Richard Clark wrote:
Cecil Moore wrote:
The Bird is supposed to measure power. The Bird's forward
power readings are in error

That has not been demonstrated by Owen's example.

Richard, if you don't understand why a 50 ohm power meter yields
erroneous readings when installed in a 75 ohm environment, I
don't know what else to tell you. What is it about using a
hammer on a screw that you don't understand?
--
73, Cecil http://www.qsl.net/w5dxp

Dave wrote:
then why are you complaining about it not showing the swr on the 75 ohm
coax?? you should know that no one in their right mind would expect it to
do that.

You at first essentially implied that a Bird wattmeter calibrated for
50 ohms yields valid readings in a 75 ohm environment. I was questioning
whether you were in your right mind or not. Others have been strangely
silent on the subject. Still others seem not to have gotten it yet.
--
73, Cecil http://www.qsl.net/w5dxp

Jim Kelley wrote:

Cecil Moore wrote:
Z0 didn't change. The load didn't change. Therefore, the
SWR didn't change. What exactly do you think changed?

I think the circuit changed. Don't you?

The circuit changed without changing the forward power,
reflected power, and SWR so nothing of interest to the
present topic (V/I ratio) changed.

Do you know what dictates the SWR in a distributed network?
Certainly not the length of the feedline or the removal
of a tuner (assuming lossless conditions).
--
73, Cecil http://www.qsl.net/w5dxp

Dave wrote:
obviously the problem here is that cecil thinks he is the only one that
knows better than to try to measure reflected power with a 50 ohm meter in a
75 ohm coax.

From your posturing, it wasn't readily apparent to me that
you were agreeing with me.
--
73, Cecil http://www.qsl.net/w5dxp

"Cecil Moore" wrote in message
. ..
Dave wrote:
obviously the problem here is that cecil thinks he is the only one that
knows better than to try to measure reflected power with a 50 ohm meter
in a 75 ohm coax.

From your posturing, it wasn't readily apparent to me that
you were agreeing with me.
--

i'm not agreeing with how you assume the 50 ohm impedance of the meter out
of the circuit. and i am not agreeing that there should be reflected power
measured by the meter in the case of the 50 ohm load on the end of a 1/2
wave 75 ohm line. obviously it will not measure any reflected power which
is perfectly correct for the whole circuit as defined and as actually
measured.

On Thu, 13 Oct 2005 19:17:23 GMT, Cecil Moore wrote:
you don't understand why a 50 ohm power meter yields erroneous readings
Any error of misunderstanding Owen's post is entirely your own.
My career in calibrating RF Wattmeters while you were flipping bits is
a good indicator of the tectonic trench you stand in.

The fact remains, there has been no error displayed (beyond the casual
5% reading error inherent in the meter) nor his results refuted as to
how much power has impinged upon the cabled load as he explicitly
described being attached to the Bird's measurement port. As there is
no other use for such a wattmeter, any appeals to the contrary are
idle chatter.

Owen's claim stands: the myth of requiring a 50 Ohm transmission line
at the measurement port of the Bird wattmeter has been debunked. This
comes as no surprise but for one poster to this board.

On Thu, 13 Oct 2005 16:45:51 +0000 (UTC), "Reg Edwards"
wrote:


Before responding I have found it necessary to have several glasses of
Australian, Banrock Station, 2004, Shiraz Mataro, Red. It can be
recommended with confidence.

Entirely OT:

Reg, you are way to impatient. There are very few Australian Shiraz
that should be taken in less than three years.

I can't recall the "instructions" on the back of the bottle that tell
one what to drink it with and when to drink it, but Banrock Station
produce "everday drinking" quality reds that should stand a few years
cellaring, but most wines in this part of the market will state "enjoy
now" for marketing purposes.

IIRC, the labels carry a bit of a story on their wildlife refuge, do
they get to tell you what to eat with it?

Anyway, now that it is open, enjoy it.

Owen
--

On Thu, 13 Oct 2005 09:53:08 -0700, Richard Clark
wrote:

Owen,

To respond to your last question:
Has anyone experimental evidence to the contrary?
is consistently NO. Your own time at the bench has already drained
the pool of ability in that regard. Your only expectation ever after
having bellied up to the bench is to watch your work being gummed to
death.


I fully expected someone to object, not only to object, but to do so
without any original experimental evidence and to devalue the
experiment that I did so that some readers who do not have even a
meager understanding of transmission line theory fall to being
convinced by whoever is most tenacious is defending their position.

However, for completeness' sake, and as no one here really understands
what accuracy is about anyway, there is one factor to be considered.
The numbers offered verge on the limit of the Bird's ability to
resolve a power anyway. There is a built in probability of ±5W of
error from the get-go, and any snake oil salesman can craft an
argument leveraging that error to prove anything. We have seen that
±5W error in the form of an argument that uses both + and - (not
simply one or the other) to please a theory.

Owen, the same experiment with a deliberate mismatch of 3:1 would be
just as effective at busting the myth AND providing data that
overwhelmed the inherent meter inaccuracy.

Indeed, and I considered a number of other experiments that did so,
but this one was based on components at hand, and should have been
easily understood by a person with the most basic understanding of
transmission line theory. It was important to surround the Bird with
line different to 50 ohms.

I expect the argument to twist an turn, to focus on everything but the
assertions that:
- there should be approximately a 50+j0 Z presented to the load side
terminals of the Bird Thruline (ie the ratio of V/I is 50+j0 where V
is the net or forward and reflected voltages, and I is the net of
forward and reflected currents);
- the Bird Thruline is a 120mm section of 50 ohm transmission line;
- in the region of the Bird Thruline sampler element, the ratio of V/I
is approximately 50+j0;
and that the observed Bird 43 readings were reasonably consistent with
those assertions.

The arguments that knowing that the Bird measurements are valid at the
point of measurement is of little value are unrelated to the issue and
a diversionary tactic, but wrong nevertheless. I won't add to the
diversion to identify them.

I will extract the essence of the analysis and write a separate web
page on it that may in the longer term assist others in their
development, go being "gummed to death" doesn't totally devalue the
information behind the case, and it might just be the cost of exposing
the proposition to review.

Thank you for your support.

Owen

PS: I am planning my next mythbusters (oh no! I hear...) Myth: SWR
meters measure SWR. Now this is not to bag SWR meters, I think that
they are very useful instruments, but they have limitations, and the
greatest problem is not the meters, but probably the knowledge base of
those (ab)using them.
--

Owen Duffy wrote:

The arguments that knowing that the Bird measurements are valid at the
point of measurement is of little value are unrelated to the issue and
a diversionary tactic, but wrong nevertheless.

Hi Owen,

If it's wrong to argue that Bird wattmeter measurements are valid at the
point of measurement, I don't wanna be right. ;-)

PS: I am planning my next mythbusters (oh no! I hear...)

Undoubtedly it's the collective "oh goody!" that you hear.

Myth: SWR
meters measure SWR. Now this is not to bag SWR meters, I think that
they are very useful instruments, but they have limitations, and the
greatest problem is not the meters, but probably the knowledge base of
those (ab)using them.

You must be new around here. :-)

73, ac6xg

On Thu, 13 Oct 2005 15:02:37 -0700, Jim Kelley
wrote:

You must be new around here. :-)

Hi Jim,

Give Owen more credit than that, after all he knows at least one
prolific poster:
I expect the argument to twist an turn, to focus on everything but the assertions

As long as you keep on the message, then that profligacy twists and
turns the postings like yellowing leaves starved of nourishment. A
faint rustle of reality causes them to fall unnoticed to the way-side.

73's
Richard Clark, KB7QHC