SWR meter vs TLI
 

I couldn't resist the temptation to copy the following messages from the UK
newsgroup.

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

The so-called swr meter is no more and no less than a simple RF
resistance
bridge which is either balanced or unbalanced. Usually the latter.


Reg,
I know you 'have a thing' about the SWR meter not measuring SWR.
I have an ohm-meter. It doesn't 'measure' ohms. It actually measures
current.
How many 'meters' actually measure what we say they measure?
Speedometer?
Flow-meter?
Odometer?
Kill-ommeter? (as pronounced by thickies - ugh!)
Methinks thou splitteth hairs.
Ian.

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

The current which is 'measured' by an ohmeter actually exists.

Yet another reason why the so-called swr meter does not measure swr is
because there is no transmission line (between meter and transmitter) on
which to measure it. SWR on it cannot not exist.

Ian, without wishing to cause the slightest offence, I'm afraid your long,
ingrained, aquaintance with the old-wives' tales surrounding swr meters is
preventing you (and others) from seeing things from a different point of
view.

The instrument is just a 4-arm RF resistance bridge, the arm subject to
variation being the input impedance of the transmission line to the antenna
which can be any Zo you like. The other 3 arms are fixed.

The 'meter' merely indicates whether or not the input impedance of the
line-to-the-antenna is some special value of ohms (usually 50) because that
is the desired transmitter load.

It won't, and cannot even, tell you what the value of that special value
actually is except under the very exceptional condition that it is exactly
correct.

And it tells you absolutely nothing else about what exists or is going on in
the station unless you deduce and add to it what you already know by other
means anyway.

My objection to current practice arises because the invalid name of the
instrument, plus all the arguments which arise in futile attempts to justify
it, cause nothing but emotional confusion amongst novices and old-wives
alike.

So why not just change the name to TLI (Transmitter Loading Indicator) and
all the confusion and arguments will cease. Novices will no longer have to
be re-educated about the true meaning and relevance of swr.

Or YOU can choose a new name if you wish and take the credit for it.

No circuit changes are needed. ;o)
---
Regards, Reg, G4FGQ

On Fri, 3 Sep 2004 00:41:25 +0000 (UTC), "Reg Edwards"
wrote a wandering communiqué, shortened
to its essence:


|So why not just change the name to TLI (Transmitter Loading Indicator)

Maybe because that description is even more arcane than SWR meter?

Reg Edwards wrote:
Yet another reason why the so-called swr meter does not measure swr is
because there is no transmission line (between meter and transmitter) on
which to measure it. SWR on it cannot not exist.

The consensus of opinion over on science.physics.electromag is
that a two foot long section of 50 ohm coax is all the length
needed to force the V/I ratio to be 50 ohms at HF - something
to do with the length Vs separation between conductors ratio.
That V/I ratio = 50 is the assumption made by the SWR meter
designer when the meter is calibrated.
--
73, Cecil http://www.qsl.net/w5dxp


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Reg Edwards wrote:
Yet another reason why the so-called swr meter does not measure swr is
because there is no transmission line (between meter and transmitter) on
which to measure it. SWR on it cannot not exist.

The consensus of opinion over on science.physics.electromag is
that a two foot long section of 50 ohm coax is all the length
needed to force the V/I ratio to be 50 ohms at HF - something
to do with the length Vs separation between conductors ratio.
That V/I ratio = 50 is the assumption made by the SWR meter
designer when the meter is calibrated.
--
73, Cecil

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

Cec, I've never before heard such a loony notion.

Your science.physics.elecromag correspondent invented the idea specially for
you and was amusing himself by pulling your leg. And now you're trying to
pull mine.
---
Reg, G4FGQ

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"Cecil Moore"
...a two foot long section of 50 ohm coax
is all the length needed to force the V/I ratio
to be 50 ohms at HF...

Of course, this is only true (in the practical sense) for that brief
interval until any reflections arrive back at the point where the
measurements are being made and all hell breaks loose. It is very obviously
all tied into the meaning of 'characteristic impedance' - there's no mystery
here.

Semantics.

There is often miscommunication(*) about the distinction between the initial
period (before the reflections arrive) and the steady state mess that arises
further along the time axis.

*These can be easily identified - even defined - as any thread that includes
more than about 20 postings by Cecil. ;-)




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"Reg Edwards" wrote (clips):
Yet another reason why the so-called swr meter does not
measure swr is because there is no transmission line
(between meter and transmitter) on which to measure it.
SWR on it cannot not exist.

The indication displayed by that meter is the result of the match of the
transmission line and antenna connected to the output of the transmitter to
the value for which the meter was designed and calibrated. Certainly it is
possible for SWR to exist in this RF system in normal use, and the meter
measures its value.

My objection to current practice arises because the invalid
name of the instrument, plus all the arguments which arise
in futile attempts to justify it, cause nothing but emotional
confusion amongst novices and old-wives alike.

The generic function of this meter is to measure the degree of match between
a source and a load. It is not a direct measure of SWR. When properly
designed, it is a measure and comparison of voltages developed by the
incident and reflected power in the system as they pass a sample point.

There may not be enough transmission line in the RF system for the standing
wave pattern itself to develop on it fully. It doesn't matter. The ratio
of forward to reflected power in the system will be the same as if there WAS
enough line, and that is what the meter measures.

The meter could be calibrated in units of return loss, reflection
coefficient, or SWR -- all of which have corresponding equivalents. A
return loss of 26.45 dB = 4.76% reflection coefficient = 1.1:1 SWR, for
example.

The historical convention for this meter is to calibrate its display in
units of SWR. Or the meter scale could just have three zones: Good - ? -
Bad, which would do away with all these troublesome technical terms and the
objections they elicit from some (nudge, nudge). No offense.

RF






Ian, without wishing to cause the slightest offence, I'm afraid your long,
ingrained, aquaintance with the old-wives' tales surrounding swr meters is
preventing you (and others) from seeing things from a different point of
view.

The instrument is just a 4-arm RF resistance bridge, the arm subject to
variation being the input impedance of the transmission line to the
antenna
which can be any Zo you like. The other 3 arms are fixed.

The 'meter' merely indicates whether or not the input impedance of the
line-to-the-antenna is some special value of ohms (usually 50) because
that
is the desired transmitter load.

It won't, and cannot even, tell you what the value of that special value
actually is except under the very exceptional condition that it is exactly
correct.

And it tells you absolutely nothing else about what exists or is going on
in
the station unless you deduce and add to it what you already know by other
means anyway.

My objection to current practice arises because the invalid name of the
instrument, plus all the arguments which arise in futile attempts to
justify
it, cause nothing but emotional confusion amongst novices and old-wives
alike.

So why not just change the name to TLI (Transmitter Loading Indicator) and
all the confusion and arguments will cease. Novices will no longer have to
be re-educated about the true meaning and relevance of swr.

Or YOU can choose a new name if you wish and take the credit for it.

No circuit changes are needed. ;o)
---
Regards, Reg, G4FGQ

Reg Edwards wrote:

W5DXP wrote:
The consensus of opinion over on science.physics.electromag is
that a two foot long section of 50 ohm coax is all the length
needed to force the V/I ratio to be 50 ohms at HF - something
to do with the length Vs separation between conductors ratio.
That V/I ratio = 50 is the assumption made by the SWR meter
designer when the meter is calibrated.

Your science.physics.elecromag correspondent invented the idea specially for
you and was amusing himself by pulling your leg. And now you're trying to
pull mine.

OK, Reg, when the conductors are 1/4 inch apart, what length
of coax is necessary for the Z0 of the coax to effect the
ratio of E-field to H-field? Those pretty smart guys over
on s.p.e say a ratio of 100:1 length/separation is plenty
enough to force the V/I ratio to be 50 ohms.

We can actually measure the V/I ratio at the input to the
SWR meter. I'll bet, when a properly calibrated 50 ohm SWR
meter is reading zero reflected power, that the V/I ratio
is indeed 50 ohms.
--
73, Cecil http://www.qsl.net/w5dxp


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Richard Fry wrote -
The generic function of this meter is to measure the degree of match
between
a source and a load.

--------------------------------------------------------

Exactly! So let's call it a TLI. Which is what it actually is. Abolish
the source of confusion and the arguments on what it does.

Of what use is the swr to anybody anyway, even when you think you know what
it is? What do you do with it? What does it tell you that you don't
already know?
---
Reg

Cecil, W5DXP wrote:
"---a two foot long section of 50 ohm coax is all the length needed to
force the V/I ratio to be 50 ohms at HF---"

At 3 MHz?

When power is applied to a transmission line, energy from the power
source doesn`t appear everywhere along the line at once. Instead, energy
travels away from the source in the form of an EM wave called the
"incident wave" arriving at various spots along the line in order and at
sequential times.The time it takes to travel through each line segment
depends on the four properties of the line, series resistance (R),
series inductance (L), shunt capacitance (C), and shunt conductance (G).

Source current will start charging the shunt capacitance of the first
line segment. It is delayed by the series inductance and resistance of
the first segment. Resistance does not directly delay current, but
limits current to the capacitace. As the shunt capacitance is charged,
the charging current tapers, but the next line segment starts charging
through its series inductance and resisitance. This energy travel
process continues sequentially throughout the line.

The value of current in an infinite line is the line voltage divided by
the line`s Zo. In a line with reflection, the current in each direction
is the voltage motivating the current in thet direction divided by Zo.

Just how short can a transmission line be and still enforce its Zo? A
1/4-wave matching section inverts impedance between its ends by
enforcing its Zo.

For Zo to equal the square root of L/C, (a resistance), XL must be much
greater than R, and XC must be much greater than G. These restrictions
impose frequency limits on Zo. And, these restrictions may place a low
frequency limit on how short a line can be and still enforce Zo.

Best regards, Richard Harrison, KB5WZI

Richard Fry wrote:
"Or the meter scale could just have three zones: Good - ? - Bad,---."

As I recall, such a meter display used to be called an "English Scale".
This may appeal to Reg.

Best regards, Richard Harrison, KB5WZI

In message , Reg Edwards
writes
Richard Fry wrote -
The generic function of this meter is to measure the degree of match
between
a source and a load.

--------------------------------------------------------

Exactly! So let's call it a TLI. Which is what it actually is. Abolish
the source of confusion and the arguments on what it does.

Of what use is the swr to anybody anyway, even when you think you know what
it is? What do you do with it? What does it tell you that you don't
already know?
---
Reg



Call it an RLR meter, which is what it IS really measuring.
Ian.
--

"Reg Edwards" wrote:
Of what use is the swr to anybody anyway, even when
you think you know what it is? What do you do with it?

You strive to minimize it.

What does it tell you that you don't already know?

You won't know anything about the degree of match between a source and its
load without a means of measuring it. It doesn't matter whether we state
the result of the measurement in units of SWR, return loss or as a
reflection coefficient -- they all give the same information, and allow the
same action to be taken as a result.

To be accurate and valid, none of these units requires measurements to be
taken with some discrete length of transmission line between the source and
the load -- including SWR.

RF

"Richard Harrison" wrote
Cecil, W5DXP wrote:
"---a two foot long section of 50 ohm coax is all the length needed to
force the V/I ratio to be 50 ohms at HF---"

At 3 MHz?

When power is applied to a transmission line, energy from the power
source doesn`t appear everywhere along the line at once.
( much clippage)
Just how short can a transmission line be and still enforce its Zo? A
1/4-wave matching section inverts impedance between its ends by
enforcing its Zo.
For Zo to equal the square root of L/C, (a resistance), XL must be much
greater than R, and XC must be much greater than G. These restrictions
impose frequency limits on Zo. And, these restrictions may place a low
frequency limit on how short a line can be and still enforce Zo.
______________

For a concept of what that length actually is in the real world, recall that
Bird Corp and others supply directional wattmeters giving reasonably
accurate measurement of forward and reflected power -- leading to an SWR
value. The coax sampling sections for RF frequencies at least as low as 540
kHz. is around 9" in length.

RF

"Reg Edwards" wrote

let's call it a TLI. Which is what it actually is. Abolish
the source of confusion and the arguments on what it does.
_____________

Afterthought... if you call it a TLI, is that really less confusing? The
term "Transmitter Loading Indicator" could apply to a way to display the
amount of power at the tx output terminals, and show nothing of the quality
of the load that is dissipating that power (e.g., the degree of match
between the source and the load).

RF

Reg,

I'm afraid you're wasting your time trying to convince mere amateurs
with your carefully reasoned and flawless logic.

Instead, I suggest you concentrate your efforts on the true
professionals out there. Surely, they'll immediately see the wisdom of
your arguments and change their careless ways.

I'm talking of course about the engineers in such unenlightened
companies as HP/Agilent, Narda, Tektronix, Wiltron/Anritsu, and their
colleagues and competitors in the U.K. They're constantly making the
same egregious error, by specifying the SWR of terminating resistors,
connectors, test equipment device inputs, and even (gasp) outputs.

Once the professionals change their ways, amateurs, copycats as they
are, will surely follow.

Good luck with your quest!

Roy Lewallen, W7EL

"SWR" has two different definitions, and Cecil is switching between them
with his usual facility.

Definition 1: the ratio of maximum to minimum voltage on a transmission
line. To measure that, you obviously need a significant length of
transmission line "for the wave to stand on" - depending on there the
maximum and minimum are, you could need anything between an electrical
quarter-wave or an electrical half-wave to locate both points with
certainty.

Cecil wrote:

The consensus of opinion over on science.physics.electromag is
that a two foot long section of 50 ohm coax is all the length
needed to force the V/I ratio to be 50 ohms at HF - something
to do with the length Vs separation between conductors ratio.

This is a side-issue, not relevant to the main discussion. I'm not sure
whether that distance should be in units of wavelengths, line diameters
or a function of both - but definitely not a simple length in feet or
metres. However, the line length required for the V/I ratio to come very
close to its characteristic value is certainly a lot less than the
length required to measure an SWR under Definition 1, so it's a
completely separate side-issue.


Cecil again:
That V/I ratio = 50 is the assumption made by the SWR meter
designer when the meter is calibrated.

That *is* relevant - but it's relevant to a different definition of SWR!

Definition 2: a mathematical function indicating how closely a given
impedance is matched to a given system reference impedance.

Under definition 2, you can measure SWR at a single point in the line.
The "given impedance" whose SWR you are measuring is the impedance
connected to the Output (or "Antenna", or "Load") side of the meter. If
50 ohms is the chosen system reference impedance, then the SWR meter is
designed, built and calibrated to indicate SWR=1 when it's terminated in
an accurate load of that impedance.


Everybody lives very comfortably with those two definitions of "SWR"
that exist side by side.

And that includes Reg - he understands transmission lines inside-out,
and two definitions of "SWR" don't trouble him in the slightest.

What Reg can't live with is that uncontrollable itch to make people jump
through hoops.



--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

Richard Fry wrote:
"For a concept of what that length actually is in the real world, recall
that Bird Corp. and others supply directional wattmeters giving
reasonably accurate measurement of forward and reflected power --
leading to SWR value."

True, and these work with mismatched loads if you have enough 50-ohm
cable connecting the wattmeter.

The Bird Model 43 wattmeter is 5.125 inches (13 cm) wide. This is the
distance between its input and output connectors. This length of "high
precision 50 ohm coaxial air line designed for insertion between the
transmitter or load" requires either some more 50-ohm line or a matched
load to enforce Zo.

IF you were to insert the Model 43 into most 75-ohm transmission
systems, the precision 50-ohm meter line of 5.125 inches would not
likely enforce the 50-ohm V/I ratio and the meter reading would be in
error. At VHF, 1/2-wave of connecting line including the Model 43
wattmeter is ideal, allowing you to insert and withdraw the meter
without affecting the match.

Best regards, Richard Harrison, KB5WZI

Somebody said, I can't tell who (or is that whom?):
Reg,
I know you 'have a thing' about the SWR meter not measuring SWR.
I have an ohm-meter. It doesn't 'measure' ohms. It actually measures
current.
How many 'meters' actually measure what we say they measure?
Speedometer?

So these used to measure magnetic drag on a conducting cup of a transmission
output monitoring rotary cable and therefore should be called a
Mdoaccoatomrc-o-meter

Flow-meter?

Lets see. One versions I am familiar with measures the number of free
balls passing a given point in a circulary disposed tube while following the
movement of a liquid and therefore is a:
Fbpagpiacdt-o-meter, I guess..


Odometer?
And this turky measures the revolutions of the first thingy.


Kill-ommeter? (as pronounced by thickies - ugh!)

Don't you mean Kill-o-meter?

Woops! Does anyone sell these meters???

Besides we say cent-a-meter


And... and... an ohm-meter is really an "incorrectly calibrated am-meter",
as is a volt-meter. Then is an am-meter a "magnetic field produced by a
coil", or an MFPBAC-o-meter ???

Methinks thou splitteth hairs.

Well...

How come we say ther-mom-eter and speed-om-eter and o-dom-eter and
comp-tom-eter, but we don't say ohm-om-eter, volt-om-eter nor am-om-eter...

And how come we say "how come" when we mean "why do"????
And why is a "K" 1000 in our world and a "K" 1024 in the digital world
except for hard drives where a meg is 1,000,000....

....and why am I here... for those who like rhetoricals.

Someone needs something serious to think/talk/post about.

Hmmmm. Me thinketh there needs to be a troll-o-meter, or would it be a
troll-om-eter...
73

Boy! spell check sure didn't like this post!
--
Steve N, K,9;d, c. i My email has no u's. and no meters either.

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"Richard Harrison"
Just how short can a transmission line be
and still enforce its Zo?

The whole thing is perfectly clear if one imagines applying a step function
(rising edge) to any short, even VERY short, length of transmission line.
The current in the short line will be equal to V/Zo - at least until the
reflections (if any) start arriving back at the input. If the line happen
to be terminated with Zo, then no reflections and I=V/Zo is the steady
state.

The only issue of shortness is that a very short line means very short time
until the reflections arrive.

The step function makes things a lot easier to understand than RF. It
'enforces' the distinction between the transient period and steady state.




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On Fri, 3 Sep 2004 17:16:48 -0300, "Another Voice" wrote:

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"Richard Harrison"
Just how short can a transmission line be
and still enforce its Zo?

The whole thing is perfectly clear if one imagines applying a step function
(rising edge) to any short, even VERY short, length of transmission line.
The current in the short line will be equal to V/Zo - at least until the
reflections (if any) start arriving back at the input. If the line happen
to be terminated with Zo, then no reflections and I=V/Zo is the steady
state.

The only issue of shortness is that a very short line means very short time
until the reflections arrive.

The step function makes things a lot easier to understand than RF. It
'enforces' the distinction between the transient period and steady state.

IMO, the length of the line is irrelevant when using a device such as the Bruene
bridge or a Bird 43. Each of those instruments are designed or adjusted to
indicate the forward or reflected power, based on three things: 1) ratio of the
foward and reflected voltages, the voltage reflection coefficient 2) the scale
numbered from 0 to 1, where 0 indicates the reflection is zero, and 1 equals
total reflection, but the significant point is that a 3:1 mismatch gives a
reflection coefficient of 0.5, which then means that the half-scale reading of
0.5 indicates the 3:1 mismatch, or a 3:1 SWR, and 3) the device is so designed
or adjusted so that the voltage ratios indicate the correct value because it's
inherent characteristic impedance, Zo, is 50 ohms.

Thus, no transmission line is necessary. For example, the device can be
connected directly to the antenna terminals, or any other device you desire to
determine the mismatch, and power it directly from the signal source--no
transmission line is needed on either port for the device to indicate the degree
of mismatch.

Walt, W2DU

"Richard Harrison" wrote
IF you were to insert the Model 43 into most 75-ohm transmission
systems, the precision 50-ohm meter line of 5.125 inches would not
likely enforce the 50-ohm V/I ratio and the meter reading would be in
error.
________________

Yet a 50 ohm RF bridge or network analyzer with a 75 ohm termination applied
directly at its output port has no trouble showing the true SWR. These
measuring devices are looking at the same transition plane from 50 to 75
ohms as the Bird 43 would see with a 75 ohm load at its output port.

If the Model 43 is unable to make an accurate measurement of this, is that
not due to reasons other than not having the right 50-ohm V/I ratio in its
line section?

RF

On Fri, 3 Sep 2004 14:14:57 +0000 (UTC), "Reg Edwards"
wrote:

|Richard Fry wrote -
| The generic function of this meter is to measure the degree of match
|between
| a source and a load.
|
|--------------------------------------------------------
|
|Exactly!

Not!

The source plays no role at all. The degree of match that is
indicated is that between the line (or system Zo) and the load Z.

A 50 ohm instrument with a 50 ohm termination shows a reflection
coefficient (or whatever mathematical equivalent you want to use) of
zero regardless of the source impedance.


|So let's call it a TLI.

Let's don't. That's just more bafflegab. Suppose the source impedance
is 25 ohm and the load is 50 ohm. By this new monstrosity of a
definition, the source should be delighted when the "transmitter
loading indicator" says---well---I'm not sure what it says, but I
think the desired number is either 0 or 1. And another source
(transmitter) with a source Z of 100 ohm should be equally happy.
Right?


|Which is what it actually is. Abolish
|the source of confusion and the arguments on what it does.


I'm all for abolishing the confusion too, so why did you add to it?

Richard Fry wrote:
The coax sampling sections for RF frequencies at least as low as 540
kHz. is around 9" in length.

The guys over on s.p.e said it has something to do with conductor
spacing Vs conductor length. They said a 100/1 ratio is plenty
long enough for Z0 to assert itself.
--
73, Cecil http://www.qsl.net/w5dxp


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"Wes Stewart" wrote (clip):

"Reg Edwards" wrote:
|Richard Fry wrote -
| The generic function of this meter is to measure
| the degree of match between a source and a load.
|--------------------------------------------------------
|Exactly!

Not!
The source plays no role at all. The degree of match that is
indicated is that between the line (or system Zo) and the load Z.
A 50 ohm instrument with a 50 ohm termination shows a reflection
coefficient (or whatever mathematical equivalent you want to use) of
zero regardless of the source impedance.
__________

I wrote "BETWEEN a source and a load," not OF the source
and a load. There is a difference.

RF

Richard Fry wrote:
"Wes Stewart" wrote (clip):

"Reg Edwards" wrote:
|Richard Fry wrote -
| The generic function of this meter is to measure
| the degree of match between a source and a load.
|--------------------------------------------------------
|Exactly!

Not!
The source plays no role at all. The degree of match that is
indicated is that between the line (or system Zo) and the load Z.
A 50 ohm instrument with a 50 ohm termination shows a reflection
coefficient (or whatever mathematical equivalent you want to use) of
zero regardless of the source impedance.
__________

I wrote "BETWEEN a source and a load," not OF the source
and a load. There is a difference.

The only difference between those two terms is that "match between" is
normal and grammatical technical usage; and "match of" ain't neither.

Wes is correct. What the meter measures is the match (expressed as
reflection coefficient, SWR, whatever) between the system Zo for which
that meter was designed and calibrated, and the load Z.

The meter measures nothing that involves the source, except the level of
RF that it supplies. It does not respond in any way whatever to the
source impedance.


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

"Ian White, G3SEK wrote
The meter measures nothing that involves the source, except
the level of RF that it supplies. It does not respond in any way
whatever to the source impedance.
_____________

Not that I said it did in my part of the thread, but nevertheless the above
statement is not strictly true. In the case where the source Z of the tx PA
does not match its load Z (which is typical), power reflected from the load
mismatch will at least partly be re-reflected from the PA -- which then
contributes to the power sensed by a "wattmeter" in the output path.

RF

Richard Fry wrote:
"Ian White, G3SEK wrote
The meter measures nothing that involves the source, except
the level of RF that it supplies. It does not respond in any way
whatever to the source impedance.
_____________

Not that I said it did in my part of the thread, but nevertheless the above
statement is not strictly true. In the case where the source Z of the tx PA
does not match its load Z (which is typical), power reflected from the load
mismatch will at least partly be re-reflected from the PA -- which then
contributes to the power sensed by a "wattmeter" in the output path.

Sorry, that statement cannot be correct. It would mean that the
impedance you measure at the near end of a transmission line (terminated
by some arbitrary load at the far end) would depend on the internal
impedance of the device that's doing the measuring - and that is not
true, either in transmission-line theory or in the real world. It is a
function only of the line and the load.

Others can probably explain why the statement is also incorrect
according to the concept of "forward and reflected power waves". Myself,
I prefer avoid that concept completely, because it so easily leads into
this kind of mess.


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

"Ian White, G3SEK"wrote:
Richard Fry wrote:
"Ian White, G3SEK wrote
The meter measures nothing that involves the source, except
the level of RF that it supplies. It does not respond in any way
whatever to the source impedance.

Not that I said it did in my part of the thread, but nevertheless the
above
statement is not strictly true. In the case where the source Z of the tx
PA
does not match its load Z (which is typical), power reflected from the
load
mismatch will at least partly be re-reflected from the PA -- which then
contributes to the power sensed by a "wattmeter" in the output path.

Sorry, that statement cannot be correct. It would mean that the
impedance you measure at the near end of a transmission line (terminated
by some arbitrary load at the far end) would depend on the internal
impedance of the device that's doing the measuring - and that is not
true, either in transmission-line theory or in the real world. It is a
function only of the line and the load. etc
____________

How, then, do you explain the "ghost image" that can occur* in analog(ue) TV
transmission systems arising from reflections at/near the antenna end of the
station's transmission line?

*with sufficient round-trip propagation time in the transmission line

RF

**** Post for FREE via your newsreader at post.usenet.com ****

"Richard Fry"
How....do you explain the "ghost image" .... TV

Sigh - moth + lamp (not you RF specifically, the newsgroup...).

LOOK - Discuss a simple step function (rising edge) - not RF. All of your
disagreements about SWR and reflections will be revealed as silly semantics
and the mixing up of the transient versus the steady state. A step function
makes it so simple that there is no room for arguments.

There is NOTHING in the endless (and now repeating) discussion other than
semantics and the above mention lack of discernment (initial transient
versus steady state).



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For those who have forgotten how or have never measured SWR.

Two separate voltage measurements are needed at two different places along
the line.
Slide the voltmeter along the line until a maximum is found.
Remember the reading, Vmax.
Slide the meter along the line again until a minimum is found.
Remember the reading, Vmin.
Before you forget, divide Vmax by Vmin.
You are left with a single number.
It has no dimensions.
It is the TRUE swr.

NOTE: In the above description and calculation there is no mention of Zo,
terminating impedance, source impedance, reflection coefficient, forward
power, reflected power, reflected volts, reflected current, Smith charts, or
conjugate matches. All these things are superflous to the determination. No
information other than the two voltage measurements is needed.

All other methods which purport to measure swr require injection of
additional information. And assumptions form an essential part of the
process. They can hardly be called swr measurements. Particularly when
they can indicate it on non-existent lines.

Richard Fry wrote:
"If the Model 43 is unable to make an accurate masuremnt of this, is
that not due to reasons other than not having the right V/I ratio in its
line section?"

Many details of desisn, construction, and application must be chosen and
executed right to get accuracy, but the line impedance is essential.

Bird can adjust the current sample to exactly equal the voltage sample,
both taken from the transmission line at any point. But it must work
with a fixed voltage to current ratio. Bird chose 50 onms.

For a directional meter, it`s necessary to respond to one direction
while rejecting the other. When power is applied to a line, the
resulting current is is in phase with the volts. On reflection, the
volts and amps in the reflected wave are 180 degrees out of phase. The
phase difference of the reflected wave is used by Bird to distinguish it
from the incident wave.

By selecting and adjusting for equal samples of volts and amps in the
forward wave, their total is 2X that of either sample. But, the samples
from the reflected wave, being equal but out of phase, cancel.

To get the value of the reflected power samples, it is only necessary to
reverse the polarity of one of the samples. They are now in phase and
the forward power samples are now out of phase and cancel.

If some other voltage to current ratio is used for the power samples
than that of the design, the samples won`t be exactly equal and
cancellation of the undesired direction does not work.

Best regards, Richard Harrison, KB5WZI

Reg Edwards wrote:
All other methods which purport to measure swr require injection of
additional information. And assumptions form an essential part of the
process. They can hardly be called swr measurements.

They can be called indirect (calculated) SWR measurements
and assumptions indeed do form an essential part of the
process. That's not at all unusual for indirect measurements.

Particularly when they can indicate it on non-existent lines.

One of the assumptions is that a transmission line exists. If
a transmission line doesn't exist, the measurement conditional
assumptions are violated, and the actual values may not be
the desired or expected results. Happens all the time with
various measuring instruments.
--
73, Cecil http://www.qsl.net/w5dxp



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Reg Edwards wrote:
For those who have forgotten how or have never measured SWR.


Hang on, Reg - didn't you spend your career working on VLF cables that
went under the ocean?


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

Richard Fry wrote:
"Ian White, G3SEK"wrote:
Richard Fry wrote:
"Ian White, G3SEK wrote
The meter measures nothing that involves the source, except
the level of RF that it supplies. It does not respond in any way
whatever to the source impedance.

Not that I said it did in my part of the thread, but nevertheless the
above
statement is not strictly true. In the case where the source Z of the tx
PA
does not match its load Z (which is typical), power reflected from the
load
mismatch will at least partly be re-reflected from the PA -- which then
contributes to the power sensed by a "wattmeter" in the output path.

Sorry, that statement cannot be correct. It would mean that the
impedance you measure at the near end of a transmission line (terminated
by some arbitrary load at the far end) would depend on the internal
impedance of the device that's doing the measuring - and that is not
true, either in transmission-line theory or in the real world. It is a
function only of the line and the load. etc
____________

How, then, do you explain the "ghost image" that can occur* in analog(ue) TV
transmission systems arising from reflections at/near the antenna end of the
station's transmission line?

*with sufficient round-trip propagation time in the transmission line

Yes, that is a true observation, just as true as the one I made... so
now you have *two* different things to explain!

The so-called SWR meter is a steady-state instrument, so it always makes
sense to use that quicker, easier way of thinking. Since you're the one
who chooses to think of this particular situation in terms of multiple
reflections, any difficulties you encounter are entirely yours.

If you ever see a conflict between two different theories that explain
the same observed facts, then there's an error somewhere. If the
multiple-reflection theory is extrapolated to infinite time, so that it
calculates results for the steady state, it *must* give identical
results to the steady-state theory. But whenever the steady-state theory
can be used, it will always get you there much more quickly.

However, when you have finally done it your way, and accounted correctly
for all the reflections and re-reflections, we can predict the outcome
with complete confidence:

1. If you sum the successive reflections correctly to infinity, and
calculate the V/I ratio and phase at the station end of the line, then
the final result will be identical to the impedance given by the
steady-state transmission-line theory. It has to be, because that single
value is the reality.

2. Somewhere in your calculations, any value that you assume for the RF
source impedance is going to cancel right out of your calculations. The
correct mathematical result *must* be independent of that value -
because, again, that's the reality.


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

On Sat, 4 Sep 2004 06:23:28 -0500, "Richard Fry"
wrote:

|"Ian White, G3SEK wrote
|The meter measures nothing that involves the source, except
|the level of RF that it supplies. It does not respond in any way
|whatever to the source impedance.
|_____________
|
|Not that I said it did in my part of the thread, but nevertheless the above
|statement is not strictly true. In the case where the source Z of the tx PA
|does not match its load Z (which is typical), power reflected from the load
|mismatch will at least partly be re-reflected from the PA -- which then
|contributes to the power sensed by a "wattmeter" in the output path.

If you have a directional wattmeter that *perfectly* senses and
displays the ratio of forward to reverse power, what difference does
the absolute power make? Hint: It doesn't.

But let's avoid going any further here. This has been argued about in
at least 10,000 other threads and has absolutely no bearing on Ian's
or my statements regarding the unimportance of source Z in the
discussion of measuring reflection coefficient.

In a laboratory environment, the source is usually well matched at the
system impedance. This is for convenience and improves accuracy only
because is takes less mathematical horsepower to remove the effects of
the "re-reflection" you speak of. This is particularly true when
highly mismatched loads are measured; and two of the most highly
mismatched loads are an open and a short, both of which are often used
as calibration standards.

Re-reflection exists and is actually an undesired thing in laboratory
measurements, primarily because most measurements are swept frequency
and the changing phase of the reflections is a pain in the ass.
Reflections are a source of *measurement error*. Removing the effects
is called *error correction* in the laboratory setting.

But I can tell you that if I calibrate a high-quality network analyzer
using a 50 ohm generator and then add a tee and second 50 ohm load to
the generator output, making it a 2:1 mismatched source, there will be
*no* significant change in the answer presented when typical
mismatches are measured.

Any differences represent measurement *error*, not measurement
*reality*. Reality is the interaction of the line and load and the
source plays *no* role.

Ian White, G3SEK wrote:



Yes, that is a true observation, just as true as the one I made... so
now you have *two* different things to explain!

The so-called SWR meter is a steady-state instrument, so it always makes
sense to use that quicker, easier way of thinking. Since you're the one
who chooses to think of this particular situation in terms of multiple
reflections, any difficulties you encounter are entirely yours.

If you ever see a conflict between two different theories that explain
the same observed facts, then there's an error somewhere. If the
multiple-reflection theory is extrapolated to infinite time, so that it
calculates results for the steady state, it *must* give identical
results to the steady-state theory. But whenever the steady-state theory
can be used, it will always get you there much more quickly.

However, when you have finally done it your way, and accounted correctly
for all the reflections and re-reflections, we can predict the outcome
with complete confidence:

1. If you sum the successive reflections correctly to infinity, and
calculate the V/I ratio and phase at the station end of the line, then
the final result will be identical to the impedance given by the
steady-state transmission-line theory. It has to be, because that single
value is the reality.

2. Somewhere in your calculations, any value that you assume for the RF
source impedance is going to cancel right out of your calculations. The
correct mathematical result *must* be independent of that value -
because, again, that's the reality.



This is correct. If you divide the formula for voltage, at any point on
a transmission line, by the formula for current, the generator impedance
cancels.
73,
Tom Donaly, KA6RUH

Pardon the extensive pasting, but it will allow a clearer post, and save a
lot of click time. / RF

"Ian White, G3SEK" wrote :
The meter measures nothing that involves the source, except
the level of RF that it supplies. It does not respond in any way
whatever to the source impedance.

Richard Fry wrote:
Not that I said it did in my part of the thread, but nevertheless the
above statement is not strictly true. In the case where the source Z
of the tx PA does not match its load Z (which is typical), power
reflected from the load mismatch will at least partly be re-reflected
from the PA -- which then contributes to the power sensed by a
"wattmeter" in the output path.

"Ian White, G3SEK" wrote :
Sorry, that statement cannot be correct. It would mean that the
impedance you measure at the near end of a transmission line
terminated by some arbitrary load at the far end) would depend
on the internal impedance of the device that's doing the
measuring - and that is not true, either in transmission-line
theory or in the real world. It is a function only of the line and
the load. etc

Richard Fry wrote:
How, then, do you explain the "ghost image" that can occur* in
analog(ue) TV transmission systems arising from reflections
at/near the antenna end of the station's transmission line?

*with sufficient round-trip propagation time in the transmission line

Ian White wrote:
Yes, that is a true observation, just as true as the one I made... so
now you have *two* different things to explain!

The so-called SWR meter is a steady-state instrument, so it always makes
sense to use that quicker, easier way of thinking. Since you're the one
who chooses to think of this particular situation in terms of multiple
reflections, any difficulties you encounter are entirely yours.

This reads to me as though you know they are there, but choose
to ignore them...?

If you ever see a conflict between two different theories that explain
the same observed facts, then there's an error somewhere.

We agree on the subject of conflict resolution, but apparently not on
the location of the error.

If the multiple-reflection theory is extrapolated to infinite time, so
that it calculates results for the steady state, it *must* give identical
results to the steady-state theory. But whenever the steady-state
theory can be used, it will always get you there much more quickly.

This is true only to the extent that all the power ever generated by the
transmitter eventually either is radiated by the antenna or is dissipated by
losses somewhere.

For simplicity, let's assume a tx with a source impedance of zero ohms feeds
a lossless transmission line of uniform impedance throughout its length to a
mismatch at the far end. The mismatch reflects a percentage of the incident
power back down the line to the tx, and continues to do so as long as the
transmitter generates power. The tx will re-reflect the reflected power
back to the far end -- in this case all of the reflected power it ever sees,
in fact. To this easily-seen, real-world reality you agreed above ("Yes,
that is a true observation, ...").

The re-reflections combine with the power generated by the tx at that
instant to create a vector sum at the sample point used by the meter. The
typical tx meter is a frequency-domain device, and cannot by itself separate
the RF output of the transmitter from re-/reflections of it. That requires
a time-domain device. So the magnitude of the transmission line samples
driving the tx RF metering circuits during normal operation under these
conditions become a function of both the source impedance and the
load impedance.

The defense rests.

RF

On Sat, 4 Sep 2004 13:59:43 +0100, "Ian White, G3SEK"
wrote:

depend on the internal impedance of the device that's doing the measuring - and that is not
true, either in transmission-line theory

Hi Ian,

I see you have yet to respond to this very matter attended to quite at
length by Chipman.

73's
Richard Clark, KB7QHC

Please see my post in this thread of 4 Sept 2004 at 16:54 UT (shown above).

RF

Richard Fry wrote:
This is true only to the extent that all the power ever generated by the
transmitter eventually either is radiated by the antenna or is dissipated by
losses somewhere.

The fly in the ointment is a definition.

If a signal generator is sourcing 100 watts and 20 watts of reflected
power is being dissipated in a circulator load resistor, we say the
source is sourcing 100 watts and 20 watts of reflected power is being
dissipated in the circulator load resistor.

If the identical thing happens in a ham transmitter, we say that the
source is sourcing 80 watts, BY DEFINITION. What's wrong with this
picture? Ham transmitters NEVER re-reflect anything, by definition.

The reason that the source impedance doesn't enter into the forward/reflected
power values is that it has been defined out of any relationship to them. By
definition, there is zero power re-reflected from a ham transmitter NO MATTER
WHAT THE IMPEDANCE OF THE HAM TRANSMITTER MIGHT BE. Never mind that we can see
those reflections with our own eyes in TV ghosting. We must be crazy because
they have been defined out of existence. How dare we have the gall to observe
them!
--
73, Cecil http://www.qsl.net/w5dxp


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