I have some thoughts concerning loading of a vertical.

Often it is impractical to use centre loading.
Assume I have an existing 20 m high vertical.
Take a wooden pole, 10 m high. Put a loading coil at top of it.
Place the wooden pole 1-2 m from the vertical.
Connect the base of the vertical to one end of the loading coil.
The other end of the coil goes to the feed point via a vertically oriented 
downlead wire/tubing.

This technique is well known as linear loading disregarding the loading coil.
Would it be worth a try or is it something I have missed?

73
Christer
SM6PXJ