Roy Lewallen wrote in news:131e0pcf2aq4g16
@corp.supernews.com:

Owen Duffy wrote:
. . .
My objective isn't so much trying to build a better loss model, but
rather to use a simple loss model and build the coefficients from
manufacturer's published data and make that available in the
calculator.
. . .

Unfortunately, manufacturer's published loss data are often quite
different than actual cable loss. Belden RG cable I measured long ago
was routinely considerably better than the spec -- apparently the spec
was dictated by the MIL SPEC, and the cable was manufactured to never
exceed it. More recently, I've found that in trying to convince rather
naive amateurs to purchase their cable, some manufacturers are claiming
considerably lower loss than the cable actually has. So the bottom line
is that manufacturer's published data are just so many numbers, and
don't necessarily have any direct relationship to any real cable.

I understand. One of the cable types that I tried to fit to the loss
model was Davis Bury Flex, and it had the worst regression errors of all
of the 90 line types that I modelled. Of course, some manufacturers data
is an extremely good fit, and I suspect that is a result of fitting their
own measurement data to the same model, then publishing points from the
modelled performance.


Careful scrutiny of real cable will also reveal that the characteristic
impedance varies quite a bit, and the velocity factor of foamed
dielectric cable is even more variable.

Agreed... but you have to start somewhere with design, and the
manufacturer's data isn't such a bad place to start. But, I hear your
point that obsessing about model accuracy isn't wise.

Owen