Hello Roger,

I hadn't thought of the loading coil that way (never too old to learn ...).
It sounds interesting to me as my loading coil consists of 3 parts :
- a 2.5mH coil at the top of the vertical part
- a big (7mH) coil that can be tapped, at the moment I am using only 0.5mH
of if (so a lot of 'useless copper' arround
- a smaller coil that can be adjusted between 0.8 and 1.3mH by sliding a
ferrite rod (now without the rod in in, so 0.8mH)
In an attempt to be smart I placed the small coil horizontal (to avoid that
the ferrite rod would 'slip away' by gravity) and the big coil vertical (to
minize coupling between the 2 coils). Both coils are on a plastic box abt.
0.4m from the ground and abt. 0.3m separated.
I wonder if I would gain anything if I placed both coils vertical ?
Also it is attempting to try the plate under the coils, for a first try I
could cover the plastic box with alu-foil (household) or is this a too thin
metal layer ?
If I find some time over the weekend I will try to do some tests.

73, Rik  ON7YD

The loading coil is not just a matching device, it is also a primary source
of vector potential and it is the vector potential, rather than its
offspring, the magnetic and electric fields, which is the fundamental
source of electromagnetic radiation.  It is therefore advantageous to place
the coil upright at the base of the antenna.  Furthermore, as a large
component of the vector potential is locally concentrated in this region,
it may be possible to gain a few dBs by placing an aluminium or copper
plate, somewhat larger than the diameter of the solenoid, a few centimetres
under the solenoid, insulated from the earth but it may be connected
directly to the TX ground terminal.