ZL2CA wrote:

The practical disadvantages of an elevated loading coil are mechanical
support needed, and it surviving high voltages on transmit, during wet
or humid weather.  I am not a supporter of using ferrite cores in

loading coils, as it could become nonlinear under transmit current. High Q air cored coils for LF loading are rather large, and heavy, hence

the practicality of using them at the bottom of the up wire ...

Agreed. The arrangement I use is about 80 turns of enamelled wire on a 3
litre cola bottle. This is quite light. It could easily go at the top of
a 'T', though its effective 'weight' would be increased whenever the
wind blew. I mount mine on top of the pole supporting the vertical part
of an 'L' (see my web site for pics and diagram).

The construction of the coil takes some ingenuity. First you need to
make it solid as in its natural state the bottle is too floppy to wind
the wire on. This is achieved by removing the lid and putting the empty
bottle in a freezer for an hour or so. The take it out and screw the lid
on. The air inside expands within a few minutes and the bottle becomes
very hard (thanks to Rik for this idea). Then, with the aid of some
plastic tape, wind as many turns as you can get on the coil. Completely
cover it with plastic tape when finished - this lasts about a year in
the sun and rain and shows no sign of insulation breakdown (it also
looks much less silly than having a cola bottle on top of the mast!).
The taped coil is now solid enough to let the air out of the bottle.

I have two other refinements: To reduce the wind resistance, I taped the
'top half' of a second bottle to the blunt end of the coil former. This
not only makes it more streamlined but makes it look less like a bottle!
And I mounted it on top of my mast by running a plastic pipe through the
bottle (through each of the bottle 'tops'). If this sounds confusing,
look at the web pics.

Certainly, it can be argued that this type of coil is not optimum as it
inevitably involves compromises, but it does work. I would like to try
increasing the inductance with ferrite which would probably give a good
inductance to weight ratio, but getting the right ferrite is not easy.

--
Mike, G3XDV
IO91VT
http://www.dennison.demon.co.uk/activity.htm