"Sal M. Onella" wrote in
:

as heat somewhere in the system. If too much is reflected back from
the antenna and dissipated within in your transmitter, the transmitter
overheats ($$$) or it reduces power to protect itself and nobody hears
you.

Here we go again!

Owen

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

as heat somewhere in the system. If too much is reflected back from
the antenna and dissipated within in your transmitter, the transmitter
overheats ($$$) or it reduces power to protect itself and nobody hears
you.

Here we go again!

Yes, this misconception will never die. Is it really worth the trouble
continually trying to contradict it?

Roy Lewallen, W7EL

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. One cannot understand these concepts
without understanding the conditions that cause
EM waves to interact.

The conditions that cause interaction between EM
waves are coherency and collinearity (in the same
direction in a transmission line).

The interaction of reflected EM waves can result
in zero or maximum reflected power being dissipated
in the transmitter - and anything in between.

The interaction of reflected EM waves at a thin-
film coating on glass can result in zero or
maximum reflected energy and anything in between
depending upon the thickness of the thin-film.
--
73, Cecil http://www.w5dxp.com

"Cecil Moore" wrote in message
news
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.

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.

"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

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

Ian White GM3SEK wrote:
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.

It is true that transmitters are dumb as a stump. However,
a human being should be smart enough to realize that the
virtual impedance, which is only a voltage to current ratio
has been replaced by an impedor with a resistor, inductor,
and/or capacitor.

The impedor *causes* the load conditions. That virtual voltage
to current ratio is a *result* and not the cause of anything.
To get down to the actual cause of the conditions, the human
being needs to know whether the load impedance is virtual or
not.

Why do you imply that a virtual impedance can *cause* the
conditions seen by a source but deny that a virtual impedance
can *cause* 100% re-reflection? Seems a contradiction.
In fact, virtual impedances cannot cause anything. The
voltage to current ratio associated with a virtual impedance
is a *result* of something physical. Choosing to ignore that
physical "something else" cause has gotten lots of folks into
logical trouble.

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.

It seems perverse to say the antenna system can be replaced
by a resistor and inductor or capacitor and nothing changes.
How about the radiation pattern? Does that change?

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.

How about the total energy in the steady-state system? The
number of joules pumped into the system during the transient
state is *exactly* the amount required to support the forward
and reflected power readings.

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.

This is based on a rather glaring rule-of-thumb assumption,
that any standing wave energy dissipated in the source was
never sourced to begin with. Born of necessity, that is a
rather rash assumption. Thus some people sweep the reflected
energy dissipated in the source under the rug and forget
about it, hoping that nobody ever lifts the rug and points
out the conservation of energy principle.

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.

Optical physicists did not have the
luxury of dealing with voltages. As a result of dealing with
power densities, they learned a lot more than RF engineers
know to this very day. Optical physicists have never asserted
that reflected waves are devoid of ExB joules/sec or that
EM waves are capable of "sloshing around".
--
73, Cecil http://www.w5dxp.com

Ian White GM3SEK wrote in
:

Ian, an excellent and quite comprehensive treatment.

Sal,

Some folk will try to distract from an adequately accurate approximation
(being the steady state solution) by wanting to descend to a time domain
solution which as you note converges to the steady state solution in
time, but is much more complex to solve.

The relevance of steady state solutions is demonstrated by the
traditional methods of designing transmission line transformers (eg
quarter wave match), stub matching schemes, the application of the Smith
chart etc. These things are only valid on applications where a steady
state solution is valid, and the widespread use of them attests to the
widespread existence of systems that are quite adequately analysed by
steady state methods.

Most ham applications are ones where the highest modulating frequency is
very small wrt the carrier frequency, and are emminently suited to steady
state analysis.

Similarly, consider that when steady state analysis is not appropriate,
then many of the devices mentioned above may be inappropriate as they
will cause distortion of the signal.

Owen

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

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