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

"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

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