Roy said -
I think your analysis isn't valid for two reasons. The first is that
you're relying on published specifications. (For example, I just
measured the Z0 of ten "50 ohm" cables of different brands and types
I
have on hand -- it ranged from 44.6 to 56.8 ohms. I already reported
a
loss measurement that was quite different from the spec.)

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

The greatest source of divergence between manufacturers'
specifications and what you think you've actually got is due to
manufacturing variation in cable dimensions.

Such as :

Inner coaxial wire diameter as it is drawn through diamond dies.
Ovality of wire drawn through worn dies.
Tightness of stranded inner conductors affecting diameter.
Diameter over the extruded polyethylene insulant. Affected by
temperature.
Off-centre eccentricity of inner conductor within the insulant.
Ovality of the polyethylene extrusion.
Diameter of braiding wires.
Tightness of braid over polyethylene.
Longitudinal tension in braid.
Tightness of copper or aluminium tapes over polyethylene.
Tightness of PVC jacket or other protection over braid or tapes.

- and a dozen other dimensional factors which I have long forgotten.

There's also variation in the conductivity of annealed copper wire and
contaminants in polyethylene due to lack of cleanliness in storage.

During manufacture, as the product is drawn through machinery,
electrical characteristics change. They can become cyclic. When
measuring long lengths small reflections can accumulate causing
attenuation versus frequency curves to exhibit a slow ripple about the
average slope of A*Sqrt(F)+B*F dB.

( The most unreliable manufacturer's specification I have seen,
associated with attenuation and power rating, allowed cable to be used
at a temperature of the melting point of polyethylene. Would burn the
skin off your hands. Testing equipment???? )

Important factors are the inevitable errors in all measuring
instruments, especially so-called SWR meters, and the delusions of
accuracy usually suffered by everybody involved. It's so easy to draw
the wrong conclusions!
----
Reg, G4FGQ