevitable stream of anecodotal evidence
that doesn't and cannot support the generality of the statement.

Owen

question about wire antenna and tuner
 

"Sal M. Onella" wrote in
:

I mistakenly put a 2m antenna on my dual band HT and tried to use it
for a short QSO on a nearby 440 repeater. The other ham said I was
barely making the repeater, while my poor HT got so hot that I could
barely hold it after a minute's use.

The antenna was wrong and the heat was real -- whatever the theory
behind it.

Let the anecodotes flow...

Your FM HT is a classic case than can be adequately represented by a steady
state analysis. Your HT was operating into a load that increased its
dissipation, but there would be almost certainly be other mismatched loads
that would decrease its dissipation... but you wouldn't notice the event,
you would likely only remember the times the HT was too hot to handle.

I await the inevitable photon explanation.

Owen

question about wire antenna and tuner
 

Owen Duffy wrote:
"Sal M. Onella" wrote in
:

I mistakenly put a 2m antenna on my dual band HT and tried to use it
for a short QSO on a nearby 440 repeater. The other ham said I was
barely making the repeater, while my poor HT got so hot that I could
barely hold it after a minute's use.

The antenna was wrong and the heat was real -- whatever the theory
behind it.

Let the anecodotes flow...

Your FM HT is a classic case than can be adequately represented by a steady
state analysis. Your HT was operating into a load that increased its
dissipation, but there would be almost certainly be other mismatched loads
that would decrease its dissipation.

The transmitter gets hot because it is operating into an incorrect load
impedance, not the 50-ohm load for which it was designed. As far as the
transmitter is concerned, that is the only problem.

What caused that incorrect load impedance is a totally different topic.

If you measured the impedance of that incorrect antenna, and then
replaced the antenna with a dummy load of the same impedance (a resistor
of the correct value, in series with an inductor/capacitor of the
correct value) then your transmitter will not know the difference. The
same value of load impedance will cause it to behave in exactly the same
way.

There are many different physical types of loads that could present
exactly the same impedance to the transmitter. These include antennas,
dummy loads and various combinations, with or without some length of
transmission line involved. So long as the load impedance presented to
the transmitter is exactly the same in all cases, the transmitter
behaves exactly the same (once it has reached steady state, after the
first few cycles of RF... more about that later).

The amount of power that the transmitter can deliver into that incorrect
load will depend on the transmitter circuit and on the value of the load
impedance - but NOT on the physical type of load.

You can measure the impedance of the load by disconnecting it from the
transmitter and connecting it to an impedance meter. (Seems obvious?
Think again - every time you make an impedance measurement, you are
using the principle that impedances of the same value are
interchangeable with no effect on steady-state operation.) If the load
happens to be an antenna and transmission line, you can use programs
like NEC and established transmission line theory to make an accurate
prediction of the load impedance. If the system happens to include an
ATU, that is just another device that modifies the load impedance
presented to the transmitter.

At that point, you're finished with antennas, transmission lines and
ATUs - once you know the load impedance they present to the transmitter,
everything else depends on the transmitter alone.

In other words, the antenna/transmission-line/ATU system can - and
wherever possible, SHOULD - be cleanly separated from transmitter
design. The separation interface is the output connector at the rear of
the transmitter.

In the huge majority of applications, both amateur and professional, it
IS possible to separate those two topics cleanly and completely. It
seems perverse to tangle them together unnecessarily.


All the above refers to the steady state, where the signal level is
constant; and if a transmission line is involved, the pattern of
standing waves is established and unchanging. For completeness, we now
need to check if anything was different during the few moments after
switch-on, while the steady-state pattern of standing waves was becoming
established. Starting from switch-on, we need to look at each of the
successive reflections and re-reflections along the transmission line,
and see how the steady state came to be.

The first thing to notice is that with the types of signals and lengths
of transmission line that we amateurs use, the steady state is
established within the first few cycles of RF, ie it all happens over
timescales much shorter than the signal's own envelope rise/decay time.
This means it is 'nice to know', but will seldom be of practical
importance.

A detailed analysis of the buildup of reflections along a transmission
line will be forced to consider reflections at the transmitter as well
as at the load - in other words, we have to specify a reflection
coefficient at *both* ends of the line. Chipman's book [1] gives a very
detailed analysis of this, and shows how the addition of voltages over
multiple reflections gives rise to a standing wave. The amplitude of the
standing wave builds up as mathematical series, in which each successive
reflection and re-reflection contribute an additional term. Some terms
add to the total while others subtract, and each successive term makes a
smaller contribution than the one before, so the series will converge
towards a constant value which represents the steady state. It should be
absolutely no surprise that, when summed to an infinite number of terms,
this series produces exactly the same results as the steady-state model
- exactly the same pattern of standing waves, and exactly the same load
impedance presented to the transmitter.

The important conclusion from this more detailed time-dependent analysis
is that re-reflections at the transmitter have NO effect on the final
steady-state pattern of standing waves. The ONLY effect of
re-reflections at the transmitter end was on the time-dependent details
of how that pattern built up, and on the final steady-state signal
levels. The magnitude of the standing waves depends on the transmitter
characteristics (in other words, on the 'signal level') but the shape of
the standing waves and their location along the transmission line
depends only on the line and the load. There are no special cases he
the same conclusion holds for all values of reflection coefficient at
the transmitter end, including 1 and 0.

Thus, even a detailed time-dependent analysis confirms that, once we
have reached the steady state, we can indeed make a clean separation
between the transmitter and its load. And since we can, we should.



[1] R A Chipman, 'Theory and Problems of Transmission Lines, Schaum's
Outline Series', McGraw-Hill. ISBN 0-07-010747-5. (Chipman isn't an
easy read, because he is Mr Meticulous who wants to tell you everything;
but you can rely on him not to cut corners.)


I await the inevitable photon explanation.

None needed. If anyone wishes to introduce additional complications
where none are necessary, then of course they're at liberty to do so.
But when invited to join in, everyone else is at liberty to decline.



--

73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

question about wire antenna and tuner
 

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

question about wire antenna and tuner
 

Sal M. Onella wrote:

It's hardly an old wives' tale.

I mistakenly put a 2m antenna on my dual band HT and tried to use it for a
short QSO on a nearby 440 repeater. The other ham said I was barely making
the repeater, while my poor HT got so hot that I could barely hold it after
a minute's use.

The antenna was wrong and the heat was real -- whatever the theory behind
it.

Once a black cat walked across the street in front of me. I had a
wonderful day! The wonderful day was real, whatever the theory behind
it. My basis for crediting the cat is just as valid as yours for
crediting "reflected energy" for the heating. And based on similar logic.

Roy Lewallen, W7EL

question about wire antenna and tuner
 

Roy Lewallen wrote:
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.
===========================
Roy ,Tnx vy much for that ,much appreciated. ( it now sits in my
'Antenna' file)

Frank GM0CSZ / KN6WH

question about wire antenna and tuner
 

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

question about wire antenna and tuner
 

Sal M. Onella wrote:
"Cecil Moore" wrote in message
. ..
Roy Lewallen wrote:
Yes, this misconception will never die. Is it really worth the trouble
continually trying to contradict it?
Not if all you do is trade one old-wives tale
for another.

It's hardly an old wives' tale.

1. The original old wives tale is that reflected power
*always* causes the source dissipation to increase,
even when it is a voltage source.

2. The subsequent old wives tale is that reflected
energy always ends up being radiated by the antenna.

Examples of decreased dissipation in the voltage source
in the presence of reflections proves #1 wrong.

Examples of increased dissipation in a current source
in the presence of identical reflections above proves
#2 wrong.

The power density in reflected waves is proportional to
ErefxHref and *CANNOT* be zero. Where that energy goes
obeys the conservation of energy principle. Anyone who
says all energy winds up being radiated by the antenna
is simply ignorant even if portraying himself as a guru.
--
73, Cecil http://www.w5dxp.com

question about wire antenna and tuner
 

Owen Duffy wrote:
It is no doubt an appealling explanation of why a PA may run hotter under
some circumstances, but it does not explain why for instance under some
circumstances, a PA may run cooler on a mismatched load.

What does explain it is the amount of destructive vs
constructive interference occurring at the source.
Assuming an unprotected source:

If the constructive interference is toward the
load, the source dissipation will decrease. If the
constructive interference is toward the source,
the source dissipation will increase. The conservation
of energy principle really does work to conserve the
ExH energy in a reflected wave.

Just because a special case results in zero dissipation
in a voltage source does not give us the permission
to make a magical leap of faith to "reflected power
doesn't exist" or "reflected power is *always*
dissipated in the load".
--
73, Cecil http://www.w5dxp.com

question about wire antenna and tuner
 

Owen Duffy wrote:
Your FM HT is a classic case than can be adequately represented by a steady
state analysis. Your HT was operating into a load that increased its
dissipation, but there would be almost certainly be other mismatched loads
that would decrease its dissipation... but you wouldn't notice the event,
you would likely only remember the times the HT was too hot to handle.

If the constructive interference is toward the source,
it will run warm. If the constructive interference is
toward the antenna, the source will run cool. Antenna
tuners cause total destructive interference toward
the source and total constructive interference toward
the load. RF energy obeys the conservation of energy
principle.

I await the inevitable photon explanation.

None needed. Interference patterns work for voltages and
that's all one needs to figure out why the source is too
hot or cool. It's a no-brainer. If the energy is going
into the antenna and not being reflected, that energy
is not being dissipated in the source. If the energy is
not going into the antenna, it doesn't take a rocket
scientist to figure out where it must be going.
--
73, Cecil http://www.w5dxp.com