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

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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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

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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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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 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

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