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

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
--

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

Yes, the thread subject was the main point. Z0's other than 50
ohms do not force the V/I ratio to 50 ohms (for matched lines)
and 2% of a 10 MHz WL of RG-213 is of a *sufficient* length to
force the V/I ratio to 50 ohms.

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.

I've already apologized for my memory being faulty on that one.
If what you want are additional apologies, you've got it. I
hereby triple dog apologize for my faulty memory. The above
statement was incomplete and was therefore wrong. What more
do you want me to say about it? It was wrong, so of course,
it "is not born out in practice".

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

What I seriously doubted being true is that a 50 ohm Bird wattmeter
has a 0.2 reflection coefficient when placed in a 75 ohm feedline.
I just don't think that 40mm (1.5") is enough length to establish a
50 ohm environment inside the Bird. But I might be wrong on that one.
I know my Autek WM-1 doesn't cause appreciable reflections when
placed in a 75 ohm feedline.

I think, from the beginning, you have confused what Reg said with
what I said. I am basically on your side in the argument.

***Additional Context***
The question of a year ago was not what is the minimum length
required to force V/I to Z0 (although that can be had from Kevin's
posting quoted from s.p.e.) I was arguing with Reg over whether
the lengths of coax into and out of *my* SWR meter forces V/I to
be 50 ohms. I use two foot lengths of RG-400. (That's ~2% of a
wavelength at 10 MHz.) Two feet is a *sufficient* length to cause
V/I to equal Z0 (in a matched line). Two feet is not the *absolute
minimum* length but is instead a *sufficient* length and the length
I use in my shack.
--
73, Cecil http://www.qsl.net/w5dxp