From G3LDO

From this it would appear that vector potential is a mathematical device

to

simplify electromagnetic calculations rather than an entity generated by

the

loading coil itself.

Exactly !   Only the maths of antennas is complicated, an engineer looks at
then in a much simpler way and ends up with the same results !
There are three fields generated by any electromagnetic system with a time
varying component.  Electric, magnetic and radiation.   Vector potential is
just a tool applied with Maxwell's equations to calculate the components of
each of these in the three dimensions.  Textbooks usually show these
equations in great detail, because most students need the maths.   The best
textbooks don't just regurgitate the same equations repeatedly, they
actually manage to explain how antennas can be made to work !

On the subject of coverings for loading coils, I have yet to find any
plastic that absorbs any energy at 137kHz (although I haven't tried any of
the black building materials that just may be loaded with carbon).  So I was
rather surprised the other day when someone mentioned that the RF had melted
their antenna covering.  Could it have been an arc over the surface caused
by wet and dirt.
 It is very straightforward to find if any material intended for an
insulator or for placing near loading coils is itself lossey.   Just put it
in a microwave cooker for a few minutes.  If it gets hot or catches fire it
may absorb something at 137k.   I've yet to find ANY plastic that gets warm.
Plywood is terrible, it got too hot to touch after 20 seconds

Andy  G4JNT