The fact is nobody knows the dielectric loss of polyethylene. It is
too small to measure samples in a bridge. The materials of which the
bridge is made have losses of the same order.

The slightest unavoidable impurities and contamination during
production cause wide variations in D.

Coaxial line Attenuation = A*Sqrt(F) + B*F

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.

I have performed this operation several times during acceptance tests
on newly laid cables. The cable insulation was mainly air-spaced with
the inner conductor being supported by polyethylene disks at 1.5"
intervals.

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

I have also measured attenuation on many miles of 1" diameter solid
polyethylene submarine cable and determined quality of the insulation
by this graphical means. It is necessary to make attenuation
measurements very accurately by the substitution method against
standard attenuators.

But for comparison, I have never measured the relative junk used by
radio amateurs.
----
Reg.