On Sat, 08 Oct 2005 20:33:14 GMT, Cecil Moore wrote:

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

Yep, it's not me, it's Reg. I have defended the Bird wattmeter
design. Reg sez one needs at least 1/4WL and preferably 1/2WL
in order to accurately ascertain the "real" SWR.

Not it was not Reg... end of the guessing game.

Having regard to the definition of VSWR (SWR), I can understand Reg's
point that the direct way to measure VSWR requires sampling voltage or
current over a quarter wave of line where you can find / observe the
actual minimum and maximum.

Having said that, there are other measurements that one can make that
allow one to reasonably predict what the VSWR would be.

Owen
--

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

Owen wrote:
"---I can understand Reg`s point that the direct way to measure VSWR
requires sampling voltage or current over a quarter wave of line where
you can find / observe the actual minimum and maximum."

Yes, and the direct way to measure MPH would be to measure the number of
miles and divide by the number of hours to get an average value. It`s
not often done that way. Maybe about as often as people find maxima and
minima on a transmission line and compute their ratio.

It is more convenient and sufficiently accurate to use indirect methods
for MPH and SWR.

Best regards, Richard Harrison, KB5WZI

In all transmission lines, including coax, there are various shapes of
transverse electric and magnetic fields that can exist for the particular
transmission line geometry. For each shape, the "propagation constant" can
be calculated. Many transmission lines (at lower frequencies) have only one
shape with propagates with low attenuation. The other shapes can exist, but
their "propagation constant" is such that they decrease exponentially with
distance. The propagation constant for each shape can be calculated, and is
often a function of frequency.
When there is a discontinuity in a line, other shapes than the usual
one must exist at the point of the discontinuity. (for example, in order to
ensure that the transverse electric field is zero the surface of a
conducting shape that is part of the line discontinuity). Thus, these other
shapes exist (at a certain amplitude) at the point of discontinuity. The
amplitude of the other shapes decreases exponentially at distances away from
the discontinuity. The rate of the fall-off will depend on the particular
shape, according to its propagation constant.
Thus, the distance needed to be back to regular old TEM propagation in a
coax will depend on the particular discontinuity, and the propagation
constants of the "higher order modes" or different field shapes, of a coax
line.
I have seen examples worked out for waveguide propagation and a step
change in waveguide width. There are probably worked examples of coax
discontinuities in the literature, also.
These non-propagating shapes are usually called " evanescent modes", and
this would be a good search term to use to investigate this further.

Cliff Curry


"Ian White G/GM3SEK" wrote in message
...
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.

Maybe the electromagnetics people have a useful way to visualize it...

Deep inside the coax, the electric field lines between the inner and outer
of the coax are exactly at right-angles to the main axis. Where that is
exactly true, you have a pure TE10 mode so it's also valid to assume that
V/I is exactly equal to Zo.

Very close to the end of the coax, the electric field lines from the
center conductor start to reach out and connect with whatever is out there
beyond the end of the shield. Then you no longer have pure TE10 and can no
longer assume that V/I=Zo.

Coming at it from the other direction, the question would be: how far into
the coax must you go before the field lines become accurately at
right-angles?

We can be sure that the field lines won't suddenly snap from being
divergent to being accurately at right-angles, so what we're really asking
is: how far before the field lines are near-enough at right angles to make
V/I=Zo a good engineering approximation?

Intuitively, the diverging field lines only seem likely to occur within a
few diameters of the end of the shield. Field lines always connect with
highly conducting surfaces at right-angles, and they won't like to be
sharply bent to run along the axis of the coax.

In other words, the effect would seem to be mainly a function of shield
diameter D. Again intuitively, I can't see where wavelength would come
into it, unless D itself is a significant fraction of the wavelength
(which is normally never true, and even microwave engineers try to avoid
it).

Following this picture of diverging field lines, there should also be a
secondary effect depending on how the inner and shield of the coax are
connected to the circuit outside.

All of this suggests that it's impossible to give a single answer that
would be valid for all cases (unless you choose a number that's so big, it
can't fail to be correct... like "120 radials" :-)

However, none of this speculation is of any practical consequence. All
practical experience indicates that if a line is so short that V/I is not
quite equal to Zo, the impedance transformation along that line will be so
small that the effect of any Zo error is lost in the noise.



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

Cliff Curry wrote:
In all transmission lines, including coax, there are various shapes of
transverse electric and magnetic fields that can exist for the particular
transmission line geometry. For each shape, the "propagation constant" can
be calculated. Many transmission lines (at lower frequencies) have only one
shape with propagates with low attenuation. The other shapes can exist, but
their "propagation constant" is such that they decrease exponentially with
distance. The propagation constant for each shape can be calculated, and is
often a function of frequency.
When there is a discontinuity in a line, other shapes than the usual
one must exist at the point of the discontinuity. (for example, in order to
ensure that the transverse electric field is zero the surface of a
conducting shape that is part of the line discontinuity). Thus, these other
shapes exist (at a certain amplitude) at the point of discontinuity. The
amplitude of the other shapes decreases exponentially at distances away from
the discontinuity. The rate of the fall-off will depend on the particular
shape, according to its propagation constant.
Thus, the distance needed to be back to regular old TEM propagation in a
coax will depend on the particular discontinuity, and the propagation
constants of the "higher order modes" or different field shapes, of a coax
line.
I have seen examples worked out for waveguide propagation and a step
change in waveguide width. There are probably worked examples of coax
discontinuities in the literature, also.
These non-propagating shapes are usually called " evanescent modes", and
this would be a good search term to use to investigate this further.


All agreed. Along with the math that Cecil has retrieved and quoted
again, everything points towards the distance in question being a
function of coax diameter only; and not wavelength.


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

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.