[Lf] raising the series coil

[Andre Kesteloot]

Mike Dennison wrote:
> 
> Dialogue between G4CNN and ZL2CA:
> 
> > > If this is so then elevating the loading coil, either by putting it high
> > up the vertical element or simply by raising it as high above the ground as
> > possible must contribute to an improved signal. Presumably as usual this will
> > appear as a reduction in the resistance at resonance. This I think will be my
> > next experiment. Alternatively a longer coil should raise the high voltage part
> > of the coil and also reduce losses. Comments?
> >
> > Lower system resistance allows more current to circulate, for a given
> > applied power.  So doing things that lowers system resistance is always "a
> > winner".  The height above ground for the loading coil is mainly a matter of
> > impacting on CURRENT DISTRIBUTION and that is reflected in a moderately higher
> > value of radiation resistance.  The radiation resistance is very difficult to
> > test, as it is of the order of milliohms versus tens of ohms for other losses.
> >
> > Mechanical support of a low loss LF loading coil is challenge, as is
> > weatherproofing (for high voltage).
> 
> I thought we had put this one to bed.
> 
> Practical experiments by several of us, including myself and ON7YD, have
> shown that in some cases raising the loading coil produces a significant
> improvement.
> 
> This is normally when local objects - houses, trees - are causing absorption,
> and when the length of the top section is small compared to the overall length.
> 
> I can go into why this works, but this has been done before. It is sufficient to
> say that under those conditions the case has been shown to be true.
> 
> What I have not yet seen is an experiment with raising the loading coil of an
> antenna way out in the open. My gut feeling is that this will still show an
> improvement.
> 
> It is not worth bothering with if the top section is large. My antenna has 14m
> vertical and three 18m parallel top wires spaced 0.5m apart.
> 
> As for mechanical problem, I have not experienced any, though I have had to
> be ingenious. Certainly, you can't put a huge coil high up, but the gain to be
> had in some cases far outweighs any reduction in coil efficiency. In the case
> of a 'T' antenna supported at the ends, the coil must be made very lightweight.
> This is not a real problem. In the case of an inverted-L there should be no
> problem at all as the coil can be mounted on top of the mast. As for
> weatherproofing, I find black plastic tape quite adequate and long-lasting.
> 
> My elevated coil uses a coke bottle as the former and is suspended
> horizontally between the mast and the antenna by some polyprop 'string'
> running through the inside of the former.
> 
> In short, if you have a small antenna with close-by trees, it is well worth going
> to the trouble of designing an elevated coil. If not, it is probably not worth it.
> 
> Mike, G3XDV (IO91VT)
> http://www.lf.thersgb.net