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