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

"Ian White wrote
Yup. It measures the reflection coefficient of whatever impedance is
connected to the port on the opposite side from the transmitter.

=====================================
No, it doesn't.

It measures the MAGNITUDE of the reflection coefficient. It discards
the information which is contained in the phase angle of the
reflection coefficient. As a consequence the only use which can be
made of the magnitude is to calculate the SWR on an imaginary 50-ohm
line. The SWR can be used to calculate the magnitude of the
reflection coefficient.
---
Reg.

Reg Edwards wrote:

"Ian White wrote
Yup. It measures the reflection coefficient of whatever impedance is
connected to the port on the opposite side from the transmitter.

=====================================
No, it doesn't.

It measures the MAGNITUDE of the reflection coefficient. It discards
the information which is contained in the phase angle of the
reflection coefficient.

Sorry, I left that important word out.


As a consequence the only use which can be
made of the magnitude is to calculate the SWR on an imaginary 50-ohm
line.

Agreed. SWR has become a number that indicates the general "goodness" of
an impedance match. It is almost always determined indirectly, by
actually measuring something else and then calculating an SWR value.

The only way to measure VSWR truly and directly is to find the points of
maximum and minimum voltage along the line, and measure the ratio of
those two voltages. That is the classical definition of VSWR, but hardly
anyone measures it that way, because it requires physical access to all
points along the line. But if they do, then...

The SWR can be used to calculate the magnitude of the
reflection coefficient.

Engineers swap freely between the different available ways of expressing
the "goodness" of an impedance match, choosing whichever one is the most
convenient (or conventional) for the application.



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

Reg Edwards wrote:
It measures the MAGNITUDE of the reflection coefficient. It discards
the information which is contained in the phase angle of the
reflection coefficient. As a consequence the only use which can be
made of the magnitude is to calculate the SWR on an imaginary 50-ohm
line.

Reg, I dug up some calculations from sci.physics.electromag
from about a year ago that indicate one foot of 50 ohm coax
on each side of the Bird is enough to make the line real,
i.e. not imaginary, and that's a conservative estimate.
--
73, Cecil http://www.qsl.net/w5dxp

On Tue, 11 Oct 2005 09:11:19 +0100, Ian White G/GM3SEK
wrote:

The subsequent conversion to VSWR is a mathematical relationship only.

Hi Ian,

This seems to be a particularly notable difference - to which
absolutely NO ONE has ever deviated from in ANY determination of SWR!

That is to say, this "mathematical" distinction that some rely on to
differentiate their arguments has not got one scintilla of difference
over any other method.

The only way to claim you "directly" measure SWR is to find some way
to place two probes of a meter along the line such that one probe goes
into the trough and the other into the peak and the meter reads SWR
directly. Unfortunately for rhetoric's sake, this STILL renders the
determination in terms of a mathematical relationship. It cannot be
escaped.

Why this keeps on being revisited must be to allow the new lurkers to
observe my correction.

73's
Richard Clark, KB7QHC

Richard Clark wrote:
That is to say, this "mathematical" distinction that some rely on to
differentiate their arguments has not got one scintilla of difference
over any other method.

I must have a dozen equations for SWR, all mathematically consistent
with each other. A lot of the math is performed by simply calibrating
a meter face. For instance, given a linear meter reading for |rho|
with full-scale equal to 1.0, SWR values can be applied to the
meter face with 3.0 at half-scale. (Ever notice how many SWR meters
have 3.0 at half scale?)
--
73, Cecil http://www.qsl.net/w5dxp