First calculate P = Q / ( R + S )

where

Q = K * Ln( 4 * H / d )

R = K * Ln( 4 * H / D )

S = Ln( D / d )

and

K = permittivity of insulating material.

H = height of wire above ground.

D = Diameter over insulating material.

d = wire diameter.

All dimensions in same units.

Now P is the ratio of wire self-capacitance with insulation, to wire
self-capacitance without insulation. It follows that -

VF = Squaroot( 1 / P ) is the velocity factor along the insulated
wire.

With ordinary antenna wire insulated up to twice the wire diameter
with PVC, the reduction in velocity from the speed of light is
insignificant and pruning can be forgotten about. It is much less than
the pruning sometimes done for end effect which is usually unecessary
anyway.

To detect the effect precision measurements are necessary unless the
insulation has a diameter of several inches or more. For the
permittivity of the insulation to have full effect it would be
necessary to completely fill the infinity of space with the insulating
material.

Terman, Brown and Klaus all forgot to mention this in their bibles.
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Reg, G4FGQ