Mythbusters: V/I ratio is forced to Z0
 

On Sat, 15 Oct 2005 14:38:15 GMT, Cecil Moore wrote:
long before you entered the thread
More guessing, and certainly poor research. As many may acknowledge,
a glance at the beginning of this thread dominates all speculation
about who entered when.

No doubt a response is in order as to what you "meant" - as is so
often the case.

V/I ratio is forced to Z0:was Mythbusters
 

Dave wrote:
"Cecil Moore" wrote:

Owen Duffy wrote:
This has nothing to do with the stated myth: Measurements with a Bird
43 of the conditions on the Thruline section are invalid unless it has
some minimum length of 50 ohm line on both sides of itself.

Would you be willing to make the same statement about an MFJ wattmeter?

now your are just trying to muddy the waters... i wouldn't trust an mfj to
measure anything!

No, I'm just trying to get back to the original discussion which
was: Do SWR meters need 50 ohm coax surrounding them to establish
the assumed 50 ohm environment? The majority of SWR meters have
no Thruline and MFJ seems to make no attempt to establish a 50
ohm environment like Bird does. What is the Z0 of a meandering wire
surrounded by an aluminum box?

The original discussion (V/I ratio is forced to Z0) had nothing to
do with Bird wattmeters. The original discussion was about SWR meters
in general (which the Bird is not). The mythbusters thing was an
interesting diversion away from the original question which remains
unanswered.
--
73, Cecil http://www.qsl.net/w5dxp

Mythbusters: V/I ratio is forced to Z0
 

Richard Clark wrote:

Cecil Moore wrote:
I guess you will take that delusion to your grave.

Yet more guessing?

It's a guess as to whether you are going to correct
your errors about thin-film coatings before you
expire. What is it about the following that you don't
understand?

www.mellesgriot.com/products/optics/oc_2_1.htm

"Thin-Film Interference"

"Thin-film coatings also rely on the principles of interference. Thin
films are dielectric or metallic materials whose thickness is comparable
to, or less than, the wavelength of light."

Speaking of 1/4WL thin-film coatings:

"Clearly, if the wavelength of the incident light and the thickness
of the film are such that a phase difference exists between reflections
of p, then reflected wavefronts interfere destructively, and overall
reflected intensity is a minimum. If the two reflections are of equal
amplitude, then this amplitude (and hence intensity) minimum will be
zero."

"Amplitude minimum equal zero" means all reflections have been eliminated.
--
73, Cecil http://www.qsl.net/w5dxp

Mythbusters: V/I ratio is forced to Z0
 

Richard Clark wrote:

Cecil Moore wrote:
long before you entered the thread

Canceled it immediately after I wrote it when I realized the
subject line had changed.

More guessing, and certainly poor research. As many may acknowledge,
a glance at the beginning of this thread dominates all speculation
about who entered when.

No doubt a response is in order as to what you "meant" - as is so
often the case.

No doubt an apology is in order. The posting no longer exists
on my news-server. I apologize that my canceled posting made
it off my news-server. Sorry, my subject line font is so small
I cannot read it without a magnifying glass.
--
73, Cecil http://www.qsl.net/w5dxp

What is a 50-ohm environment. ???
 

- or even more vague - an assumed 50-ohm environment?

Or an evironment of any other impedance.

An engineering definition please! In plain English.

Then perhaps I can understand what you are all waffling about.

Thank you.
----
Reg, G4FGQ.

V/I ratio is forced to Z0:was Mythbusters
 

"Cecil Moore" wrote in message
om...
Dave wrote:
"Cecil Moore" wrote:

Owen Duffy wrote:
This has nothing to do with the stated myth: Measurements with a Bird
43 of the conditions on the Thruline section are invalid unless it has
some minimum length of 50 ohm line on both sides of itself.

Would you be willing to make the same statement about an MFJ wattmeter?

now your are just trying to muddy the waters... i wouldn't trust an mfj
to measure anything!

No, I'm just trying to get back to the original discussion which
was: Do SWR meters need 50 ohm coax surrounding them to establish
the assumed 50 ohm environment? The majority of SWR meters have
no Thruline and MFJ seems to make no attempt to establish a 50
ohm environment like Bird does. What is the Z0 of a meandering wire
surrounded by an aluminum box?

The original discussion (V/I ratio is forced to Z0) had nothing to
do with Bird wattmeters. The original discussion was about SWR meters
in general (which the Bird is not). The mythbusters thing was an
interesting diversion away from the original question which remains
unanswered.
--

an interesting diversion until everyone was convinced that you were off the
wall. now its time to re-open a dead thread???.

as far as cheap swr meters, the daiwa, swan, and mfj manuals all require 50
ohm coax 'for accurate readings'... a joke by any standard of measurement
for that type of instrument. but just to put this one to rest quickly... i
set up my tdr and ran some quick measurements. this tdr will resolve a 6"
75 ohm jumper in the 25' or so of 50 ohm test cable that i used. i measured
an mfj-815b and a daiwa ns-660pa and they are indistiguishable from the 50
ohm line. so the answer is yes, they do internally look like a 50 ohm line
section.

What is a 50-ohm environment. ???
 

"Reg Edwards" wrote in message
...
- or even more vague - an assumed 50-ohm environment?

Or an evironment of any other impedance.

An engineering definition please! In plain English.

Then perhaps I can understand what you are all waffling about.

Thank you.

don't bother, its a figment of cecil's imagination and creative vocabulary.

Mythbusters: V/I ratio is forced to Z0
 

On Sat, 15 Oct 2005 16:18:22 GMT, Cecil Moore wrote:
Yet more guessing?
It's a guess
and it took 32 lines to say that? I can understand why editors dump
your submissions.

What is a 50-ohm environment. ???
 

Reg Edwards wrote:
- or even more vague - an assumed 50-ohm environment?
Or an evironment of any other impedance.

The transmission line reflection model tells us that the Z0
of a transmission lines forces the following relationship.

Vfor/Ifor = Vref/Iref = Z0

A "50 ohm environment" used in the context of the previous
discussion would be one in which the above relationship
is forced on the system at certain points in the system.

600 ohm transmission line going from a tuner/balun to an
antenna establishes a 600 ohm environment for the signals
on the transmission line. An SWR meter calibrated for 600
ohms will indicate the actual SWR.

About a year ago, based on a discussion that you and I were
having, I asked the sci.physics.electromag newsgroup how
long a piece of RG-213 coax has to be to establish the
above relationship. The answer was that the non-TEM product
terms decrease at about 1/e every two inches for RG-213.

The RG-400 coax leads going to and from my SWR meter are
two feet each. So I asserted to you that my SWR meter
was reading the actual SWR in the middle of that run of
RG-400. I think you disagreed with my assertion but I
cannot remember for sure.
--
73, Cecil http://www.qsl.net/w5dxp

V/I ratio is forced to Z0:was Mythbusters
 

"Dave" wrote in message
. ..

"Cecil Moore" wrote in message
om...
Dave wrote:
"Cecil Moore" wrote:

Owen Duffy wrote:
This has nothing to do with the stated myth: Measurements with a Bird
43 of the conditions on the Thruline section are invalid unless it has
some minimum length of 50 ohm line on both sides of itself.

Would you be willing to make the same statement about an MFJ wattmeter?

now your are just trying to muddy the waters... i wouldn't trust an mfj
to measure anything!

No, I'm just trying to get back to the original discussion which
was: Do SWR meters need 50 ohm coax surrounding them to establish
the assumed 50 ohm environment? The majority of SWR meters have
no Thruline and MFJ seems to make no attempt to establish a 50
ohm environment like Bird does. What is the Z0 of a meandering wire
surrounded by an aluminum box?

The original discussion (V/I ratio is forced to Z0) had nothing to
do with Bird wattmeters. The original discussion was about SWR meters
in general (which the Bird is not). The mythbusters thing was an
interesting diversion away from the original question which remains
unanswered.
--

an interesting diversion until everyone was convinced that you were off
the wall. now its time to re-open a dead thread???.

as far as cheap swr meters, the daiwa, swan, and mfj manuals all require
50 ohm coax 'for accurate readings'... a joke by any standard of
measurement for that type of instrument. but just to put this one to rest
quickly... i set up my tdr and ran some quick measurements. this tdr will
resolve a 6" 75 ohm jumper in the 25' or so of 50 ohm test cable that i
used. i measured an mfj-815b and a daiwa ns-660pa and they are
indistiguishable from the 50 ohm line. so the answer is yes, they do
internally look like a 50 ohm line section.


ah, good cecil hasn't replied yet... sorry, i should have put this in the
same message, but the sun was trying to come out and i had to take advantage
of it. we haven't seen the sun here for the last 10 days which is the only
reason i was bored enough to bother getting into one of these 'discussions'
again.

now for the other part of the story. cecil states that a 50 ohm
'environment' must be established around these meters to get proper
readings. this 'environment' requires some unknown length of 50 ohm cable
on both sides of the meter. i can disprove this one easily... i again used
the two meters above. first, attach 50 ohm dummy load to the meter, set
transmitter to 100w forward on the meter, read reverse... there is always
some small movement, but not much more than a needle width on either one.
now, get 1/2 wave of 75 ohm cable, attach between the dummy load and the
meter antenna output... again adjust for 100w forward and voila, same needle
wiggle as before. what does this mean? well, if indeed the meter was
relying on the length of 50 ohm coax on the output to 'establish the
environment' as cecil seems to think is necessary then the 75 ohm cable
should have upset this 'environment' and caused an incorrect reading...
since the reading was correct for a 50 ohm meter then it must be that the
required length of 50 ohm cable is zero.

now of course cecil will rant and rave and say the meter is reading
incorrectly since there is obviously some reflected power in the 75 ohm
cable. and he will be correct on that point, however, once you change the
reference Zo to 50 ohms inside the meter there is no reflected power to
measure since the load presented at the end of the 75 ohm line is 50 ohms.
QED.

V/I ratio is forced to Z0:was Mythbusters
 

Dave wrote:
an interesting diversion until everyone was convinced that you were off the
wall. now its time to re-open a dead thread???.

No, an interesting diversion that has been resolved. Time to return
to the real issue.

as far as cheap swr meters, the daiwa, swan, and mfj manuals all require 50
ohm coax 'for accurate readings'... a joke by any standard of measurement
for that type of instrument. but just to put this one to rest quickly... i
set up my tdr and ran some quick measurements. this tdr will resolve a 6"
75 ohm jumper in the 25' or so of 50 ohm test cable that i used. i measured
an mfj-815b and a daiwa ns-660pa and they are indistiguishable from the 50
ohm line. so the answer is yes, they do internally look like a 50 ohm line
section.

Try it with 75 ohm coax. I suspect they will be equally indistinguishable
from 75 ohm line.
--
73, Cecil http://www.qsl.net/w5dxp

What is a 50-ohm environment. ???
 

Dave wrote:
don't bother, its a figment of cecil's imagination and creative vocabulary.

Actually, the question logically follows from the reflection model.
Under what boundary conditions does a piece of transmission line
force Vfor/Ifor = Vref/Iref = Z0 to a specified accuracy?
--
73, Cecil http://www.qsl.net/w5dxp

V/I ratio is forced to Z0:was Mythbusters
 

Dave wrote:
however, once you change the
reference Zo to 50 ohms inside the meter there is no reflected power to
measure since the load presented at the end of the 75 ohm line is 50 ohms.

The question remains: Does the MFJ's physical design ensure the
*physical* reference Z0 is 50 ohms or would it perform just as
well in a 75 ohm enviornment simply by recalibrating it for
75 ohms?

In other words, it is sampling a voltage and current completely
divorced from any *physical* reference Z0. The actual reference
depends upon an arbitrary setting of a calibration cap which
has no effect at all on the *physical* reference Z0.
--
73, Cecil http://www.qsl.net/w5dxp

What is a 50-ohm environment. ???
 

On Sat, 15 Oct 2005 17:52:47 GMT, Cecil Moore wrote:
I cannot remember for sure.
Let's see, you know the news group, you know the context, you know the
time, you know the correspondents, you may have even "quoted" the
material (now truly suspect) - but sadly you forget the details that
are somehow unretrievable from a simple google search with such a
wealth of key terms and specific constraints.

And from this you've been sowing an itinerant thought as proof....

How is this distinguished from trolling?

V/I ratio is forced to Z0:was Mythbusters
 

On Sat, 15 Oct 2005 18:25:02 GMT, Cecil Moore wrote:
a calibration cap which has no effect at all on the *physical* reference Z0.

If it had an effect, the Z would change, wouldn't it? The cap has a
Z, that much is sure. The purpose of the cap is not what you ascribe
to it - but that is par for the course as this inclusion quoted above
has no meaning either in or out of context.

V/I ratio is forced to Z0:was Mythbusters
 

"Cecil Moore" wrote in message
m...
Dave wrote:
an interesting diversion until everyone was convinced that you were off
the wall. now its time to re-open a dead thread???.

No, an interesting diversion that has been resolved. Time to return
to the real issue.

as far as cheap swr meters, the daiwa, swan, and mfj manuals all require
50 ohm coax 'for accurate readings'... a joke by any standard of
measurement for that type of instrument. but just to put this one to
rest quickly... i set up my tdr and ran some quick measurements. this
tdr will resolve a 6" 75 ohm jumper in the 25' or so of 50 ohm test cable
that i used. i measured an mfj-815b and a daiwa ns-660pa and they are
indistiguishable from the 50 ohm line. so the answer is yes, they do
internally look like a 50 ohm line section.

Try it with 75 ohm coax. I suspect they will be equally indistinguishable
from 75 ohm line.

ok, just for you i swapped all the cables around. and you are wrong, it is
not indistinguishable from 75 ohms, it is easily picked out as a 50 ohm
section.

What is a 50-ohm environment. ???
 

"Cecil Moore" wrote in message
m...
Dave wrote:
don't bother, its a figment of cecil's imagination and creative
vocabulary.

Actually, the question logically follows from the reflection model.
Under what boundary conditions does a piece of transmission line
force Vfor/Ifor = Vref/Iref = Z0 to a specified accuracy?

then look up article 3.19 in 'fields and waves in communications
electronics' which is where they derive the fringe effects for a step change
in spacing between two plates. this is where they refer you for calculating
the effects of evanescent modes from a more complex problem stating that the
results are identical with the static case in 3.19... in the derivation of
the complex case of a step change in a cable they show that a single lumped
capacitance added at the step is an adequate representation of the
discontinuity when calculating the evanescent modes below cutoff. in 3.19
if you disregard the fields along the length of the step it ends up in an
equation: Z=h/pi(exp(pi*W/V0)-1-pi*W/V0_j*pi) the important part is
obvioulsy the decay factor in the exponential which goes as exp(-x/h) where
h is the separation in the planes... or you get one 1/e reduction for each
distance equal to the spacing which in rg-58 or rg-8x that i used is
something like 1mm or less.. so how far down do you want to be? in 4.5mm
you are down to 1% which is well under the accuracy of these cheap meters,
and much less than the length of even the connectors on the meters.

Mythbusters: V/I ratio is forced to Z0
 

On Sat, 15 Oct 2005 17:57:35 GMT, Cecil Moore wrote:


Try it with 75 ohm coax. I suspect they will be equally indistinguishable
from 75 ohm line.

Cecil, you have got everyone else running around based on your
apparent misconceptions.

Is it not time you put in some time on the experimental side to
support / validate your conceptual contribution?

Enough of the "I suspect", "I seriously doubt". Where does that fit in
scientific method... you are not the eminent professor running a bunch
of PHD students around.

Owen
--

V/I ratio is forced to Z0:was Mythbusters
 

Richard Clark wrote:
On Sat, 15 Oct 2005 18:25:02 GMT, Cecil Moore wrote:
a calibration cap which has no effect at all on the *physical* reference Z0.

If it had an effect, the Z would change, wouldn't it? The cap has a
Z, that much is sure. The purpose of the cap is not what you ascribe
to it - but that is par for the course as this inclusion quoted above
has no meaning either in or out of context.

Here's a schematic of an MFJ SWR meter at the bottom:

http://www.mfjenterprises.com/man/pdf/MFJ-816.pdf

What do you think is the purpose of the 10pf variable cap
if not to vary the voltage in the voltage divider?

The question is: Between the "transmitter" terminal and
the "antenna" terminal, what determines the physical
characteristic impedance of the sampling circuit?
--
73, Cecil http://www.qsl.net/w5dxp

V/I ratio is forced to Z0:was Mythbusters
 

Dave wrote:
ok, just for you i swapped all the cables around. and you are wrong, it is
not indistinguishable from 75 ohms, it is easily picked out as a 50 ohm
section.

What causes that?
--
73, Cecil http://www.qsl.net/w5dxp

Mythbusters: V/I ratio is forced to Z0
 

I always see a few that delight in attacking MFJ. Find me another company
that produces products for a reasonable price for the Ham.

Do they get any props at all, hell no. Some idiot ****es and moans that the
MFJ $19.95 what ever is less accurate than their favorite $350.00 supplier.

Why would you not "trust" an MFJ to measure anything?

If you can't give a specific reason to avoid an MFJ product, then what is
gained by your supercilious remark.


"Dave" wrote in message
...

"Cecil Moore" wrote in message
om...
Owen Duffy wrote:
This has nothing to do with the stated myth: Measurements with a Bird
43 of the conditions on the Thruline section are invalid unless it has
some minimum length of 50 ohm line on both sides of itself.

Would you be willing to make the same statement about an MFJ wattmeter?

now your are just trying to muddy the waters... i wouldn't trust an mfj to
measure anything!

What is a 50-ohm environment. ???
 

Dave wrote:
... or you get one 1/e reduction for each
distance equal to the spacing which in rg-58 or rg-8x that i used is
something like 1mm or less.. so how far down do you want to be?

Is that 1/e reduction for power or voltage?

in 4.5mm
you are down to 1% which is well under the accuracy of these cheap meters,
and much less than the length of even the connectors on the meters.

Kevin Rhodes said, "... the lowest order undesired mode should reduce
intensity by a factor of 1/e in about 2.03"..." for Rg-213. 2.03" is
a lot greater than the spacing in RG-213.
--
73, Cecil http://www.qsl.net/w5dxp

Mythbusters: V/I ratio is forced to Z0
 

Owen Duffy wrote:
Is it not time you put in some time on the experimental side to
support / validate your conceptual contribution?

Maybe you can help. I have an MFJ-259B antenna analyzer and a
dual-trace 100 MHz o'scope. Let's say I have an unmarked 2 inch
piece of coax and I don't know the characteristic impedance.

Using the MFJ-259B and o'scope, how could I ascertain the Z0 of
that 2 inch piece of coax? This is similar to the problem of
determining the physical Z0 of a poorly designed SWR meter.
--
73, Cecil http://www.qsl.net/w5dxp

V/I ratio is forced to Z0:was Mythbusters
 

On Sat, 15 Oct 2005 20:37:11 GMT, Cecil Moore wrote:
http://www.mfjenterprises.com/man/pdf/MFJ-816.pdf
What do you think is the purpose of the 10pf variable cap
if not to vary the voltage in the voltage divider?

You don't know, do you? :-)
It adjusts the frequency correction at the high end of the meter's
frequency range. Too bad this version of the manual doesn't lay it
out for Xeroxing - many of their other manuals do. This is a
commonplace of the Bruene design.

The question is: Between the "transmitter" terminal and
the "antenna" terminal, what determines the physical
characteristic impedance of the sampling circuit?

You don't know that one either, do you?

The turns ratio and the 82 Ohm load resistor IS the Z of sampling
circuit; the capacitor provides the e phase reference and tracking
across frequency. It is very lightly loading as a series load by
design and as evidenced by Dave's measurements.

Odd that you couldn't have said this yourself - rather ordinary stuff.

What is a 50-ohm environment. ???
 

On Sat, 15 Oct 2005 20:49:42 GMT, Cecil Moore wrote:
in 4.5mm
you are down to 1% which is well under the accuracy of these cheap meters,
and much less than the length of even the connectors on the meters.
Kevin Rhodes said, "... the lowest order undesired mode should reduce
intensity by a factor of 1/e in about 2.03"..." for Rg-213. 2.03" is
a lot greater than the spacing in RG-213.
so much for relying on an obscure poster quoted indirectly by
paraphrase to a new context. Such is third hand information.

What is a 50-ohm environment. ???
 

On Sat, 15 Oct 2005 19:02:58 -0000, "Dave" wrote:


"Cecil Moore" wrote in message
om...
Dave wrote:
don't bother, its a figment of cecil's imagination and creative
vocabulary.

Actually, the question logically follows from the reflection model.
Under what boundary conditions does a piece of transmission line
force Vfor/Ifor = Vref/Iref = Z0 to a specified accuracy?

then look up article 3.19 in 'fields and waves in communications
electronics' which is where they derive the fringe effects for a step change
in spacing between two plates. this is where they refer you for calculating
the effects of evanescent modes from a more complex problem stating that the
results are identical with the static case in 3.19... in the derivation of
the complex case of a step change in a cable they show that a single lumped
capacitance added at the step is an adequate representation of the
discontinuity when calculating the evanescent modes below cutoff. in 3.19
if you disregard the fields along the length of the step it ends up in an
equation: Z=h/pi(exp(pi*W/V0)-1-pi*W/V0_j*pi) the important part is
obvioulsy the decay factor in the exponential which goes as exp(-x/h) where
h is the separation in the planes... or you get one 1/e reduction for each
distance equal to the spacing which in rg-58 or rg-8x that i used is
something like 1mm or less.. so how far down do you want to be? in 4.5mm
you are down to 1% which is well under the accuracy of these cheap meters,
and much less than the length of even the connectors on the meters.

Dave, I note the work above estimating the rate of decay of the effect
of the discontinuity, and we have seen another analysis of the decay
rate (mis)quoted.

Doesn't the impact of the discontinuity at a distance x depend on the
magnitude of the products of the discontinuity times the attenuation
at distance X.

These discussions do not seem to have attempted to estimate the
magnitude of the products of the discontinuity.

Think about all the places where we do not preserve nominal Zo, and
whether significant power is diverted to other than the dominant mode
and subsequently lost. Look inside a HF transceiver, it is full of
discontinuities between the PA collector and the coax socket. Design
practice for HF transceivers does not consider potential losses from
evanescent modes.

If there were significant dissapative losses from such
discontinuities, don't all the texts on stub tuner design need to be
rewritten.

Where is the experimental evidence that significant power is diverted
in practical circuits and transmission line discontinuities?

My experiment reported at the beginning of the thread could be varied
to include another Bird at the dummy load, and to compare the power
loss between Birds with the expected cable loss (in the dominant mode)
to indicate whether there was another dissapative loss mechanism at
work.

I won't waste the time on the experiment because I anticipate that
instrument errors and cable specification errors would swamp any
likely effect of the discontinuities. IMHO, a well designed experiment
would require more accurate measurement of the dominant mode loss of
the 75 ohm cable, and more precise and accurate power measurement
equipment.

To demonstrate that point, I have just performed a test at 10.1MHz
with the Bird 43 and the 5.27m of 9275 on the load side with o/c load.

The measured return loss was -0.36dB. (Always seems un-natural
specifying a loss as -dB). I estimate the return loss should have been
-0.25dB (using the calculator at http://www.vk1od.net/tl/tllce.php),
leaving an unaccounted for -0.11dB. The differences are smaller than
the tolerances / accuracy of the equipment.

Perhaps it does demonstrate that the loss caused by the discontinuity
is not significant in terms of the specified accuracy of the Bird 43.

Owen
--

What is a 50-ohm environment. ???
 

"Cecil Moore" wrote in message
m...
Dave wrote:
... or you get one 1/e reduction for each distance equal to the spacing
which in rg-58 or rg-8x that i used is something like 1mm or less.. so
how far down do you want to be?

Is that 1/e reduction for power or voltage?

the equation gives the potential and flux functions, so its the
voltage/current essentially. no body calculates power functions unless they
are doing something strange.


in 4.5mm you are down to 1% which is well under the accuracy of these
cheap meters, and much less than the length of even the connectors on the
meters.

Kevin Rhodes said, "... the lowest order undesired mode should reduce
intensity by a factor of 1/e in about 2.03"..." for Rg-213. 2.03" is
a lot greater than the spacing in RG-213.
--
who is kevin rhodes? what reference books is he cited in as calculating
that and using what method? and that seems like an awfully long distance
for a fringe effect from a change in diameter of the conductors... but that
is just a gut feeling based on my understanding of the field equations.

V/I ratio is forced to Z0:was Mythbusters
 

"Cecil Moore" wrote in message
m...
Dave wrote:
ok, just for you i swapped all the cables around. and you are wrong, it
is not indistinguishable from 75 ohms, it is easily picked out as a 50
ohm section.

What causes that?
--

obviously between the tx and ant terminals it looks like a 50 ohm
transmission line.

ok, you guys have fun now, the rain is over and i probably won't be around
to argue with cecil as much the next few days. i trust someone will keep an
eye on him and make sure he doesn't twist the problem around so it can only
be solved with s parameter analysis referencing that optics web site
formulas he likes so much.

What is a 50-ohm environment. ???
 

"Owen Duffy" wrote in message
...
On Sat, 15 Oct 2005 19:02:58 -0000, "Dave" wrote:


"Cecil Moore" wrote in message
. com...
Dave wrote:
don't bother, its a figment of cecil's imagination and creative
vocabulary.

Actually, the question logically follows from the reflection model.
Under what boundary conditions does a piece of transmission line
force Vfor/Ifor = Vref/Iref = Z0 to a specified accuracy?

then look up article 3.19 in 'fields and waves in communications
electronics' which is where they derive the fringe effects for a step
change
in spacing between two plates. this is where they refer you for
calculating
the effects of evanescent modes from a more complex problem stating that
the
results are identical with the static case in 3.19... in the derivation of
the complex case of a step change in a cable they show that a single
lumped
capacitance added at the step is an adequate representation of the
discontinuity when calculating the evanescent modes below cutoff. in 3.19
if you disregard the fields along the length of the step it ends up in an
equation: Z=h/pi(exp(pi*W/V0)-1-pi*W/V0_j*pi) the important part is
obvioulsy the decay factor in the exponential which goes as exp(-x/h)
where
h is the separation in the planes... or you get one 1/e reduction for each
distance equal to the spacing which in rg-58 or rg-8x that i used is
something like 1mm or less.. so how far down do you want to be? in 4.5mm
you are down to 1% which is well under the accuracy of these cheap meters,
and much less than the length of even the connectors on the meters.

Dave, I note the work above estimating the rate of decay of the effect
of the discontinuity, and we have seen another analysis of the decay
rate (mis)quoted.

Doesn't the impact of the discontinuity at a distance x depend on the
magnitude of the products of the discontinuity times the attenuation
at distance X.

These discussions do not seem to have attempted to estimate the
magnitude of the products of the discontinuity.

Think about all the places where we do not preserve nominal Zo, and
whether significant power is diverted to other than the dominant mode
and subsequently lost. Look inside a HF transceiver, it is full of
discontinuities between the PA collector and the coax socket. Design
practice for HF transceivers does not consider potential losses from
evanescent modes.

If there were significant dissapative losses from such
discontinuities, don't all the texts on stub tuner design need to be
rewritten.

Where is the experimental evidence that significant power is diverted
in practical circuits and transmission line discontinuities?

My experiment reported at the beginning of the thread could be varied
to include another Bird at the dummy load, and to compare the power
loss between Birds with the expected cable loss (in the dominant mode)
to indicate whether there was another dissapative loss mechanism at
work.

I won't waste the time on the experiment because I anticipate that
instrument errors and cable specification errors would swamp any
likely effect of the discontinuities. IMHO, a well designed experiment
would require more accurate measurement of the dominant mode loss of
the 75 ohm cable, and more precise and accurate power measurement
equipment.

To demonstrate that point, I have just performed a test at 10.1MHz
with the Bird 43 and the 5.27m of 9275 on the load side with o/c load.

The measured return loss was -0.36dB. (Always seems un-natural
specifying a loss as -dB). I estimate the return loss should have been
-0.25dB (using the calculator at http://www.vk1od.net/tl/tllce.php),
leaving an unaccounted for -0.11dB. The differences are smaller than
the tolerances / accuracy of the equipment.

Perhaps it does demonstrate that the loss caused by the discontinuity
is not significant in terms of the specified accuracy of the Bird 43.

Owen
--
yes, this is true also. the initial amplitude of the evanescent modes is
proportional to the size of the discontinuity. when going from 50 to 75
ohms that is a relatively small discontinuity to start with and the modes
decay exponentially from that starting point. there are probably bigger
losses due to interconnections between components and the connectors on the
case than are caused by the actual change in cable impedance.

What is a 50-ohm environment. ???
 

On Sat, 15 Oct 2005 20:49:42 GMT, Cecil Moore wrote:


Kevin Rhodes said, "... the lowest order undesired mode should reduce
intensity by a factor of 1/e in about 2.03"..." for Rg-213. 2.03" is
a lot greater than the spacing in RG-213.

Cecil,

Go back and read your own post:


You quoted Kevin Rhodes:

(1/45m)**2 = (1/.203")**2 + kz**2
Clearly, kz must be imaginary to make this work. thus an
evanescant, non-propagating wave:
kz**2 = (1/45m)**2 - (1/.203")**2
To the accuracy used to date, the first term on the right
is negligible, so the decay rate, alpha, can be estimated:
alpha**2 = - (kz)**2 = (1/2.03")**2

How does rearranging the terms here increase 0.203" to 2.03"?

In any event 0.203" is an overestimate of the spacing, 0.203" is
actually half the overall diameter of RG213.

I don't really understand this mathematics, do you understand the
thing you are citing?

Owen
--

What is a 50-ohm environment. ???
 

On Sat, 15 Oct 2005 21:20:16 GMT, Owen Duffy wrote:

Look inside a HF transceiver, it is full of
discontinuities between the PA collector and the coax socket. Design
practice for HF transceivers does not consider potential losses from
evanescent modes.

Hi Owen,

Some of the minor points here would be, to use the term you use
yourself later in your post, swamped. The swamping contributors are
generally attended to in design, and those corrections draw in and
encompass these smaller points.

A lot of this is handled by the low pass filtering following the final
stage's Z transformer in a transistor set.

If there were significant dissapative losses from such
discontinuities, don't all the texts on stub tuner design need to be
rewritten.

These would be called mismatch loss, which is not dissipative, but
rather what power was not delivered, that could have been if not in
the face of the discontinuity. Think of it as a loss of opportunity,
not a loss down the drain. Consider that same low pass filter
following the transformer. It withholds the power found in the
harmonics, and yet few filters need as much cooling as do the finals
transistors that are arcin' and sparkin' with those transients.

The low pass filter is a cheap alternative to linearizing the finals
stage and thus reducing those spurs. This would run the finals at a
lower gain however (to be put into what Bode calls the noise gain, or
negative feedback), and you would need more dollar investment for the
same power out.

Where is the experimental evidence that significant power is diverted
in practical circuits and transmission line discontinuities?

A practical example can be found in any RADAR system. You have ATR
tubes that perform power steering in waveguides. The list goes on
with lots of goodies (can anyone explain the Magic-T?) as RADAR is
particularly theatrical in this arena.

Every connection to a waveguide uses the physics of discontinuity to
suppress leakage. Examine the choke fittings, they are series shorted
tuned cavities used to bridge joints that necessarily have some
prospect of not maintaining entirely mating interfaces.

Perhaps it does demonstrate that the loss caused by the discontinuity
is not significant in terms of the specified accuracy of the Bird 43.

And then there is that distinct possibility. In such cases you design
the test to discard that perturbation by increasing the effect being
investigated. This is why I suggested a large mismatch would render
this chatter about 4W as being inconsequential.

73's
Richard Clark, KB7QHC

What is a 50-ohm environment. ???
 

On Sat, 15 Oct 2005 15:01:45 -0700, Richard Clark
wrote:

On Sat, 15 Oct 2005 21:20:16 GMT, Owen Duffy wrote:


Where is the experimental evidence that significant power is diverted
in practical circuits and transmission line discontinuities?

A practical example can be found in any RADAR system. You have ATR
tubes that perform power steering in waveguides. The list goes on
with lots of goodies (can anyone explain the Magic-T?) as RADAR is
particularly theatrical in this arena.

Every connection to a waveguide uses the physics of discontinuity to
suppress leakage. Examine the choke fittings, they are series shorted
tuned cavities used to bridge joints that necessarily have some
prospect of not maintaining entirely mating interfaces.


Yes, I should have qualified the statement to scope it at HF. I am
aware of the risk of excitation of undesirable modes in waveguide and
the need for mode traps to deal with them where their propagation is
undesirable.

I am not questioning whether physical discontinuities give rise to
electrical changes that can be explained or modelled with by lumped
constants or excitation of other propagation modes, I am questioning
whether the effects are significant in practical applications below
microwave frequencies, and especially at HF.

Owen
--

V/I ratio is forced to Z0:was Mythbusters
 

Richard Clark wrote:

On Sat, 15 Oct 2005 20:37:11 GMT, Cecil Moore wrote:
http://www.mfjenterprises.com/man/pdf/MFJ-816.pdf
What do you think is the purpose of the 10pf variable cap
if not to vary the voltage in the voltage divider?

You don't know, do you? :-)
It adjusts the frequency correction at the high end of the meter's
frequency range.

From page 27-8 ARRL Antenna Book 15th edition. "Capacitive
voltage dividers, C1-C3 and C2-C4, are connected across the
line to obtain equal-amplitude voltages in phase with the
line voltage, the division ratio being adjusted so that
these voltages match the voltage drops across R1 and R2
in amplitude."

In other words, it is the main Z0 calibration setting that
sets the sampled voltage equal to the sampled current while
driving a dummy load equal to Z0. Frequency response is
a completely secondary consideration.

If the range of C1 and C2 are great enough, the wattmeter
could be calibrated for 75 ohms rather than 50 ohms.

The question is: Between the "transmitter" terminal and
the "antenna" terminal, what determines the physical
characteristic impedance of the sampling circuit?

It is very lightly loading as a series load by
design and as evidenced by Dave's measurements.

Exactly how much effect does that light loading have on
the primary voltage/current amplitude and phase? Enough
to be detectable if the V/I ratio is not 50 ohms?
--
73, Cecil http://www.qsl.net/w5dxp

What is a 50-ohm environment. ???
 

Richard Clark wrote:
so much for relying on an obscure poster quoted indirectly by
paraphrase to a new context. Such is third hand information.

Which is better? Third hand or under hand? :-)
--
73, Cecil http://www.qsl.net/w5dxp

What is a 50-ohm environment. ???
 

On Sat, 15 Oct 2005 23:01:02 GMT, Cecil Moore wrote:

Owen Duffy wrote:
How does rearranging the terms here increase 0.203" to 2.03"?

Already caught that boo-boo. Correcting it brings Kevin's math
in line with Dave's.

So you were aware of the apparent defect in the work you were citing
as recently as less than two hours ago, and it took someone else to
notify the defect here?

Owen
--

What is a 50-ohm environment. ???
 

Owen Duffy wrote:
If there were significant dissapative losses from such
discontinuities, don't all the texts on stub tuner design need to be
rewritten.

We're not talking about losses, Owen. We are talking about the
changing relationships between V and I at an impedance discontinuity.
Why do you think they call them discontinuities?
--
73, Cecil http://www.qsl.net/w5dxp

What is a 50-ohm environment. ???
 

Dave wrote:
who is kevin rhodes?

The guy from sci.physics.electromag whom I quoted previously
in the "V/I forced to Z0" thread. It appears that he
accidentally replaced the conductor spacing of 0.203" with
2.03" in his calculations. If that is corrected, his values
tend to agree with yours given the different conductor
spacing between RG8X and RG213.

So can we say, the lowest order undesired mode should reduce
intensity by a factor of 1/e in about one conductor spacing,
1/e^2 in two conductor spacings, etc.?
--
73, Cecil http://www.qsl.net/w5dxp

V/I ratio is forced to Z0:was Mythbusters
 

Dave wrote:
obviously between the tx and ant terminals it looks like a 50 ohm
transmission line.

So I repeat, what causes that characteristic? Is there
some coax inside the MFJ box?
--
73, Cecil http://www.qsl.net/w5dxp

What is a 50-ohm environment. ???
 

Owen Duffy wrote:
How does rearranging the terms here increase 0.203" to 2.03"?

Already caught that boo-boo. Correcting it brings Kevin's math
in line with Dave's.
--
73, Cecil http://www.qsl.net/w5dxp

What is a 50-ohm environment. ???
 

"Owen Duffy" wrote in message
...
On Sat, 15 Oct 2005 23:01:02 GMT, Cecil Moore wrote:

Owen Duffy wrote:
How does rearranging the terms here increase 0.203" to 2.03"?

Already caught that boo-boo. Correcting it brings Kevin's math
in line with Dave's.

So you were aware of the apparent defect in the work you were citing
as recently as less than two hours ago, and it took someone else to
notify the defect here?

Owen
--

ugh! all of this was over a slipped decimal point??? so we are down to .2"
transition, which pretty much agrees with the one i came up with, and which
basically means that by the time you are out of the connector shell you are
back at Z0. and since the meter takes its own 50 ohm 'environment' with it
for sampling it is reading every thing exactly as it should... and exactly
as has been measured... and there is no requirement for some particular
length of 50 ohm coax on either side of a meter... what a waste of a
perfectly good argument, you better apologize big time for this one cecil!