On Tue, 05 Sep 2006 22:52:47 GMT, Owen Duffy wrote:

....
It is true that the reflected wave component is not constant with
displacement for a lossy line. The amplitude of the reflected wave
component decreases exponentially from the load end towards the source
end, and the rate at which it decreases is given by the line's
"matched line loss". The phase of the reflected wave component also
changes with displacement (distance along the line).

The amplitude of the forward wave component decreases exponentially
from the source end towards the load end, and the rate at which it
decreases is given by the line's "matched line loss". The phase of the
forward wave component also changes with displacement.
....
On practical lines at HF and above with practical loads, the SWR
decreases smoothly from the load to the source because of line loss,
the lower the loss, the lower the rate at which SWR falls as you
approach the source.
....

I have plotted theoretical forward and reflected power for a detailed
model of RG58C/U at 100MHz with a severely mismatched load to
illustrate the underlying thing that is being measured by a
reflectometer.

The plots are at http://www.vk1od.net/lost/NN7K.htm .

Comments?

Owen
--

On Wed, 06 Sep 2006 07:36:13 GMT, Owen Duffy wrote:

I have plotted theoretical forward and reflected power for a detailed
model of RG58C/U at 100MHz with a severely mismatched load to
illustrate the underlying thing that is being measured by a
reflectometer.

The plots are at http://www.vk1od.net/lost/NN7K.htm .

Comments?

Hi Owen,

Interpretative comment - You state here "theoretical" plots of a
model. At the reference you state "indicated forward power, reflected
power." To me this implies data taken, not generated from a model's
formula - but my interpretation may be a stretch.

Data source comment - However, you go to rather elaborate lengths to
specify a load of 2+0.5j (sic) and a line of 50.36571-j0.17872. What
was the point of injecting reactances here if you simply wish to
illustrate the degradation of SWR with feedline loss? They add to the
implication of data taken, not generated - except for 5 place
resolution at 100 MHz which is (excuse me) absurd.

Elaboration of the implications of interpretation and data source
comment - Further implying this was data taken at the bench is the
declaration "THE REFLECTOMETER IS NOT CALIBRATED FOR THE LINE Zo."
Again, my presumption circuits peg on that one because the reactances
are so easily removed from emphatic concern if you are in control of
the "virtual" workspace.

Informational comment - What is supposed to be the total length of
line? Was it matched at its input, or simply fed by a transmitter?

Picky comment - Graph font sizes are a squint too small.

Last, really picky comment - Your copyright notice at the bottom is
off by a year.

73's
Richard Clark, KB7QHC

On Wed, 06 Sep 2006 04:45:57 GMT, Cecil Moore
wrote:

Jim - NN7K wrote:
... but it is
beyond the ability to have MORE power returned
to the source, than the source provided

Did you know a reflection coefficient can be greater
than 1.0?

It is true that reflection coefficient can be greater than 1.0.

The reflection coefficient *CANNOT* be greater than 1.0 where Zo is
purely resistive.

So, where a sampler is calibrated (nulled) on a purely resistive load
(eg 50+j0) as is most commonly done, it should never show a reflection
coefficient greater than 1. A reflectometer calibrated to a resistive
load and that shows a "reflected" reading greater than the "set"
reading is inaccurate / defective / a poor design.

Owen
--

Owen Duffy wrote:
The reflection coefficient *CANNOT* be greater than 1.0 where Zo is
purely resistive.

Yes, a reflection coefficient greater than one is caused
by the interaction of the Z0 reactance and the load reactance
in an effect resembling resonance.
--
73, Cecil http://www.w5dxp.com

On Wed, 06 Sep 2006 10:36:52 -0700, Richard Clark
wrote:

On Wed, 06 Sep 2006 07:36:13 GMT, Owen Duffy wrote:

I have plotted theoretical forward and reflected power for a detailed
model of RG58C/U at 100MHz with a severely mismatched load to
illustrate the underlying thing that is being measured by a
reflectometer.

The plots are at http://www.vk1od.net/lost/NN7K.htm .

Comments?

Hi Owen,

Hi Richard,



Interpretative comment - You state here "theoretical" plots of a
model. At the reference you state "indicated forward power, reflected
power." To me this implies data taken, not generated from a model's
formula - but my interpretation may be a stretch.

For avoidance of any doubt, it is entirely theoretical. Line
characteristics are based on an RLGC model of the line derived from
Belden's published specs for 8262 (RG58C/U).

Firstly, I should not have said "indicated forward power, reflected
power" as the "indicated" value is a voltage or current, ie the square
root of power, it is the deflection on the indicator meter if you
like, the meter current on the reflectometer. I have changed the text.

The meaning "indicated" is that it is the voltage or current driving a
reflectometer meter, calculated from the sampler and line conditions.


Data source comment - However, you go to rather elaborate lengths to
specify a load of 2+0.5j (sic) and a line of 50.36571-j0.17872. What
was the point of injecting reactances here if you simply wish to
illustrate the degradation of SWR with feedline loss? They add to the
implication of data taken, not generated - except for 5 place
resolution at 100 MHz which is (excuse me) absurd.

The stated line Zo precision is a result of lazy cutting and pasting,
and it should not be seen to imply that level of accuracy. I have
changed it.

The load (2+0.5j) is relatively unimportant, but it was a load to
create a high VSWR, higher than would be practical for such a feedline
at that frequency, extreme to satisfy the people who assert strange
things can only be observed at extreme VSWR. The reactance is just a
non zero value for no other reason than to test the models behaviour
on a complex load.

What is significant, is the load VSWR is very high at ~25, I would
suggest higher than practical, and therefore extreme.


Elaboration of the implications of interpretation and data source
comment - Further implying this was data taken at the bench is the
declaration "THE REFLECTOMETER IS NOT CALIBRATED FOR THE LINE Zo."
Again, my presumption circuits peg on that one because the reactances
are so easily removed from emphatic concern if you are in control of
the "virtual" workspace.

A reality of lossy lines is that Zo is not purely resistive.

A source of instrument error is a sampler that is calibrated for a
nominal Zo (being Ro). Fig 1 shows the modelled VSWR (which is what
accounts for increased loss, impedance transformation etc), and Fig 2
shows the calculated indication for a reflectometer calibrated or
nulled at 50+j0. There is a small difference, and the difference
becomes minute at lower VSWR.


Informational comment - What is supposed to be the total length of
line? Was it matched at its input, or simply fed by a transmitter?

The x scale is in wavelengths (as noted on the page), Frequency is
100MHz, line length covered by the graph is therefore about 9m.


Picky comment - Graph font sizes are a squint too small.

Ok, it is a quick cut and paste from Mathcad. For a permanent article,
I would try to do better, for I too have aging eyesight!


Last, really picky comment - Your copyright notice at the bottom is
off by a year.

Thanks, I have fixed the footer (which is common for most of the
site).

The graphs are a quick and dirty presentation, but it is a very
detailed transmission line model underneath. I acknowledge there are
all sorts of error sources in real life, but keeping sight of the
underlying phenomena helps to flag whether measurements look invalid.

Back to the original proposition that asserted that changing feedline
length can vary measured VSWR dramatically, and that shortening a
feedline might improve VSWR, there are reasons why that might happen,
but it is not explained solely by what is happening on the inside of
the feedline. A possible explanation for example is that the feedline
is not decoupled and carries common mode current, and that changing
the feedline length changes the load impedance seen by the load end of
the transmission line.

So, enough from me, can someone produce details of a scenario with a
very high load VSWR where there are multiple sweet spots with 1:1 VSWR
on a practical line? ("VSWR" here means a property of the line at that
point, not what might be read by an unsuitable or defective
instrument, and an extremely long line is not "practical".)

Thanks...
Owen
--

Thanks, Owen, and all and sorry to cause so much
grief. I fall on my sword! Have believed 99.44%
of what was in the old VHF'er Magazine, as was
started by a consortium of many engineers ,
originally by Doug Demaw, W8HHS, as editor, and
finally Loren Parks,K7AAD, an ex engineer from
Tektronix, and covered the Gamut of VHF, UHF, and
Microwave-- to early satelites and Moonbounce.
Was instrumental to me getting on air, many
moons ago. Tho much I've learned (and much I've
forgotten!) over the years. I have built tuned
coaxial finals, ect., and from former job,
haveing to use tuned lines (mainly,to couple
cavities together , and then tune them to
use on same antenna. IF those lines changed,
the things detunewhen you remove your test
equipment! Also, I get a little bull headded
after all theses years! Stuck on Stupid, if you
might! Owen- your charts are quite illuminating!
and, finely, The meter referred to , if memory
serves correct, was made by SWAN, just before
they went out of business (made for the "CB"
trade, more than amateur radio. and the lines
used in it were on a Printed Circuit board!
and this probably before they knew how to get
the correct impedence , on a p.c. board!
I shall return to my lair-- Jim NN7K


Owen Duffy wrote:
On Wed, 06 Sep 2006 04:45:57 GMT, Cecil Moore
wrote:

Jim - NN7K wrote:
... but it is
beyond the ability to have MORE power returned
to the source, than the source provided
Did you know a reflection coefficient can be greater
than 1.0?

It is true that reflection coefficient can be greater than 1.0.

The reflection coefficient *CANNOT* be greater than 1.0 where Zo is
purely resistive.

So, where a sampler is calibrated (nulled) on a purely resistive load
(eg 50+j0) as is most commonly done, it should never show a reflection
coefficient greater than 1. A reflectometer calibrated to a resistive
load and that shows a "reflected" reading greater than the "set"
reading is inaccurate / defective / a poor design.

Owen
--

On Wed, 06 Sep 2006 23:24:45 GMT, Jim - NN7K wrote:

Thanks, Owen, and all and sorry to cause so much
grief. I fall on my sword!

Jim,

I have been following this thread for some time and wish to complement
you on your response. Normally, on the NG egos seem bigger than brains
and seldom does anyone admit making an error. You are to be
complemented for you forthrightness. It speaks highly of your
character - a rare quality in this day and age.

Very 73,
Danny, K6MHE

On Wed, 06 Sep 2006 23:24:45 GMT, Jim - NN7K wrote:

Thanks, Owen, and all and sorry to cause so much
grief. I fall on my sword! Have believed 99.44%

No need for that Jim, the discussion has challenged your thinking (and
mine), and that is part of the learning process.

of what was in the old VHF'er Magazine, as was

IMHO, it is not a very good article.

Forward Voltage, Forward Current, Reflected Voltage, Reflected
Current, Current, Voltage, Zo, VSWR, impedance and propagation
constant are all relevant, related but different and the article
doesn't adequately draw the distinction.

Indicators might sample one or more of voltage, or current, or
directional power. They are all different, a voltage sensor or a
current sensor alone at a single point will not allow you to determine
VSWR, a pair of directional power sensors will. Your Lecher lines
example needs a voltage or current sensor, not a directional power
sensor (which won't work), whereas a VSWR meter needs a pair of
directional power sensors, not a voltage or current sensor (which
won't work).

My view is that the article is imprecise, confused and contains bad
advice. It does play to one of the archetypal myths of ham radio!

I am considering whether I should create some more graphs of current,
voltage, and impedance etc that illustrate how those behave, and
incorporate it in a more complete permanent article on my web site.
(IIRC there were some graphs of voltage, current, phase in Fred
Terman's book, but I am thinking of going a little beyond that.)

Thanks for the response Jim.

73
Owen
--