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

On Tue, 11 Oct 2005 10:43:00 +0100, Ian White G/GM3SEK
wrote:

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.


To be picky, in most implementations, its response is a function of
the forward or reflected power provided that Zo is real, and the
magnitude of the complex reflection coefficient can be calculated from
those measurements.

Owen
--

"Owen Duffy" wrote
To be picky, in most implementations, its response is a function of
the forward or reflected power provided that Zo is real, and the
magnitude of the complex reflection coefficient can be calculated
from
those measurements.

================================
Owen,

Forward and especially reflected power are even more imaginary than
the SWR on a non-existent 50-ohm line.

The only use for forward and reflected power is to calculate the
magnitude of the reflection coefficient. And the only use for the
reflection coefficient is to calculate the imaginary SWR. And the only
use . . . . .

I understand meter manufacturers provide graphs, which, if you don't
know how to use a pocket calculator, will do the calculations for you.
But you will still go round in circles.
----
Reg.

Reg Edwards wrote:
Forward and especially reflected power are even more imaginary than
the SWR on a non-existent 50-ohm line.

My old Heathkit HM-15 SWR meter samples the peak forward and
reflected currents. A pot is used to set the meter to full
scale using a voltage proportional to the peak forward current.

When the voltage proportional to the peak reflected current is
then switched into the meter circuit, it reads SWR from the
precalibrated scale which is linear with |rho|, i.e. at half-scale,
SWR=3 and |rho|=(3-1)/(3+1)=0.5
--
73, Cecil http://www.qsl.net/w5dxp

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

"Cecil Moore" wrote in message
.. .
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

i like imaginary!

On Tue, 11 Oct 2005 15:40:55 -0000, "Dave" wrote:
i like imaginary!
fantasy is more appropriate.

On Tue, 11 Oct 2005 15:37:51 GMT, Cecil Moore wrote:
I dug up some calculations from sci.physics.electromag
which you recite here; then in sci.physics.electromag you can quote
their use by authorities (sic both times) in
rec.radio.amateur.antenna....

This appeal is called a circle of friendship - not evidence.