Cecil Moore wrote:

And if you will remember, the original question didn't involve the
SWR meter at all. The question was: Are my 2-foot sections of RG-400
connected to my SWR meter long enough to ensure that the SWR meter
reading is valid for the coax? Remember that argument?

The threads for the past few days have all diverged from that
original question which was:

How long must the 50 ohm coax connected to the SWR meter be
for the SWR meter to report a valid SWR *on that coax*? The
answer obviously cannot be zero length.

Cecil,

What has become quite clear from this lengthy thread and the experiments
reported is that what you seek is impossible. The design and physical
configuration of the Bird 43, and probably most or all simple SWR
meters, is such that the measurement is completely unaware of the
external "environment". The Bird 43 correctly reports system mismatch,
such as that from a load that differs from 50 ohms, but it ignores any
artificial mismatch from the adjacent coax.

A report of the valid SWR *on that coax* will happen only by
coincidence. The meter does not care about the length. Zero is just as
good an answer as any other length.

And who would really care to know such a thing? The unknown of interest
is generally a load or matching device, not the improper coax.

If you really need to know the "valid SWR" on the mismatched coax you
are going to need a different technique and instrument.

73,
Gene
W4SZ

Gene, W4SZ wrote:
"The unknown of interest is generally a load or matching device, not the
improper coax."

Bird says:
"The Model 43 THRULINE Directional Wattmeter is a portable
insertion-type instrument which accurately measures forward or reflected
power in coaxial transmission lines under any load condition."

I accept the above description as I`ve seen confirmation under many
conditions.

Bird also says:
"Line section:
The line section is a high precision 50 ohm coaxial air line designed
for transmission line insertion between the transmitter and the antenna
or load. A socket is provided in each line section for the plug-in
element with the desired power rating and frequency range."

The width of the instrument including connectors is given as 4.25
inches. This makes the preciaion coax section longer than its width or
spacing. I conclude from this that it is capable of enforcing its 50
ohms.

Bird mentions cable lengths in regard to VHF measurements. They suggest
that insertion and removal of the 43 with random length cables attached
may affect the load match unless it is made exactly a total of
1/2-wavelength. In the 1/2-wave case, insertion and removal have no
effect on the load match. I`ve done this countless times and attest to
its effectiveness.

If there were other possible problems with Bird cable lengths, they
would have appeared during the past 50 years and been reported.

Best regards, Richard Harrison, KB5WZI

Gene Fuller wrote:
What has become quite clear from this lengthy thread and the experiments
reported is that what you seek is impossible.

Not true. The question has been answered and it surprised me. Only
a half inch or so of ordinary 50 ohm coax is required to ensure the
SWR on that half inch of coax is the same as the SWR reported by
the SWR meter.

I didn't ask the original question in a very understandable
manner because I don't know of anyone who understood my original
question.

And who would really care to know such a thing?

History:
Reg and I were arguing about a year ago whether two feet of RG-400
was long enough to ensure that the SWR reported by the SWR meter
was the same as the SWR on the RG-400. I said it was but had no
references that covered the subject so had no proof. There was
no answer to my question forthcoming from this newsgroup at the
time, so I asked the question over on sci.physics.electromag. I
received an answer on that newsgroup but, unfortunately, didn't
question the math. The answer was one foot of RG-213 but the answer
was off by a magnitude due to an accidental shift in a decimal point.
The correct answer was about an inch to reduce the undesired mode
by a factor of 1/e^5, i.e. 1/0.2=5 where 0.2 is the conductor
spacing, e.g. estimated for RG-213.

The error was caused by using 2" instead of 0.2" as the
conductor spacing.

I mistakenly ran with the one foot answer but, wrong as it was,
proved that my two feet of RG-400 was long enough. Now Dave has
set the record straight and caused the decimal point error to
be exposed by Owen and me at about the same time.

The correct answer is surprising to me. 1/2 inch of ordinary
50 ohm coax is enough to ensure a 50 ohm environment in that
1/2 inch piece of coax, i.e. to ensure Vfor/Ifor=Vref/Iref=50.

Nobody else may care, but I learned a lot and thank everyone
who contributed.
--
73, Cecil http://www.qsl.net/w5dxp

On Mon, 17 Oct 2005 16:27:33 GMT, Cecil Moore wrote:
I didn't ask the original question in a very understandable manner
makes sense so far, and consistent too.
because I don't know of anyone who understood my original question.
well -um- it's still consistent

"Cecil Moore" wrote

I mistakenly ran with the one foot answer but, wrong as it was,
proved that my two feet of RG-400 was long enough.

=======================================

Cecil, bearing in mind Lord Kelvin, can you say what the error in the
measured value of the SWR, or the reflection coefficient, would be if
the (sufficient) two feet of RG-400 was reduced to 1" inch.
----
Reg.

Reg Edwards wrote:
"Cecil Moore" wrote
I mistakenly ran with the one foot answer but, wrong as it was,
proved that my two feet of RG-400 was long enough.

Cecil, bearing in mind Lord Kelvin, can you say what the error in the
measured value of the SWR, or the reflection coefficient, would be if
the (sufficient) two feet of RG-400 was reduced to 1" inch.

According to Dave's calculations, 1" of 50 ohm coax will
result in a negligible error in the SWR on that 1" of coax.
--
73, Cecil http://www.qsl.net/w5dxp

On Mon, 17 Oct 2005 18:49:49 GMT, Cecil Moore wrote:
According to Dave's calculations, 1" of 50 ohm coax will
result in a negligible error in the SWR on that 1" of coax.
Negligible for you, is brighter than the sun for everyone else.

No numbers, hmm?

Richard Clark wrote:

Cecil Moore wrote:
According to Dave's calculations, 1" of 50 ohm coax will
result in a negligible error in the SWR on that 1" of coax.

Negligible for you, is brighter than the sun for everyone else.

When reflections are completely canceled, appear to have
disappeared from everyone else's sight, and you say they are
brighter than the sun (for you), who am I to argue? You have
created your own unreality and everyone who understands the
physics of 1/4WL thin-films in this universe laughs at you
every time you post that "brighter than the sun" nonsense.
In your calculations, you superposed powers which is not
a valid thing to do and you apparently haven't even
comprehended the error of superposing powers.

No numbers, hmm?

Dave provided the numbers. Check his posting. Simply stated:
The unwanted mode is attenuated by 1/e with each increment
of length equal to the spacing between the two conductors.
If the conductor spacing is 0.1", the unwanted mode is down
to 0.00000001% in one inch. That's a big surprise to me but
I don't see any reason not to believe it.
--
73, Cecil http://www.qsl.net/w5dxp

On Mon, 17 Oct 2005 19:13:38 GMT, Cecil Moore wrote:
everyone who understands the
physics of 1/4WL thin-films in this universe laughs at you
every time you post that "brighter than the sun" nonsense.
Just how big IS your universe?