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
--

Owen Duffy wrote:
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.

If there is 75 ohm coax on the input of the Bird, the reflected
power reported by the Bird on the coax will be off by an infinite
percent. That's pretty inaccurate.
--
73, Cecil http://www.qsl.net/w5dxp

On Wed, 12 Oct 2005 02:25:33 GMT, Cecil Moore wrote:

Owen Duffy wrote:
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.

If there is 75 ohm coax on the input of the Bird, the reflected
power reported by the Bird on the coax will be off by an infinite
percent. That's pretty inaccurate.

The Bird does not measure or report the conditions on the coax, it
measures and reports the conditions in the immediate region of the
sampling element which is some 40mm inside the Thruline coupler
section.

Owen
--

Owen Duffy wrote:
The Bird does not measure or report the conditions on the coax, it
measures and reports the conditions in the immediate region of the
sampling element which is some 40mm inside the Thruline coupler
section.

I don't know what this argument is all about. Consider the
following:

XMTR---75 ohm coax---Bird---75 ohm load

Are you saying the Bird's placement will result in a reflection
coefficient of 0.2? I seriously doubt that is true.
--
73, Cecil http://www.qsl.net/w5dxp

On Wed, 12 Oct 2005 04:06:07 GMT, Cecil Moore wrote:
I seriously doubt that is true.
We are genuinely grateful that you don't list all the combinations and
permutations of those topics of invention you "seriously doubt" being
true...
.... or false.

On Wed, 12 Oct 2005 04:06:07 GMT, Cecil Moore wrote:

Owen Duffy wrote:
The Bird does not measure or report the conditions on the coax, it
measures and reports the conditions in the immediate region of the
sampling element which is some 40mm inside the Thruline coupler
section.

I don't know what this argument is all about. Consider the
following:

XMTR---75 ohm coax---Bird---75 ohm load

Are you saying the Bird's placement will result in a reflection
coefficient of 0.2? I seriously doubt that is true.

I don't have the equipment at hand to do that experiment, but I have
done another experiment.


XMTR -- 2m of RG58 -- Bird43 -- 1.2m of RG213 Bird43 -- 20m RG6 (75
ohms) -- antenna.

1.2m of 50 ohm coax between the Birds is 4.2% of an electrical
(wavelength.)

I have made measurements with only one 100W slug which is moved from
instrument to instrument.

The tx was adjusted to 100W forward on the first instrument.

Both instruments read 100W forward.

Both instruments read 2W reflected.

When I swap the instruments around, I get the same results. It is only
a simple test, but I am not convinced that measurements from one
position are signficantly different to the other position, despite the
transmission line "environment" being different.

I am not surprised that both instruments read similarly, despite the
fact that one doesn't have any 50 ohm coax on the load side of itself,
whereas the other one does.

Owen
--

Cecil Moore wrote:


If there is 75 ohm coax on the input of the Bird, the reflected
power reported by the Bird on the coax will be off by an infinite
percent. That's pretty inaccurate.

Why wouldn't the meter correctly indicate the reflection resulting from
the mismatch between the 50 ohm wattmeter and the 75 ohm transmission line?

ac6xg

Jim Kelley wrote:

Cecil Moore wrote:
If there is 75 ohm coax on the input of the Bird, the reflected
power reported by the Bird on the coax will be off by an infinite
percent. That's pretty inaccurate.

Why wouldn't the meter correctly indicate the reflection resulting from
the mismatch between the 50 ohm wattmeter and the 75 ohm transmission line?

The question implies that the mismatch would cause appreciable
reflections. I have not witnessed that happening at HF but
perhaps others have. On MFJ meters, for instance, the one inch
wire through the ferrite toroid probably wouldn't cause an
appreciable mismatch.
--
73, Cecil http://www.qsl.net/w5dxp