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

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

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
--

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.
--
73, Cecil http://www.qsl.net/w5dxp

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

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

Ian White G/GM3SEK wrote:
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.

Please forgive my previous senior moment.
It was ~2% of a wavelength at 10 MHz for RG-213.
It appears that one foot of coax on each side of
a Bird wattmeter is enough to establish Z0 at
50 ohms which forces Vfor/Ifor=Vref/Iref=50,
the necessary Bird boundary conditions.
--
73, Cecil http://www.qsl.net/w5dxp