On Jun 11, 12:09*pm, Roy Lewallen wrote:

But why would you expect the "reflected power" to have a different
effect when the transmission line is a quarter wavelength than when it's
a half wavelength?

If reflected power is fictitious, and the number wavelengths of
transmission line of any random impedance compared to the load
connected to it makes no difference in the load seen by the
transmitter, the output power produced by the transmitter, and the
power dissipated in the far-end termination, then what is the reason
you chose a 1/2 wavelength of transmission line in your quoted post?

RF

On Jun 11, 12:29*pm, Richard Fry wrote:

If reflected power is fictitious, etc

Followup: Those denying the existence of reflected signals within an
antenna system may wish to view the measurement of such signals, at
the link below.

http://i62.photobucket.com/albums/h8...easurement.gif

RF

On Jun 11, 12:09*pm, Roy Lewallen wrote:
But why would you expect the "reflected power" to have a different
effect when the transmission line is a quarter wavelength than when it's
a half wavelength?

Good Grief! Because the interference has the opposite result between
the two examples?
--
73, Cecil, w5dxp.com

On Jun 11, 12:28*pm, K1TTT wrote:
and that is part of the problem in here... 'you call it'. *what is the
definition of 'virtual impedance' in the ieee dictionary?

"The IEEE Dictionary" has no such definition. Those are my words
adopted from "Reflections", by Walter Maxwell, and designed to
differentiate between the (A) and (B) definitions of "impedance" in
"The IEEE Dictionary". Calling the V/I (B) definition of impedance,
"virtual", is much more descriptive than calling it "the (B)
definition".

Walt is arguing that the impedance of an RF source is "non-
dissipative". Ratios of V/I are non-dissipative if they exist devoid
of an impedor. Walt adopted the word "virtual" from the optics
"virtual image". It is an image that is not really there in reality. A
virtual impedance would therefore be the image of an impedor that is
not really there.

I'm not hung up on the word "virtual". What adjective would you use to
differentiate between a dissipative impedor and a V/I non-dissipative
impedance? I am not trying to be difficult - just trying to
communicate. I'm willing to adopt any convention that you suggest for
the duration of this discussion.
--
73, Cecil, w5dxp.com

On Jun 11, 12:56*pm, Richard Fry wrote:
Followup: *Those denying the existence of reflected signals within an
antenna system may wish to view the measurement of such signals, at
the link below.

Or at this link. Scroll down to "Using Dielectric Beamsplitters to
find the "missing energy" in destructive interference".

http://www.teachspin.com/instruments...eriments.shtml

I guarantee that every optical physicist who is reading this thread is
laughing at the ignorance of the alleged RF gurus.
--
73, Cecil, w5dxp.com

On Jun 11, 12:09*pm, Roy Lewallen wrote:

Can you write an equation giving the supposed dissipation caused
by "reflected power" as a function of transmission line length?

I'll just quote the following from Chapter 24-11 of REFERENCE DATA FOR
RADIO ENGINEERS, 1975 Edition:

QUOTE
A load of 0.4-j2000 ohms is fed through a length of RG-218/U cable at
a frequency of 2.0 MHz. What are the input impedance and efficiency
for a 24-foot length and a 124-foot length?

(Omitting the number crunching shown...)

Tabulating the results,

Input
Length Impedance Efficiency Loss
(feet) (ohms) ( % ) ( dB )

24 0.106-j95 1.1 19.6
124 1.8+j55 0.03 35

The considerably greater loss for 124 feet compared with 24 feet is
because the transmission passes through a current maximum, where the
loss per unit length is much higher than at the current minimum.
END QUOTE

As the input Z for each of these lengths is not the same as their 0.4-
j2000 ohm termination, this shows the dependence of the performance of
such systems on the electrical lengths of the transmission line in
use.

I realize we have strayed into the real world here, because RG-218/U
transmission line is not lossless even when perfectly matched at both
ends.

RF

Richard Fry wrote in news:908e1320-965a-4e11-b02b-
:

On Jun 11, 6:16*am, Owen Duffy wrote:

... The transmitter output power is probably different ...

Thank you, Owen.

Do your comments apply to a transmitter designed/adjusted for, and
expecting a 50 + j 0 ohm load?

In answer to the question you directly asked, yes. But that says nothing
of whether such a transmitter, in the general case, is well represented
by a Thevenin equivalent circuit with Zeq=50+j0.

As you know, there is a proposition that a transmitter "designed/adjusted
for, and expecting a 50 + j 0 ohm load" can be well represented by a
Thevenin equivalent circuit and naturally has Zeq=50+j0. However, that
proposition is easily proven wrong by valid experiments in the real
world, and those of us who have done such experiments are disinclined to
accept the proposition.


IOW, if the net output power of such a transmitter (which equals that
dissipated in the load) probably is different with such a mismatch, do
you expect the reason for that to be related to "reflected power?"

Average power at the transmitter terminals is given by the average value
of the instantaneous product of v and i over time. There is no need for
"net" in the calculation. The average power delivered by the transmitter
will depend on the load impedance (ie the complex ratio of v/i) at its
load terminals, but that dependence is not well predicted in the general
case by a Thevenin equivalent circuit.

I gave a link to a simple experiment to test Zeq of a tx, and that uses
equipment found in most ham shacks. The article is at
http://vk1od.net/blog/?p=1028 .

Owen

Richard Fry wrote in news:510d2db0-df70-4433-a216-
:

On Jun 11, 12:09*pm, Roy Lewallen wrote:

Can you write an equation giving the supposed dissipation caused
by "reflected power" as a function of transmission line length?

I'll just quote the following from Chapter 24-11 of REFERENCE DATA FOR
RADIO ENGINEERS, 1975 Edition:

QUOTE
....
The considerably greater loss for 124 feet compared with 24 feet is
because the transmission passes through a current maximum, where the
loss per unit length is much higher than at the current minimum.
END QUOTE

The author acknowledges that loss under standing waves is not uniform,
that in this case, it is higher in the region of a current maximum. That
is simply explained the I^R effects on the conductors (which account for
most of the loss in most practical coax cables at that frequency).

If you treated the forward and reflected waves as travelling
independently, and each attenuated by the matched line loss
characteristic of the line, you would get a different (and incorrect)
result. It happens that we sometimes make that approximation, and under
many practical scenarious, it gives a result with acceptable error, it is
nevertheless less accurate because the model is poorer.


As the input Z for each of these lengths is not the same as their 0.4-
j2000 ohm termination, this shows the dependence of the performance of
such systems on the electrical lengths of the transmission line in
use.

Yes, it does, and not simply on the VSWR as often inferred.

But that does not change the transmitter's behaviour in its output power
being sensitive the the impedance at its load terminals, however that
impedance is derived, and your response has not answered Roy's question,
well at least in my mind.


I realize we have strayed into the real world here, because RG-218/U
transmission line is not lossless even when perfectly matched at both
ends.

Indeed, under some circumstances, the line can have less loss when not
perfectly matched. Yes, throw those treasured graphs of ExtraLoss vs VSWR
away, they depend on assumptions not usually stated.

Cases similar to the example you quoted can be solved using TLLC at
http://www.vk1od.net/calc/tl/tllc.php . An interesting case is to explore
the loss in 1m of RG58C/U at 3.6MHz with 50+j0, 500+j0, and 5+j0 loads.
The latter two are VSWR=10, but have quite different loss. The first two
cases demonstrate that VSWR does not necessarily cause extra loss
(compared to the matched line case).

Owen

On Jun 11, 7:19*pm, Cecil Moore wrote:
On Jun 11, 12:28*pm, K1TTT wrote:

and that is part of the problem in here... 'you call it'. *what is the
definition of 'virtual impedance' in the ieee dictionary?

"The IEEE Dictionary" has no such definition. Those are my words
adopted from "Reflections", by Walter Maxwell, and designed to
differentiate between the (A) and (B) definitions of "impedance" in
"The IEEE Dictionary". Calling the V/I (B) definition of impedance,
"virtual", is much more descriptive than calling it "the (B)
definition".

Walt is arguing that the impedance of an RF source is "non-
dissipative". Ratios of V/I are non-dissipative if they exist devoid
of an impedor. Walt adopted the word "virtual" from the optics
"virtual image". It is an image that is not really there in reality. A
virtual impedance would therefore be the image of an impedor that is
not really there.

what is an 'impedor' in this context? that is a relatively rarely
used term in circuit and wave analysis, but is generically defined as
anything that has an impedance. that doesn't seem to fit your
definition though if you can qualify an impedance as non-dissipative
if they don't have one.


I'm not hung up on the word "virtual". What adjective would you use to
differentiate between a dissipative impedor and a V/I non-dissipative
impedance? I am not trying to be difficult - just trying to
communicate. I'm willing to adopt any convention that you suggest for
the duration of this discussion.

the ieee dictionary qualifiers of dissipative and non-dissipative seem
adequate to me. no need to make up any other terms or qualifiers.

On Jun 11, 4:29*pm, K1TTT wrote:
what is an 'impedor' in this context?

Hopefully, the same IEEE Dictionary definition as any other context:
"impedor - a device, the purpose of which is to introduce impedance
into an electric circuit." Note that it has a material existence.

the ieee dictionary qualifiers of dissipative and non-dissipative seem
adequate to me. *no need to make up any other terms or qualifiers.

OK, I will change "virtual impedance" to "non-dissipative impedance"
although if the resistance is zero, that still doesn't solve the
semantic problem. The word "virtual" as used by Walter Maxwell over
the past half-century conveys the meaning as well as any other words,
IMO.

The fact remains that a dissipative impedor is something that exists
in the material world and can cause an outcome. A non-dissipative
impedance is a *result* of a superposed V/I ratio, not a cause of
anything.

Roy once challenged me to detect the difference between a 50 ohm
antenna and a 50 ohm dummy load. I said, "Simple, use a field strength
meter."
--
73, Cecil, w5dxp.com