On Sat, 2 Jul 2005 11:15:29 +0000 (UTC), "Reg Edwards"
wrote:

....

Noted.

The most accurate way to estimate D at HF is to measure attenuation
versus frequency over a wide frequency range on several miles of solid
polyethylene coaxial line.

Then separate the constants A and B by plotting on graph paper
Attenuation/Sqrt(F) versus Sqrt(F). and then do a few simple
calculations.

Ok, as I stated earlier, I am working from published specs rather than
my own measurements, and I understand there are issues about the
reliability of published specs.

I didn't plot the values on graph paper to find A and B (or k1 and
k2), but used a least squares regression to find A and B (and r**2,
the correlation coefficient as an indicator of the quality of the
fit).

What I was trying to do was to develop an RLGC model for the line from
published figures (attenuation at spot frequencies, nominal Ro, vf),
and I can do that. The issue about the dielectric came up in trying to
reconcile the G / D / k2 figures with the knowledge that the cable had
PE dielectric and should have had D=2e-4 (I stand corrected on my
misreading of the value 2e-5 from my ITT Handbook by Wes - thanks).

The purpose of the model and validation are for my online transmission
line loss calculator.

The notes to the calculator show the modelled loss vs the data points
in one of the graphs at http://www.vk1od.net/tl/tllc.php .

D can vary noticeably from one length of cable to another manufactured
from a different batch of nominally identical materials.


Ok, noted. I have added a few words to the explanatory notes on the
calculator.

Thanks for the help all.

Owen
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