Mythbusters: V/I ratio is forced to Z0
 

On Sat, 15 Oct 2005 03:37:16 GMT, Cecil Moore wrote:

Owen Duffy wrote:

Cecil Moore wrote:
and not to be a myth at all. There's 104.17 watts of forward power
through the Bird and 4.17 watts of reflected power back through the
Bird. Why does the Bird ignore those actual power values?

I did not report or even measure such a thing.

Since I realized the Bird forms a Z0-match at its output that
statement should be ammended to say: There 104.17 watts of forward

Why the belated revelation of the Bird 43 coupler Zo? In a post on 9
Oct on the earlier thread I stated:

"My suggestion is that the sampler inside a Bird 43 coupler section is
sufficiently far inside the 50 ohm coupler line to provide
measurements within the instrument's stated accuracy of what is
happening within the 50 ohm coupler, irrespective of whether, for
instance, a 75 ohm line is attached to the coupler on the load side."

You must have read it, you responded to it with your message
.

Perhaps the honest way to deal with the situation is to acknowledge
that your statement quoted at the top of this post was just not true,
rather than to edit the words to shift the context to somewhere else
where it might be true.

It would be gracious of you to return to the myth, read it carefully
and think about it, and to respond restricting yourself to the myth as
stated, without qualification or obfuscation, is it a myth YES or NO.

To save you the hassle of finding it, here is:

The myth: Measurements with a Bird 43 of the conditions on the
Thruline section are invalid unless it has some minimum length of 50
ohm line on both sides of itself.

Myth or not, YES or NO?

Owen
--

Mythbusters: V/I ratio is forced to Z0
 

Is that BTW?
Yes - dear oh dear - pot calling kettle black :)


It appears to have a flat section of line that is parallel to the coax
centre conductor and is presumably capacitively and inductively
coupled, and it uses some form of frequency compensation to give it
broadband response. You rotate the sampler element for measurement of
the opposite direction.

Have played with that type of Bird. Worked well as I recall. Have also
made a Bruene type - fairly constant readings over wide frequency range.
Yes 40m is a bit ordinary at the mo for short distance.
Cheers,
Alan

What is a 50-ohm environment. ???
 

OK, Cec, I have a better idea of what you mean by '50-ohm
environment'.

You refer to: -

Vfor/Ifor = Vref/Iref = Z0

with which I agree.

But Zo is a pure resistance and the relationship can be true only when
there is no phase shift between Vfor/Ifor and Vref/Iref.

Which means, in the ideal meter, the voltage tap-off and current
tap-off points must be identically located within the instrument.

I would now like to go off in a slightly different tack to these
extensive threads.

There is a length of conductor between the input and output terminals
of the meter. It cannot be avoided. For obvious reasons related to the
high-frequency response and accuracy, the Zo environment along this
conductor has to be maintained as good as is reasonably possible. (
With the common or garden SWR meter nobody bothers very much. It
hardly matters anyway.)

The location of the voltage and current tapping points along the
conductor doesn't matter two hoots. What matters is the distance
beween them because of the phase difference. Small errors due to
misplacement are unavoidable and have to be lived with.

Getting to the point of this message: - The length of transmission
line inside the meter and the unknowns regarding 'Zo environment' play
no part either in operation or analysis of the meter.

The story that the length of line inside the meter is used to detect
and measure standing wave ratios is just another old-wives' tale which
confuses CB-ers, novices and professional engineers alike.

( By the way, if there is a short transmission line of any sort inside
the instrument, its length can be used to set the measuring
sensitivity. The longer the line the greater the sensitivity. But the
longer the line the greater the measuring errors and the worse the
frequency response.)
----
Reg, G4FGQ.

===========================================
"Cecil Moore" wrote

The transmission line reflection model tells us that the Z0
of a transmission lines forces the following relationship.

Vfor/Ifor = Vref/Iref = Z0

A "50 ohm environment" used in the context of the previous
discussion would be one in which the above relationship
is forced on the system at certain points in the system.

600 ohm transmission line going from a tuner/balun to an
antenna establishes a 600 ohm environment for the signals
on the transmission line. An SWR meter calibrated for 600
ohms will indicate the actual SWR.

About a year ago, based on a discussion that you and I were
having, I asked the sci.physics.electromag newsgroup how
long a piece of RG-213 coax has to be to establish the
above relationship. The answer was that the non-TEM product
terms decrease at about 1/e every two inches for RG-213.

The RG-400 coax leads going to and from my SWR meter are
two feet each. So I asserted to you that my SWR meter
was reading the actual SWR in the middle of that run of
RG-400. I think you disagreed with my assertion but I
cannot remember for sure.
--
73, Cecil http://www.qsl.net/w5dxp

V/I ratio is forced to Z0:was Mythbusters
 

On Sat, 15 Oct 2005 22:57:40 GMT, Cecil Moore wrote:

Dave wrote:
obviously between the tx and ant terminals it looks like a 50 ohm
transmission line.

So I repeat, what causes that characteristic? Is there
some coax inside the MFJ box?

I've been reading this thread while biting my tongue concerning the Zo of the
SWR measuring device. IMO the basis for the Zo of the device is being
overlooked, although Cecil is coming the closest to describing it.

We are discussing the Bruene version of the device in which the inductance
derives a sample of the current and C1 derives a sample of the voltage. When C1
is adjusted to obtain the correct voltage for the voltage-current ratio to equal
Zo at that point, such as 50 ohms, the device provides the correct readings when
the line impedance on the load side is Zo. The line impedance Zo on the input
side is irrelevant.

If C1 is adjusted to obtain the correct voltage for the voltage-current ratio to
equal Zo of 75 ohms it will provide the correct reading when the line impedance
Zo on the load side is 75 ohms.

The line impedance of either the input or load transmission line is irrelevant
to the basis for the Zo of the device, therefore a length of line is unnecessary
to establish a Zo environment, the current sample and voltage sample establish
it..

One can terminate the device with a lumped impedance and get the same answer as
with a transmssion line whose input impedance is the same as the lumped
impedance.

Walt, W2DU

Mythbusters: V/I ratio is forced to Z0
 

Owen Duffy wrote:

Cecil Moore wrote:
Since I realized the Bird forms a Z0-match at its output ...

Perhaps the honest way to deal with the situation is to acknowledge
that your statement quoted at the top of this post was just not true,
rather than to edit the words to shift the context to somewhere else
where it might be true.

I already admitted that, Owen, when I said the Bird developes
a Z0-match at its terminals (above). If you will tell me how
many times I have to admit I was wrong, I will prepare the
appropriate posting.

The myth: Measurements with a Bird 43 of the conditions on the
Thruline section are invalid unless it has some minimum length of 50
ohm line on both sides of itself.

Myth or not, YES or NO?

Myth YES. I don't know in how many ways I have to say that.
Would me writing an admission in my own blood help calm you down?
--
73, Cecil http://www.qsl.net/w5dxp

V/I ratio is forced to Z0:was Mythbusters
 

Walter Maxwell wrote:
The line impedance of either the input or load transmission line is irrelevant
to the basis for the Zo of the device, therefore a length of line is unnecessary
to establish a Zo environment, the current sample and voltage sample establish
it..

Walt, there's one thing I don't understand about what Dave reported.
As I understand it, these were the test setups.

TDR---50 ohm coax---MFJ---50 ohm coax---50 ohm load

The TDR doesn't see the MFJ wattmeter.

TDR---75 ohm coax---MFJ---75 ohm coax---75 ohm load

The TDR sees the MFJ wattmeter.

If the MFJ in the second setup were recalibrated for 75 ohms,
would the TDR not see it?

************************************************** **************

The threads for the past few days have all diverged from the
original question which was:

How long must the 50 ohm coax connected to the SWR meter be
for the SWR meter to report a valid SWR on that coax? The
answer obviously cannot be zero length.
--
73, Cecil http://www.qsl.net/w5dxp

What is a 50-ohm environment. ???
 

Reg Edwards wrote:
Which means, in the ideal meter, the voltage tap-off and current
tap-off points must be identically located within the instrument.

In my MFJ, the toroid and calibration cap are about 1/2" apart.

The story that the length of line inside the meter is used to detect
and measure standing wave ratios is just another old-wives' tale which
confuses CB-ers, novices and professional engineers alike.

And if you will remember, the original question didn't involve the
SWR meter at all. The question was: Are my 2-foot sections of RG-400
connected to my SWR meter long enough to ensure that the SWR meter
reading is valid for the coax? Remember that argument?

The threads for the past few days have all diverged from that
original question which was:

How long must the 50 ohm coax connected to the SWR meter be
for the SWR meter to report a valid SWR *on that coax*? The
answer obviously cannot be zero length.
--
73, Cecil http://www.qsl.net/w5dxp

What is a 50-ohm environment. ???
 

"Cecil Moore" wrote in message
. ..
Reg Edwards wrote:
Which means, in the ideal meter, the voltage tap-off and current
tap-off points must be identically located within the instrument.

In my MFJ, the toroid and calibration cap are about 1/2" apart.

The story that the length of line inside the meter is used to detect
and measure standing wave ratios is just another old-wives' tale which
confuses CB-ers, novices and professional engineers alike.

And if you will remember, the original question didn't involve the
SWR meter at all. The question was: Are my 2-foot sections of RG-400
connected to my SWR meter long enough to ensure that the SWR meter
reading is valid for the coax? Remember that argument?

and the answer is .2"

What is a 50-ohm environment. ???
 

Dave wrote:
"Cecil Moore" wrote:
And if you will remember, the original question didn't involve the
SWR meter at all. The question was: Are my 2-foot sections of RG-400
connected to my SWR meter long enough to ensure that the SWR meter
reading is valid for the coax? Remember that argument?

and the answer is .2"

Not for 6" diameter hard line. :-)
--
73, Cecil http://www.qsl.net/w5dxp

Mythbusters: V/I ratio is forced to Z0
 

On Sun, 16 Oct 2005 14:44:09 GMT, Cecil Moore wrote:
Myth or not, YES or NO?
Myth YES.

Third-hand knowledge, Junk or Treasure?