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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

my SWR reading
 

"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

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There
are no waves of average power bouncing back and forth on a
transmission
line.
************************************************** *******
You really mean that Roy, or am I misreading?
I agree that no 'average' value of power is reflecting, but with a
mismatch at the antenna terminals, voltage/current is definitely
sloshing back and forth on the line making standing waves at .25L
intervals that we can physically probe and measure...

denny / k8do

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

RD

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Denny wrote:
There
are no waves of average power bouncing back and forth on a
transmission
line.
************************************************** *******
You really mean that Roy, or am I misreading?
I agree that no 'average' value of power is reflecting, but with a
mismatch at the antenna terminals, voltage/current is definitely
sloshing back and forth on the line making standing waves at .25L
intervals that we can physically probe and measure...

It depends upon one's definition of "power". A purist
will argue that it is not power until it is dissipated
and steady state reflected power is not dissipated
until power is removed from the source.

The question is: Are the ExB watts associated with a
reflected energy wave defined as "power" or not.
--
73, Cecil http://www.w5dxp.com

my SWR reading
 

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

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

my SWR reading
 

"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

my SWR reading
 

Denny wrote:
There
are no waves of average power bouncing back and forth on a
transmission
line.
************************************************* ********
You really mean that Roy, or am I misreading?
I agree that no 'average' value of power is reflecting, but with a
mismatch at the antenna terminals, voltage/current is definitely
sloshing back and forth on the line making standing waves at .25L
intervals that we can physically probe and measure...

Waves of voltage, yes [1].

Waves of current, yes [1].

Waves of average power, no.

History repeats itself in this discussion, so someone is sure to come up
with that snappy line about: "Feel the dummy load on the reflected power
port of a broadcast TX! If there's no such thing as reflected power, how
come that load gets hot?"

That's sloppy thinking. You are not looking directly at the main
transmission line - you're looking at the output of a directional
coupler that has been inserted into that line. Without the coupler,
there's nowhere to connect that load.

That coupler is a transducer. It samples signals from the main line, and
then it does things to them.

A directional coupler does NOT sample waves of power from the main line.
It samples the voltage and current on the main line, adds the reflected
components in phase, and delivers them to the output port labeled
'Reflected'. Connect a dummy load there, and sure enough it will get
hot... but that's only through the action of the coupler.

The challenge is still on the mat from several previous rounds: can
anyone provide a fully detailed description of the operation of a
directional coupler, circulator or similar device, based ONLY on waves
of forward and reflected power? It can be done easily using forward and
reflected waves of voltage and/or current, but that approach is
off-limits for this challenge. Believers in waves of power shouldn't
need it.



[1] Real hams can blow their own holes in the side of the coax - we
don't need no broadcast TX :-)


--

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

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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

my SWR reading
 

Ian White GM3SEK wrote:
Waves of voltage, yes [1].

Voltage is proportional to the E-field.

Waves of current, yes [1].

Current is proportional to the H-field

Waves of average power, no.

ExH = joules/sec = watts. Are watts the the dimensions
of power? Are the E-field and H-field usually given in
RMS (average) values? Can ExH be considered as the
average power in an EM wave? Are you arguing that
watts are not necessarily power? Are you arguing
that it is not power until it is dissipated?

It can be done easily using forward and
reflected waves of voltage and/or current, but that approach is
off-limits for this challenge. Believers in waves of power shouldn't
need it.

"Waves of power" is just a semantic strawman designed
to elicit an emotional response. Anyone using the term
loses technical credibility. It is akin to using the
'N' word to describe race.

How about believers of EM waves containing energy passing
a point? e.g. joules/sec = watts measured at a point?

All this is explained in joules/sec = watts without referring
to volts or amps in my energy analysis article at:

http://www.w5dxp.com/energy.htm

Note that I do not mention "waves of power" anywhere in
my article. What I do talk about is forward energy waves
and reflected energy waves the average value of which can
be measured at a point in joules/sec, i.e. watts.
--
73, Cecil http://www.w5dxp.com