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The coax need not be any specific length, just long enough to make a "good"
measurement (low enough in frequency). Z = SQRT (L/C) applies to any
length, so long as the L and C are measured over the same length
transmission line. When I made a careful measurement of a roughly 28-MHz
"length" of parallel conductor windowline, I did not get the 450-ohm result
as advertised. I got something around 410 to 420 ohms. A online
discussion ensued where several participants brought out the fact that
there is available "windowline" anywhere from 400 ohms through 470 as
common offerings. I then checked DavisRF where I bought it some 9 years
ago. Sure enough, they do sell the "advertised" 470 ohm window but also
other values of impedance as well. I also "exercised" several online
applets which purport to calculate the impedance of parallel conductors
given the appropriate parameters. Of course, I did not measure the
relative dielectric constant of the spacers. I took "their word" for that
parameter. I could have done the measurement at the measurement frequency,
but didn't want to make yet another fixture.

At Phillips Lab where I worked before moving to N. Colorado with HP, we
were required to maintain 0.5 dB consistency between setups (HPPM
instrumentation). That is a monumental task if you've ever been there!!
No Pasternack connectors were allowed in the setups, just expensive
*machined* HP connectors and adaptors. Won't go into details, but once
things were set up the previous evening, no one was permitted to enter the
setup other than to remove our focus target for the video cameras the next
morning. Even a minor move of the coax cables in the setup (generally 7/8
hardline of corragated hardline) would change things.

Personally, I wouldn't be too concerned with your 51.067-ohm result against
50.000-ohms and your disagreement against the published value of pF/foot.
Did you measure the frequency, L, C, and the length of your coax (to
determine pF/foot) to three significant figures which your impedance
shows? What are your error bars on all your measurements? Would you
subject your conclusions to a rigorous analysis using the tools of the
calculus of variations? I'm too old (and wise) for that at this point.

Given my write-up on measuring L and C, I would augment based on additional
posts, to properly establish the 1/8-wavelength frequency for the L and C
measurements. At least that unquestionably pegs the measurement frequency
with a known technique.

Dave - W?LEV

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*Dave - W?LEV*
*Just Let Darwin Work*