[Lf] More on LF antenna]

[Andre' Kesteloot]

Mike Dennison wrote:

> 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