"Denny" wrote:
Assuming the transmitter does not fold back, the 11% reflected power
will not change the signal strength... The reflected 11% will be 88%
radiated on the return trip (minus any line losses) and 88% of that on
the next round trip, etc... So, in the end the decrease in
transmitted power is only a fraction of 1% for feedlines with low
losses to start with ...

Now, if we take transmitter foldback into consideration, ...
____________

If, as reported, more than 99% of the output power of a transmitter
ultimately is absorbed/radiated even by a mismatched antenna system, then
why would a transmitter need power foldback for such loads?

RF

Richard Fry wrote:

If, as reported, more than 99% of the output power of a transmitter
ultimately is absorbed/radiated even by a mismatched antenna system,
then why would a transmitter need power foldback for such loads?

Egad, here we go again.

Foldback has nothing to do with "reflected power". It's simply that a
mismatch results in higher voltage or current at the output which could
damage the output device or circuitry. That's why foldback is used.

And, for that matter, "reflected power" isn't radiated *or* absorbed by
the transmitter. The transmitter produces power which is sent to the
antenna. All the power produced by the transmitter arrives at the
antenna less whatever is lost as heat in the transmission line. There
are no waves of average power bouncing back and forth on a transmission
line. Mathematically separating the power moving down the line into
"forward" and "reverse" components doesn't mean that waves of average
power actually exist.

Roy Lewallen, W7EL

Roy Lewallen wrote:
Foldback has nothing to do with "reflected power". It's simply that a
mismatch results in higher voltage or current at the output which could
damage the output device or circuitry. That's why foldback is used.

If the source is connected to a transmission line, the
mismatch results from a virtual impedance other than the
one for which the transmitter was designed. The virtual
impedance seen by the source is (Vfor+Vref)/(Ifor+Iref)
where Vfor is the forward voltage phasor and Vref is the
reflected voltage phasor. |Vfor|*|Ifor|=Pfor and
|Vref|*|Iref|=Pref If it were not for reflections,
the source would see Z0.

DEVIATIONS AWAY FROM Z0 ARE *CAUSED* BY REFLECTIONS!
Deviations away from the design impedance are what
causes foldback.

There are no waves of average power bouncing back and forth
on a transmission line.

One way for that to be true is for reflected energy waves
to contain zero energy but any rational person knows that
cannot be true. Reflected waves consist of an E-field and
an H-field whose ratio is Z0. ExB is watts. Watts are the
unit of power.

Do you really think that the EM waves bouncing back from
your mirror into your eyeballs while you shave contain
ExB = zero watts?
--
73, Cecil http://www.w5dxp.com

"Roy Lewallen" wrote:

All the power produced by the transmitter arrives at the antenna
less whatever is lost as heat in the transmission line. There are no waves
of average power bouncing back and forth on a
transmission line. Mathematically separating the power moving
down the line into "forward" and "reverse" components doesn't mean that
waves of average power actually exist.
____________

Roy, I have been involved with the evaluation and repair of FM and TV
broadcast antenna systems where the initial problem was a failure in the
antenna, which then produced a high mismatch between it and the main
transmission line.

The allegedly non-existent nodes along the transmission line for this
condition did a fine job of melting holes in the inner conductor and Teflon
insulators of 3-1/8" OD (and larger) rigid transmission line, at
1/2-wavelength intervals over a considerable length of that line.

What other phenomenon do you believe caused such a result?

RF

Correction: substitute the word "loop" for "node."

RD

Richard Fry wrote:
Correction: substitute the word "loop" for "node."

"Anti-node" will also work.
--
73, Cecil http://www.w5dxp.com

"Richard Fry" wrote in message
...
"Roy Lewallen" wrote:

All the power produced by the transmitter arrives at the antenna
less whatever is lost as heat in the transmission line. There are no
waves of average power bouncing back and forth on a
transmission line. Mathematically separating the power moving
down the line into "forward" and "reverse" components doesn't mean that
waves of average power actually exist.
____________

Roy, I have been involved with the evaluation and repair of FM and TV
broadcast antenna systems where the initial problem was a failure in the
antenna, which then produced a high mismatch between it and the main
transmission line.

The allegedly non-existent nodes along the transmission line for this
condition did a fine job of melting holes in the inner conductor and
Teflon insulators of 3-1/8" OD (and larger) rigid transmission line, at
1/2-wavelength intervals over a considerable length of that line.

What other phenomenon do you believe caused such a result?

RF

Hi Richard

I recognize that you address your question to Roy, so forgive me for
breaking in.
It seems clear that the power is generated at the "source" and disipated
at the "load" and that between the source and the load, only disipative
components will exist.
I would ask "what component along the transmission line between the source
and load can *inrease* power?".

As for the melting of condutors at 1/2 wave intervals, I attribute that to
high current density related to a low impedance at that point. The damage
may actually be related to the high impedance on the line which caused the
voltage to rise too high.

Jerry

Jerry Martes wrote:
As for the melting of condutors at 1/2 wave intervals, I attribute that to
high current density related to a low impedance at that point.

Because that impedance is virtual, it is a *result*
and not a *cause*. The *cause* of the melting is the
in-phase addition of the forward current and reflected
current whose phasor sum is a maximum at points 1/2 WL
apart. Denying that reflected energy exists will not
keep the wire from melting.

The low virtual impedance mentioned above is the
*result* of:

Z = (Vfor+Vref)/(Ifor+Iref)

at the points where the two voltages are out of phase
and the two currents are in phase.
--
73, Cecil http://www.w5dxp.com

Richard Fry wrote:
"Roy Lewallen" wrote:

All the power produced by the transmitter arrives at the antenna
less whatever is lost as heat in the transmission line. There are no
waves of average power bouncing back and forth on a
transmission line. Mathematically separating the power moving
down the line into "forward" and "reverse" components doesn't mean
that waves of average power actually exist.
____________

Roy, I have been involved with the evaluation and repair of FM and TV
broadcast antenna systems where the initial problem was a failure in the
antenna, which then produced a high mismatch between it and the main
transmission line.

The allegedly non-existent nodes along the transmission line for this
condition did a fine job of melting holes in the inner conductor and
Teflon insulators of 3-1/8" OD (and larger) rigid transmission line, at
1/2-wavelength intervals over a considerable length of that line.

What other phenomenon do you believe caused such a result?

Let's suppose for a moment that the holes were melted by reflecting
waves of average power. Why do they repeat every half wavelength? Do the
waves of average power have a phase angle such that they reinforce
periodically? As an engineer, you of course know that the average of a
periodic function is the integral of that function taken over one
period, divided by the period. How then can average power have a phase
angle? Or do the waves not have a phase angle but rather change
amplitude as they travel? If so, what is the mechanism by which they do?
Can you write the equations showing the power at each point along the
line and how it can be greater at half wavelength intervals?

In contrast, the existence of traveling and standing waves of voltage
and current have long been established. You can find a rigorous analysis
of their behavior in a vast number of textbooks. Given the load and
transmission line impedances, you can very quickly calculate, even by
hand and without the use of a computer, the current and voltage at any
point along the line. Unless the line is perfectly matched, there will
be repeating points of high current and of high voltage. Depending on
the nature of the conductor and insulator, either or both of these can
cause localized heating. In the example you gave, the damage is almost
certainly caused by high current rather than high voltage. If you'll
provide me with the impedance of the load and the impedance and velocity
factor of the cable, I'll show that the high current points fall at the
points where the damage occurred. If you tell me the transmitter power
output, I'll also tell you what the current was at those points. Can you
do the same for your theory of power nodes resulting from bouncing waves
of average power? Anyone else having a basic understanding of
transmission line operation can explain your cable damage without any
necessity to imagine bouncing waves of average power.

If you insist on believing that the damage was caused by traveling waves
of average power, please provide an explanation of how these waves
interact to create more power at some points than others. Because power
is the rate of transfer or dissipation of energy, the power into any
point has to equal the sum of the power dissipated at that point and the
power leaving that point, unless that point contains some mechanism to
store energy. Your analysis has to be consistent with this in order to
avoid violating the law of conservation of energy.

I can provide a detailed mathematical quantitative analysis of the
nature of traveling voltage and current waves which explain the
phenomenon you cite. I'm looking forward to your corresponding
mathematical explanation of the phenomenon using traveling average power
waves.

Roy Lewallen, W7EL

Roy Lewallen wrote:
In contrast, the existence of traveling and standing waves of voltage
and current have long been established.

How can those waves exist without energy? In particular,
how can those waves exist without their energy components
passing a point and being measured in joules/sec?

I can provide a detailed mathematical quantitative analysis of the
nature of traveling voltage and current waves which explain the
phenomenon you cite.

What happens when you calculate ExH in watts? Is it zero?
If not, your whole argument falls apart.
--
73, Cecil http://www.w5dxp.com