V/I ratio is forced to Z0:was Mythbusters
 

On Sat, 15 Oct 2005 22:36:11 GMT, Cecil Moore wrote:

these voltages match the voltage drops across R1 and R2
in amplitude.

There is NO R1 and/or R2

This is familiar bait and switch methods so common to your trolling.
Changing the schematics to suit the argument can make any answer come
alive.

Like I said, if you were already familiar with this, you would have
said so and skipped all this stageshowing.

In other words, it is the main Z0 calibration setting

An interpretation spanning as many documents as it takes to assemble
all the words xeroxed to this posting.

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

Notably, there are no C1 and/or C2. Even more, if there were, any
competent tech in this group would be aware that it also would take a
similar shift in value of your R1 and/or R2. In fact, this
overstatement by calling out BOTH Cs needing change begs the chuckle
reflex being stifled.

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?

[clue: this is in the portion of my original post that you omitted.]

I would normally ask my students "What do you think?" but there is a
danger of that here, with so little obtained in return for so much
said.

Rather, I will offer "Think about it. How much power does it take to
swing a 1mA meter even full scale when there is a 500KOhm calibration
resistor (imagine that, do we assign THAT as the Z?) limiting it."

Something about I²R comes to mind for max power (300W) which works out
to half a watt out of 300 supplied. A simple, back-of-the-napkin
computation suggests less than 1%. A similar study of the actual load
of 82 Ohms needs only be cast back through the N² turns ratio. Seeing
that it, too, is probably a quarter Watt resistor that we are still
talking about less than 1% consumption.

So, anyway you slice and dice it without actually counting the turns
ratio, the tech at the bench would easily offer the meter injects only
half an ohm or less series resistance into the line.

What is a 50-ohm environment. ???
 

On Sat, 15 Oct 2005 22:37:18 GMT, Cecil Moore wrote:
Which is better? Third hand or under hand? :-)
If you can't get it across the plate, neither.

What is a 50-ohm environment. ???
 

On Sat, 15 Oct 2005 22:55:14 GMT, Cecil Moore wrote:
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.
Nice thing about third hand quotes, you can blame them for your
transcription errors and they can't defend themselves.

What is a 50-ohm environment. ???
 

Owen Duffy wrote:
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?

You should have been able to figure it out from the posting times.
I reported the problem at 5:55pm, after you had reported it but
before I had read your posting which reported it. You've heard of
two ships passing in the night? Ain't usenet great?
--
73, Cecil http://www.qsl.net/w5dxp

What is a 50-ohm environment. ???
 

Dave wrote:
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!

I do apologize for the slip of the decimal point and for not finding it
before today but I think you guys completely missed the context of the
original argument and instead went off on several interesting tangents.

Now back to the original context: As Reg pointed out, you can't have an
SWR on a feedline that doesn't exist so how much 50 ohm feedline must
exist *external to the SWR meter* to be able to report a valid 50 ohm SWR
reading *on that coax*? Turns out to be around an inch, more or less, and
that is a good thing to know. Thanks for answering the question even if
in a time consuming way. If you had simply said that 0.2" of 50 ohm
coax is required to establish a 50 ohm environment *external to the SWR
meter*, we could have started arguing from that point and it would have
saved a lot of time and effort.
--
73, Cecil http://www.qsl.net/w5dxp

V/I ratio is forced to Z0
 

Richard Clark wrote:

Cecil Moore wrote:
these voltages match the voltage drops across R1 and R2
in amplitude.

There is NO R1 and/or R2

I gave the reference, Richard, and am looking at it right now.
Page 27-9 of the 15th edition of the ARRL Antenna Book. R1 and
R2 are 10 ohms each.
--
73, Cecil http://www.qsl.net/w5dxp

What is a 50-ohm environment. ???
 

Richard Clark wrote:
Nice thing about third hand quotes, you can blame them for your
transcription errors and they can't defend themselves.

The evidence is always on Google.
--
73, Cecil http://www.qsl.net/w5dxp

What is a 50-ohm environment. ???
 

On Sun, 16 Oct 2005 04:20:58 GMT, Cecil Moore wrote:
Nice thing about third hand quotes, you can blame them for your
transcription errors and they can't defend themselves.
The evidence is always on Google.
Three degrees of separation between a poor third hand quote and its
validation.

What is a 50-ohm environment. ???
 

On Sun, 16 Oct 2005 04:16:56 GMT, Cecil Moore wrote:
but I think you guys completely missed the context
What is the validity of context when it is buried in error? This is
this is the pollution of argument that discards proper method and
accuracy to prove absolutist principles.

The point you miss is that when the errors are corrected, they
invalidate your premise - context is quite obviously out the window at
that point.

V/I ratio is forced to Z0
 

On Sun, 16 Oct 2005 04:20:01 GMT, Cecil Moore wrote:
I gave the reference, Richard, and am looking at it right now.
On Sat, 15 Oct 2005 20:37:11 GMT, Cecil Moore wrote:
Here's a schematic of an MFJ SWR meter at the bottom:
http://www.mfjenterprises.com/man/pdf/MFJ-816.pdf
Like I said, your common technique of bait and switch to please your
argument. This technique is so shop-worn that you couldn't even
respond in the same thread - what a wheeze. Please pretend you don't
understand... again. This is probably another reason why the editors
dump your submissions.

Mythbusters: V/I ratio is forced to Z0
 

On Sat, 15 Oct 2005 03:37:16 GMT, Cecil Moore wrote:

Owen Duffy wrote:

Cecil Moore wrote:
and not to be a myth at all. There's 104.17 watts of forward power
through the Bird and 4.17 watts of reflected power back through the
Bird. Why does the Bird ignore those actual power values?

I did not report or even measure such a thing.

Since I realized the Bird forms a Z0-match at its output that
statement should be ammended to say: There 104.17 watts of forward

Why the belated revelation of the Bird 43 coupler Zo? In a post on 9
Oct on the earlier thread I stated:

"My suggestion is that the sampler inside a Bird 43 coupler section is
sufficiently far inside the 50 ohm coupler line to provide
measurements within the instrument's stated accuracy of what is
happening within the 50 ohm coupler, irrespective of whether, for
instance, a 75 ohm line is attached to the coupler on the load side."

You must have read it, you responded to it with your message
.

Perhaps the honest way to deal with the situation is to acknowledge
that your statement quoted at the top of this post was just not true,
rather than to edit the words to shift the context to somewhere else
where it might be true.

It would be gracious of you to return to the myth, read it carefully
and think about it, and to respond restricting yourself to the myth as
stated, without qualification or obfuscation, is it a myth YES or NO.

To save you the hassle of finding it, here is:

The 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.

Myth or not, YES or NO?

Owen
--

Mythbusters: V/I ratio is forced to Z0
 

Is that BTW?
Yes - dear oh dear - pot calling kettle black :)


It appears to have a flat section of line that is parallel to the coax
centre conductor and is presumably capacitively and inductively
coupled, and it uses some form of frequency compensation to give it
broadband response. You rotate the sampler element for measurement of
the opposite direction.

Have played with that type of Bird. Worked well as I recall. Have also
made a Bruene type - fairly constant readings over wide frequency range.
Yes 40m is a bit ordinary at the mo for short distance.
Cheers,
Alan

What is a 50-ohm environment. ???
 

OK, Cec, I have a better idea of what you mean by '50-ohm
environment'.

You refer to: -

Vfor/Ifor = Vref/Iref = Z0

with which I agree.

But Zo is a pure resistance and the relationship can be true only when
there is no phase shift between Vfor/Ifor and Vref/Iref.

Which means, in the ideal meter, the voltage tap-off and current
tap-off points must be identically located within the instrument.

I would now like to go off in a slightly different tack to these
extensive threads.

There is a length of conductor between the input and output terminals
of the meter. It cannot be avoided. For obvious reasons related to the
high-frequency response and accuracy, the Zo environment along this
conductor has to be maintained as good as is reasonably possible. (
With the common or garden SWR meter nobody bothers very much. It
hardly matters anyway.)

The location of the voltage and current tapping points along the
conductor doesn't matter two hoots. What matters is the distance
beween them because of the phase difference. Small errors due to
misplacement are unavoidable and have to be lived with.

Getting to the point of this message: - The length of transmission
line inside the meter and the unknowns regarding 'Zo environment' play
no part either in operation or analysis of the meter.

The story that the length of line inside the meter is used to detect
and measure standing wave ratios is just another old-wives' tale which
confuses CB-ers, novices and professional engineers alike.

( By the way, if there is a short transmission line of any sort inside
the instrument, its length can be used to set the measuring
sensitivity. The longer the line the greater the sensitivity. But the
longer the line the greater the measuring errors and the worse the
frequency response.)
----
Reg, G4FGQ.

===========================================
"Cecil Moore" wrote

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
 

On Sat, 15 Oct 2005 22:57:40 GMT, Cecil Moore wrote:

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?

I've been reading this thread while biting my tongue concerning the Zo of the
SWR measuring device. IMO the basis for the Zo of the device is being
overlooked, although Cecil is coming the closest to describing it.

We are discussing the Bruene version of the device in which the inductance
derives a sample of the current and C1 derives a sample of the voltage. When C1
is adjusted to obtain the correct voltage for the voltage-current ratio to equal
Zo at that point, such as 50 ohms, the device provides the correct readings when
the line impedance on the load side is Zo. The line impedance Zo on the input
side is irrelevant.

If C1 is adjusted to obtain the correct voltage for the voltage-current ratio to
equal Zo of 75 ohms it will provide the correct reading when the line impedance
Zo on the load side is 75 ohms.

The line impedance of either the input or load transmission line is irrelevant
to the basis for the Zo of the device, therefore a length of line is unnecessary
to establish a Zo environment, the current sample and voltage sample establish
it..

One can terminate the device with a lumped impedance and get the same answer as
with a transmssion line whose input impedance is the same as the lumped
impedance.

Walt, W2DU

Mythbusters: V/I ratio is forced to Z0
 

Owen Duffy wrote:

Cecil Moore wrote:
Since I realized the Bird forms a Z0-match at its output ...

Perhaps the honest way to deal with the situation is to acknowledge
that your statement quoted at the top of this post was just not true,
rather than to edit the words to shift the context to somewhere else
where it might be true.

I already admitted that, Owen, when I said the Bird developes
a Z0-match at its terminals (above). If you will tell me how
many times I have to admit I was wrong, I will prepare the
appropriate posting.

The 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.

Myth or not, YES or NO?

Myth YES. I don't know in how many ways I have to say that.
Would me writing an admission in my own blood help calm you down?
--
73, Cecil http://www.qsl.net/w5dxp

V/I ratio is forced to Z0:was Mythbusters
 

Walter Maxwell wrote:
The line impedance of either the input or load transmission line is irrelevant
to the basis for the Zo of the device, therefore a length of line is unnecessary
to establish a Zo environment, the current sample and voltage sample establish
it..

Walt, there's one thing I don't understand about what Dave reported.
As I understand it, these were the test setups.

TDR---50 ohm coax---MFJ---50 ohm coax---50 ohm load

The TDR doesn't see the MFJ wattmeter.

TDR---75 ohm coax---MFJ---75 ohm coax---75 ohm load

The TDR sees the MFJ wattmeter.

If the MFJ in the second setup were recalibrated for 75 ohms,
would the TDR not see it?

************************************************** **************

The threads for the past few days have all diverged from the
original question which was:

How long must the 50 ohm coax connected to the SWR meter be
for the SWR meter to report a valid SWR on that coax? The
answer obviously cannot be zero length.
--
73, Cecil http://www.qsl.net/w5dxp

What is a 50-ohm environment. ???
 

Reg Edwards wrote:
Which means, in the ideal meter, the voltage tap-off and current
tap-off points must be identically located within the instrument.

In my MFJ, the toroid and calibration cap are about 1/2" apart.

The story that the length of line inside the meter is used to detect
and measure standing wave ratios is just another old-wives' tale which
confuses CB-ers, novices and professional engineers alike.

And if you will remember, the original question didn't involve the
SWR meter at all. The question was: Are my 2-foot sections of RG-400
connected to my SWR meter long enough to ensure that the SWR meter
reading is valid for the coax? Remember that argument?

The threads for the past few days have all diverged from that
original question which was:

How long must the 50 ohm coax connected to the SWR meter be
for the SWR meter to report a valid SWR *on that coax*? The
answer obviously cannot be zero length.
--
73, Cecil http://www.qsl.net/w5dxp

What is a 50-ohm environment. ???
 

"Cecil Moore" wrote in message
. ..
Reg Edwards wrote:
Which means, in the ideal meter, the voltage tap-off and current
tap-off points must be identically located within the instrument.

In my MFJ, the toroid and calibration cap are about 1/2" apart.

The story that the length of line inside the meter is used to detect
and measure standing wave ratios is just another old-wives' tale which
confuses CB-ers, novices and professional engineers alike.

And if you will remember, the original question didn't involve the
SWR meter at all. The question was: Are my 2-foot sections of RG-400
connected to my SWR meter long enough to ensure that the SWR meter
reading is valid for the coax? Remember that argument?

and the answer is .2"

What is a 50-ohm environment. ???
 

Dave wrote:
"Cecil Moore" wrote:
And if you will remember, the original question didn't involve the
SWR meter at all. The question was: Are my 2-foot sections of RG-400
connected to my SWR meter long enough to ensure that the SWR meter
reading is valid for the coax? Remember that argument?

and the answer is .2"

Not for 6" diameter hard line. :-)
--
73, Cecil http://www.qsl.net/w5dxp

Mythbusters: V/I ratio is forced to Z0
 

On Sun, 16 Oct 2005 14:44:09 GMT, Cecil Moore wrote:
Myth or not, YES or NO?
Myth YES.

Third-hand knowledge, Junk or Treasure?

V/I ratio is forced to Z0:was Mythbusters
 

On Sun, 16 Oct 2005 14:58:52 GMT, Cecil Moore wrote:
How long must the 50 ohm coax connected to the SWR meter be
for the SWR meter to report a valid SWR on that coax? The
answer obviously cannot be zero length.
Already back peddling I see.

What is a 50-ohm environment. ???
 

"Cecil Moore" wrote in message
t...
Dave wrote:
"Cecil Moore" wrote:
And if you will remember, the original question didn't involve the
SWR meter at all. The question was: Are my 2-foot sections of RG-400
connected to my SWR meter long enough to ensure that the SWR meter
reading is valid for the coax? Remember that argument?

and the answer is .2"

Not for 6" diameter hard line. :-)

yeah, in your dreams your hardline as 6" in diameter! its probably not even
6" long!

by the time you put enough adapters on it to get it down to your so-239 or n
connector on your cheap swr meter it will be.

Mythbusters: V/I ratio is forced to Z0
 

Richard Clark wrote:
Cecil Moore wrote:
Myth YES.

Third-hand knowledge, Junk or Treasure?

Actually, it doesn't rise to the definition of a myth.
It wasn't a popular belief. :-)
--
73, Cecil http://www.qsl.net/w5dxp

V/I ratio is forced to Z0:was Mythbusters
 

On Sun, 16 Oct 2005 14:58:52 GMT, Cecil Moore wrote:

Walter Maxwell wrote:
The line impedance of either the input or load transmission line is irrelevant
to the basis for the Zo of the device, therefore a length of line is unnecessary
to establish a Zo environment, the current sample and voltage sample establish
it..

Walt, there's one thing I don't understand about what Dave reported.
As I understand it, these were the test setups.

TDR---50 ohm coax---MFJ---50 ohm coax---50 ohm load

The TDR doesn't see the MFJ wattmeter.

The TDR doesn't see the wattmeter because there is no discontinuity.

TDR---75 ohm coax---MFJ---75 ohm coax---75 ohm load

The TDR sees the MFJ wattmeter.

It seems to me the amount of the discontinuity the wattmeter calibrated to 50
ohms the TDR sees would be determined by the tightness of coupling of the
sampling inductance and capacitance to the main line.

If the MFJ in the second setup were recalibrated for 75 ohms,
would the TDR not see it?

I believe the TDR would not see it.

************************************************* ***************

The threads for the past few days have all diverged from the
original question which was:

How long must the 50 ohm coax connected to the SWR meter be
for the SWR meter to report a valid SWR on that coax? The
answer obviously cannot be zero length.

Cecil, as I explained earlier, if we're considering the Bruene type SWR
indicator, the setting of the sampling cap in relation to the inductive sampling
determines the measurement reference, not the length or Zo of the coax. So I
repeat, with this SWR indicator zero coax length is sufficient. It will produce
the same answer with a lumped impedance as the load as it will with the same
impedance appearing as the input impedance of a coax.

Walt, W2DU

Mythbusters: V/I ratio is forced to Z0
 

Owen Duffy wrote:
The 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.

Myth or not, YES or NO?

Actually, upon closer examination, the definition of "myth"
proves the above is NOT a myth. It is not a traditional story.
It is not a parable allegory. It is not a popular belief.

So, no Owen, it is not a myth.
--
73, Cecil http://www.qsl.net/w5dxp

V/I ratio is forced to Z0:was Mythbusters
 

On Sun, 16 Oct 2005 10:43:18 -0400, Walter Maxwell
wrote:

We are discussing the Bruene version of the device in which the inductance
derives a sample of the current and C1 derives a sample of the voltage. When C1
is adjusted to obtain the correct voltage for the voltage-current ratio to equal
Zo at that point, such as 50 ohms, the device provides the correct readings when
the line impedance on the load side is Zo. The line impedance Zo on the input
side is irrelevant.

Hi Walt,

You might force what appears to be SWR meter action, but what you
describe above is not sufficient to offer the original accuracy; and
at that point why bother putting a screwdriver in and twisting things?
To accomplish the purpose being discussed takes TWO changes (of the
THREE variables of design) as I have already discussed.

Except under the most fortuitous of circumstances, there is nothing
inside the meter that even closely approaches Z0 - or has to.

As an aside to your comments and directed to the novitiates, any
number of Bruene variants can be found where those variants offer less
than optimal performance. However, quality equipment offers that the
line transiting between input and output connectors and thru the
sensing core IS Z0. This consideration takes so little effort, that
those designs that do not are probably suffering other short-cuts away
from intelligence. This opprobrium goes equally for arguments that
presume there is no line either.

73's
Richard Clark, KB7QHC

V/I ratio is forced to Z0:was Mythbusters
 

Walter Maxwell wrote:

Cecil Moore wrote:
How long must the 50 ohm coax connected to the SWR meter be
for the SWR meter to report a valid SWR on that coax? The
answer obviously cannot be zero length.

Cecil, as I explained earlier, if we're considering the Bruene type SWR
indicator, the setting of the sampling cap in relation to the inductive sampling
determines the measurement reference, not the length or Zo of the coax. So I
repeat, with this SWR indicator zero coax length is sufficient. It will produce
the same answer with a lumped impedance as the load as it will with the same
impedance appearing as the input impedance of a coax.

But Walt, there no SWR possible on a zero length line. I think it
was Reg who said that first. I can now see how the initial confusion
arose and why the thread drifted.

Source---75 ohm line---SWR meter---load

The SWR meter will not report the actual SWR on the external 75 ohm line.

Source--75 ohm line--50 ohm line--SWR meter--load

The SWR meter will report the actual SWR on the external 50 ohm coax if
the external 50 ohm coax is _______ long.

Turns out the answer is a lot smaller than I previously thought.
The answer is less than one inch for the most popular 50 ohm coax's.

I didn't mean for the original question to be a trick question but it
apparently turned out that way.
--
73, Cecil http://www.qsl.net/w5dxp

Mythbusters: V/I ratio is forced to Z0
 

On Sun, 16 Oct 2005 17:43:40 GMT, Cecil Moore wrote:
Cecil Moore wrote:
Myth YES.
Third-hand knowledge, Junk or Treasure?
Actually, it doesn't rise to the definition of a myth.
It wasn't a popular belief. :-)
Junk³

Mythbusters: V/I ratio is forced to Z0
 

On Sun, 16 Oct 2005 14:44:09 GMT, Cecil Moore wrote:
Myth YES.
On Sun, 16 Oct 2005 17:49:44 GMT, Cecil Moore wrote:
not a myth.
must be a democrat from Massachusetts running for president.

Mythbusters: V/I ratio is forced to Z0
 

On Sun, 16 Oct 2005 17:49:44 GMT, Cecil Moore wrote:

Owen Duffy wrote:
The 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.

Myth or not, YES or NO?

Actually, upon closer examination, the definition of "myth"
proves the above is NOT a myth. It is not a traditional story.
It is not a parable allegory. It is not a popular belief.

So, no Owen, it is not a myth.

Cecil, this is the opposite to your stated YES opinion just three
hours ago.

I am beginning to form the view that your opinion is just the opposite
of that expressed in the last posting you read, that you will argue
for arguments sake. When I look for consistency in you postings, it is
that they are consistently arguments against other propositions, even
if it means you have to change your own position to remain opposed. To
be kind, the devil's advocate.

Owen
--

What is a 50-ohm environment. ???
 

Cecil Moore wrote:

And if you will remember, the original question didn't involve the
SWR meter at all. The question was: Are my 2-foot sections of RG-400
connected to my SWR meter long enough to ensure that the SWR meter
reading is valid for the coax? Remember that argument?

The threads for the past few days have all diverged from that
original question which was:

How long must the 50 ohm coax connected to the SWR meter be
for the SWR meter to report a valid SWR *on that coax*? The
answer obviously cannot be zero length.

Cecil,

What has become quite clear from this lengthy thread and the experiments
reported is that what you seek is impossible. The design and physical
configuration of the Bird 43, and probably most or all simple SWR
meters, is such that the measurement is completely unaware of the
external "environment". The Bird 43 correctly reports system mismatch,
such as that from a load that differs from 50 ohms, but it ignores any
artificial mismatch from the adjacent coax.

A report of the valid SWR *on that coax* will happen only by
coincidence. The meter does not care about the length. Zero is just as
good an answer as any other length.

And who would really care to know such a thing? The unknown of interest
is generally a load or matching device, not the improper coax.

If you really need to know the "valid SWR" on the mismatched coax you
are going to need a different technique and instrument.

73,
Gene
W4SZ

What is a 50-ohm environment. ???
 

Gene, W4SZ wrote:
"The unknown of interest is generally a load or matching device, not the
improper coax."

Bird says:
"The Model 43 THRULINE Directional Wattmeter is a portable
insertion-type instrument which accurately measures forward or reflected
power in coaxial transmission lines under any load condition."

I accept the above description as I`ve seen confirmation under many
conditions.

Bird also says:
"Line section:
The line section is a high precision 50 ohm coaxial air line designed
for transmission line insertion between the transmitter and the antenna
or load. A socket is provided in each line section for the plug-in
element with the desired power rating and frequency range."

The width of the instrument including connectors is given as 4.25
inches. This makes the preciaion coax section longer than its width or
spacing. I conclude from this that it is capable of enforcing its 50
ohms.

Bird mentions cable lengths in regard to VHF measurements. They suggest
that insertion and removal of the 43 with random length cables attached
may affect the load match unless it is made exactly a total of
1/2-wavelength. In the 1/2-wave case, insertion and removal have no
effect on the load match. I`ve done this countless times and attest to
its effectiveness.

If there were other possible problems with Bird cable lengths, they
would have appeared during the past 50 years and been reported.

Best regards, Richard Harrison, KB5WZI

Mythbusters: V/I ratio is forced to Z0
 

Owen Duffy wrote:

Cecil Moore wrote:
Actually, upon closer examination, the definition of "myth"
proves the above is NOT a myth. It is not a traditional story.
It is not a parable allegory. It is not a popular belief.

So, no Owen, it is not a myth.

Cecil, this is the opposite to your stated YES opinion just three
hours ago.

Yes, when I'm wrong about something, I change my mind. What do
you do when you are wrong about something?

Your "myth" is false but it doesn't rise to the definition of a
myth so *YOU WERE ABSOLUTELY WRONG TO CALL IT A MYTH*. I was wrong
but so were you, so we're even.

Get it? If not, close your eyes and concentrate on your legs.
:-) :-) :-) :-) :-) :-) :-) :-) :-) :-) :-) :-) :-) :-) :-) :-)
--
73, Cecil http://www.qsl.net/w5dxp

Mythbusters: V/I ratio is forced to Z0
 

"Cecil Moore" wrote in message
t...
Owen Duffy wrote:

Cecil Moore wrote:
Actually, upon closer examination, the definition of "myth"
proves the above is NOT a myth. It is not a traditional story.
It is not a parable allegory. It is not a popular belief.

So, no Owen, it is not a myth.

Cecil, this is the opposite to your stated YES opinion just three
hours ago.

Yes, when I'm wrong about something, I change my mind. What do
you do when you are wrong about something?

hey cecil, what happened??? worldradio pull your 'series' after only one
installment?

What is a 50-ohm environment. ???
 

Gene Fuller wrote:
What has become quite clear from this lengthy thread and the experiments
reported is that what you seek is impossible.

Not true. The question has been answered and it surprised me. Only
a half inch or so of ordinary 50 ohm coax is required to ensure the
SWR on that half inch of coax is the same as the SWR reported by
the SWR meter.

I didn't ask the original question in a very understandable
manner because I don't know of anyone who understood my original
question.

And who would really care to know such a thing?

History:
Reg and I were arguing about a year ago whether two feet of RG-400
was long enough to ensure that the SWR reported by the SWR meter
was the same as the SWR on the RG-400. I said it was but had no
references that covered the subject so had no proof. There was
no answer to my question forthcoming from this newsgroup at the
time, so I asked the question over on sci.physics.electromag. I
received an answer on that newsgroup but, unfortunately, didn't
question the math. The answer was one foot of RG-213 but the answer
was off by a magnitude due to an accidental shift in a decimal point.
The correct answer was about an inch to reduce the undesired mode
by a factor of 1/e^5, i.e. 1/0.2=5 where 0.2 is the conductor
spacing, e.g. estimated for RG-213.

The error was caused by using 2" instead of 0.2" as the
conductor spacing.

I mistakenly ran with the one foot answer but, wrong as it was,
proved that my two feet of RG-400 was long enough. Now Dave has
set the record straight and caused the decimal point error to
be exposed by Owen and me at about the same time.

The correct answer is surprising to me. 1/2 inch of ordinary
50 ohm coax is enough to ensure a 50 ohm environment in that
1/2 inch piece of coax, i.e. to ensure Vfor/Ifor=Vref/Iref=50.

Nobody else may care, but I learned a lot and thank everyone
who contributed.
--
73, Cecil http://www.qsl.net/w5dxp

Mythbusters: V/I ratio is forced to Z0
 

Dave wrote:
hey cecil, what happened??? worldradio pull your 'series' after only one
installment?

Katrina was a monster hurricane whose effects extended
all the way to California.
--
73, Cecil http://www.qsl.net/w5dxp

What is a 50-ohm environment. ???
 

On Mon, 17 Oct 2005 16:27:33 GMT, Cecil Moore wrote:
I didn't ask the original question in a very understandable manner
makes sense so far, and consistent too.
because I don't know of anyone who understood my original question.
well -um- it's still consistent

What is a 50-ohm environment. ???
 

"Cecil Moore" wrote

I mistakenly ran with the one foot answer but, wrong as it was,
proved that my two feet of RG-400 was long enough.

=======================================

Cecil, bearing in mind Lord Kelvin, can you say what the error in the
measured value of the SWR, or the reflection coefficient, would be if
the (sufficient) two feet of RG-400 was reduced to 1" inch.
----
Reg.

What is a 50-ohm environment. ???
 

Reg Edwards wrote:
"Cecil Moore" wrote
I mistakenly ran with the one foot answer but, wrong as it was,
proved that my two feet of RG-400 was long enough.

Cecil, bearing in mind Lord Kelvin, can you say what the error in the
measured value of the SWR, or the reflection coefficient, would be if
the (sufficient) two feet of RG-400 was reduced to 1" inch.

According to Dave's calculations, 1" of 50 ohm coax will
result in a negligible error in the SWR on that 1" of coax.
--
73, Cecil http://www.qsl.net/w5dxp

What is a 50-ohm environment. ???
 

On Mon, 17 Oct 2005 18:49:49 GMT, Cecil Moore wrote:
According to Dave's calculations, 1" of 50 ohm coax will
result in a negligible error in the SWR on that 1" of coax.
Negligible for you, is brighter than the sun for everyone else.

No numbers, hmm?