Owen Duffy wrote:
Cecil, do you have some quantitative explanation / support for this?

Nope, but there were no disagreeing postings.

I am not asking whether or not field conditions (and V/I on the
conductors) immediate to the discontinuity are not Zo of either of the
lines, just where has the 2% of a wavelength come from?

As I remember it came from the spacing between conductors Vs wavelength.
The spacing between conductors is about 0.1 inches for RG-58. How many
times that value would you think it would take for a transmission line
to force its Z0 upon the signals? At 10 MHz, 2% of a wavelength (24 inches)
is about 250 times the spacing between conductors.
--
73, Cecil http://www.qsl.net/w5dxp

On Sat, 08 Oct 2005 00:25:46 GMT, Cecil Moore wrote:

Owen Duffy wrote:
Cecil, do you have some quantitative explanation / support for this?

Nope, but there were no disagreeing postings.

I am not asking whether or not field conditions (and V/I on the
conductors) immediate to the discontinuity are not Zo of either of the
lines, just where has the 2% of a wavelength come from?

As I remember it came from the spacing between conductors Vs wavelength.
The spacing between conductors is about 0.1 inches for RG-58. How many
times that value would you think it would take for a transmission line
to force its Z0 upon the signals? At 10 MHz, 2% of a wavelength (24 inches)
is about 250 times the spacing between conductors.

It seems different people have this conceptual model of "a
transmission line forcing its Z0 upon the signals" in a gradual way,
though differing propositions for the length of line that does not
behave as predicted by Zo.

An extension of that thinking is in the proposition that I have seen
that a Bird 43 cannot give valid readings unless there is at least a
quarter wave of 50 ohm line on each side of itself. In this case, the
magnitude of significantly affected line seems to be 25%, someone
else's is 2%, can they both be correct?

It seems to me that apart from the region of the significant
distortion of the fields local to some kind of discontinuity, that the
fields further along the line at a distance from the discontinuity
large compared to the dimension of the discontinuity (which will often
be the conductor spacing) should be as constrained by the physical
parameters of the line (V/I=Zo for each travelling wave).

In the case of the Bird 43, I suggest that if had, say, at 1MHz, 75
ohm line and a 75 ohm load on the load side, that the V/I raio for the
travelling waves in the region of the sampling element would be so
close to 50 ohms as to not materially affect the accuracy of
measurements on the 50 ohms coupler section, irrespective of the fact
that the sampling element has only 0.02% of a wavelength of 50 ohm
line on its load side.

(For avoidance of doubt, nothing in the foregoing is to imply the Bird
43 would be directly measuring or indicating the conditions on the 75
ohm line.)

Owen
--

Owen Duffy wrote:
An extension of that thinking is in the proposition that I have seen
that a Bird 43 cannot give valid readings unless there is at least a
quarter wave of 50 ohm line on each side of itself. In this case, the
magnitude of significantly affected line seems to be 25%, someone
else's is 2%, can they both be correct?

I think if you will recheck that posting you will find the assertion
was that a Bird 43 cannot give valid readings by sampling at a point.
The line must be at least 1/4WL, and preferably 1/2WL, so that voltage
maximums and minimums will exist and can be measured.

And that 2% of a wavelength is from my faulty memory. I'll try to
Google and find the exact quotation.
--
73, Cecil http://www.qsl.net/w5dxp

"Cecil Moore" wrote in message
t...
Owen Duffy wrote:
An extension of that thinking is in the proposition that I have seen
that a Bird 43 cannot give valid readings unless there is at least a
quarter wave of 50 ohm line on each side of itself. In this case, the
magnitude of significantly affected line seems to be 25%, someone
else's is 2%, can they both be correct?

I think if you will recheck that posting you will find the assertion
was that a Bird 43 cannot give valid readings by sampling at a point.
The line must be at least 1/4WL, and preferably 1/2WL, so that voltage
maximums and minimums will exist and can be measured.

And that 2% of a wavelength is from my faulty memory. I'll try to
Google and find the exact quotation.
--
73, Cecil http://www.qsl.net/w5dxp

i want to see a quote from a manufacturer's or good laboratory manual for
that 1/4 or 1/2 wave thing on the bird also.

Dave wrote:

"Cecil Moore" wrote in message
et...
I think if you will recheck that posting you will find the assertion
was that a Bird 43 cannot give valid readings by sampling at a point.
The line must be at least 1/4WL, and preferably 1/2WL, so that voltage
maximums and minimums will exist and can be measured.


i want to see a quote from a manufacturer's or good laboratory manual for
that 1/4 or 1/2 wave thing on the bird also.


Cecil was quoting someone else there, and is completely innocent :-)


Here's how the Bird 43 measures VSWR. It contains a pair of needle-fine
voltage probes, powered by small explosive charges. When coax is
connected at either side, it fires those probes out into the coax until
it finds a voltage maximum and a voltage minimum. Then it computes the
Voltage Standing Wave Ratio and a recoil mechanism reels the probes back
in. It's so slick, it all happens before you even know it.

Warning: when handling a Bird 43, keep all sensitive parts more than
1/2WL from those sockets!


An alternative possibility is that the Bird 43 does give valid readings
by sampling at the point where it physically is.


--
73 from Ian G/GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

Ian White, G/GM3SEK wrote:
"An alternative possibility is that the Bird 43 does give valid readings
by sampling at the point where it physically is."

Bird claims + or - 5% of Full Scale accuracy for the Model 43.

Why is there power from the reverse direction for a Bird Model 43 to
indicate? There is no second generator sending power in the peverse
direction. The reverse r-f comes from a reflection. The coax enforces a
voltage to current ratio equal to Zo in each direction of flow. Zo is 50
ohms in the Model 43.

Reflection does a peculiar thing. It produces a 180-degree phase
reversal between a wave`s voltage and current. Bird uses the fact that
the current is in-phase with the voltage in one direction of travel and
out-of-phase in the opposite direction of travel to distinguish between
the two directions.

To distinguish, Bird takes a voltage sample and a current sample at the
same point in a 50 ohm line. These two samples are scaled and calibrated
to produce identical deflections of the power indicator.

Out-of-phase samples thus cancel leaving the in-phase samples to produce
double the deflection either would produce alone. This deflection is
carefully calibrated in watts.

Reversing the direction of the wattmeter element, reverses the sense of
the direction indicated and reverses the direction in which the samples
of voltage and current cancel.

The Bird has been satisfactory for about a half century.

Best regards, Richard Harrison, KB5WZI

Richard Harrison wrote:
Ian White, G/GM3SEK wrote:
"An alternative possibility is that the Bird 43 does give valid readings
by sampling at the point where it physically is."

Sorry, Richard, apparently my attempt at irony fell flat. Let me put it
another way:

The instrument can only make measurements at the point on the line where
it physically IS. Therefore the Bird 43 cannot be measuring "SWR" by
sampling the maximum and minimum voltages at locations further up and
down the line.

Therefore it follows that the instrument must actually be measuring
something else... namely, what you described in your follow-up:

Why is there power from the reverse direction for a Bird Model 43 to
indicate? There is no second generator sending power in the peverse
direction. The reverse r-f comes from a reflection. The coax enforces a
voltage to current ratio equal to Zo in each direction of flow. Zo is 50
ohms in the Model 43.

Reflection does a peculiar thing. It produces a 180-degree phase
reversal between a wave`s voltage and current. Bird uses the fact that
the current is in-phase with the voltage in one direction of travel and
out-of-phase in the opposite direction of travel to distinguish between
the two directions.

To distinguish, Bird takes a voltage sample and a current sample at the
same point in a 50 ohm line. These two samples are scaled and calibrated
to produce identical deflections of the power indicator.

Out-of-phase samples thus cancel leaving the in-phase samples to produce
double the deflection either would produce alone. This deflection is
carefully calibrated in watts.

Reversing the direction of the wattmeter element,

reverses the polarity of the current sample, while not affecting the
voltage sample...

and reverses the direction in which the samples
of voltage and current cancel.


Yup. It measures the reflection coefficient of whatever impedance is
connected to the port on the opposite side from the transmitter. This
measurement is made at one physical point along the line.

The subsequent conversion to VSWR is a mathematical relationship only.


The Bird has been satisfactory for about a half century.

As I've often said before, you don't need to defend the Bird 43 to me.
I own and use one, and admire the design. It only needs to be defended
from weird notions about how it works.



--
73 from Ian G/GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek

On Sat, 8 Oct 2005 14:20:46 -0000, "Dave" wrote:
i want to see a quote from a manufacturer's or good laboratory manual for
that 1/4 or 1/2 wave thing on the bird also.

Hi Dave,

Don't hold your breath waiting for that Baloney sandwich to be made.

73's
Richard Clark, KB7QHC

"Richard Clark" wrote in message
...
On Sat, 8 Oct 2005 14:20:46 -0000, "Dave" wrote:
i want to see a quote from a manufacturer's or good laboratory manual for
that 1/4 or 1/2 wave thing on the bird also.

Hi Dave,

Don't hold your breath waiting for that Baloney sandwich to be made.

73's
Richard Clark, KB7QHC

i'm not, just bored sitting in here watching it rain. over 2" so far today.

On Sat, 08 Oct 2005 14:13:02 GMT, Cecil Moore wrote:

Owen Duffy wrote:
An extension of that thinking is in the proposition that I have seen
that a Bird 43 cannot give valid readings unless there is at least a
quarter wave of 50 ohm line on each side of itself. In this case, the
magnitude of significantly affected line seems to be 25%, someone
else's is 2%, can they both be correct?

I think if you will recheck that posting you will find the assertion

Cecil, it is someone else who has on a number of occasions suggested
the quarter wave thing in email correspondence, and here in postings.

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.

The measurements of what is happening within the Bird 43 coupler could
then be used to model what is happening on the adjacent line, having
regard for any Zo changes, loss, length etc.

Owen
--