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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"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

"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

"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

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

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.
--

Ian Jackson wrote:
Call it an RLR meter, which is what it IS really measuring.

How about an "SVI", Superposed Voltage-sample(+/-)Current-sample,
named for the math function that it is performing?
--
73, Cecil http://www.qsl.net/w5dxp


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"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 Fry wrote:
"It doesn`t matter whether we state the result of measurement in units
of SWR, return loss, or as a reflection coefficient -- they all give the
same information -- ."

Correct. The units above are fungible. All are an expression of the
mismatch of a load to the Zo of the transmission line.

With a Bird wattmeter, the reflection coefficient (rho) is the sq. rt.
of the reflected power divided by the forward power.

SWR = 1+rho / 1-rho

Return loss in dB = 20 log (rho)

Return loss in dB = 10 log (Ref.Pwr./Fwd.Pwr.)

Rho = (ZL/Zo)-1 / (ZL/Zo)+1

None of the expressions above include the source Z, therefore it does
not apply.

Best regards, Richard Harrison, KB5WZI