"Cecil Moore" wrote in message
. com...
Owen Duffy wrote:
I fully expected someone to object, not only to object, but to do so
without any original experimental evidence ...

Owen, there was absolutely no reason for anyone else to do any
additional experiments. Your own experiment proved that the Bird
wattmeter was ignoring the 4.1667 watts of reflected energy flowing
through it when installed in the 75 ohm environment. You proved
exactly the opposite of what you were trying to prove.

ah, but the sensor isn't in a 75 ohm environment, it is in a 50 ohm
environment inside the meter and sees a 50 ohm load.

I did not know that red wine caused gout! :-)

It's port wine and pheasant which causes gout.

Richard Clark wrote:
On Fri, 14 Oct 2005 00:51:02 GMT, Cecil Moore wrote:


Sorry, I am just quoting Owen's results.

There is nothing in his entire scope of postings that presents:

There was 4.1667 watts of reflected energy flowing back through the Bird.

Which is absurd.

The SWR on the 75 ohm line is 1.5:1. If 100 watts is delivered to
the load, there's 4.1667 watts of energy reflected from the load
and flowing back through the Bird. The Bird doesn't see it because
the Bird is calibrated for the wrong Z0.

Owen has demonstrated quite clearly that your assertion

It takes a certain length of feedline to establish a Z0 environment

is blarney from one end of your feedline to the other.

Really? They why bother with characteristic impedance at all?
If Z0 doesn't establish a Z0 environment, then all transmission
lines are just alike and transmission theory is hogwash.
--
73, Cecil http://www.qsl.net/w5dxp

Reg Edwards wrote:
But it appears to me that ALL SWR meters work on the principle of a
common, identical, take off point for both voltage and current
samples.

There are a number of different designs. One very popular class of
designs samples the voltage and current essentially at a point. If
the device is in a 50 ohm environment and the two samples are equal
in magnitude and phase, there is no reflected power. If the device
is in a non-50 ohm environment and the two samples are equal in
magnitude and phase, reflected power exists at the sample point even
though the device reports none.

Therefore, anything inside the meter which could constitute a
transmission line, no matter how long or short, if it separates the
two effective take-off points, it is a small source of error and its
actual length otherwise plays no part in the measuring process.

( I mean its actual length is not included in the basic design
calculations, except perhaps in rare instances to minimise the error.)

( The error arises from a phase-shift error in tapping off the current
sample. There may be no problem with defining the location of the
voltage sample. All depends on meter construction and the tapping
methods which vary but do not affect the basic design principle of a
common tapping point.)

I am not trolling. Please read very carefully. Try to limit yourself
to HF and VHF. Do you agree? Yes or No?

All of the above is why this class of wattmeters, including the Bird,
does not establish a 50 ohm environment merely by being inserted in
a non-50 ohm system. The Bird samples voltage and current essentially
at a point, assumes it is in a 50 ohm environment, and reports the
power reading. If it is not in a 50 ohm environment, the power reading
that it reports will be wrong as Owen's experiment proved.
--
73, Cecil http://www.qsl.net/w5dxp

Dave wrote:

"Cecil Moore" wrote:
I don't think you understand what I was saying. It takes a
certain length of feedline to establish a Z0 environment
for the forward and reverse traveling TEM waves, i.e. to
force Vfor/Ifor = Vref/Iref = Z0.

and what the experiment pointed out was that the length of feedline attached
is immaterial, the 50ohm environment is established within the meter itself
and it doesn't care what the length of feedline attached is.

The experiment proved that the 50 ohm environment is NOT established
because the Bird readings were wrong. The Bird reported 100 watts
forward power where the actual forward power was 104.1667 watts. The
Bird reported zero reflected power where the reflected power was
4.1667 watts. Thus the experiment proves my point. The Bird does not
yield accurate results when embedded in an environment other than
50 ohms.

All the Bird does is sample the voltage and current essentially at
a point. If the ratio of net voltage to net current happens to be
50 ohms, the Bird will report a matched condition, no matter what
the Z0 of the transmission line and no matter how much reflected
power actually exists at the measurement point. For instance:

XMTR--balun--1/2WL 450 ohm line--x--1/2WL 450 ohm line--50 ohm load

The SWR on the line is 9:1. The reflected power is 64% of the forward
power. If forward power equals 100 watts, reflected power will be
64 watts at point 'x'. Yet a Bird wattmeter inserted at point 'x'
will read zero reflected power. The environment is clearly 450 ohms.
The reflected energy doesn't tuck tail and run away when the Bird
is inserted. The Bird is simply giving the wrong reading for reflected
power because all it is looking at is voltage and current at a point.
This is such elementary transmission line stuff that I cannot believe
there is any argument about it.

The insertion of a Bird wattmeter in the middle of a non-50 ohm
environment does NOT eliminate the reflected energy that exists
before the insertion.
--
73, Cecil http://www.qsl.net/w5dxp

Dave wrote:

"Cecil Moore" wrote:
Yes, that's exactly what I said in the other posting. But some
people seem to believe that inserting a Bird into a transmission
line with a Z0 other than 50 ohms magically changes it to a 50
ohm environment. The 40mm of transmission line inside the Bird
is supposed to accomplish that miracle.

yep, that is true, and that is what the experiment shows. the 50 ohm load,
even though it is caused by a 75 ohm line, is far enough away from the
sensor that it sees it as 50 ohms. so the 'miracle' length is obviously
less than 40mm.

So a one inch piece of 50 ohm coax forces the ratio of net voltage
to net current to be 50 ohms? Of course it does. You had me going
there for awhile. I didn't realize until now that you were pulling
my leg all the while.
--
73, Cecil http://www.qsl.net/w5dxp

Dave wrote:
ah, but the sensor isn't in a 75 ohm environment, it is in a 50 ohm
environment inside the meter and sees a 50 ohm load.

Again, 40mm is not enough to establish a 50 ohm environment
when embedded within a 75 ohm environment. The Bird is simply
sampling voltage and current without knowing the reference.
If the Bird were recalibrated for 75 ohms, it would read the
correct values in a 75 ohm environment.
--
73, Cecil http://www.qsl.net/w5dxp

"Cecil Moore" wrote in message
m...
Reg Edwards wrote:
But it appears to me that ALL SWR meters work on the principle of a
common, identical, take off point for both voltage and current
samples.

There are a number of different designs. One very popular class of
designs samples the voltage and current essentially at a point. If
the device is in a 50 ohm environment and the two samples are equal
in magnitude and phase, there is no reflected power. If the device
is in a non-50 ohm environment and the two samples are equal in
magnitude and phase, reflected power exists at the sample point even
though the device reports none.

Therefore, anything inside the meter which could constitute a
transmission line, no matter how long or short, if it separates the
two effective take-off points, it is a small source of error and its
actual length otherwise plays no part in the measuring process.

( I mean its actual length is not included in the basic design
calculations, except perhaps in rare instances to minimise the error.)

( The error arises from a phase-shift error in tapping off the current
sample. There may be no problem with defining the location of the
voltage sample. All depends on meter construction and the tapping
methods which vary but do not affect the basic design principle of a
common tapping point.)

I am not trolling. Please read very carefully. Try to limit yourself
to HF and VHF. Do you agree? Yes or No?

All of the above is why this class of wattmeters, including the Bird,
does not establish a 50 ohm environment merely by being inserted in
a non-50 ohm system. The Bird samples voltage and current essentially
at a point, assumes it is in a 50 ohm environment, and reports the
power reading. If it is not in a 50 ohm environment, the power reading
that it reports will be wrong as Owen's experiment proved.

ah, but if the bird does establish a 50 ohm 'environment' within its 40mm
housing then the reading that was actually obtained is indeed correct. so
now, how else could you read zero reflected power at that point if it didn't
establish that environment? what other evidence to you have that 40mm is
too short? if 40mm is too short then why do we worry at all about impedance
bumps in connectors that are 20mm or so long??? if 20mm is enough to see a
bump then that is obviously affecting the 'environment' on the line.

"Cecil Moore" wrote in message
m...
Dave wrote:

"Cecil Moore" wrote:
Yes, that's exactly what I said in the other posting. But some
people seem to believe that inserting a Bird into a transmission
line with a Z0 other than 50 ohms magically changes it to a 50
ohm environment. The 40mm of transmission line inside the Bird
is supposed to accomplish that miracle.

yep, that is true, and that is what the experiment shows. the 50 ohm
load, even though it is caused by a 75 ohm line, is far enough away from
the sensor that it sees it as 50 ohms. so the 'miracle' length is
obviously less than 40mm.

So a one inch piece of 50 ohm coax forces the ratio of net voltage
to net current to be 50 ohms? Of course it does. You had me going
there for awhile. I didn't realize until now that you were pulling
my leg all the while.
--

yep, sounds good to me. now, someone please put a bird meter on a tdr and
see what the characteristic impedance of it is internally. then put it on a
network analyzer and see if it does indeed transform the 'environment' from
75 to 50 ohms in its 40mm length. oh and while you are at it calculate the
s factors for all the transitions, those will come up in the discussion here
shortly.

Dave wrote:
ah, but if the bird does establish a 50 ohm 'environment' within its 40mm
housing then the reading that was actually obtained is indeed correct. so
now, how else could you read zero reflected power at that point if it didn't
establish that environment?

Would you like for me to go through the math of a voltage and
current sampled at a point without disturbing anything?

Assume the voltage is sampled such that 100v of net voltage
is sampled as 10 volts.

Assume the current is sampled such that two amps of net current
is sampled as 10 volts.

If these samples are in phase, adding them will result in 20v.
That equals 200 watts of forward power on the meter.

Subtracting them will result in zero volts. That equals zero
watts of reflected power on the meter.

The sampled net voltage and net current determine the 50 ohm
calibration. No 50 ohm environment required.

If the sample point is in 75 ohm coax with no reflections, the
sample voltage will be 12.3 volts and the sample current will
8.165 volts. Subtracting those sample values doesn't yield zero
so 41.35 watts of reflected power will be reported where none
actually exists.

So unless the mere writing of my words on this newsgroup
establishes a 50 ohm environment, the Bird doesn't need to
establish a 50 ohm environment to obtain the same above
results.

If the Bird indeed did control the environment into which
it was inserted, it would be violating a design goal for
measuring instruments and Bird wouldn't be selling many
of those highly intrusive devices. Bird wants the meter to
be as unobtrusive as possible and it is.
--
73, Cecil, http://www.qsl.net/w5dxp