On Wed, 12 Oct 2005 02:03:10 GMT, Cecil Moore wrote:
What length of 50 ohm coax needs to be attached to the Bird
input and output to ensure that a 50 ohm environment is
present?
None.

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
--

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

I don't have any argument with your results. Try this instead.

XMTR -- 2m of RG58 -- Bird43 -- 1.2m of RG6 Bird 43 -- 20m RG6 -- antenna

The second Bird will NOT indicate the actual SWR on the RG6.
--
73, Cecil http://www.qsl.net/w5dxp

On Wed, 12 Oct 2005 05:36:38 GMT, Cecil Moore wrote:

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

I don't have any argument with your results. Try this instead.

XMTR -- 2m of RG58 -- Bird43 -- 1.2m of RG6 Bird 43 -- 20m RG6 -- antenna


I won't waste the time, I can predict that they will most likely be
different. Transmission line theory tells me that the Z in the region
of each Bird will be different, and that will probably result in a
different indicated VSWR.

It is a sidetrack, just noise.

The measurements I reported were identical, although one Bird was
surrounded by 50 ohm line, and the other had 75 ohm line on one side
of it. The 75 ohm line did not cause a measureable difference in meter
readings in that simple trial.

Owen
--

Owen Duffy wrote:
It is a sidetrack, just noise.

The main point of the discussion has been: An SWR meter
calibrated for Z0=50 ohms will not accurately report the
actual SWR on a line that is not Z0=50 ohms.

Of the four following configurations, in a lossless situation,
which one accurately reports the SWR on both sides of the SWR
meter?

1. XMTR---50 ohm coax---SWR meter---50 ohm coax---load

2. XMTR---50 ohm coax---SWR meter---75 ohm coax---load

3. XMTR---75 ohm coax---SWR meter---50 ohm coax---load

4. XMTR---75 ohm coax---SWR meter---75 ohm coax---load
--
73, Cecil http://www.qsl.net/w5dxp

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

On Wed, 12 Oct 2005 11:56:44 GMT, Cecil Moore wrote:

Owen Duffy wrote:
It is a sidetrack, just noise.

The main point of the discussion has been: An SWR meter
calibrated for Z0=50 ohms will not accurately report the
actual SWR on a line that is not Z0=50 ohms.


No Cecil, the thread subject and the quote of your text in the first
message of the post was the main point of discussion:

The transmission line length must only be long enough such that
the V/I ratio is forced to the Z0 value. According to some pretty
smart guys I asked, that's about 2% of a wavelength.

It seems that the statement you have quoted is not born out in
practice, though I note that you "seriously doubt that is true".

I have found out what I needed to know, thanks... Owen
--