Jim Kelley wrote:


Cecil Moore wrote:

Here's the example sans the Bird. Between the tuner output
and the load, where exactly is the actual SWR = 1:1 and where
exactly is the actual SWR = 2.25:1? Answer: nowhere!

XMTR--tuner---1 WL 75 ohm coax---50 ohm load

And the answer would be different still in this circuit.

XMTR---1/4 WL 75 ohm coax---50 ohm load

No, it wouldn't. The answer is exactly the same. *NOWHERE* is
the SWR 1:1 or 2.25:1. In both examples, the SWR on the coax
is 1.5:1. The SWR is *always* set by the relationship of Z0
to the load. In both examples above, that relationship is
*identical*.

Amazin', what happens when you change the circuit!

Z0 didn't change. The load didn't change. Therefore, the
SWR didn't change. What exactly do you think changed?
--
73, Cecil http://www.qsl.net/w5dxp

"Cecil Moore" wrote in message
. ..
Jim Kelley wrote:


Cecil Moore wrote:

Here's the example sans the Bird. Between the tuner output
and the load, where exactly is the actual SWR = 1:1 and where
exactly is the actual SWR = 2.25:1? Answer: nowhere!

XMTR--tuner---1 WL 75 ohm coax---50 ohm load

And the answer would be different still in this circuit.

XMTR---1/4 WL 75 ohm coax---50 ohm load

No, it wouldn't. The answer is exactly the same. *NOWHERE* is
the SWR 1:1 or 2.25:1. In both examples, the SWR on the coax
is 1.5:1. The SWR is *always* set by the relationship of Z0
to the load. In both examples above, that relationship is
*identical*.

Amazin', what happens when you change the circuit!

Z0 didn't change. The load didn't change. Therefore, the
SWR didn't change. What exactly do you think changed?
--
73, Cecil http://www.qsl.net/w5dxp

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.

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.

Dave wrote:
i'm not agreeing with how you assume the 50 ohm impedance of the meter out
of the circuit.

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 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.

I never said it "should" measure that reflected power. What I
said is if it doesn't measure the reflected power that exists,
it is not giving a valid measurement of reflected power. A properly
calibrated meter will indeed measure the reflected power that exists.
--
73, Cecil http://www.qsl.net/w5dxp

"Cecil Moore" wrote in message
om...
Dave wrote:
i'm not agreeing with how you assume the 50 ohm impedance of the meter
out of the circuit.

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.

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

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? I also think that if you
change the circuit, it's possible to measure the effect of that change.
The meter measures what takes place at its insertion point in the
circuit. What you seem to be upset about is that it might not in every
case accurately display the conditions at some arbitrary position away
from its insertion point. Like within a shorted quarterwave stub for
example. Are you still unwilling to accept that the meter itself can
present a perturbation?

ac6xg

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

Cecil Moore wrote:

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).

Well, it's certainly true that both circuits are missing the 50 ohm
impedance discontinuity in the middle, which is at least one of the
"present topics" (and my point).

I'll just go ahead and say what you really want me to say:

You're good enough, you're smart enough, and doggone it, people like you!

:-)

ac6xg