The thickness of insulation on an antenna wire does not entirely
reduce noise due to (charged particle) precipitation.

There are two capacitances involved - the self-capacitance of
individual isolated particles and the much greater capacitance of the
insulating material between its surface and the wire.

When it impacts with the thick insulation, the electric charge on a
particle redistributes itself between antenna capacitance and particle
capacitance.

Only a very small fraction of particle volts is transferred to the
antenna via antenna capacitance. The result is a reduction in
receiver noise.

For given particle volts, size of particle, thickness of antenna wire
insulation, permittivity of insulation, the noise induced in the
receiver is calculable.

As Roy would insist on, to prove the point, all you have to do is
measure the average volts on a particle of average diameter.

And the best way of doing that is to work backwards from the S-meter.
;o)

A nice, low noise antenna can be constructed by removing everything
from the inner conductor of a 1" diameter coaxial cable except the
solid polyethylene insulation.

This is an expensive operation, likely in the UK to of use only for 15
minutes every few years. So nobody bothers.
----
Reg.