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

Cecil Moore wrote:
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.

The Bird doesn't require any upstream and downstream boundary
conditions. You can insert the instrument between any source impedance
and any load impedance, and what it reports is entirely about the load
impedance, unaffected by the source impedance.

However, it was scaled and calibrated assuming a 50 ohm system reference
impedance, so in order to read correctly, it requires you to agree that
your system reference impedance is 50 ohms too.

The element is trying to sample the voltage and current at a single
point along the instrument's internal line. Because that line is
physically quite long, it is built as an accurate 50-ohm line so that
the instrument will cause minimal disturbance when inserted somewhere
along a 50-ohm cable.


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

Ian White G/GM3SEK wrote:
The Bird doesn't require any upstream and downstream boundary
conditions.

When Bird requires a 50 ohm environment, they are requiring
50 ohm boundary conditions for the reading to be valid. If
you install the Bird in a 450 ohm environment on both sides
of the wattmeter, for instance, it will NOT read a valid forward
power and reflected power. In a matched-line 450 ohm environment
with absolutely zero reflected power, the Bird will indicate an
SWR of 9:1, a |rho| of 0.8 and a ratio of reflected power to
forward power of 0.64 even when the reflected power is zero.
--
73, Cecil http://www.qsl.net/w5dxp

On Tue, 11 Oct 2005 21:25:53 GMT, Cecil Moore wrote:

Ian White G/GM3SEK wrote:
The Bird doesn't require any upstream and downstream boundary
conditions.

When Bird requires a 50 ohm environment, they are requiring
50 ohm boundary conditions for the reading to be valid. If
you install the Bird in a 450 ohm environment on both sides
of the wattmeter, for instance, it will NOT read a valid forward
power and reflected power. In a matched-line 450 ohm environment
with absolutely zero reflected power, the Bird will indicate an
SWR of 9:1, a |rho| of 0.8 and a ratio of reflected power to
forward power of 0.64 even when the reflected power is zero.

(I am assuming your 450 ohm line to be an unbalanced line, impractical
as that is, but the issues of balance to unbalanced transition are
just noise to the discussion.)

Is this about whether the Bird readings are correct for the conditions
on the Bird Thruline, or whether the meter readings are extensible to
the adjacent transmission line without further interpretation /
modelling?

The Bird readings should be correct for the conditions on the Bird
Thruline. You can safely extend those measurements literally to the
adjacent line where the adjacent line is the same as the Bird Thruline
and of negligible loss. In other cases, knowing the line parameters,
you may be able to use the measurements to some extent to calculating
some conditions on the other line.

Though the Bird readings in your example for Forward and Reflect Power
cannot be assumed valid for the adjacent line, the net power should be
correct.

I don't think anyone is suggesting that the Bird could be used in a
general sense to estimate the VSWR on your 450 ohm line.

Owen
--

Owen Duffy wrote:
I don't think anyone is suggesting that the Bird could be used in a
general sense to estimate the VSWR on your 450 ohm line.

I thought that was the subject of the discussion.
--
73, Cecil http://www.qsl.net/w5dxp

On Wed, 12 Oct 2005 02:04:52 GMT, Cecil Moore wrote:

Owen Duffy wrote:
I don't think anyone is suggesting that the Bird could be used in a
general sense to estimate the VSWR on your 450 ohm line.

I thought that was the subject of the discussion.

From an earlier post:

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
--

Cecil Moore wrote:
Ian White G/GM3SEK wrote:
The Bird doesn't require any upstream and downstream boundary
conditions.

When Bird requires a 50 ohm environment, they are requiring
50 ohm boundary conditions for the reading to be valid.

No, they're not. They are requiring a 50-ohm system reference impedance.

What you call the "impedance environment" consists of physical things
like the source impedance, line impedance and load impedance. You're
confusing those with the system reference impedance, which something
completely different.

System reference impedance is purely a matter of definition. The most
common choice is 50 ohms... and by definition, that means 50 ohms
exactly.

Having made that choice, then you obviously design and calibrate your
instruments to give correct readings in an impedance environment that is
as close to your chosen reference impedance as you can practically make
it.

Your example shows the difference between impedance environment and
reference impedance most clearly.

If
you install the Bird in a 450 ohm environment on both sides
of the wattmeter, for instance, it will NOT read a valid forward
power and reflected power. In a matched-line 450 ohm environment
with absolutely zero reflected power, the Bird will indicate an
SWR of 9:1, a |rho| of 0.8 and a ratio of reflected power to
forward power of 0.64 even when the reflected power is zero.

You have changed the impedance environment to 450 ohms, and that's
fine... but all of the Bird's readings are perfectly correct if the
system reference impedance remains defined at 50 ohms. The reason why
say they are incorrect is that you also changed your definition of
system reference impedance to 450 ohms, without acknowledging that you
did it.

It's like doing a financial calculation without mentioning that you
switched into another base currency... darn right the results are not
valid.


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

Ian White G/GM3SEK wrote:

You have changed the impedance environment to 450 ohms, and that's
fine... but all of the Bird's readings are perfectly correct if the
system reference impedance remains defined at 50 ohms.

I have changed the system reference impedance to 450 ohms. Assuming
a tube PA with a pi-net output, 50 ohms doesn't exist anywhere anymore.
The system reference impedance is no longer 50 ohms so the Bird wattmeter
is being abused and misused. You can do the same thing by using a DC
voltmeter on an RF voltage or by using a hammer on a screw. If you
want to know the SWR on 450 ohm line, use a 450 ohm SWR meter.
--
73, Cecil http://www.qsl.net/w5dxp