Owen Duffy wrote:
Owen Duffy wrote in
:


This sets me thinking of a way to calculate a lower frequency limit to
the loss model when I generate it, so that I can store that limit in
the database and prevent calculation below that frequency.


I have just analysed the tllc database contents to find cases where the
modelled error is more than 10% different to the data points on which the
regression was based.

There are a few cases, they are all copper clad steel inner conductors
(some of the RG6, RG59, RG174, RG316). I need to implement a lower
frequency limit for model validity for each cable type.

An alternative approach to retain some lower frequency results is to use a
cubic spline interpolation... but it has its own problems.

Not the least of which is that you (philosophically) want a model that
is based on the underlying physics (which the sqrt(f), f model is)..
The problem comes in because sqrt(f) doesn't model skin effect at low
frequencies very well, when the skin depth becomes an appreciable
fraction of the conductor diameter (because the conductor is no longer a
thin wall tube).. the cladding just throws another wrench into the works.

What might work is if you look at the generic curves for Rac/Rdc for
round and tubular conductors. The analytical formulation is quite
complex, but I'm pretty sure there's a simple polynomial approximation.



Jim